The Cassini-Huygens Visit to Saturn An Historic Mission to the Ringed Planet [Aufl. 2015] 9783319076072, 9783319076089, 3319076078, 3319076086

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The

CASSINI-HUYGENS CASSINI–HUYGENS Visit visit to

SATURN AN HISTORIC MISSION TO THE RINGED PLANET

Michael Meltzer

The Cassini-Huygens Visit to Saturn An Historic Mission to the Ringed Planet

Michael Meltzer

The Cassini-Huygens Visit to Saturn An Historic Mission to the Ringed Planet

Michael Meltzer Oakland, CA, USA

SPRINGER-PRAXIS BOOKS IN SPACE EXPLORATION

ISBN 978-3-319-07607-2 ISBN 978-3-319-07608-9 (eBook) DOI 10.1007/978-3-319-07608-9 Springer Cham Heidelberg New York Dordrecht London Library of Congress Control Number: 2014953013 © Springer International Publishing Switzerland 2015 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Cover design: Jim Wilkie Project copy editor: David M. Harland Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Author’s preface

“The only way to discover the limits of the possible is to venture past them into the impossible.” – NASA Administrator Michael Griffin, 20081 The Cassini-Huygens mission pushed the limits of what was possible, plunging deep into a planetary system so very different from our own. From the distant Saturn system, Cassini-Huygens sent back extraordinary data and superb images that dramatically expanded our understanding of the solar system. Carolyn Porco, the Imaging Science Team leader, asserted that this mission to Saturn, as well as other robotic space journeys, were “part of a bigger human journey: a voyage … to get a sense of our cosmic place, to understand something of our origins and how we, living on Earth, came to be.”2 Cassini-Huygens has made a noble attempt to do just this. Building on the shoulders of the Pioneer, Voyager, and Galileo endeavors, Cassini-Huygens traveled 3.5 billion kilometers (2.2 billion miles) to reach Saturn, carrying 18 sophisticated scientific experiments as well as a probe that it dispatched to the surface of Titan, a Saturnian moon that is larger than the planet Mercury.3 Over 5,000 people from 17 countries (including the U.S.) contributed to the project. Thirty-three U.S. states participated in making Cassini-Huygens a smashing success. Moreover, this flagship mission took an important step towards answering a question that is central to our weltanschauung, our perception of the universe: are we, the human race and all that is alive on our planet, alone in the cosmos? Or are we joined by other forms of life on other worlds?

WHO THIS BOOK IS WRITTEN FOR? This book is targeted for three communities: space historians, engineers who design and operate spacecraft, and planetary scientists. I have taken great pains to make this book valuable to all three of these audiences. The book traces the evolution of the mission, starting with first conceptions of how the outer solar system should be explored. The book also v

vi Author’s preface chronicles the long and rather tortuous route to international approval and eventual funding, and the subsequent conflicts with a new NASA Administrator on how the mission should be designed and with advocate groups in the U.S. and other countries on the use of plutonium fuel for shipboard power. It analyzes the decision-making structure that arose to manage research time aboard the Cassini-Huygens spacecraft, and why the decisions were so much more complex than on similar missions such as Galileo. Several chapters are devoted to the engineering challenges of developing such a complicated spacecraft designed to carry out so broad a spectrum of research tasks. The detail devoted to these chapters is necessary, I believe, in order to communicate the magnitude of the achievement. The engineering chapters also provide a record of how problems were solved and the contributions of many different organizations were integrated into a coherent operation. The final chapters of the book focus primarily on the enormous science return of the mission, a return that has dramatically altered many of our views on the nature of the Saturnian system, and by extension, on the natures of other gas giant systems. These chapters also seek to convey the sheer magnificence and beauty of the system, the interactive character of its mother planet, satellites, rings, and magnetosphere, and its dissimilarity with our own planetary system.

CASSINI AND HUYGENS: RENAISSANCE MEN In studying the lives of Giovanni Domenico Cassini and Christiaan Huygens, I am struck by the range of their interests and the diversity of their contributions to science. The Dutch scientist Christiaan Huygens built some of the best telescopes of the day, and discovered Saturn’s largest moon Titan in 1655. Examining the “strange arms”4 of the planet that had been reported decades earlier by Galileo Galilei (who thought they might be two large moons), Huygens realized they were actually part of a thin flat ring surrounding the entire planet.5 Huygens also devised better ways of grinding and polishing telescope lenses and patented the first pendulum clock, thus greatly increasing the accuracy of time measurement. The Italian astronomer and mathematician Giovanni Domenico Cassini made important observations of Saturn, Jupiter, Venus, and Mars. He discovered Saturn’s icy moons Iapetus, Rhea, Tethys, and Dione. In 1675 he determined that the ring around Saturn consisted of an outer and inner ring separated by a darker band, now known as the Cassini Division.6 Furthermore, he correctly “presumed that Saturn’s rings were composed of myriads of small particles,”7 although it was another two centuries until Scottish physicist James Clerk Maxwell proved this to be the case. Cassini directed the Paris astronomical observatory and founded a dynasty of four astronomers in that city whose work spanned centuries.

EXAMPLES OF WHAT THE MISSION ACHIEVED The impressive range of scientific contributions emerging from the Cassini-Huygens international exploration effort are in some ways reminiscent of its namesakes’ many and varied contributions. It was the last in an era of multi-billion dollar expeditions that extensively

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explored celestial bodies throughout our solar system. The Cassini-Huygens project was also a proving ground for new observational technologies and for a novel spacecraft design approach. Finally, it was “emblematic of international cooperation and shared investment in space exploration,”8 creating a new template for mission organization. In fact, it was the strong international support for Cassini-Huygens that probably kept it alive during adverse times in its development. The Cassini-Huygens spacecraft that took off from the Cape Canaveral complex in Florida on 15 October 1997 was conceived, designed, and built in order to provide “unprecedented information on the origin and evolution of our solar system.”9 The data that its suite of instruments collected have certainly achieved this, and much more as well. In particular, Cassini-Huygens has provided valuable insights into how the chemical building blocks of life are formed. Science returns and engineering achievements The spacecraft had a daunting task – to travel between the planets for nearly seven years, whipping twice by Venus, then by Earth and finally by Jupiter to gain speed from those bodies’ gravitational fields and attain its ultimate objective of the Saturn system. There, Cassini-Huygens initiated its multi-year study of the mother planet and its rings, satellites, dust, and magnetic field.10 The mission had to overcome not only enormous technical challenges to be successful, but also political and societal ones. Successful negotiations with the U.S. Congress, the European Space Agency (ESA), and the Italian Space Agency (ASI) ensured funding of well over $3 billion to make the trip to Saturn a reality. This took an extensive international cooperative effort that included a program to justify the use of plutonium aboard the spacecraft. Cassini-Huygens was one of the largest interplanetary vessels ever launched, only exceeded by the two Phobos craft that the Soviet Union sent to Mars in the late 1980s, neither of which was entirely successful. When Cassini-Huygens was fully fueled, it weighed 6.1 tons (5,574 kilograms) and the mass of its fuel was more than the entire mass of the Galileo and Voyager spacecraft combined.11 Cassini-Huygens’ initial mission was to study the Saturn system for four years. This included examining the planet’s atmosphere and magnetic field, its extensive rings, and its many moons. These observations revealed a planet with “huge columns of thunderstorms … producing lightning bolts 10,000 times stronger than the most powerful on Earth”12 and superfast winds reaching speeds of 1,800 kilometers (1,118 miles) per hour near its equator.13 The mission was a rare opportunity to gain insight into major scientific questions about the creation of the solar system and the conditions that led to life on Earth, in addition to numerous questions specific to the Saturnian system.14 Among the key scientific goals was a thorough characterization of the large moon Titan, thought to resemble a frigid, primordial Earth. Like the early Earth, Titan has a nitrogen-rich atmosphere. Complex organic molecules constitute the haze that obscures its surface from view. These molecules must eventually fall to the surface in the same way that organic molecules fell from Earth’s sky at the time life originated on our planet. For this reason, understanding the chemistry of Titan’s atmosphere and surface may be key to understanding the evolution of early life on Earth.15

viii Author’s preface The study of Titan was in part accomplished by ESA’s Huygens Probe, released from the main Cassini Orbiter to descend by parachute through Titan’s atmosphere, using its instruments to directly sample and observe the atmosphere to determine its composition and characteristics. The results were astonishing. To everyone’s delight, the Probe continued to operate on the surface of the moon. In addition to the Huygens Probe, the mission’s Titan studies were carried out by a radar on the Cassini Orbiter that beamed signals in through the opaque atmosphere. The reflected signals were processed into images of an amazing landscape.16 Imaging radar has been used for many other planetary exploration applications as well, such as for mapping cloud-covered regions of the Earth where other instruments cannot “see” the surface. Radar was also used on NASA’s Magellan spacecraft to produce a global terrain map of cloud-shrouded Venus.17 Saturn’s rings were another principal target for study. Explorations by the two Voyager spacecraft showed that the rings are composed of thousands of individual rings, and made largely of ice particles ranging in size “from sugar grains to small houses.”18 Slight color variations indicated the presence of rocky material as well. The flyby Voyager observations showed a wide range of unexplained phenomena in the rings, including various wave patterns, small and large gaps, clumping of material, and small “moonlets”19 embedded in the rings. These discoveries helped set the research agenda for Cassini-Huygens. One of the most interesting phenomena that has been detected is the strange “housecleaning” action of the A-ring, which soaks up material gushing from the fountains of Saturn’s tiny ice moon Enceladus, 100,000 kilometers (62,000 miles) away.20 Long-term, close-up observations of the rings by the Cassini Orbiter are helping to resolve whether the rings are material left over from Saturn’s primordial creation or remnants of one or more moons shattered either by comet or meteor strikes or by tidal disruption effects.21 Detailed studies of Saturn’s rings provide important data for theories of the origin and evolution of the dust and gas from which our solar system’s planets first formed. Studies of the Saturnian system might also provide insight into the larger disk systems in our universe such as our own spiral galaxy, the Milky Way. The orientation of Saturn’s ring plane relative to the Sun varies during the planet’s orbit. The changing angle of sunlight incident on the rings dramatically alters their visibility. Cassini-Huygens’ arrival at Saturn in 2004 was timed for optimum viewing of the rings, during a period when they were well-illuminated by sunlight. Upon arrival at Saturn, the tilt of the ring plane and resulting illumination angle allowed Cassini-Huygens’ instruments an unsurpassed view of the ring disk. Orbiting Saturn, the spacecraft was able to detect small moonlets inside the rings, determine the composition of the particles, determine the effects of magnetic field interaction with the rings, and conduct intensive observations of the ring dynamics.22 Cassini-Huygens examined many of Saturn’s moons besides Titan, and one of the most interesting turned out to be tiny Enceladus, made almost entirely of water ice with relatively few signs of impacts, indicating a largely young and active surface. Enceladus was one of the mission’s major surprises. A 500 kilometer satellite that space scientists had once thought was just an inert ball of ice and rocks,23 proved to be a tremendously vibrant little sphere with “water spewing out of it.”24 The presence of liquid water is one

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of the requirements for life as we know it, so its discovery on Enceladus caused a stir in the space science community. The Cassini Orbiter then analyzed the possible sources of heat that could melt the interior, searched the moon for geyser-like water-and-ice volcanoes,25 and “tasted” the particles that were being ejected, finding them to contain surprising amounts of organic materials (another requirement for life) that were some 20 times the density expected.26 The moon Iapetus was another rewarding subject of study because of its unique surface: half the moon is covered with a snow-bright substance, while the other half is covered with something as dark as asphalt that is thought to be a complex organic material. Cassini-Huygens helped determine the satellite’s surface composition and discover what the dark material is and whether it came from within the moon or was deposited from another source.27 The mission also found an equatorial mountain range twice the height of Mount Everest on the heavily cratered moon. In addition, it found evidence that Saturn’s moon Phoebe “is an outsider that wandered in from deep space and was captured by the planet’s gravity.”28 The surface of this strange moon is more diverse than any other body in the solar system except Earth. And one more of the many phenomena Cassini-Huygens discovered was a “Trojan moon,”29 a 3 mile wide midget that orbits Saturn in harmony with the larger moon Dione. Such moons are known to exist only in the Saturn system. These and other scientific returns and engineering achievements are examined in depth in subsequent chapters. The benefits of Cassini-Huygens were not limited, however, to the capabilities of the spacecraft and its discoveries. Also of note were the many ways in which the mission impacted an audience much wider than simply the space science community. Technological benefits from the mission Challenging scientific enterprises often result in technological advances which are applicable to other, unrelated fields. Such was the case with Cassini-Huygens, whose development efforts generated a range of benefits for industry, business, and the environment, including: •

• • • •

The computerized resource trading system to resolve the conflicting cost, data rate, and electrical power needs for the spacecraft’s science instruments and other subsystems. This tool has been utilized by California’s South Coast Air Quality Management District (AQMD) in its implementation of market-based regulation of air pollution.30 A solid-state recorder with no moving parts. It has found applications in a variety of fields from aerospace to consumer electronics to the entertainment industry. Integrated circuit advances such as new application-specific integrated circuits (ASIC) that can replace one hundred or more traditional chips and thereby reduce mass.31 A solid-state power switch for eliminating transient current surges and extending parts’ lifetimes and efficiencies.32 Inertial reference unit gyros with greater reliability and less vulnerability to mechanical failure because they contain no moving parts.33

These and other innovations that emerged from the mission’s engineering efforts have found use on other NASA projects, in other U.S. agencies, and in American industry. Technological spinoffs from Cassini-Huygens are discussed in detail later in this book.

x Author’s preface Cassini-Huygens’ partnership with academic research institutions Roughly one-quarter of the budget for developing the instruments for Cassini-Huygens went to leading academic research institutions across the U.S., such as the Applied Physics Laboratory of Johns Hopkins University, University of Chicago, University of Colorado, University of Arizona, and University of Iowa. The advanced remote-sensing instruments Cassini-Huygens carried were of higher resolution and operated at higher data rates than previous instruments used on missions to the outer planets. Researchers and graduate students working on the state-of-the-art devices devoted as much as half of their entire science and technology careers towards the development, operation, and data return of these instruments.34 Maintaining a unique R&D capability Cassini-Huygens was a major element in the U.S.’s highly productive program of exploring the solar system with robotic spacecraft. It employed the unique skills of more than 800 individuals in science, technology, and related positions at NASA’s Jet Propulsion Laboratory (JPL), and distributed additional work among more than 2,200 others in academia, business, and industry in 33 states.35 International cooperation Cassini-Huygens was a cooperative venture of NASA, ESA, and ASI and formed a vital foundation upon which future U.S.-European space science can be based. In fact, fiscal restrictions in both the U.S. and Europe make the merging of planetary exploration programs increasingly attractive. Cassini-Huygens was an opportunity for spacefaring nations to share in both the investment and the science return of the most ambitious and challenging of explorations.36 European contributions The value of the European contributions to the Cassini-Huygens mission totaled $660 million. Europe’s cooperation and its scientific and engineering expertise were of enormous benefit to the mission’s success. International partnerships greatly expanded the scientific depth and breadth of the activities. Foremost in Europe’s input to the mission was development and operation of the Huygens Probe that was provided primarily by ESA, and the Cassini Orbiter high-gain antenna supplied by ASI.37 More than 135 scientists and a similar number of engineers from sixteen European countries supported the hardware, software, and scientific analysis of the twelve Orbiter and six Probe investigations. Continued exploration of the solar system will require similar international cooperation.38 The international nature of the mission turned out to have an additional benefit as well, and a very major one. The cross-border sharing of responsibilities appealed to the U.S. Congress. In fact, Cassini-Huygens “likely would have fallen prey to budget cuts if not for the emphasis on space exploration as a venue for international cooperation.”39 This will be discussed in more detail in Chapter 2, in the sections on 1990s congressional negotiations and on Administrator Dan Goldin.

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THEMES EXPLORED IN THIS BOOK Issues of risk To make Cassini-Huygens a reality, NASA had to confront a difficult conflict in U.S. society: our need to explore new frontiers versus our aversion to risk. NASA astronaut Michael Foale has written of “the public expectation for success”40 in all endeavors, and the shock that ensues when major efforts such as space missions fail. This conflict has grown more, not less, severe during the decades since our space program began. Contrast the outrage that swept the U.S. when several NASA Mars missions of the 1990s failed, versus the attitude of NASA Administrator James Webb decades earlier regarding the Mercury Redstone flights: “We must keep the perspective that each flight is but one of many milestones we must pass. Some will completely succeed in every respect. Some … will fail. From all of them will come mastery of the vast new space environment on which so much of our future depends.”41 Webb’s perspective was not shared by all. For instance, William Coughlin, the editor of the aerospace journal Missiles and Rockets, savagely attacked the failures of JPL on the Ranger lunar exploration program, calling it a disgrace. He referred to the Ranger program itself as a “lunar debacle.”42 This was after the failure of Ranger 6’s television telemetry system to transmit high resolution, close-up photographs of the lunar surface.43 But key members of Congress had views that were far more forgiving. George P. Miller, chairman of the House of Representatives Committee on Science and Astronautics, maintained his faith in NASA and JPL. He considered the accuracy of the Ranger 6 flight path and the craft’s overall performance to have rendered the mission “an accomplishment of the first order,”44 despite the failure of its television system. He understood, it seemed, that some failures were necessary to eventually achieve fully successful lunar exploration missions, and recognized the significance of the fact that most of the spacecraft’s systems had indeed functioned correctly. Representative Joseph Karth, subcommittee chairman of the House Committee on Science and Astronautics, was another Congress person who accepted a certain risk of failure in order to achieve lofty objectives. He firmly supported continuation of the Ranger program, and held that if even two out of the three remaining Ranger missions were successful, the program would have been well worth all the money spent on it. U.S. citizens have perceived different types of risks on different NASA missions. When Alan Shepard entered his Mercury capsule on 5 May 1961 to be launched atop a Redstone rocket, many people feared that he might not return to Earth alive. On CassiniHuygens, however, the perceived risks were of different types. The technical risks were huge: would this enormously expensive mission actually make it to Saturn in working condition and fulfill its many objectives, under the guidance of a control crew located 750 million miles away? Thousands of people in this country also saw a frightening risk to human life and health. Cassini-Huygens carried over 70 pounds of plutonium onboard, and the fear was that, if the spacecraft crashed on Earth, this dangerous material would be spread far and wide. While years of detailed analyses by NASA have demonstrated that the devices carrying the plutonium are extremely rugged and resistant to releases of this substance, even under severe accident conditions,45 many people argued the mission was too dangerous to launch and the government was not telling the public about the true risks involved.46

xii Author’s preface Influence of previous missions A recurring theme throughout the book is the relationship of the final configuration of the Cassini-Huygens mission to the problems and successes of its predecessors. The book seeks to communicate how the Cassini-Huygens mission is both a unique achievement in its own right and the evolutionary product of earlier undertakings, in particular the Galileo, Voyager, and Pioneer expeditions to the outer solar system. Of special interest are the influences that the serious problems encountered by the Galileo mission to Jupiter had on the design of the Cassini-Huygens spacecraft and its operations. Adaptability to unforeseen problems Cassini-Huygens’ success depended on its ability to adapt quickly and effectively to factors such as equipment malfunctions and breakdowns in communication between spacecraft and Earth. The mission team’s philosophy of designing redundancy into a broad range of spacecraft components and systems proved invaluable for keeping the ship running during a long, distant mission and keeping the scientific discovery data flowing to the NASA and ESA teams. This book discusses a collection of different incidents in which redundancy was key in working through serious issues. The book also discusses the adaptability of the Cassini-Huygens mission team itself in responding to unforeseen situations. One example of an ingenious way the team addressed a potentially mission crippling issue, that is examined in detail later in the book, concerned a Doppler shift communications-link problem that threatened to severely impair the data flow from the Huygens Titan Probe. Cross-border challenges The abovementioned communications-link problem was an example of an issue that can arise in a complex mission involving numerous cultures and ways of conducting business, often without the necessary transparency. The root causes of this and other serious incidents, such as the near-loss of vital data during a measurement of Titan’s winds, arose in part because of the enormous difficulty of melding the contributions of multiple countries and organizations into a coherent whole.

DATA SOURCES USED FOR THIS BOOK One of my major research activities was obtaining archival material from the NASA Historical Reference Collection (NHRC) and Jet Propulsion Laboratory (JPL). In particular, these contained invaluable sources of correspondence between mission managers, engineers, NASA administrators, and government officials, including congressional staff. These archives also supplied numerous operations manuals as well as meeting and operations reports. Some archival material was also obtained from Ames Research Center (ARC), Glenn Research Center (GRC), and ESA and ASI sources. GRC had data on launch vehicle design and testing as well as launch operations. NASA’s online Planetary

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Data System (PDS) archive also had useful information that included mission profiles, planning documents, and graphics. Oral historical data was obtained in numerous interviews from NASA and mission managerial staff, especially JPL, ESA, and ASI managers, as well as from mission scientists, engineers, and operations staff. Extensive searches of the existing literature were performed to gather information about all phases of the mission, from initial conception through the science return. Relevant publications included not only technical papers and books, but also those pertaining to the organizational and management challenges of Cassini-Huygens. Documents regarding the use of plutonium aboard the spacecraft were obtained from the U.S. Environmental Protection Agency and Department of Energy. The above sources were supplemented with numerous articles in popular journals, newspapers, and other periodicals.

OVERVIEW OF THE BOOK’S PARTS AND CHAPTERS Part I traces Cassini-Huygens’ evolution from initial concepts of how to explore the outer solar system through the mission’s eventual funding and subsequent attempts to cancel it. Chapter 1 focuses on the political, scientific, and societal motivations for developing Cassini-Huygens, and outlines the project’s history from outer solar system exploration concepts in the 1950s through the initial vision formulated in the 1970s and the development of a mission aimed at exploring Saturn and its rings, satellites, fields, and particles. This chapter accents the multi-country, collaborative nature of the mission, and in particular how the perceived benefits of international cooperation helped keep the mission alive in the face of NASA budget limitations. Europe’s evolving role in collaborations with the U.S. is documented, from being a junior contributor on early projects through to the NASA-ESA-ASI partnership of the Cassini-Huygens mission. Challenges to international cooperation are examined, such as NASA’s difficulty in making long-term, multi-year fiscal commitments, due to its annual dependence on the budget assigned by Congress; and the complicated task of manufacturing critical sections of the spacecraft on different continents and ensuring they all interface correctly. The different funding processes used by NASA and ESA to secure support for their shares of the mission are also analyzed. Chapter 2 discusses how NASA, ESA and ASI formed a powerful international coalition to develop the Cassini-Huygens mission in the face of U.S. congressional budget reductions and repeated threats of mission cancellation. Also examined is the complex relationship that NASA Administrator Dan Goldin had with the mission. This chapter reviews the debate as to the most effective means of exploring the outer solar system. The aim of Part II is to convey what an achievement it was to develop such a complicated, capable spacecraft. Chapter 3 details the design history of the Cassini spacecraft, examining how its design was influenced by Galileo and other mission experiences. The part that ASI played in developing key equipment for the Orbiter is examined, and the capabilities of the spacecraft’s twelve scientific experiments are discussed. Chapter 4 reviews the development of the Huygens Probe for Titan, the largest Saturnian moon, and analyzes its science experiments. The chapter discusses the complex relationships and cooperation between NASA, which would launch the spacecraft and have responsibility

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for the entire Cassini-Huygens mission, and ESA, which oversaw the design and fabrication of the Probe. Chapter 5 analyzes the task of interfacing components produced by NASA, ESA, and ASI. Characteristics of the Titan 4B/Centaur launch vehicle are also studied, as well as Lewis (Glenn) Research Center contributions to the vehicle and to launch operations. Chapter 6 reviews arguments for and against the controversial use of plutoniumpowered radioisotope thermoelectric generators (RTG) for onboard electricity and radioisotope heater units (RHU) for temperature control. Risk analyses performed in order to assess the dangers of using RTGs on Cassini-Huygens are discussed, along with the worstcase scenarios that might have occurred had the spacecraft crashed. The chapter considers the legal challenges and demonstrations organized to stop the use of plutonium fuel, discusses myths and truths about various plutonium isotopes, characterizes the isotopes used on Cassini-Huygens, and analyzes the safety features employed. It also discusses the political challenges of obtaining plutonium 238 for future outer solar system missions, either by buying it from another country, or by restarting manufacturing operations in the U.S. Part III focuses on the flybys, gravity assists, and scientific observations that CassiniHuygens carried out during its voyage from Earth to Saturn. It also discusses the Probe mission to Titan after arrival at Saturn, and the features and dynamics of the Orbiter’s tour of the Saturnian system, including the organizational structure that achieved this success. Chapter 7 examines the characteristics of the spacecraft’s trajectory, including gravity assists from Venus, Earth, and Jupiter, as well as the trajectory correction maneuvers that were needed to reach Saturn and satisfy mission objectives. The joint observations of Jupiter by Cassini-Huygens and Galileo are examined, and Cassini-Huygens’ arrival at the Saturn system, including its one and only flyby of the moon Phoebe, are reviewed. Chapter 8 discusses a crisis involving the radio link between the Probe and the Orbiter that was discovered during the cruise to Saturn, and how it was related to the complexities of effective communication on a large international mission with many players. Chapter 9 provides details of the Huygens Probe’s mission, including operations to transport it to Titan, establishment of the Orbiter-Probe relay radio link, and the penetration of the moon’s atmosphere. The Doppler Wind Experiment problem and its root causes are examined. Finally, the science return from the Probe’s mission is discussed. Chapter 10 details the Prime Mission tour conducted by the Orbiter, the Cassini Equinox Mission, and the Solstice Mission, including descriptions of Saturn orbits, Titan flybys, close and distant flybys of other satellites, and observations of the ring system and magnetosphere. This chapter discusses the process of choosing trajectories and assigning research opportunities. Planning and management issues on the Galileo mission to Jupiter are contrasted with those on Cassini-Huygens. The orbital dynamics for Cassini-Huygens and how it employed Titan gravity assists for fuel conservation and guidance are examined, as well as planetary protection issues for bodies that might harbor life. Finally, the impact of a management change on the Cassini-Huygens team is examined. Part IV reviews the mammoth science return from Cassini-Huygens’ observations of Saturn and its magnetosphere (Chapter 11), its myriad rings (Chapter 12), its icy moons, in particular the active satellite Enceladus (Chapter 13), and its giant veiled moon Titan (Chapter 14). This part of the book documents the leap in understanding of the Saturnian

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system provided by Cassini-Huygens over that from the Pioneer and Voyager flybys and from Earth-based studies. Moreover, this section explains how these new findings have rewritten the textbooks on the meteorological and magnetic processes operating on Saturn, the sculptural and multivariate natures of its myriad rings, and the startling connections between the mother planet and its satellites, rings and magnetosphere.

Michael Meltzer Oakland, CA, USA

REFERENCES 1. Michael Griffin, “Speech by NASA Administrator Michael Griffin: The Magic of Science,” referring to the observations of Arthur C. Clarke, http://www.spaceref.com/news/viewsr. html?pid=29217, 33rd International Conference on Infrared, Millimeter, and Terahertz Waves, California Institute of Technology (Caltech), Pasadena CA (15 Sep. 2008). 2. Carolyn Porco, “Fly Me to the Moons of Saturn,” http://www.ted.com/talks/view/id/178, speech to the TED Conference, Monterey CA, 7 March 2007. 3. NASA-JPL, “Mission Overview: Quick Facts,” http://saturn.jpl.nasa.gov/mission/quickfacts/, accessed 28 Mar. 2010; CNN, “Last of the Mega-Missions,” http://www.cnn.com/SPECIALS/ space/cassini/mission/, Cable News Network, 1997. 4. Charley Kohlhase, “Meeting with a Majestic Giant: The Cassini Mission to Saturn,” Planetary Report (July 1993):5, NHRC 17908 Cassini Probe (81–97 Aug). 5. ESA, “Solving the Puzzles of Saturn and Titan,” http://www.esa.int/SPECIALS/CassiniHuygens/SEM3782VQUD_0.html, accessed 26 Jan. 2009. 6. ESA, “Solving the Puzzles of Saturn and Titan.” Astronomers later determined that Saturn was really encircled by a system of several distinct rings 7. SEDS, “Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712),” https://www.forum. seds.org/messier/xtra/Bios/cassini.html, Messier Catalog, Students for the Exploration and Development of Space (SEDS), 17 Sep. 2006, accessed 23 Apr. 2008. 8. Anne Eisele, “Cassini Mission Marks Last of Big Spenders,” Space News, 6–12 Oct. 1997, p. 4, NHRC 17908 Cassini Probe (81–97 Aug). 9. NASA, “Cassini,” Flight Project Data Book, 1995, p. 121, 14294 Cassini/Saturn/Huygens Probe; Background, Fact Sheets & Brochures (1979–1996). 10. NASA-JPL, “The Cassini Spacecraft and Huygens Probe,” JPL Cassini public information, March 1996, 14294 Cassini/Saturn/Huygens Probe; Background, Fact Sheets & Brochures (1979–1996). 11. NASA-JPL, “Cassini Spacecraft and Huygens Probe.” http://saturn.jpl.nasa.gov/files/space_ probe_fact.pdf (JPL 400–777, May 1999). 12. Space Today Online, “Current Events at Saturn,” http://www.spacetoday.org/SolSys/Saturn/ CassiniSaturnNews.html, Exploring Saturn, spacetoday.org (20 Sep. 2005). 13. Jim Wilson (ed.), “A Gas-Giant with Super-Fast Winds,” http://www.nasa.gov/mission_pages/ cassini/whycassini/planet.html, NASA Cassini-Huygens Mission to Saturn Web site, 26 May 2004. 14. JPL, “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL; ScienceDaily, “Cassini Flyby Of Saturn Moon Offers Insight Into Solar System History,” http://www.sciencedaily.com/

xvi Author’s preface releases/2008/10/081006180819.htm (7 Oct. 2008); NASA-JPL, “Saturn Propellers Reflect Solar System Origins,” http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20100708/ (8 July 2010); C.D. Parkinson et al., “Enceladus: Cassini Observations and Implications for the Search for Life,” Astronomy and Astrophysics 463 (2007):353–357. 15. ESA, “Life on Titan?” http://www.esa.int/esaMI/Cassini-Huygens/SEM696HHZTD_0.html, accessed 12 May 2011; NASA-JPL, “Is Titan Considered a Likely Place to Look for Life?” http:// saturn.jpl.nasa.gov/faq/FAQTitan/, accessed 12 May 2011. 16. Planetary Society, “Cassini RADAR,” http://www.planetary.org/explore/topics/cassini_huygens/instrument_radar.html, accessed 8 July 2009; Tony Freeman, “What is Imaging Radar?” http://southport.jpl.nasa.gov/desc/imagingradarv3.html, NASA/JPL Imaging Radar Program, 26 January 1996; Margaret Cheney, “Radar Imaging,” http://www.ima.umn.edu/imaging/T9.1923.05/activities/Cheney-Margaret/radartutorialpart1.pdf, 18 Sept. 2005. 17. NASA-JPL, “Magellan Mission to Venus,” http://www2.jpl.nasa.gov/magellan/. 18. JPL, “Scientific Benefits.” 19. O. C. Winter et al., “Moonlets Wandering on a Leash-Ring,” http://www.agnld.uni-potsdam. de/~frank/Winter_07.pdf, Mon. Not. R. Astron. Soc. (2007):L1; Jim Schefter, “Return to Saturn,” Popular Science (Jan. 1982):54. 20. Bill Steigerwald (Goddard Space Flight Center), “Saturn Has a ‘Giant Sponge,” JPL news release, 5 Feb. 2008. 21. Robin M. Canup, Origin of Saturn’s Rings and Inner Moons by Mass Removal from a Lost Titan-sized Satellite,” http://www.nature.com/nature/journal/vaop/ncurrent/full/nature09661. html, Nature (online version published 12 Dec. 2010). 22. NASA-JPL, “Cassini-Huygens Mission to Saturn,” http://saturn.jpl.nasa.gov/files/mission_fact. pdf, Cassini Program Outreach, JPL 400–776 5/99; NASA-JPL, “Scientific Benefits of the Cassini Mission,” Nov. 1993. 23. Bob Mitchell, interview by author, JPL, Pasadena CA, 5 Feb. 08. 24. Mark Dahl, interview by author, NASA HQ, Washington D.C., Sept. 07. 25. NASA-JPL, “Scientific Benefits of the Cassini Mission,” Nov. 1993. 26. Carolina Martinez and Dwayne Brown, “Cassini Tastes Organic Material at Saturn’s Geyser Moon,” JPL news release 2008–050, 26 Mar. 2008. 27. Dale P. Cruikshank, “Hydrocarbons on Saturn’s satellites Iapetus and Phoebe,” Icarus 193(2) (Feb. 2008):334–343; Bill Arnett, “Iapetus,” http://nineplanets.org/iapetus.html, updated 13 May 2005; NASA-JPL, “Scientific Benefits of the Cassini Mission,” Nov. 1993. 28. Space Today Online, “Current Events at Saturn,” http://www.spacetoday.org/SolSys/Saturn/ CassiniSaturnNews.html, Exploring Saturn, spacetoday.org (20 Sep. 2005). 29. Ibid. 30. “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL. 31. “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL. 32. “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL. 33. “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL.

Author’s preface

xvii

34. “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL. 35. “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL. 36. “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL. 37. “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL. 38. “Scientific Benefits of the Cassini Mission,” Insert A in “Integrated JPL General Comments Cassini Draft Environmental Impact Statement (10/1 9193 SAIL Draft),” in Richard J. Spehalski memos, November 1993, JPL-Cassini CASTL; Robert T. (Bob) Mitchell email to author, 16 Feb. 2010. 39. CNN, “Last of the Mega-Missions,” http://www.cnn.com/SPECIALS/space/cassini/mission/, Cable News Network, 1997. 40. Michael Foale, “Remarks,” p. 258 in Risk and Exploration: Earth, Sea, and the Stars, NASA SP-2005-4701, 2005. 41. James E. Webb statement on 1 May 1961, “Official NASA Biography,” http://www.astronautix. com/astros/webb.htm, in Mark Wade, Encyclopedia Astronautica, accessed 30 Jan. 08. 42. R. Cargill Hall, “The Worst of Times,” Chapter 16 in Lunar Impact: A History of Project Ranger (Washington D.C.: NASA History Division SP-4210, 1977, updated 27 November 2006). 43. Robert J. Parks and H. M. Schurmeier, “Ranger to the Moon,” Engineering and Science XXVIII(I) (1964):8. 44. Hall, “The Worst of Times.” 45. U.S. Department of Energy, “What is an RTG?” (and other questions), Sept. 1997, 14294 Cassini/Saturn/Huygens Probe; Background, Fact Sheets & Brochures (1979–1996). 46. Kathy Sawyer, “Plutonium-Powered Cassini Revives Spacecraft Debate,” Wash. Post, 12 Oct. 1997, p. A22, NHRC 5132 Cassini (Sept. 1997-Oct. 14, 1997).

Contents

Author’s preface ........................................................................................................ Part I

Creating a new expedition to Saturn

1

Conceiving and funding the mission ............................................................... 1.1 The path to Cassini-Huygens ..................................................................... 1.2 Partnering with Europe .............................................................................. 1.3 Congressional negotiations ........................................................................

2

Building an international partnership and preventing mission cancellation .......................................................................................... 2.1 NASA-ESA-ASI mission planning activities ............................................ 2.2 Nineteen-nineties impacts of congressional budget reductions and the threat of mission cancellation ........................................................ 2.3 Change of Administrator: Goldin replaces Truly .......................................

Part II

3

v

3 4 17 19 27 27 30 36

Designing, fabricating, and integrating the Cassini-Huygens space vessel

Constructing the Cassini Orbiter .................................................................... 3.1 From Galileo to Cassini-Huygens.............................................................. 3.2 Developing a mission to Saturn ................................................................. 3.3 Final design of the Cassini Orbiter ............................................................ 3.4 Spacecraft performance predictions ........................................................... 3.5 Improvements over Galileo ........................................................................ 3.6 The science instrument selection process .................................................. 3.7 Optical remote sensing instruments: Seeing the visible and invisible ....... 3.8 Fields, particles, and waves instruments ....................................................

49 50 51 57 68 70 72 74 81

xix

xx

Contents 3.9 3.10

Microwave remote sensing instruments ................................................... Building flight instruments at JPL: Inappropriate competition with outside organizations?...................................................................... 3.11 Principal Investigator versus facility instruments .................................... 3.12 The spacecraft: Our eyes, hands, legs, and brains at Saturn ....................

93 94 96

4

The Titan Huygens Probe................................................................................. 4.1 Phases of the Huygens Probe mission ..................................................... 4.2 Huygens Probe development ................................................................... 4.3 Selection procedures for the Probe’s science package ............................. 4.4 The Huygens Probe’s Suite of instruments .............................................. 4.5 Probe testing, integration, and release To NASA.....................................

107 107 109 119 120 130

5

Integrating the Cassini Orbiter, Huygens Probe, and Titan/Centaur launch vehicle ................................................................... 5.1 Orbiter integration .................................................................................... 5.2 Probe integration ...................................................................................... 5.3 The assembly and testing process ............................................................ 5.4 Environmental testing .............................................................................. 5.5 Risk issues in shipping the spacecraft to Kennedy Space Center ............ 5.6 Preparing the spacecraft for flight ............................................................ 5.7 The launch vehicle ................................................................................... 5.8 Final launch preparation ..........................................................................

139 139 141 142 142 146 148 149 151

6

Using plutonium to run a spacecraft ............................................................... 6.1 Why NASA uses radioisotope thermoelectric generators (RTG) for shipboard power................................................................................. 6.2 Why didn’t NASA use solar power on Cassini-Huygens?....................... 6.3 Radioisotope heater units (RHU): a second plutonium application......... 6.4 The politics of obtaining plutonium 238 .................................................. 6.5 Dangers and safety features of RTGs....................................................... 6.6 Public opposition to the use of plutonium ............................................... 6.7 Attempts to stop the launch ...................................................................... 6.8 The launch ................................................................................................ 6.9 Twenty-first century RTG issues ..............................................................

Part III 7

91

157 157 158 159 160 162 164 167 169 169

From Earth to Saturn

The interplanetary journey .............................................................................. 7.1 The journey begins ................................................................................... 7.2 The cruise phase ....................................................................................... 7.3 On to Earth ............................................................................................... 7.4 Huygens Probe activities during the cruise .............................................. 7.5 The Jupiter flyby: Partnering with Galileo ............................................... 7.6 Last leg of the cruise ................................................................................ 7.7 Arrival at Saturn .......................................................................................

181 181 182 193 195 197 201 203

Contents xxi 8

9

10

How a few people can make a big difference: The Doppler shift problem that nearly ended the Huygens mission ........................................... 8.1 The Probe-Orbiter transmitter link........................................................... 8.2 The structure of Probe transmissions ....................................................... 8.3 Enquiries into the Doppler problem......................................................... 8.4 Saving the mission ...................................................................................

213 213 216 216 218

The Titan Huygens Probe mission ................................................................... 9.1 From Earth to Titan: The Cruise, separation, and coast phases ............... 9.2 Orbiter activities during the Probe’s coast phase ..................................... 9.3 The Probe’s encounter with Titan: Entry, descent, and surface operations ............................................................................. 9.4 What did the Huygens Probe tell us about Titan? .................................... 9.5 Significance of Huygens Probe data ........................................................

224 226 236

The Saturn tour: Decision-making processes, trajectory design, and changes of management ............................................................................ 10.1 Choosing trajectories and assigning time on the spacecraft .................... 10.2 Using Titan as the “Tour Engine” for changing spacecraft orbits............ 10.3 The Cassini-Huygens Prime Mission tour ............................................... 10.4 The Cassini Equinox Mission .................................................................. 10.5 The Solstice Mission ................................................................................

241 241 246 248 251 254

Part IV

221 222 224

A great natural laboratory

11

The mother planet and its magnetosphere ..................................................... 11.1 Atmospheric structure, temperature, and gas composition ...................... 11.2 Wind and storm characteristics ................................................................ 11.3 Lightning discharges ................................................................................ 11.4 Origin and structure of Saturn’s liquid and solid regions ........................ 11.5 Internal and external rotational characteristics ........................................ 11.6 The magnetosphere .................................................................................. 11.7 A laboratory for understanding stellar explosions ................................... 11.8 Looking beyond Saturn: Studies of the heliosphere ................................

263 264 267 270 272 273 275 279 279

12

The ring system ................................................................................................. 12.1 Modern ring scientists .............................................................................. 12.2 Characteristics of the ring system ............................................................ 12.3 Saturn’s equinox: Viewing rings edge-on ................................................

285 286 288 313

13

The icy moons .................................................................................................... 13.1 The Satellite Orbiter Science Team.......................................................... 13.2 Two-faced Iapetus .................................................................................... 13.3 Tethys: Signs of a tumultuous past .......................................................... 13.4 Enceladus: Water jets and a possible ocean ............................................ 13.5 Hyperion: Spongy and small....................................................................

321 322 322 327 329 337

xxii

Contents 13.6 13.7 13.8 13.9

Mimas: The bull’s-eye moon ................................................................... Rhea ......................................................................................................... Dione ........................................................................................................ Summary of moons discovered to date ....................................................

339 341 342 345

14

Titan observations by the Cassini Orbiter ...................................................... 14.1 The Voyager legacy .................................................................................. 14.2 Studies of Titan’s atmosphere .................................................................. 14.3 Exploring Titan’s surface from orbit ........................................................ 14.4 Titan: A model for the future earth?.........................................................

357 357 358 364 380

15

Conclusions ........................................................................................................ 387 15.1 Themes running through the book ........................................................... 388 15.2 The remaining years of the mission ......................................................... 391

Appendix: Breakdown of mission costs................................................................... About the author ....................................................................................................... Image credits ............................................................................................................. Index ...........................................................................................................................

395 397 399 403

Part I

Creating a new expedition to Saturn Part I traces Cassini-Huygens’ evolution from initial concepts of outer solar system exploration in the 1950s through the development of an international collaboration aimed at exploring Saturn and its ring system, moons, fields, and particles. This part considers the basic question of how best to explore the outer solar system.

1 Conceiving and funding the mission “The Cassini-Huygens mission will probably help answer some of the big questions … about origins and where we came from and where life came from.” – Robert (Bob) Mitchell, Program Manager of the Cassini-Huygens Mission1

The mysteries of Saturn, its rings, its fields and particles, and its moons, have enticed and perplexed scientists for many years. The Cassini-Huygens mission sought to shed light on these mysteries by exploring the entire Saturnian system in greater depth than had ever been attempted before, using the largest and most sophisticated interplanetary vehicle that NASA had ever built or launched. This book examines all aspects of the project: its conception and planning; the political processes, engineering, and development necessary to make it a reality; its 2.2 billion mile (3.5 billion kilometer) journey to the ringed planet; and what it found there. This chapter begins with early visions of how the outer solar system should be explored and examines how they evolved into the Cassini-Huygens mission. What is most interesting to me are the factors that played key roles in creating the mission. The commitment of articulate, influential scientists of vision was required, and it was essential that they represented not only the U.S., but also the European space community. Fear of losing our world leadership in space exploration was a strong motivator in convincing Congress and the White House to undertake this mission. The outspoken support of a national heroine and cultural icon was also valuable, as were the political advantages of carrying out a major flagship effort in close partnership with Europe. And underlying all these factors was simply our basic curiosity about what goes on out there, in the distant part of our solar system where gas giants dwell.

© Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_1

3

4 Conceiving and funding the mission 1.1

THE PATH TO CASSINI-HUYGENS

“Only when we have flown missions to every part of the solar system will we have the vital statistics of all its components and be able to turn back the pages of the book of cosmology to the origins of our own world, and perhaps of the universe itself.” – Arthur C. Clarke2 1.1.1

Early work on outer planet missions

Soon after NASA was formed, its scientists began to envision what outer solar system explorations, still many years in the future, might look like. In 1959, JPL scientist Ray Newburn, Jr. developed several mission concepts for investigating the portion of our solar system beyond Mars. He foresaw deep space flights that would pass only through interplanetary space, making observations and measurements as they went. Flybys, on the other hand, would have brief encounters with planets. Although the data collected would be restricted by limits on time, flyby missions could study several different planets during one trip. Flyby data might be compared to observations taken through the windows of a tourist bus: brief, but varied. Orbiter missions would involve long-term trajectories around target planets and permit in-depth data collection. Finally, planetary entry and lander missions would penetrate planetary atmospheres, enabling observations that could not be obtained by missions flying above the atmosphere.3 In 1965, Caltech graduate student Gary Flandro made a calculation that proved important for the feasibility of missions to Saturn and other outer planets. Flandro used the work of Michael Minovitch, a graduate student from University of California Los Angeles, who had been investigating a spacecraft mission strategy called “gravity-assist,” in which a planet’s gravitational field is used to modify a craft’s trajectory. Minovitch’s and Flandro’s work demonstrated that one spacecraft could use gravity assists to visit Jupiter, Saturn, Uranus and Neptune on one mission, if the planets were aligned just so. Gravity assists allow far more to be accomplished in a mission with a given amount of fuel and launch energy, because such maneuvers can greatly augment a spacecraft’s velocity and kinetic energy and increase the amount of mass that can be flown.4 Throughout NASA’s years of space travel, gravity assists have significantly reduced the fuel and vehicle weight requirements needed to reach the outer planets. Both the Pioneer and Voyager programs depended on these maneuvers to accomplish all that they did.5 Serious planning for specific outer solar system missions began only a little more than a decade after Newburn’s conceptualizations. By the early 1970s, with Mars and Venus already being explored by flyby and orbiter spacecraft, many astronomers identified outer solar system targets for exploration. In 1972, NASA released a Space Vehicle Design Criteria Monograph that envisioned the environment that a spacecraft would encounter at Saturn, including aspects of its gravity field, charged particles, ring particles, and atmospheric structure and composition.6 Scientists of the time also focused particular attention on Saturn’s largest satellite, Titan.7 It is a moon large enough to hold onto its own atmosphere and was especially fascinating because Earth-based observations suggested that its surface might be “covered with higher hydrocarbon compounds of the type which were the

1.1

The path to Cassini-Huygens

5

basic building blocks of life on our planet.”8 Furthermore, observations suggested that Titan might have a surprisingly warm atmosphere 20 times more massive than that of Mars, although not made up of oxygen and nitrogen like ours, but rather of methane and hydrogen. Titan’s environment was suggestive of Earth’s several billion years ago, during the early stages of the evolution of life. Titan thus might yield information on the types of primordial material from which the solar system formed, as well as the “nature of chemistry which led to the origin and evolution of life on the planet earth.”9

1.1.1.1

The Space Science Board’s 1975 report

A guide for Saturn mission planning of the late 1970s and the 1980s was the Space Science Board10 (SSB) of the U.S. National Academy of Sciences’ Report on Space Science 1975. The Board adopted the recommendations made by its Committee on Planetary and Lunar Exploration (COMPLEX) regarding goals for future missions. Noting that the Pioneer 11 and Voyager 1 and 2 flyby missions, if successful, would complete a reconnaissance phase of Saturn investigation, COMPLEX envisioned a range of objectives for subsequent expeditions:11 • • • • • •

1.1.1.2

Intensive investigation of Saturn’s atmosphere Determination of satellite surface chemistry and properties Ring particle analyses Intensive examination of Titan Atmospheric dynamics and structure investigations Comparative planetology of the satellites.

The Saturn orbiter/dual probe study

In 1977, NASA initiated a Saturn Orbiter/Dual Probe (or “SOP2”) Study,12 a joint effort between NASA’s Jet Propulsion Laboratory (JPL) and Ames Research Center to define the science objectives and instrumentation required for a Saturn mission. Management responsibilities for the study were given to JPL, while Ames’ task was definition of the entry probes. The investigation was conducted in a manner similar to “Jupiter Orbiter with Probe,”13 the conceptual study that eventually led to NASA’s Galileo mission to Jupiter.

1.1.1.3

The Saturn system conference

NASA held a conference in February 1978 in Reston, Virginia in order to provide comprehensive scientific input to those scientists and engineers working on plans for the SOP2 mission, and toward this end, produced a 400-plus page “compendium of the present knowledge of Saturn, its satellites, its rings, and its magnetosphere.”14 The document also described the current state of Saturn mission planning and the expected state of knowledge once the Voyagers flew by the planet in 1980 and 1981. The next phase of exploration would logically be one in which long-duration studies of the Saturn system would be made.15 This suggested an orbiting rather than flyby spacecraft with the propulsive

6

Conceiving and funding the mission

capability to tour the Saturn system’s many points of interest, as well as one or more probes to provide in situ measurements of the most compelling targets, in particular Titan and Saturn themselves. One of the mission strategies discussed at great length at the conference was SOP2, which encompassed atmospheric probe exploration of both Saturn and Titan, as well as a Saturn orbiter performing multiple satellite encounters.16 At the time of this conference, a popular vision for the SOP2 spacecraft was to closely model its orbiter on the Galileo Jupiter orbiter that was then being designed, and to derive the two SOP2 probes from that mission’s probe.17 This vision would change radically before the final design for the spacecraft was chosen.

1.1.1.4

The Martin Marietta Titan probe study

NASA contractor Martin Marietta also helped define a Saturn mission, and in 1978 produced a briefing18 as part of a Titan probe study that raised a range of questions regarding mission concepts.19 Some questions pertained to the moon’s atmosphere and its implications for the design of an appropriate probe. NASA needed to know what Titan’s atmospheric pressure was. The Agency thought that the Voyager flyby spacecraft, both of which were launched in the summer of 1977, would be able to determine the surface pressure to within a few percent. These observations would be supplemented with ground-based measurements. Besides affecting probe design, the atmospheric pressure would also influence mission operations and descent time. For instance, a thin atmosphere model with a surface pressure of 17 millibars (mb) or 17 thousandths the pressure at Earth’s surface would result in a descent of 30 minutes, while a thick atmosphere model with a pressure as high as 21 bars predicted that the descent would last between 4 and 8 hours. The final Martin Marietta study considered three classes of Titan atmospheric entry probes: one that would conduct atmospheric science only, another that would land and carry out limited surface science, and a third that would perform extended mission studies on the moon’s surface. The study concluded that the middle option of limited surface exploration was a feasible and worthwhile, yet reasonably low risk alternative that could be performed without the need to develop new technology, and indeed could use hardware inherited from other probe missions.20 As will be shown in later chapters, the favored Martin Marietta alternative closely resembled the actual course of the Huygens Probe mission. 1.1.2

The Voyager missions

Voyager 1 traveled past Saturn on 12 November 1980 and Voyager 2 flew by on 25 August 1981,21 approaching to within 100,000 kilometers (60,000 miles). Although Pioneer 11 had visited Saturn several years earlier, it was the Voyagers, according to Andy Ingersoll, a meteorologist at California Institute of Technology (Caltech), that really “changed our perceptions”22 of the planet. The Voyagers were equipped with so many more instruments and so many more capabilities per instrument, that their dramatic observations of the Saturn system awed scientists around the world and led to demands for a follow-up.

1.1

The path to Cassini-Huygens

7

A close flyby of Titan revealed the moon to be larger than the planet Mercury and veiled by a thick layer of photochemical haze. The atmosphere is so dense that at the surface the pressure is 60% greater than on Earth’s surface.23 Voyager data revealed that the mother planet Saturn was no less fascinating. On Earth, winds are driven by solar energy, so it might be expected that this would be the case with other planets. At Saturn’s distance from the Sun, where there is only about 1% the solar energy per unit area experienced on our planet, the winds ought to be much gentler. But they’re not! They blow far stronger.24 Near Saturn’s equator, the Voyagers measured wind speeds of 500 meters per second (1,100 miles an hour). Furthermore, Saturn is the only planet less dense, overall, than water. This means that “if a lake could be found large enough, Saturn would float in it.”25 Voyager observations of Saturn’s ring system revealed even greater surprises as well as very perplexing puzzles. The spacecraft found unexplained gaps in the ring system, as well as sections that are subdivided into strands that appear to be braided together in places. There are also “spokes” on the ring system, so named because they resemble the spokes of a bicycle wheel. Intriguingly, some of the spokes, those thought to have formed most recently, appear to co-rotate with the planet’s magnetic field.26 Voyager observations indicated a complex planet-moon-ring system that needed to be further explored. The resolve to return to Saturn thus arose naturally from the science return of these missions. It was shortly after the Voyager flybys that an influential alliance arose between three scientists, one from the U.S., one from France, and one of Chinese origin. 1.1.3

The Horizon2000 programme

In the early 1980s, ESA established the Horizon2000 programme, which aimed at using “a modest but predictable science budget as daringly as possible, by defining ambitious missions affordable over a large period of time, spanning from 1985 to 2005.”27 Horizon2000 envisioned major “cornerstone missions” as well as medium-sized endeavors with more focused objectives and smaller budgets. What eventually became the Titan probe effort was the first of Horizon2000’s medium-sized mission concepts. As described in the next section, the first proposals for a probe to Titan were submitted in 1982 by an international team of scientists in response to a general call by ESA for potential Horizon2000 projects. 1.1.4

A special collaboration campaign for the Saturn mission

Tobias (Toby) Owen, an atmospheric scientist from the U.S., met French planetary scientist Daniel Gautier in the early 1970s. They became friends and so did their families. Owen later said that their strong personal relationship augmented all they were able to accomplish. He once explained that, in regard to developing space missions, “you have to overcome a lot of inertia and absorb a lot of defeats yet keep going. Agencies will not do that, only people who care enough.”28 Owen envisioned a new regime of space exploration by many different nations all working together. Although he was ultimately frustrated by how separately NASA and ESA operated in exploring space, his collaboration with Gautier yielded some very positive results, especially regarding Cassini-Huygens.

8 Conceiving and funding the mission The first attempts to sell a return-to-Saturn-and-Titan concept did not meet with success. France’s National Center for Space Exploration (CNES) told Gautier that such a project would be far too costly unless he could find partners. Owen received a similar response from NASA. The Agency was actually thinking about sending an orbiter to study Saturn, but believed adding a Titan probe would be too expensive. Nevertheless, Owen and his colleagues thought that a Galileo-style probe would fit the bill nicely. Such a probe had already been designed and built by NASA’s Ames Research Center and vetted at Jupiter. Owen did not understand why NASA went on to reject the use of such a probe, but suspected that rivalry between JPL and Ames, the developer of the Galileo probe, might have been the deciding factor.29 In the early 1980s, Owen became chairman of NASA’s Solar System Exploration Committee (SSEC), which was concentrating at the time on outer planet mission plans. Its vision was to explore Saturn as a logical step after Jupiter and to include both a Titan probe/radar mapper and a Saturn probe, as well as an orbiter. Flying separate missions to Saturn and Titan was also considered. At the same time as Owen and SSEC were developing exploration strategies, Daniel Gautier was preparing a similar study in response to ESA’s call for mission proposals from the European space science community. He had been approached by Wing Ip, a Taiwanese plasma scientist working at Germany’s Max Planck Institute, who had an interest in outer planet missions, and the two joined efforts to prepare a proposal. Ip also spent considerable effort eliciting support from the European space science community for a Saturn orbiter mission which would, among other things, further examine the planet’s magnetosphere. In addition, Ip conceived of naming the mission “Cassini” after the Renaissanceera Italian scientist who had been invited to France to run the Observatoire de Paris. 1.1.5

The International Solar Polar Mission

The momentum for an international mission to Saturn appeared to be building, but then the U.S. made a decision that dealt a severe blow to Europe’s trust in our country’s space program and in the wisdom of partnering with it. NASA had agreed to partner with ESA in a two-spacecraft project, the International Solar Polar Mission (ISPM) that would simultaneously explore different high-latitude regions of the Sun. ESA had invested heavily in the effort. But then in 1981, impacted by the reductions in space funding that President Ronald Reagan and his Administration demanded, the U.S. decided to cancel construction of the NASA spacecraft for this mission. The European spacecraft did eventually launch, however, under the mission title of Ulysses.30 President Reagan was not by any means an enemy of the U.S. space program. He did express interest in revitalizing it, although he never made the space program a vital national goal as it had been during the 1960s under Presidents Kennedy and Johnson. Reagan believed that major government spending reductions were needed in many sectors to counteract the expanding economic problems that had begun in the 1970s, and his austerity measures significantly impacted NASA’s activities as well as those of other agencies.31 George Keyworth, Science Advisor and Director of the Office of Science and Technology Policy under the Reagan Administration,32 was heavily involved in the fiscal restrictions imposed on NASA, and even recommended that “all new planetary space missions for at least the next decade”33 be halted. He favored a policy shift away from

1.1

The path to Cassini-Huygens

9

planetary exploration and toward experiments launched by the Space Shuttle, such as putting a space telescope in orbit. When NASA decided to cancel construction of its ISPM spacecraft, it did so “without bothering to inform its European partners,”34 who first heard of the decision from the media. This oversight added insult to injury. Many ESA personnel became very hesitant to enter into another major endeavor with NASA, which they now viewed as an untrustworthy partner. Their dissatisfaction over the ISPM incident has lasted many years. To this day, according to Torrence Johnson, “the Europeans have not let us forget”35 our country’s behavior. In fairness, however, it must be stated that the U.S. did not totally pull out of ISPM/Ulysses. The U.S. provided key aspects of the mission, including some of the science instruments, the launch and subsequent operational support. 1.1.6

The Gautier-Ip proposal to ESA

Gautier and Ip led a consortium of scientists in the preparation of the Saturn mission proposal and presented it to ESA in November 1982. Similar to the NASA vision, the consortium recommended combining a spacecraft that would orbit Saturn with one that would descend to the surface of Titan, whose atmosphere hid the frigid landscape of its surface. Furthermore, the consortium suggested that this ambitious mission be carried out in collaboration with NASA.36 The original European concept of the mission had been to use a spare Giotto spacecraft to fly to Saturn, with the U.S. furnishing a Titan probe. Giotto was the ESA craft under development for launch in 1985 for a flyby of Halley’s Comet in 1986. After this success, Giotto went on to encounter comet Grigg-Skjellerup in 1992.37 According to Owen, the Gautier-Ip proposal turned out to be a pivotal document for building European – and eventually U.S. – support for the mission. One of the proposal’s strengths was that it effectively communicated the roles that different scientific communities would play in the mission. These communities included those studying planetary surfaces, planetary atmospheres, icy satellites, cosmic dust, plasma phenomena, magnetospheres, the solar wind, and the atmosphere, surface, and interior of the moon Titan. Exobiologists would also be needed, to investigate the possibility of an environment in the Saturn system supporting life. By linking a variety of constituencies to the success of the mission, the Gautier-Ip proposal won over a broad spectrum of European scientists because they could all envision how they would be included in the effort. Early in the planning process, the international Saturn mission built a strongly loyal bloc of support from European space scientists of many different stripes. Many U.S. science personnel also backed the mission, due in part to Toby Owen’s efforts.38 This broad base of support would be sorely needed later in the approval process, when the mission came under severe danger of being canceled. 1.1.7

NASA’s reaction to an international Saturn mission concept

Although scientists from the U.S. and many other countries backed the European mission concept, Owen’s presentation of it to NASA was not warmly received. NASA considered such a mission grandiose, requiring more funding than was available. Owen surmised that NASA also felt uneasy about partnering with foreign entities that operated differently than

10

Conceiving and funding the mission

it did, lest the Agency not be able to exert the control over the mission that it desired. Owen persisted, however, and so did Ip and Gautier in selling the mission to their organizations. All three men shared a deep belief in the benefits of international cooperation in space and in this particular mission.39 Owen argued persuasively to NASA that because of the international element, the envisioned mission to Saturn was now affordable, so why couldn’t it be considered by SSEC as a serious option for future exploration? NASA finally conceded that such a mission might have some value, and in 1983 SSEC published a report which recommended the Agency analyze an international mission of this sort. The report suggested that the mission’s core program should include a Titan Probe with a Radar Mapper that could peer through the moon’s clouds, and ought to consider a Saturn Orbiter for later implementation.40 It is interesting that at this point in the planning, such a Saturn project fit into the category of “low- and moderate cost missions.”41 By the time that Cassini-Huygens launched, it was anything but that. 1.1.8

The Joint Working Group and the ESA-NASA study

In June 1982 the Space Science Committee of the European Science Foundation (ESF) and the Space Science Board of the U.S. National Academy of Sciences (NAS) set up a Joint Working Group (JWG) to further investigate how Europe and the U.S. might cooperate on planetary science projects. The recommendations of the JWG were expected to carry considerable weight. NASA and the U.S. Congress did not typically back a science proposal that had not been blessed by NAS. But Owen encountered difficulties in convincing the JWG to support his ideas, even though he had friends and colleagues in NAS. Eugene Levy, who was the U.S. chair of the JWG, personally favored another mission, the Mars surface rover. He thought that Saturn’s distance from Earth was a disadvantage; it would require seven years for a spacecraft to travel there. Owen’s arguments, however, impressed Hugo Fechtig, the JWG’s European delegation chair. Owen’s mission vision was supported by his influential friends Fred Scarf, an authority on space plasma physics who worked for TRW’s Space Systems Group, and Hal Masursky, a geologist and planetary scientist from the U.S. Geological Survey’s Branch of Astrogeology Studies.42 The JWG eventually decided to include in its recommendations a Saturn Orbiter and Titan Probe, both of which were to be based on the design of the Galileo spacecraft. The JWG study was followed up in 1984 and 1985 by a joint assessment by the relevant space agencies, ESA and NASA. This effort included heavy participation by engineers because it was vital to establish the technical feasibility of carrying out the scientists’ ideas. In February 1986, ESA’s Science Programme Committee (SPC) approved the conducting of an initial mission analysis. As proposed by Wing Ip, the mission was officially called “Cassini.” The initial analysis was termed a “Phase A study” and was to start in 1987.43 1.1.9

Threats to U.S. leadership in space

In the wake of the Challenger tragedy of January 1986, the Space Shuttle fleet was grounded and with it, all JPL-developed spacecraft. In June 1986, NASA eliminated use of the Centaur upper-stage rocket as a booster for Space Shuttle payloads, due to the risks involved with carrying a hydrogen-fueled device in the Shuttle’s cargo bay. This action

1.1

The path to Cassini-Huygens

11

threatened several missions designed to employ the Centaur, including Galileo, Ulysses, and the voyages that would use the Mariner Mark II interplanetary platform (discussed later in this chapter). Unfortunately, Cassini was one of those voyages. NASA’s resources over the next several years were directed toward analyzing the causes of the Challenger crash and returning the Shuttle to space. This turned out to be a longer process than first imagined. The Shuttle remained grounded for over two and a half years, and flights did not resume until September 1988. Planetary Shuttle missions were not launched until 1989, when Magellan and Galileo finally took off. The 12 year hiatus in planetary missions since the Voyagers were launched in 1977 had impacted NASA’s leadership role in space exploration. Lew Allen, Director of JPL, summed up the dark feelings of many space scientists and engineers when he commented that “the decade of the ’80s turned out to be a dry hole. … Things have gone very sour indeed.”44 The erosion of NASA’s international leadership role in space was emphasized by Soviet successes such as two Venera missions which entered orbit around Venus in 1983 to map its surface using radar, and two VeGa spacecraft that deployed landers and balloons there in 1985 en route to a flyby of Halley’s Comet in 1986. In 1985, Lew Allen mentioned his “rising concern about the Soviet space program, which appears to be doing the American program two years before the Americans.”45 This concern sparked a series of high-level national efforts to deal with the future of our space program. One was a White House panel, the National Commission on Space. In 1985 the White House chose Thomas Paine, NASA’s former Administrator, as its chair. He was tasked to prepare a report on the future of space exploration, and the Commission’s report, Pioneering the Space Frontier, was released in May 1986.46 1.1.10

The Paine Commission report

The 1986 Paine Commission report proclaimed a lofty mission for the U.S., namely, “To lead the exploration and development of the space frontier, advancing science, technology, and enterprise, and building institutions and systems that make accessible vast new resources and support human settlements beyond Earth orbit …”47 One of the key phrases is “lead the exploration.” The report clearly advocated that with our country’s “pioneer heritage, technological preeminence, and economic strength, it is fitting that we should lead the people of this planet into space.”48 It also recognized the critical role that the U.S. government needed to play in supporting such exploration. The report proposed a three-part agenda, the first two elements of which pertained directly to planetary exploration: • • •

Advancing understanding of our planet, solar system, and universe Exploring, prospecting, and settling the solar system Stimulating space enterprises for the direct benefit of people on Earth.

Specific actions are recommended later in the report, two of which relate directly to missions such as Cassini: • •

Study the evolution of the solar system by analyzing samples from selected planets Search for evidence that life exists – or has existed – beyond Earth, by studying other bodies of the solar system.

12

Conceiving and funding the mission

The Paine Commission report carried a good deal of influence with the space science community and with Congress, not only because of what it said, but also because of where it originated – in the executive branch of the government. The report clearly recognized the international competition which was beginning to challenge the U.S. role in space. This consisted not only of competition from the USSR with defense and its political implications, but also growing commercial competition from Western Europe and Japan. In particular, Europe was already operating the Ariane launch vehicle in competition with the Space Shuttle as a means of launching satellites.49 The report concluded that the U.S. would need a long-term effort in order to meet these challenges. Technology development and long-range exploratory missions were needed, and this required sustained, multi-year government financial support. While the report recognized that other countries were providing the U.S. with increasingly stiff competition, the Paine Commission solidly endorsed U.S. partnering with other nations on mutually beneficial projects because this was the way to realize our goals in space sooner and less expensively, and to “create the kind of international environment most conducive to an expansive space program conducted in accordance with American values.”50 Maintaining such a network of international cooperation would prove to be a key factor in the early 1990s for keeping the Cassini project alive. # After the release of the Paine Commission report the Cassini project development process continued to move forward, with advances taking place on both sides of the Atlantic. The Space Science Advisory Committee (SSAC) of ESA endorsed the envisioned mission to Saturn, and in November 1986 an executive proposal was approved by the Science Programme Committee to proceed with the Phase A study. NASA Administrator James Fletcher also strongly supported the mission as part of the U.S.’s return to a dominant position in space exploration. A key action that he took toward this end was to appoint astronaut and physicist Sally Ride in August 1986 as his Special Assistant for Strategic Planning, charging her with “preparing a new blueprint for NASA’s future.”51 Her committee produced an influential report that, in a more specific way than the Paine Commission report, tied the execution of outer planet and other space missions to the basic aims of the U.S. space program as well as to national security goals. 1.1.11

Sally Ride’s report

Sally K. Ride, who flew on several Space Shuttle missions, as well as serving on the Presidential Commissions that investigated both the Challenger and the Columbia disasters, gave a strong wakeup call to the United States in her August 1987 report to the NASA Administrator, NASA Leadership and America’s Future in Space, and accompanied this document with a testimony before the House Subcommittee on Space Science and Applications.52 Ride underlined in her report that although “In the 1960s and 1970s, planetary exploration was a vital and important component of the United States space program”53 for maintaining our country’s undisputed leadership in nearly all civilian space endeavors, our preeminent position was in serious danger of disappearing. We had not launched a new planetary mission in a decade.

1.1

The path to Cassini-Huygens

13

This sorry state of affairs not only ran counter to NASA’s proud history, but it also had national security implications. The Ride report mentioned as an example the Reagan Administration’s 1982 National Security Decision Directive that proclaimed it necessary to “maintain United States space leadership.”54 Such preeminence was important for a variety of reasons that ranged from photoreconnaissance capabilities for collecting intelligence and monitoring arms control agreements, to colonization and exploitation of other celestial bodies, to a range of classified tasks. President John F. Kennedy once said, “We have vowed that we shall not see space filled with weapons of mass destruction, but with instruments of knowledge and understanding. Yet the vows of this Nation can only be fulfilled if we in this Nation are first, and, therefore, we intend to be first … the world’s leading space-faring nation.”55 Ride pointed out that to maintain space leadership, the U.S. needed to commit to ambitious goals capable of inspiring others, both domestically and internationally, and to actively demonstrate the capabilities to meet those goals. These capabilities had to have two distinct attributes: scientific research and technology development. Our program needed to expand the nation’s understanding of space and the space environment, as well as its capacity to explore and operate in that environment.56 The United States therefore had to accomplish “feats which demonstrate prowess, inspire national pride, and engender international respect and a worldwide desire to associate with U.S. space activities.”57 This had to be accomplished by building on NASA’s tradition of unparalleled solar system exploration. One key aspect of these efforts was to further investigate the outer planets. NASA needed to go beyond what the Pioneer and Voyager programs achieved and beyond what the planned Galileo program hoped to accomplish. This meant exploring Saturn and its largest moon, Titan. This satellite was an especially worthwhile target for a cutting edge mission because the organic chemistry occurring there provided “the only planetary-scale laboratory for studying processes that may have been important in the prebiotic terrestrial atmosphere.”58 Ride challenged United States’ pride and competitiveness when she proposed a very ambitious set of explorations for the coming years. One key component of these was a mission to Saturn that sought to achieve considerably more than the Titan Probe with Radar Mapper effort that was suggested in 1983 by the Solar System Exploration Committee. Ride’s vision included an orbital spacecraft and not one, but three probes: one would be launched toward Titan to enter and study its atmosphere, another would make a semi-soft landing on the surface of Titan, and a third would investigate the atmosphere of Saturn.59 # During the year following publication of the Ride report, in 1987 and 1988, NASA carried out key definition work on the Cassini Saturn Mission, and in particular on the Mariner Mark II spacecraft that would carry the Cassini package. The Mark II was designed to have multi-mission capability, in that it was to be adaptable at low cost to a wide variety of planetary missions, including those to comets, main-belt asteroids, and the outer planets.60 It would not only carry the Cassini spacecraft, but would also be employed on another effort: the Comet Rendezvous Asteroid Flyby (CRAF) Mission. In early 1988, NASA decided to combine CRAF and Cassini into a single Mariner Mark II program and to

14

Conceiving and funding the mission

submit this double mission to Congress as a new start in fiscal year 1990.61 However, the battle to launch these two missions did not go smoothly, and NASA and the space science community’s negotiations with Congress will be discussed in depth later in this chapter. Ride was a national hero whose words carried considerable weight. Her report, like the Paine Commission report, proved valuable in focusing national space policy, influencing Congress, and reawakening U.S. planetary exploration efforts. NASA Administrator James Fletcher foresaw the considerable influence of Ride’s study, assuring her when the report was publicly released on 17 August 1987 that she had “contributed strongly to a process that will determine the goals and directions of the nation’s civil space activities well into the next century.”62 1.1.12

The Cassini Phase A study

In November 1987, the Cassini Saturn Orbiter and Titan Probe Phase A study began, as approved by ESA’s Science Programme Committee in 1986. The analysis was carried out by a European industrial consortium led by Marconi Space Systems and was completed in September 1988. It developed possible scientific objectives and a spacecraft technical design, with emphasis on the Titan Probe system. From their inception, Cassini mission concepts had a strong international aspect. This was reflected by the membership of the Joint Science Working Group (JSWG) which supported Cassini’s Phase A activities. Daniel Gautier of France, Wing Ip of Germany and Toby Owen of the U.S. served as the lead scientists, and the JSWG included the following participants:63 • • • • • • • • • • • • • • • • •

M. Allison, Goddard Institute for Space Studies, New York, U.S. S. Bauer, Karl Franzens Universitiit, Graz, Austria M. Blanc, Centre de Recherches en Physique de l’Environnement, St. Maur, France S. Calcutt, Department of Atmospheric Physics, Oxford, U.K. J. Cuzzi, NASA Ames Research Center, Moffett Field CA, U.S. M. Fulchignoni, Universita La Sapienza, Rome, Italy D. Gautier, Observatoire de Paris, Meudon, France D. Hunten, University of Arizona, Tucson AZ, U.S. W. Ip, Max Planck Institute fur Aeronomie, Katlenburg-Lindau, Germany T. Johnson, Jet Propulsion Laboratory, Pasadena CA, U.S. H. Masursky, U.S. Geological Survey, Flagstaff AZ, U.S. P. Nicholson, Cornell University, Ithaca NY, U.S. T. Owen, State University of New York, Stony Brook NY, U.S. R. Samuelson, NASA Goddard Space Flight Center, Greenbelt MD, U.S. F. Scarf, TRW, Redondo Beach CA, U.S. E. Sittler, NASA Goddard Space Flight Center, Greenbelt MD, U.S. B. Swenson, NASA Ames Research Center, Moffet Field CA, U.S.

The Cassini mission proposed by the Phase A study represented “a natural extension of the reconnaissance/first exploration of the Saturn system carried out so successfully by the Pioneer 11 (1979) and Voyager 1 and 2 (1980–81) flybys.”64 It was meant to accomplish far more than flybys could, although only one planetary probe, rather than the three envisioned in the Ride report, would be employed. The probe part of the spacecraft would descend for

1.1

The path to Cassini-Huygens

15

three hours into the atmosphere of Titan, making a series of detailed measurements on the way down. The orbiting section of the spacecraft would perform a four year, approximately 36-orbit tour of the Saturnian system, continuously changing its path and ending in a highly inclined, nearly polar orbit, that would allow a thorough examination of the system’s moons, rings and magnetosphere. The Phase A study envisioned a project management structure similar to what was finally implemented. A NASA project manager would be responsible for the overall mission as well as associated NASA resources and major requirements, such as the launch vehicle, the orbiter section, tracking and data acquisition technologies, spacecraft integration operations, and flight operations. The ESA project manager would direct development of the Titan probe system and be the formal contact for all matters concerning it.65 This initial plan for Cassini accurately represented much of the actual mission’s operations, but what it did not envision was the essential part that the Italian Space Agency (ASI) would play in spacecraft development (discussed later in the book). One of the most valuable contributions of the Phase A study was to envision, in some detail, a set of scientific objectives that the mission would strive to fulfill. These included objectives not only at Saturn but also at Jupiter, during an asteroid flyby, and on the long journey from Earth. These objectives are listed in Table 1.1. Much more detailed treatments of these objectives are included in several later chapters of the book. Table 1.1. Cassini science objectives envisioned by the Phase A study.66 Objectives in the Saturnian system, the primary target of Cassini Titan • Determine abundances of atmospheric constituents • Establish isotope ratios for abundant elements • Develop scenarios of Titan’s formation and evolution • Observe vertical and horizontal distributions of trace gases • Search for more complex organic molecules • Investigate energy sources for atmospheric chemistry • Model photochemistry of Titan’s stratosphere • Study formation and composition of aerosols • Measure winds and global temperatures • Investigate cloud physics, general circulation, and seasonal effects in Titan’s atmosphere • Search for lightning discharges • Determine surface topography and the composition • Infer Titan’s internal structure • Investigate the upper atmosphere, its ionization, and its role as a source of neutral and ionized material for the magnetosphere of Saturn Saturn • Determine atmosphere’s temperature field, cloud properties, and composition • Measure global wind field, including wave and eddy components • Observe cloud features and processes • Infer internal structure and rotation of the deep atmosphere • Study diurnal variations and magnetic interactions with ionosphere • Provide scenarios for formation and evolution of Saturn • Investigate sources and morphology of Saturn’s lightning (continued)

16

Conceiving and funding the mission Table 1.1. (continued)

Rings • Study ring configurations and dynamical processes (gravitational, viscous, erosional, electromagnetic) responsible for ring structure • Map composition and size distribution of ring material • Investigate interrelation of rings and satellites, including embedded satellites • Determine dust and meteoroid distribution both in the vicinity of the rings and in interplanetary space • Study interactions between rings and Saturn’s magnetosphere, ionosphere, and atmosphere Icy satellites • Determine general characteristics and geological histories • Define mechanisms modifying crusts and surfaces • Investigate compositions and distributions of surface materials, particularly dark, organicrich materials and low melting point condensed volatiles • Constrain bulk compositions and internal structures • Investigate interactions with magnetosphere and ring systems, including possible gas injections into magnetosphere Magnetosphere of Saturn • Determine configuration of nearly axially symmetric magnetic field • Analyze relation to modulation of Saturn Kilometric Radiation (SKR) • Determine current systems, composition, sources and sinks of magnetosphere-charged particles • Investigate wave-particle interactions and dynamics of dayside magnetosphere and magnetotail • Investigate interaction with solar wind, satellites, and rings • Study effect of Titan’s interaction with solar wind and magnetospheric plasma • Investigate Titan’s atmosphere and exosphere interactions with surrounding plasma Targets of opportunity Asteroid flyby • Investigate an asteroid not seen by previous missions, possibly a new class of asteroids • Characterize global properties, determine composition and morphology, investigate regolith Jupiter system • Extend study period of atmospheric dynamics and variable satellite phenomena, specifically Io’s volcanism, beyond the Galileo mission • Infer global atmospheric thermal structure and composition with instrumentation not carried by Galileo • Explore the dusk side of the magnetosphere and intermediate regions of the planet’s magnetotail • Obtain high resolution images of Io’s plasma torus Cruise science • Improve sensitivity of interstellar ion composition measurements by three orders of magnitude • Investigate behavior of the solar wind during solar minimum • Search for gravitational waves • Extend studies of interplanetary dust out to the orbit of Saturn

1.1.13

What scientists hoped to learn from the Saturn mission

The mission envisioned by U.S. and European scientists was an ambitious one, meant to “venture to the outer solar system to study the rich diversity of the Saturn system.”67 As underlined in Table 1.1, this included examination of Saturn’s rings, satellites, fields,

1.2 Partnering with Europe

17

particles, and magnetosphere, and the atmosphere and surface of its largest moon, Titan. The mission was to generate data that would help tell the story of the solar system’s origin and evolution. Our solar system has spent most of its 4.6 billion year lifetime recovering from the tumultuous violence of its formation. But virtually all evidence of this early period has been obliterated on Earth and the other terrestrial planets (Mars, Venus, and Mercury) through continuous changes in their surfaces by both internal and external processes. Outer solar system planets such as Saturn, however, have been subjected to relatively few modifications. In fact, since Saturn is so massive and thus has quite a strong gravitational field, it has probably retained almost all of the primordial material from which it was created, and thus ought to contain “a representative sample of the original nebula” that provided the material for the planets.68 Scientists believe that Saturn’s composition and chemistry must contain clues to the early solar system’s evolutionary processes, including the formation of the planets. The dynamics of Saturn’s rings were also expected to yield valuable information. The interactions of the rings’ uncountable numbers of small orbiting bodies were thought to resemble those of planetesimals in our young solar system, which led to planetary accretion. Another area of study related to the solar system’s molecular evolution. How did the system’s chemical makeup evolve as the original interstellar cloud transformed into a solar nebula and then into planets?69 Earth’s primitive atmosphere may well have been similar to Titan’s present-day atmosphere. If so, the synthesis of organic compounds in Titan’s atmosphere could improve our understanding of the origin of life on Earth.70 Another goal of the mission was to study Saturn’s atmosphere to obtain data that could be compared with that of Jupiter supplied by the Galileo spacecraft. Scientists envisioned that the comparison of these two gas giant planets would contribute “far more to our understanding of planetary origin and evolution than could be gained through the study of either planet alone.”71

1.2

PARTNERING WITH EUROPE

Sally Ride’s 1987 report envisioned the mission to Saturn as a solo effort by NASA, but there were those in the Agency who understood the importance of international collaborations. In 1988 Lennard Fisk, NASA Associate Administrator for Space Science and Applications, foresaw the advantages of a joint NASA and ESA mission and contacted his counterpart at ESA, Roger Bonnet. ESA was at that time in the process of choosing its next major scientific project from five short-listed candidate missions. Fisk strongly urged Bonnet to partner with NASA on the Cassini mission by developing a probe that would descend through the atmosphere of Titan. NASA, in turn, would provide the Saturn Orbiter and would launch the combined spacecraft from Earth.72 Right up until ESA’s final choice of mission, those who favored a joint Cassini mission with NASA had to fight off fierce competition from those who preferred the Vesta project, a pair of space voyages that would investigate various asteroids and comets over a 5 year period. Before this competition was decided, however, NASA submitted a budget request to the White House’s Office of Management and Budget (OMB) for development of a key

18

Conceiving and funding the mission

piece of Cassini mission technology, hoping that the White House would endorse the request and submit it to Congress in January 1989 for inclusion in the fiscal year 1990 budget. NASA’s budget request asked for funds to cover development of an orbiting space vehicle. The spacecraft to journey to and ultimately orbit Saturn would be NASA’s newly developed Mariner Mark II interplanetary platform (technical and historical details of the Mark II are given in Chapter 2). It would carry the Titan probe and relay the radio data from the probe to Earth, and would also conduct extensive observations of the Saturnian system.73 ESA chose its next major space mission in two steps. The first took place at an October 1988 meeting of ESA’s Space Science Advisory Committee (SSAC) held in Bruges, Belgium at a movie theater which, later in the day, would be showing the movie Who Killed Roger Rabbit? The SSAC session was attended by well over 300 scientists and engineers, and seven mission proposals were presented. Two of the five short-listed proposals were in serious contention for ESA approval: Cassini and Vesta. Daniel Gautier and Wing Ip from Europe, as well as Michel Blanc, another French scientist, and Toby Owen from the U.S., had been caucusing effectively for Cassini, but there were other top scientists who were making strong cases for Vesta. This mission had been conceived by France’s National Center for Space Studies (CNES) and the USSR’s Intercosmos space agency, and was envisioned as a follow-up to the 1986 flybys of Halley’s Comet by ESA’s Giotto and the USSR’s VeGa spacecraft. ESA participation in Vesta was heavily championed by CNES and its Director General, Frederic d’Allest. This was at least partially because of the strong French policy to augment its close partnership with the Soviets. This alliance was supported not only by d’Allest but also by President François Mitterrand, the latter mentioning on a November 1988 visit to the Soviet launch center at Baikonour that it had been an important topic in his meetings with President Mikhail Gorbachev.74 Gautier commented in an interview with the author that a mission to Saturn had relevance for “almost all species of science.”75 Those wanting to study the planet’s atmosphere, its ring system, fields and particles, or satellites could find a place on the mission team. This was a major reason why the mission concept garnered the support of so many people in Europe. At ESA’s October 1988 Bruges meeting, the attendees strongly endorsed the Cassini mission, the European agency’s first major step in planetary exploration. The Bruges meeting also set the tone for international cooperation and sharing of capabilities that continued throughout the mission. One of the stipulations agreed in Bruges was that both U.S. and European scientists were to be invited to share each other’s instruments on both the Cassini Orbiter and the Titan Probe. This was carried out. U.S. scientists worked with all six of the science instruments on the Probe, while European scientists made use of all twelve of those on the Orbiter. And there were serious efforts throughout the mission to keep this international participation alive and well.76 The second stage of ESA’s approval process took place the next month in Paris at a meeting of the agency’s Science Programme Committee (SPC). While the Bruges meeting was attended by technical personnel – scientists and engineers – SPC was a more politically oriented body. It strongly endorsed the mission.77 According to the Cassini mission plan, the science experiments to be carried out at Saturn would be allocated by open competition, and NASA would provide launching systems for the spacecraft.78 During the selection process, ESA named its probe “Huygens,” in honor of the discoverer of Titan. Different people had put forth this name at various times. Carl Sagan proposed it in

1.3

Congressional negotiations 19

letters to Gautier and others. Roger Bonnet mentioned that he thought of it at the Bruges meeting. It was also suggested by the Swiss delegation on the SPC. The full name of the mission, Cassini-Huygens, was a strong reminder of the European roots of Saturn research in Italy, France, and the Netherlands.79 NASA was becoming more sensitive to the strain that had developed between the American and European space programs as a result of European perceptions that it had not been treated as an equal partner during previous collaborations. Cross-border challenges such as the above arose from attempts to meld different cultures, having different imperatives, into efficient missions. Other such challenges are discussed throughout this book. Perhaps the worst crisis between the European and American agencies occurred in 1981 when NASA decided to cancel its International Solar Polar Mission without a priori discussions with ESA, its partner.80 NASA officials and advisors involved in promoting and planning the Cassini project tried to improve negative perceptions by stressing their desire to evenly share any scientific and technology benefits resulting from the mission. In part, this newfound spirit of cooperation with Europe was being driven by a sense of competition with the USSR, which had begun to cooperate more closely with Europe as ESA drew further away from NASA.81 ESA hoped that the U.S. Congress would decide to fund Cassini, but cognizant of the “potential pitfalls of depending too heavily on transatlantic collaboration,”82 had a backup plan. If U.S. congressional approval was not forthcoming for Cassini, then ESA intended to reexamine the other four project options that had been considered by the SPC in December 1989.

1.3

CONGRESSIONAL NEGOTIATIONS

With the support for renewing planetary exploration given by the White House-sponsored Paine Commission report and with the specific recommendations for a Saturn mission in the Sally Ride report, it was not surprising that Congress viewed NASA’s Cassini mission proposal favorably – at least initially. For instance at an August 1988 Senate hearing, Michigan Senator Donald Riegle, chairman of the Subcommittee on Science, Technology, and Space, affirmed that his committee considered the CRAF83/Cassini mission “NASA’s highest priority space science initiative,”84 and should be included in the fiscal year 1990 NASA budget request. Georgia Senator Wyche Fowler Jr. added his support for “prompt initiation of the CRAF/Cassini mission,” asserting that when it and missions such as the Hubble Space Telescope, the Magellan mission to Venus, and the Ulysses mission to study the Sun were launched, the U.S. “will have reasserted our international leadership in the field.”85 NASA seemed to be “riding on a wave of goodwill in Congress, driven by a fear that the Soviet space program is far ahead of the United States’.”86 By the time of the above Senate hearing, the House of Representatives had already included in its fiscal year 1989 NASA authorization bill87 a mandate for NASA to proceed with a new start for CRAF/ Cassini.88 Support continued from both houses of Congress. In a July 1989 statement on the floor of the Senate touting NASA’s history of space travel, Massachusetts Senator John Kerry made the point that “Exploring the planets captures the imagination of all Americans,

20

Conceiving and funding the mission

but especially the imagination of the young. We learn about our universe, and we learn about our Earth.”89 Also on the floor of the Senate, California Senator Alan Cranston reminded his audience of the unique opportunities that missions such as Cassini offered for cooperation with other nations, underlining that “last year the European Space Agency agreed to join the Cassini program – a project to send a spacecraft to survey Saturn, its rings, and several moons. Such joint ventures allow us to expand and share our knowledge and work with all nations.”90 He went on to discuss how space journeys were important not only for our national pride, but also for our economy, in that “Our space policy plays an important role in maintaining [our] technological edge,” which was “our strongest competitive advantage with our trading partners.”91 1.3.1

House of Representatives actions: H.R. 1759

The bill H.R. 1759 passed the U.S. House of Representatives on 21 September 1989.92 Four days later, the House sent an announcement to the Senate stating that this was “an act to authorize appropriations to the National Aeronautics and Space Administration”93 that included approval to initiate the Cassini project. The House requested Senate concurrence with the bill, which had been passed by unanimous consent.94 1.3.2

Senate actions: S. 916

The Senate looked favorably on the Cassini mission, but this mission was only one item in the House’s proposed NASA Authorization Act H.R. 1759. Months earlier, the Senate had proposed its own authorization bill, S. 916, sponsored by Senator Al Gore Jr. of Tennessee, introduced to the Senate on 3 May 1989, and referred to the Committee on Commerce.95 Senator Gore explained that this bill gave the members of the Senate its Commerce Committee’s “best estimate of the real fiscal year 1990 budget requirements of NASA and the committee’s best judgment as to the mix of programs that are required to retain U.S. technological leadership in space.”96 By October 1989, the Senate Committee on Commerce finished developing a version of S. 916 that it supported, then sent it on to the Senate floor for approval. The Senate passed the bill on 9 November 1989 and delivered it to the House on 13 November.97 1.3.3

The cost containment requirement

The Senate was concerned about controlling mission expenses, and so included in S. 916 a key requirement that the CRAF/Cassini mission implement a cost containment plan. This was in response to Congressional direction regarding NASA’s fiscal year 1990 budget,98 that the Agency submit, with its fiscal year operating plan, projected annual funding requirements for each year of CRAF/Cassini development, and that this report be updated twice annually on each subsequent fiscal year. A further cost control measure required NASA to cap its spending for developing the mission at $1.6 billion total, and if at any time the total budget appeared heading to exceed this amount, then the Agency was required to descope the mission to fit into the funding limit.99

References 21 1.3.4

The House Response to S. 916

The House’s Committee on Science, Space, and Technology responded to S. 916 on 19 November 1989 by introducing a new NASA authorization bill, H.R. 3729, to the House floor and asking for its immediate consideration. In an effort at compromise, H.R. 3729 included many provisions that were in the Senate authorization bill.100 After this bill was passed by the House and placed before the Senate, that body did not respond favorably. The objections were not over the Cassini mission, but over other matters such as expendable launch vehicles, financial accounting practices for the Space Shuttle, and the value of agency-wide multi-year funding authorizations. Senators Fritz Hollings of South Carolina and Al Gore proposed Amendment 1208 to H.R. 3729. Both the unamended bill and the amendment touted the value of international cooperation in the exploration of the universe and set aside $1.6 billion to cover development, launch, and first 30 days of operations of the CRAF/Cassini mission. Both documents required NASA to submit a cost containment plan to the House and Senate by 31 January 1990, and updates by 31 July and 31 January of each succeeding year until such funds were expended.101 Effective cost containment was to depend on rigorous resource planning and project management, tracking, and planning. It was to include periodic, in-depth cost reviews involving teams external to the project. The plan also demanded “resiliency … to allow for descoping”102 if unexpected, expensive issues arose. The Senate passed this amended bill.103 NASA developed a cost containment plan and, although both houses of Congress had not yet agreed on an authorization bill for the agency, began sending biannual status reports to both the Senate and House of Representatives.104 The House did not agree with the Senate-amended version of H.R. 3729, and on 28 February 1990 requested a conference with the Senate.105 Agreement between the two houses was still a long way off. On 9 March 1990, the Senate introduced yet another authorization bill, S. 2287.106 An amended version of this bill eventually passed both the Senate and the House on 25 October 1990. Finally, on 16 November 1990, President George Bush signed the bill and it became Public Law Number 101–611, still with a CRAF/Cassini budget of $1.6 billion for development, launch, and 30 days of operations, and still with the requirement for a cost containment plan.107 This was a compromise bill that contained sufficient provisions desired by the House for that body to pass the measure. While most of these provisions did not pertain directly to CRAF/Cassini, one that did was Sec. 114, requiring a “Study on International Cooperation in Planetary Exploration.” This provision had been in a House version of the appropriations bill H.R. 5649: National Aeronautics and Space Administration Multiyear Authorization Act of 1990.108

REFERENCES 1. Michael Coren, “Huygens to Plumb Secrets of Saturn Moon,” http://www.ufocasebook.com/ titanssecrets.html, 13 January 2005. 2. Arthur C. Clarke with Chesley Bonestell, Beyond Jupiter: The Worlds of Tomorrow, (Boston: Little Brown, 1972).

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3. R.L. Newburn, “Scientific Considerations,” in Exploration of the Moon, the Planets, and Interplanetary Space, ed. Albert R. Hibbs, JPL Report No. 30–1 (Pasadena, CA: JPL, 30 April 1959). 4. Michael A. Minovitch, The Determination and Characteristics of Ballistic Interplanetary Trajectories Under the Influence of Multiple Planetary Attractions, Technical Report No. 32–464 (California: JPL, 31 Oct. 1963), p. viii; Michael A. Minovitch, Utilizing Large Planetary Perturbations for the Design of Deep-Space, Solar-Probe, and Out-of-Ecliptic Trajectories, JPL Technical Report No 32–849 (California: JPL, 15 Dec. 1965), p. 67; Gary Flandro, “Utilization of Energy Derived from the Gravitational Field of Jupiter for reducing Flight Time to the Outer Solar System,” JPL Space Programs Summary 4, no. 37–35 (31 Oct. 1965):22; Michael Meltzer, Mission to Jupiter: A History of the Galileo Project” (Washington D.C.: NASA SP-2007-4231), pp. 17–18; Bob Mitchell review of manuscript, Feb. 2011. 5. Richard A. Wallace, “Background of the SOP2 Mission,” in Donald M. Hunten and David Morrison (eds.), The Saturn System (Washington D.C.: NASA Conference Publication 2068, 1978), p. 4. 6. Frank Don Palluconi et al., “The Planet Saturn (1970),” http://ntrs.nasa.gov/archive/nasa/casi. ntrs.nasa.gov/19720022190_1972022190.pdf, NASA Space Vehicle Design Criteria Monographs, SP-8091 (June 1972). This document reflects published information available in mid-1970 regarding Saturn. 7. Calvin P. Myers et al., “The Environment of Titan (1975),” NASA Space Vehicle Design Criteria (Environment), SP-8122 (July 1976), NHRC 010122, Saturn Titan Mission. 8. S. Ichtiaque Rasool, “Satellites of the Outer Planets (Titan),” unpublished manuscript labeled ‘layman’s version,’ 24 Oct. 1972, NHRC 010122 Saturn Titan Mission. 9. Rasool, “Satellites.” 10. The Space Science Board, also known as the Space Studies Board, dates back to the beginning of the U.S. space program. The SSB was formed during spring of 1958 with the task of surveying the scientific aspects of the space exploration. More information on SSB can be found at http://history.nasa.gov/sputnik/20fe.html and in the Archives of the National Academy of Sciences-National Research Council. 11. Donald M. Hunten, “Conclusions and Recommendations: Exploration of the Saturn System,” in Donald M. Hunten and David Morrison (eds.), The Saturn System (Washington D.C.: NASA Conference Publication 2068, 1978), p. 407. 12. Daniel H. Herman memo, “Saturn Orbiter/Dual Probe Study,” 21 Nov. 1977, NHRC 010122 Saturn Titan Mission. 13. Ibid. 14. Donald M. Hunten and David Morrison (eds.), The Saturn System (Washington D.C.: NASA Conference Publication 2068, 1978), p. 1. 15. Richard P. Rudd, “Saturn Orbiter Dual Probe Mission,” in Donald M. Hunten and David Morrison (eds.), The Saturn System (Washington D.C.: NASA Conference Publication 2068, 1978), p. 345. 16. Lawrence Colin, “Outer Planet Probe Missions, Designs and Science,” in Donald M. Hunten and David Morrison (eds.), The Saturn System (Washington D.C.: NASA Conference Publication 2068, 1978), p. 372. 17. David Morrison, “Galileo Orbiter Spacecraft and Instrumentation,” and Lawrence Colin, “Outer Planet Probe Missions, Designs and Science,” in Donald M. Hunten and David Morrison (eds.), The Saturn System (Washington D.C.: NASA Conference Publication 2068, 1978), pp. 361, 372–374, 379. 18. Martin Marietta, “Study of Entry and Landing Probes for Exploration of Titan,” midterm briefing, 27 October 1978, as reported in Michael R. Helton memo to Deputy Director, Planetary

References 23

19. 20.

21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

38.

39. 40. 41. 42. 43. 44.

Division, “Titan Atmosphere Issues Relating to Titan Probe Planning,” 15 Dec. 1978, NHRC 010122 Saturn Titan Mission. Michael R. Helton memo to Deputy Director, Planetary Division, “Titan Atmosphere Issues Relating to Titan Probe Planning,” 15 Dec. 1978, NHRC 010122 Saturn Titan Mission. Martin Marietta Corporation, “Study of Entry and Landing Probes for Exploration of Titan,” MCR-79-512, NASA-CR-152275, N79-23868, 31 March 1979, NHRC 14294 Cassini/Saturn/ Huygens Probe; Background, Fact Sheets and Brochures (1979–1996). JPL, “Planetary Voyage,” http://voyager.jpl.nasa.gov/science/planetary.html, 23 March 2004, accessed 7 July 2008. Andy Ingersoll interview, Rome, Italy, by author, 12 June 08. JPL, “Titan,” in Voyager – The Interstellar Mission, http://voyager.jpl.nasa.gov/science/saturn_titan.html, last updated January 14, 2003, accessed 7 June 08. Andy Ingersoll interview, Rome, Italy, by author, 12 June 08. JPL, “Saturn,” in Voyager – The Interstellar Mission, http://voyager.jpl.nasa.gov/science/ saturn.html, last updated January 14, 2003, accessed 7 July 08. JPL, “Saturn’s Rings,” in Voyager – The Interstellar Mission, http://voyager.jpl.nasa.gov/ science/saturn_rings.html, last updated January 14, 2003, accessed 8 July 08. Anne Marie Schipper and Jean-Pierre Lebreton, “The Huygens Probe—Space History in Many Ways,” Acta Astronautica 59 (2006):319–334. Bram Groen and Charles Hampden-Turner, The Titans of Saturn (Singapore: Marshall Cavendish Business and London: Cyan Communications Limited, 2005), p. 15. Groen and Hampden-Turner, p. 15. Paolo Ulivi with David Michael Harland, Robotic Exploration of the Solar System: Part 2 – Hiatus and Renewal, 1983–1996 (Springer Praxis, Praxis; 1 edition (Nov. 2008):313. Meltzer, Mission to Jupiter, pp. 48–49. “Key Reagan Administration Officials,” http://www.reagan.utexas.edu/archives/reference/ keyofficials.html, Ronald Reagan Presidential Library Archives, U. of Texas, accessed 14 July 08. Meltzer, Mission to Jupiter, p. 52. Groen and Hampden-Turner, pp. 15–18. Torrance Johnson interview, Rome, Italy, by author, 11 June 08. ESA/NASA, Cassini: Report on the Phase A Study, doc. no. SCI(88)5 (Oct. 88):i, JPL/Cassini CASTL. ESA, “Giotto,” http://sci.esa.int/science-e/www/area/index.cfm?fareaid=15, accessed 8 July 08; ESA, “Giotto – Grigg-Skjellerup,” http://sci.esa.int/sciencee/www/object/index.cfm? fobjectid=31877, accessed 8 July 08. D. Gautier and W.H. Ip, “Project Cassini: A Saturn Orbiter/Titan Probe Mission Proposal,” http:// www.springerlink.com/content/v810rnm273n14k39/fulltext.pdf, Origins of Life and Evolution of Biospheres 14(1–4) (The Netherlands: Springer, Dec. 1984):802–803; Groen and HampdenTurner, pp. 18–19. Groen and Hampden-Turner, p. 17. ESA, Announcement of Opportunity—Cassini Mission: Huygens Probe, Annex A, Cassini Phase A Report + Addendum, ESA SCI(89)2 (October 1989). Charles Redmond, “Planetary Exploration Through Year 2000 Outlined,” NASA News, release no. 83–50, 17 Apr. 1983, NHRC 17908 Cassini Probe (‘81-’97 Aug.). Groen and Hampden-Turner, pp. 17–18, 200. ESA/NASA, Cassini: Report on the Phase A Study, doc. no. SCI(88)5 (Oct. 88):i, JPL/Cassini CASTL; Groen and Hampden-Turner, pp. 19–20. Peter J. Westwick, Into the Black: JPL and the American Space Program, 1976–2004 (Yale University Press, 2007), pp. 177–178.

24 Conceiving and funding the mission 45. Westwick, Into the Black, p. 179. 46. NASA, “Thomas O. Paine,” http://history.nasa.gov/Biographies/paine.html, updated 22 October 2004. 47. U.S. National Commission on Space, Pioneering the Space Frontier, (New York: Bantam Books, May 1986). 48. Ibid. 49. Ibid. 50. Ibid. 51. David S.F. Portree, “Challengers,” chapter 8 in Humans to Mars: Fifty Years of Mission Planning, 1950–2000, http://history.nasa.gov/monograph21/Chapter%208.pdf, NASA monographs in aerospace history #21, SP-2001-4521 (Washington D.C.: NASA, February 2001), p. 69. 52. Portree, “Challengers,” p. 69. 53. Sally K. Ride, NASA Leadership and America’s Future in Space, http://history.nasa.gov/ riderep/cover.htm, NASA History Division, Aug. 1987, updated 22 February 2006, p. 50. 54 U.S. National Security Decision Directive Number 42, “National Space Policy,” http://www. hq.nasa.gov/office/pao/History/nsdd-42.html, 4 July 1982. 55. John F. Kennedy Address at Rice University, 12 September 1962. 56. Ride, NASA Leadership, 57. Ibid., p. 11. 58. Ibid., p. 27. 59. Ride, NASA Leadership. 60. A.V. Diaz et al., “New Approaches to Planetary Exploration: Spacecraft and Information Systems Design,” Acta Astronautica 13 (April 1986):185–196. 61. ESA/NASA, Cassini. 62. Mark Carreau, “Sally Ride is Leaving NASA After Making Major Contributions,” Houston Chronicle, 21 Sept. 1987. 63. ESA/NASA, Cassini: Report on the Phase A Study, doc. no. SCI(88)5 (Oct. 88):i, JPL/Cassini CASTL. 64. ESA/NASA, Cassini: Report on the Phase A Study, doc. no. SCI(88)5 (Oct. 88):1, JPL/Cassini CASTL. 65. ESA/NASA, Cassini: Report on the Phase A Study, doc. no. SCI(88)5 (Oct. 88):120, JPL/ Cassini CASTL. 66. ESA/NASA, Cassini: Report on the Phase A Study, doc. no. SCI(88)5 (Oct. 88):120, JPL/ Cassini CASTL. 67. NASA-JPL, “Outward to the Beginning,” JPL 400–341, June 1988, 14294 Cassini/Saturn/ Huygens Probe; Background, Fact Sheets & Brochures (1979–1996). 68. NASA-JPL, “Outward.” 69. NASA-JPL, “Outward.” 70. NASA-JPL, “Outward.” 71. NASA-JPL, “Outward to the Beginning,” revision 2, JPL 400–341, June 1991, 14294 Cassini/ Saturn/Huygens Probe; Background, Fact Sheets & Brochures (1979–1996). 72. L.A. Fisk letter to Roger Bonnet, 21 October 1988, NHRC 14552 ESA-U.S. Cooperation; David Dickson, “Europeans Decide on a Trip to Saturn,” Science 242 (9 Dec. 1988):1375–1376. 73. Dickson, “Europeans Decide on a Trip to Saturn,” p. 1375. 74. Dickson, “Europeans Decide on a Trip to Saturn,” pp. 1375–1376; Groen and HampdenTurner, pp. 20–21. 75. Daniel Gautier telephone interview, 19 July 2008, by author.

References 25 76. JPL, Minutes of Project Science Group Meeting, Rome, Italy, June 2008; Groen and HampdenTurner, p. 21. 77. David Dickson, “Europeans Decide on a Trip to Saturn,” Science 242 (9 Dec. 1988):1375– 1376; Gautier interview. 78. Kathy Sawyer, “Tight Budget Pinching Space Probe; European Agency Expected to Seek Joint U.S. Mission to Saturn,” Washington Post (24 Nov. 1988); David Dickson, “Europeans Decide on a Trip to Saturn,” Science 242 (9 Dec. 1988):1375–1376. 79. J.-P. Lebreton and D.L. Matson, “The Huygens Probe: Science, Payload and Mission Overview,” ESA Bulletin No. 92, November 1997; Groen and Hampden-Turner, p. 21. 80. Roger-Maurice Bonnet, “Evolution of International Cooperation in Space Science,” http:// www7.nationalacademies.org/ssb/IGY_Beckman_Bonnet_remarks.pdf, National Academies Space Studies Board, p. 3. 81. Dickson, “Europeans Decide on a Trip to Saturn,” p. 1376. 82. Dickson, “Europeans Decide on a Trip to Saturn,” p. 1375. 83. CRAF – the Comet Rendezvous-Asteroid Flyby mission – was to use the same Mariner Mark II type of interplanetary spacecraft as Cassini (in order to save on development costs). NASA requested that these two endeavors be funded as one mission. 84. U.S. Senate, “National Aeronautics and Space Administration Authorization,” Congressional Record – Senate, Vol. 134 No. 118, 100th Cong. 2nd Sess., 134 Cong Rec S 11213, 9 August 1988. 85. Both quotes in the sentence are from U.S. Senate, “National Aeronautics and Space Administration Authorization.” 86. Science Magazine, “Research and the ‘Flexible Freeze,’” Science 242 (9 Dec. 88):1370. 87. National Aeronautics and Space Administration Authorization Act, Fiscal Year 1989, which became Public Law No. 100–685 on 17 Nov. 1988. 88. U.S. Senate, “National Aeronautics and Space Administration Authorization,” Congressional Record – Senate, Vol. 134 No. 118, 100th Cong. 2nd Sess., 134 Cong Rec S 11213, 9 August 1988. 89. Congressional Record – Senate, 101st Cong. 1st Sess., 135 Cong Rec S 8431, Vol. 135 No. 98, 20 July 1989. 90. Congressional Record – Senate, 101st Cong. 1st Sess., 135 Cong Rec S 8432, Vol. 135 No. 98, 20 July 1989. 91. Both quotes in the sentence are from Congressional Record – Senate, 101st Cong. 1st Sess., 135 Cong Rec S 8428, Vol. 135 No. 98, 20 July 1989. 92. Congressional Record, “National Aeronautics and Space Administration Multiyear Authorization Act of 1989,” 101st Cong. 1st Sess., 135 Cong Rec H 5819, Vol. 135 No. 122, 21 September 1989. 93. Congressional Record, “Messages from the House” (Senate - September 25, 1989), p. S11759. 94. Congressional Record, “Measures Referred” (Senate - September 25, 1989), p. S11759. 95. Library of Congress, “S.916,” http://thomas.loc.gov/cgi-bin/bdquery/z?d101:s.00916:, Thomas, accessed 13 May 2008. 96. Congressional Record, “National Aeronautics and Space Administration Authorization Act, Fiscal Year 1990” (Senate - November 09, 1989), p. S15410. 97. Library of Congress, “S.916” (Summary of all Congressional actions), http://thomas.loc.gov/ cgi-bin/bdquery/z?d101:SN00916:@@@X, Thomas, accessed 13 May 2008. 98. House Report 101–297, a conference report accompanying H.R. 2916, the Veterans Affairs and Housing and Urban Development, and Independent Agencies (VA-HUD-IA) Appropriations Act, 1990.

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99. NASA, “National Aeronautics and Space Administration Comet Rendezvous Asteroid Flyby (CRAF)/Cassini Cost Containment Plan - Second Biannual Status Report, August 1990,” attached to Richard H. Truly letter to Bill Nelson, 31 August 1990, NHRC 17908 Cassini Probe (81–97 Aug.). 100. Congressional Record, “National Aeronautics and Space Administration Multiyear Authorization Act of 1989,” (House of Representatives - 20 November 1989), p. H9128. 101. Congressional Record, “Hollings (and Gore) Amendment No. 1208” (Senate - 21 November 1989), p. S16840. 102. Committee on Commerce, Science, and Transportation, “Hearings Before the Subcommittee on Science, Technology, and Space,” Senate Hearing 101–981, 101st Congress, 9 March 1990, pp. 22–24. 103. Congressional Record, “National Aeronautics and Space Administration Authorization Act” (Senate - 21 November 1989) p. S16590. 104. NASA, “National Aeronautics and Space Administration Comet Rendezvous Asteroid Flyby (CRAF)/Cassini Cost Containment Plan/Report, March 1990,” attached to Richard H. Truly letter to Albert Gore Jr., 12 Apr. 1990, NHRC 17908 Cassini Probe (81–97 Aug.); NASA, “National Aeronautics and Space Administration Comet Rendezvous Asteroid Flyby (CRAF)/ Cassini Cost Containment Plan, Second Biannual Status Report, August 1990,” attached to Richard H. Truly letter to Bill Nelson, 31 Aug. 1990, NHRC 17908 Cassini Probe (81–97 Aug.) 105. Congressional Record, “Appointment of Conferees on H.R. 3729, National Aeronautics and Space Administration Multiyear Authorization Act of 1989,” (House of Representatives - 28 February 1990), p. H515. 106. “S. 2287: National Aeronautics and Space Administration Authorization Act, Fiscal Year 1991,” http://www.govtrack.us/congress/bill.xpd?bill=s101-2287, govtrack.us, accessed 17 May 2011. 107. “S.2287: National Aeronautics and Space Administration Authorization Act, Fiscal Year 1991,” 101st Congress; Library of Congress, “S. 2287 - All Congressional Actions,” Thomas, accessed 14 May 2008. 108. Congressional Record, “Explanation of S. 2287,” National Aeronautics and Space Administration Authorization Act, Fiscal Year 1991 (House of Representatives - October 25, 1990).

2 Building an international partnership and preventing mission cancellation This chapter describes how NASA, the European Space Agency, and the Italian Space Agency constructed a mutually beneficial international coalition to develop CassiniHuygens in the face of U.S. congressional budget reductions and repeated threats of mission cancellation. The chapter also analyzes the fiery relationship that Dan Goldin, one of NASA’s most colorful Administrators, had with the mission and considers the basic question of how best to explore the outer solar system.

2.1

NASA-ESA-ASI MISSION PLANNING ACTIVITIES

While the two houses of Congress were conducting their lengthy debates over the form of the NASA Authorization Bill, NASA and ESA were moving ahead on several fronts with Cassini-Huygens mission planning activities. A draft JPL/ESA Cassini/Huygens Project Implementation Plan was released in June 1989.1 In October 1989, over a year before the authorization bill was signed by President Bush and the final memorandum of understanding (MOU) was signed, ESA and NASA released a joint announcement of opportunity inviting scientific investigations on both the Cassini Orbiter and Huygens Probe. The two sets of scientific experiments were addressed by both agencies, working in close coordination with each other as well as with the various European state agencies which were providing funding for specific hardware. Probe and Orbiter selections were announced by ESA and NASA, respectively, in September and November 1990.2 The instruments that were chosen by this means are described later in the book. In June 1990, JPL and ESA issued the jointly developed Cassini/Huygens Project Implementation Plan (CHPIP),3 as required by the as-yet unfinished MOU between NASA and ESA. This described how the Huygens Probe was to be integrated with the Cassini Orbiter, and included the basic management and operations structure as well as details of the handling and distribution of data generated during the mission. #

© Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_2

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Building an international partnership…

During this time of congressional negotiations and mission planning, NASA also developed important MOUs with its international partners: one with ESA, and one with ASI. 2.1.1

The ESA memorandum of understanding

In December 1990, the month after Cassini’s authorization was signed into law as part of Public Law Number 101–611, NASA signed a key MOU with ESA for a joint venture to the Saturn system. This established a basis for cooperation that had been discussed for years, but not formalized. The Orbiter that NASA was to build would circle the planet, while ESA’s Probe would descend through Titan’s atmosphere and analyze its composition. The MOU also delineated specifics of the Cassini-Huygens spacecraft launch, which was to use a Titan 4 expendable rocket.4 The mission was now called Cassini-Huygens to honor the two seventeenth-century astronomers who made pioneering telescopic observations of the ringed planet. Upon signing the MOU, ESA Director General Jean-Marie Luton joined NASA Administrator Richard H. Truly5 in wishing this “ambitious and historic mission to the outer worlds”6 complete success. U.S. cooperation with Europe on space missions made sense for several reasons. It helped create a positive image of – and continued – goodwill toward the U.S., reinforcing the perception of our open relations and sharing of scientific data with other nations. Such cooperation with ESA on missions such as Cassini-Huygens, in which numerous European nations participated, encouraged European unity and strengthened U.S. ties to those nations. From a broad perspective, the sharing of space technology and solar system exploration was a tool of diplomacy that served U.S. foreign policy goals. Such international projects also “expanded the investment for any space project beyond that committed by the United States”7 and helped create new markets for U.S. aerospace industries. Another advantage of international cooperation that proved to be of particular benefit to the Cassini-Huygens mission in the early 1990s is that contracts in place with foreign nations help “insulate [U.S.] space projects from drastic budgetary and political changes.”8 Roger Launius of the Division of Space History at the National Air and Space Museum, and formerly NASA’s Chief Historian, commented on the “notoriously rambunctious and shortsighted” nature of U.S. politics, but also noted that “neither U.S. diplomats nor politicians relish”9 the international incidents that can result from abortive program changes impacting our partner nations in negative ways; a good example being our sudden cancellation of the International Solar Polar Mission and the vitriol that this triggered from Europe. A previously agreed-upon cooperative project with foreign entities might, in fact, make the difference between letting the project continue, instead of canceling it for short-term political and budgetary pressures. International partners, in other words, can act as stabilizing influences for space missions, “in essence a bulwark to weather difficult domestic storms.”10 This was the case for the 1980s Space Station Freedom program, even though it ultimately did not get completed. Responding to President Reagan’s invitation, Canada, Japan and ESA had all joined the Freedom program and were given important roles in its design, development, and operational phases. According to Andrew Stofan, NASA Associate Administrator for the Space Station, it was of consequence to signal to

2.1

NASA-ESA-ASI mission planning activities 29

international partners that “the U.S. commitment … is firm and that their own budgetary and intellectual resources can safely and productively be devoted to a cooperative … program.”11 Support for international partnering on missions was not a NASA-wide position by any means though. For instance, a 25 May 1982 briefing for NASA Headquarters officials captured a major misgiving on foreign involvement in the space station with the opinion that major international space projects just resulted in U.S. technology leakage abroad. In addition, there was serious concern that international involvement was inconsistent with possible U.S. military utilization of the space station.12 2.1.2

The ASI memorandum of understanding

In parallel with the NASA-ESA conceptualization of the Cassini-Huygens mission there were discussions between NASA and the Italian Space Agency (ASI). Besides being a major player in ESA and contributing to the instrumentation aboard the Titan Huygens Probe, Italy had a strong space research-related relationship with NASA as well as with the U.S. Air Force Office of Scientific Research dating back almost to the start of the Space Age. This included joint Italian-U.S. sounding rocket activities and efforts such as the San Marco Project in which Italian satellites were launched using rockets furnished by the U.S.13 ASI drew on this relationship to make its own bilateral arrangement with NASA. ASI saw advantages in using its own areas of expertise to augment Cassini Saturn Orbiter capabilities beyond what NASA alone was able to fund. Such actions could promote the Italian science community’s interests as well as those of the country’s technological industries.14 An August 1990 letter from NASA’s Director of International Relations Peter G. Smith to ASI President Luciano Guerriero detailed the deliverables that ASI would provide for the Cassini Orbiter if the bilateral agreement were to be formalized. The deliverables included: • • • •

Titan radar mapper (CTRM), which would be able to “see” through the satellite’s clouds and characterize its surface Visual and infrared mapping spectrometer (VIMS), for gathering data on the composition of moon surfaces, rings, and the atmospheres of Saturn and Titan15 Four-band (S, X, Ku, Ka) high-gain antenna (HGA) Radio science subsystem (RSS), which would interface with the HGA for signal transmission and reception.16

Although the 1990 letter from Smith to Guerriero stated that NASA and ASI had agreed to this bilateral project, it was not until June 1993 that Italy’s Minister of Universities and Scientific and Technological Research, Umberto Colombo, whose ministry oversaw ASI, approved the MOU and formalized the agreement. He also approved ASI’s plans to obtain a loan that would ensure the approximately $100 million needed to sign Cassini hardware development contracts with Italian industry. Colombo’s approval was not by any means an assured action, for it came only “after a week of uncertainty during which the Minister bluntly told U.S. Embassy officials that, given the uncertainty that the U.S. created

30 Building an international partnership… regarding Italian and European participation in the Space Station program, Italy would not be able to determine whether funding was available to proceed with Italy’s part of the Cassini program for two to three months.”17 Although there were hard feelings on the Italian side over U.S. actions in the past, ASI President Guerriero did assure NASA that Cassini-Huygens was a “joint NASA/ASI/ESA program of great interest and importance to Italy.”18 The Cassini-Huygens MOU between NASA and ASI was signed by ASI on 22 June 1993, eight days after it was signed by NASA.19

2.2

NINETEEN-NINETIES IMPACTS OF CONGRESSIONAL BUDGET REDUCTIONS AND THE THREAT OF MISSION CANCELLATION

When President Bush signed NASA authorization bill S. 2287 on 16 November 1990 and it became Public Law Number 101–611, Cassini-Huygens passed a major milestone. However, it was not long before the mission again encountered funding troubles. One of these occurred in summer 1991, when the House of Representatives threatened to shut down the Solid Rocket Motor Upgrade (SRMU) program. Under an agreement between NASA and the Air Force,20 the Air Force was to provide the Cassini-Huygens launch vehicle to NASA, which was responsible for the overall success of the mission, including integrating the spacecraft with the launch vehicle and approving the final launch.21 The Advanced Solid Rock Motor (ASRM) was to add 25% to the existing motor’s capability, but its development was plagued with problems and this concerned many in Congress. The House Appropriations defense subcommittee stated in its fiscal year 1992 report that “it was clearly inclined to terminate”22 the program. 2.2.1

The importance of the SRMU program

If the House made good on its threat to cancel the SRMU program, it would cause a series of consequences that could have ended up adding hundreds of millions of dollars to Cassini-Huygens mission costs. CRAF/Cassini program manager Howard T. Wright explained that without the upgrade to the motor, NASA would have to find new trajectories for both the CRAF and Cassini spacecraft. “We have to make an extra gravity assist, either at the Earth or Venus, and that extends by one year or more the mission duration.”23 The mission’s planned launch dates would also be delayed by about a year. This would make it impossible for CRAF/Cassini to stay within its $1.6 billion congressionally mandated funding cap. Loss of the SRMU program would have a heavier impact on Cassini-Huygens than CRAF. Many different trajectories could have been selected for CRAF, for there was a range of asteroids and comets that the spacecraft could choose from in order to fulfill its mission objectives. But the timing of the Cassini-Huygens launch was more critical, because the opportunity to use Jupiter as a gravity assist might be lost by a launch delay. Jupiter and Saturn were rapidly separating from each other. In Howard Wright’s words, “When they get far enough apart it doesn’t pay to use Jupiter as a gravity assist.”24 The

2.2 Nineteen-nineties impacts of congressional budget reductions… 31 critical launch limit was October 1997; a Jupiter gravity assist would not be beneficial if Cassini-Huygens were to be launched after that time.25 The SRMU program, though threatened, was still alive in the fall of 1991. In fact, Congress increased its proposed funding $115 million above the White House’s fiscal year 1992 budget request – for a total of $465 million. It did not hurt the program to have the strong support of Representative Jamie Whitten, an influential Democrat from Mississippi and chairman of the appropriations committee. The plant that would build the ASRM was in Yellow Creek, which was within Representative Whitten’s district.26 2.2.2

Challenges to CRAF/Cassini

Congress continued to support CRAF/Cassini through the fall of 1991, but with a new condition: NASA must persuade Germany to help finance launch operations.27 Congress was pressuring NASA to not request real growth over the previous year’s budget of more than 5%. In the 1980s, Congress had regularly approved budget increases that were above inflation. But now, dealing with recession, the White House instructed NASA not to expect increases above inflation for the foreseeable future.28 The White House had also mandated the Agency not to seek a total budget of more than $15 billion. NASA Administrator Richard Truly, who called this an “extremely constrained”29 budget, understood that certain sacrifices would have to be made. The single biggest cut that he proposed would do away with the SRMU program. He chose to drop this program, which was currently $469 million, because it didn’t appear to be absolutely necessary. Improvements made to the existing Solid Rocket Boosters (SRB) after the Challenger accident were working well and seemed to be adequate. Truly believed, however, that the CRAF mission had to be canceled to satisfy budget limits. But he expended great efforts to save the Cassini-Huygens mission, appealing all the way to the White House for its continued support. The CRAF mission had run into unforeseen expenses, in particular the escalating cost of one of its key technologies, the comet nucleus penetrator. Initially estimated at $22 million to build, the projected cost eventually rose as high as $120 million. This was too much of an increase and prompted NASA to cancel the instrument’s development. Without the comet nucleus penetrator, NASA questioned the justification for the mission. It did this even though a 1992 report by the Space Studies Board (SSB) of the National Academy of Sciences found that the CRAF mission still had significant scientific merit.30 NASA chose to eliminate CRAF, which helped the Agency meet congressional budget restrictions on mission expenditures. NASA saved “between $150 million and $200 million” by cancelling CRAF, according to NASA’s program manager for the mission Howard T. Wright.31 SSB was dismayed by this action, but recognized that it was necessary, since anticipated resources would probably not be adequate to cover both the CRAF and Cassini missions.32 In President Bush’s fiscal year 1993 budget request issued on 29 January 1992, he proposed NASA’s cancellation of both the CRAF mission and the SRMU program, although he asked for an additional $700 million over the Agency’s current budget of $14.3 billion. Even with this 5% increase, the $15 billion NASA would receive was far less than Truly originally requested.33

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In the fall of 1991, the White House had also pushed for cancellation of Cassini-Huygens, but NASA successfully defended against that move.34 But future funding was by no means assured. The mission was “on thin ice”35 within both the Bush Administration and Congress. In its fiscal year 1993 budget request, the Administration had ordered an in-depth review of Cassini-Huygens, calling for a reassessment of its technical and schedule risks.36 The Office of Management and Budget’s (OMB) science and space branch chief, Norine Noonan, explained to the Agency’s Space Science and Applications Advisory Committee (SSAAC) that this review might well lead to Cassini-Huygens’ termination. One of the main concerns was that with the shutting down of the SRMU program, Cassini-Huygens would have to take off in October 1997, very near the end of its launch window. Any further delays would push it out of this window and make it impractical to use a Jupiter gravity assist. Noonan thought this was likely to happen. In her words, “We are five years from launch, and I ask you to name me one big mission from NASA that has come in five years away that has actually come in on schedule …”37 Noonan’s statement was challenged by James Burch of San Antonio’s Southwest Research Institute, which was constructing Cassini-Huygens’ plasma spectrometer instrument. Burch cited the Upper Atmosphere Research Satellite as a mission that NASA produced on schedule and budget within five years of a major funding shift. Noonan vigorously defended her position, however, asserting that “all decisions to cut NASA programs were taken in collaboration with top NASA management.”38 She also reminded SSAAC members that “the Administration budget cuts were only the opening move in the annual budget battle,”39 warning that NASA would face a difficult battle on Capitol Hill as the year’s budget caps began to constrain programs. She had concerns that Cassini-Huygens might not survive those constraints. Even before the release of President Bush’s budget request in January 1992, the SSB’s Committee on Planetary and Lunar Exploration (COMPLEX) was planning a review of the Cassini-Huygens mission. This review was part of the Committee’s continuing assessment of the responsiveness of NASA missions to COMPLEX’s science objectives for exploring the solar system. While SSB recognized that even scientists were subject to the impacts of large budget deficits, the Board underlined the consequences of impairing or eliminating the Cassini mission. The COMPLEX review explained that the “Saturn system, with its interacting system of magnetic fields, plasmas, rings, and moons, [was] an ideal laboratory for many of the physical processes believed to be important in the formation and present-day dynamics of our solar system and of planetary systems of other stars,”40 and that because of the travel time to the outer solar system of seven or more years, it was critical not to interrupt development, or delay the launch, of the CassiniHuygens mission. Furthermore, the Cassini mission had to proceed without delay “in order to benefit from the extremely favorable orientation of Saturn’s rings at the spacecraft’s projected arrival in 2004.”41 President Bush did strongly support some kinds of missions, particularly human endeavors to the Moon and Mars. The Administration also requested $125 million – a huge increase over the $38 million that Congress had appropriated – for NASA to work on the National Launch System, a joint effort with the Department of Defense to develop the next generation of launch systems. It was estimated that this program would eventually cost more than $10 billion.42

2.2 Nineteen-nineties impacts of congressional budget reductions… 33 As discussed earlier, cancellation of the SRMU program would significantly impact Cassini-Huygens scheduling and operations. NASA had also planned to use this upgraded rocket to assist the Space Shuttle launch the elements of the Freedom space station later in the decade. NASA Administrator Richard Truly had wanted considerably more than the $15 billion that the Agency received for fiscal year 1993, and had met in late December 1991 with new White House Chief of Staff Samuel Skinner to lobby for an increase, but was unsuccessful. One of the reasons Truly thought NASA needed more funds was that $15 billion left no resources for the SRMU. His only alternative to canceling the rocket motor upgrade would have been to severely cut space science research, which had considerable support in Congress. The White House wanted to avoid such a confrontation. It seemed to Truly that canceling the SRMU program was necessary, and he expressed this at his White House meeting with Skinner. In the end, the SRMU program did indeed go forward in time for Cassini-Huygens to use the upgraded motor, but at the time of Truly’s meeting with Skinner, it appeared that canceling the program was the best alternative.43 2.2.3

The need for downscoping

Cutting the SRMU and CRAF programs were only some of the measures that appeared necessary at the time for NASA to meet its budget constraints. The Agency eventually decided to reduce the cost and size of Cassini-Huygens, as well as an environmental satellite program and an orbiting X-ray telescope. Conceived during a time of NASA budget growth, Cassini-Huygens (and other space science missions) now had to contend not only with a recession, but also with development of the space station, an orbiting X-ray observatory (later dubbed Chandra), and the Earth Observing System satellites.44 The word to cut back Cassini-Huygens was delivered to project personnel from NASA Associate Administrator Lennard Fisk. The task assigned to Cassini Orbiter engineers was to trim, if possible, 20% of the craft’s 6,600 kilogram (14,500 pound) mass. According to Dennis Matson, Cassini Project Scientist, two downsizing scenarios were considered: (1) remove a number of the spacecraft’s science instruments; or (2) eliminate the mechanisms that allowed them to rotate and point at a target, independent of the spacecraft’s orientation.45 The latter option would render the logistics far more complex for a dozen science teams to take their measurements. Few if any personnel realized just how much more complex these operations would become, but at least the range of different scientific observations possible with a full suite of instruments would not be reduced. This proved to be a key factor in deciding the issue. COMPLEX’s vision for investigating the Saturnian system and its reviews of CassiniHuygens provided guidance on the best way to downsize that mission. The multidisciplinary, multiple objective Saturn mission that COMPLEX envisioned in its 1986 document A Strategy for Exploration of the Outer Planets: 1986–1996, and that it considered to be “the highest priority for outer planet exploration in the next decade,”46 required a large and varied suite of instruments to analyze the assorted characteristics of the entire Saturn system. In March 1992 Louis J. Lanzerotti, chair of the SSB, wrote to Fisk. Lanzerotti agreed with this guidance and recommended that downsizing be carried out in a manner that retained the maximum science content possible. He affirmed that Cassini-Huygens’ planned configuration of instruments was “highly responsive to the scientific priorities set

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out in its [1986] report. … The instrument payload that has been tentatively selected, the mission plan that has been outlined, and the spacecraft that is being developed together provide an excellent opportunity to advance our understanding of Saturn and its satellites, rings, and magnetosphere.”47 While some modifications to the spacecraft would be necessary, the guidance of the letter report was to downscope in a manner that still allowed as wide a spectrum of scientific enquiry as could be retained. Keeping all of Cassini-Huygens’ research instruments was the way to accomplish this. Both JPL and Cassini-Huygens project management participated in the discussion of how to downsize the mission, and they indeed chose the second option mentioned above, which was to eliminate the mechanisms that allowed scientific instruments to rotate and point at a target, independent of the spacecraft’s orientation,48 but save all of the mission’s science instruments. Mission engineers hoped to achieve the 1,300 kilogram reduction requirement by paring only 250 kilograms (550 pounds) from the actual spacecraft hardware. The remaining mass would be cut from the propellant load. Richard Spehalski, Cassini-Huygens’ new project manager, recognized that reducing propellant would negatively impact the Orbiter’s maneuvering capabilities once it had reached Saturn, but fully understood the NASA budget constraints and endorsed the downsizing strategy.49 NASA was successful in trimming down the Cassini-Huygens spacecraft, thereby reducing its cost by $250 million and making it light enough to be launched using a standard Air Force Titan 4 system, without need of the upgraded solid rocket motors whose development budget had been cut. But NASA paid a price for its economies. Most of the hardware reduction was attained by eliminating the scan platform for remote sensing instruments, the turntable for fields and particles instruments, and an articulating radar antenna. These sophisticated mechanisms would have enabled a range of research groups to simultaneously maneuver their instruments and carry out their observations. Their elimination made it very much more difficult to operate the spacecraft. The fields and particles instruments would have had 360-degree viewing all the time. An articulating radar antenna would have enabled both radar and remote sensing observations of Titan and the icy moons to be made simultaneously. And a scan platform would have allowed the high-gain antenna to be Earth-pointed while the remote sensing instruments took data, increasing Cassini’s total data return and science return, especially around key events such as close flybys.50 Nevertheless, Associate Administrator Fisk and researchers were pleased that all twelve of the Orbiter’s instruments were to be retained, along with ESA’s Huygens Probe and its complement of instruments. The hitch was that the Orbiter’s science instruments now had to be rigidly attached to the main body of the spacecraft. Thus, the entire Orbiter would have to turn back and forth in order to point an instrument toward its target. Simultaneous measurements taken by two different instruments would often not be possible. And during a part of each Saturn orbit, the spacecraft would have to turn yet again to point its antenna back toward Earth and transmit data.51 Mark Dahl, the mission’s program executive at NASA Headquarters, commented that the personnel involved with the spacecraft’s redesign did not envision the level of complexity that would result from making the science instruments “body-fixed”52 rather than mounted on rotating platforms. For instance, besides having to move the entire spacecraft each time a science observation was to be carried out, tremendous care had to be taken to make sure that the vessel’s Sun sensors and star scanners, which provided critical information on spacecraft attitude, were pointed correctly and that the radiators, whose job it was

2.2 Nineteen-nineties impacts of congressional budget reductions… 35 to cool the detectors in the VIMS and CIRS instruments,53 were not pointed toward the Sun; that might have caused overheating problems. Also, because of the time involved in reorienting the spacecraft whenever a measurement was to be performed, and because orienting one instrument favorably often meant that another instrument could not take the observations its team wanted, the total amount of science that could be performed was necessarily reduced. Radar and camera imaging, for instance, could not be performed at the same time on the same target. For radar observations to be taken, the spacecraft’s antenna dish had to be pointed at the object, but this meant that the cameras were probably pointed in the wrong direction for their observations. With a traditional independently rotating scan platform, the situation would have been different. Additional burdens were placed on mission staff as a result of eliminating the rotating platforms. Krishan Khurana, a UCLA geophysicist on the magnetometer team, estimated that he needed to sit on three or four Cassini telephone conferences per week, each lasting one to two hours, in order to map out spacecraft operations two to three weeks ahead. Moreover, members of his team also needed to spend four to six hours per week on this planning work. That is a lot of time out from research activities. In comparison, on the Galileo Jupiter mission, which had both rotating and stationary sections, Khurana only “spent fifteen or twenty minutes a week discussing what observations we were going to get in the next two or three weeks.”54 Body-fixing the scientific instruments to the Orbiter did indeed impose large, unwelcome increases in personnel time, as well as significant added expense to the mission. But the bottom line was, the original breadth of scientific enquiry could still be carried out, and this was enormously important. As Lanzerotti and Joseph A. Burns, chairman of COMPLEX, wrote to Fisk: “The necessity of pointing the entire spacecraft, rather than just a scan platform, means that observations take longer. … On the other hand. … No Saturnian science objectives are lost [author’s italics]. … Even though fewer data will be taken per orbit, any set of observations needed to address a particular scientific question can be planned.”55

2.2.3.1

An omen of things to come?

Reducing the size and capital cost of a large spacecraft was one way to help satisfy budget constraints, but some members of the space science community were worried that something more dramatic was occurring. The shutdown of CRAF and the near-cancellation of Cassini-Huygens were really indicators that “the flagship concept of robotic planetary exploration”56 was ending. OMB expressed its opinion in its fiscal year 1993 budget document when it said that with current budget limitations, it was unlikely that multibillion dollar exploration missions that often spanned a decade or more could continue. Smaller, less costly endeavors should be emphasized instead. An editorial in Space News expressed distress over the OMB statement, making the point that NASA’s grand era of large robotic missions, which included such past successes as Galileo, Pioneer, and Voyager, might be ending because of a knee-jerk reaction to the tight budgets of the 1990s rather than in response to a well-examined “national decision by the Congress and the science community together with the White House.”57 The economic and scientific advisability of large flagship missions would become an especially poignant issue during the tenure of NASA’s new Administrator, Dan Goldin.

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2.3

CHANGE OF ADMINISTRATOR: GOLDIN REPLACES TRULY

In the early 1990s, NASA Administrator Richard Truly found himself increasingly at odds with the National Space Council, a White House committee headed by Vice President Dan Quayle, whom President Bush had designated to be in charge of the Administration’s space policies. Truly “had been resisting White House policy on a number of fronts”58 and Quayle considered him to be less than cooperative. The National Space Council as well as Congress and the American public saw much that was wrong about NASA. To some it was “a bloated bureaucracy pursuing missions that took too long, cost too much, and used technology that was old by the time it was put into space.”59 Disasters such as Challenger and high-profile missions such as the Hubble Space Telescope, with its technical delays, budget issues, and blurred vision, added to the perception that NASA was inept. The Agency needed a major overhaul and Truly did not appear able do it. The bickering and battling among NASA’s programs and field centers was a problem that Truly did not solve, nor could he head off Congress’s micromanagement of his budget. The time had come for a change in leadership. In 1991, Vice President Quayle and Presidential Chief of Staff John Sununu set out to remove Truly. They secured endorsements for this action from three former NASA Administrators. Members of the National Space Council deemed Truly to be “too committed to NASA’s tradition of large and costly missions.”60 They favored replacing him with Lieutenant General James Abrahamson, who had been the first Director of President Reagan’s Strategic Defense Initiative (SDI), nicknamed “Star Wars,” because they approved of SDI’s organizational approach. SDI managers had spearheaded development of the Clementine lunar mission, which cost only $80 million and moved “from design work to liftoff in just 22 months.”61 Although a software error prematurely terminated the mission, it was still seen as an example of how NASA might conduct its missions. But replacing Truly with Abrahamson did not fly with Democrats in Congress, who opposed SDI and did not want the space agency run by a “Star Warrior.” The National Space Council conducted a search for a manager who would be acceptable to it as well as to Congress and came up with Dan Goldin, Vice President and General Manager of TRW Space and Technology Group in Redondo Beach, California. While Goldin had little experience in the world of Washington politics, he was a rising star at TRW with a reputation as a dynamic, innovative leader and an advocate of small satellites. Though reluctant, in 1992 President Bush forced Truly to resign and turned to Goldin. The National Space Council believed that Goldin could give NASA the shaking up and realigning it needed. In particular, Goldin was willing, as President Bush demanded, to align NASA with the grim budget limitations of the 1990s. Goldin understood from Truly’s fate that “to get along with political masters, he would have to go along with budgetary reality.”62 Goldin’s strategy, however, was to have NASA control the program cuts that would be necessary, rather than let the White House and Congress impose their priorities.63 To gain this control for his Agency, Goldin had to convince President Bush and Congress that he was committed to their goals and was not simply another NASA advocate paying lip service to what his bosses wanted to hear.

2.3

Change of Administrator

37

It is sometimes forgotten that Goldin began his career at NASA’s Glenn Research Center. This preceded his 25 year tenure in industry, working with TRW. His entire career involved exploring and exploiting outer space. As NASA Administrator, he affirmed that while he was quite critical of NASA’s way of doing business, he was unconditionally prospace. But he was also committed to employing his industrial experience in making NASA operate far more efficiently than it had been doing. 2.3.1

Dan Goldin’s position on the Cassini-Huygens mission

“A combination of visionary, reformer, and hatchetman, Goldin wasted no time in making his presence felt.” – W. Henry Lambright64 With the appointment of Daniel S. Goldin to the position of NASA Administrator on 1 April 1992, the Agency’s analyses of its programs intensified, with an eye toward reducing mission operating costs.65 Goldin quickly organized review teams that identified ways of operating the programs faster, better, and cheaper, without compromising safety.66 The basic thrust of faster-better-cheaper was to find creative approaches to cut costs, improve performance with novel technological approaches, and accept a greater risk of failure in return for flying more missions.67 For instance, Goldin introduced design approaches from the microelectronics industry, requesting that project managers miniaturize components in order to reduce both spacecraft size and costs. And he introduced industry techniques for downsizing, or “reengineering” organizations. He also tasked project managers to meet their goals with fewer staff. Above all else, Goldin sought to avoid repeating the mistakes of Truly, who had “stubbornly championed bigger appropriations and resisted cuts.”68 Goldin expressed his sentiments quite strongly regarding the Cassini-Huygens project that was then under development, as well as other large missions, when he bemoaned that “the world’s finest research organization [NASA] had been reduced to two or three big programs, that everyone fed off. … I was prepared to recommend to the President that we cancel [CassiniHuygens] and start another bunch of programs.”69 Cassini-Huygens was on Goldin’s hit list. In his view, smaller, cheaper spacecraft that were developed more quickly and flown more frequently would give a superior science return compared to the spectacular, multifaceted craft of the past. NASA’s style was to load many experiments onto a small number of spacecraft. Because of the large scale of each such mission, it took a long time to develop the spacecraft. By the time one was launched, the technology it incorporated had become obsolete. But with so much invested in each spacecraft, NASA could ill afford to lose even one of them. Consequently, even though the 1990s was a time of an exciting “explosion in technology,”70 the Agency had become suffocatingly risk-averse, choosing ultra-reliable technology over more innovative, cutting edge designs.71 Goldin viewed this conservative approach as a “death knell for future space exploration.”72 As Goldin put it years later in his commencement address to the graduates of the Massachusetts Institute of Technology Class of 2001, he advocated that they go forth “with imagination, ingenuity and audacity”73 and try to change the

38 Building an international partnership… world. He approved of scientists and engineers taking risks in their technology choices and research goals, even if doing so endangered the mission. He wanted NASA to develop more missions than they currently did, but with significantly lower budgets and quicker development times. In one light, Goldin can be viewed as a bold risk-taker, ready to face mission failures in order to push the envelope and achieve superior results. But as we will see below, he was in another sense quite risk averse. Goldin’s desire for a faster, better, and cheaper approach to space exploration did not begin at the time he became NASA Administrator. Back when he was at TRW he disagreed with a NASA design for an Earth Observing System that would employ a complex, multi-instrument craft orbiting our planet. Goldin saw this spacecraft as too big and too expensive, and instead proposed a system of small payloads circling Earth. Lennard Fisk, who had been in charge of the project at NASA, threw out the proposal because he believed that small payloads would not be able to supply the near-simultaneous readings from different instruments as were required for scientific purposes. According to Groen and Hampden-Turner in The Titans of Saturn, Goldin never forgave Fisk for that. When Goldin took over at NASA, Fisk, who was one of its Associate Administrators, became “an early casualty”74 who got transferred to a position without budget authority. He later resigned. If Goldin, while he was at TRW, developed an animosity toward Fisk, these sentiments were apparently not returned by Fisk at that time. Just the opposite, in fact, for Fisk has stated, “I thought [Goldin] was wonderful. I thought he was the most responsive contractor that I had.” Fisk went on to affirm that he believed Goldin to be a really competent guy. But after Goldin became NASA Administrator, Fisk began viewing him quite differently, as a man who was very good at pleasing those he worked for (including, eventually, Vice President Al Gore) because “he answer[ed] all their desires and [was] very helpful.” On the other hand, people who worked for Goldin often found him difficult, to say the least. In Fisk’s opinion, “enormous damage was done to NASA during [Goldin’s] time as Administrator,” especially due to “the talent drain that took place under his Administration.”75 NASA lost vital experience and capability during Goldin’s tenure, according to Fisk. Fisk also had an idea why Goldin forced him out of NASA. For years Fisk had a contentious relationship with the White House’s National Space Council, chaired by Vice President Quayle. The Council did not like many things that Fisk was doing, and he believes that when Goldin was hired, one of his mandates was to get rid of Fisk. This did not prove easy, for Fisk was “too well connected and powerful, with congressional support.”76 So Goldin promoted him to the position of the Agency’s chief scientist, but without portfolio (or, as mentioned above, budget authority).77 Goldin and his review teams called for fundamental project management changes that would streamline NASA operations and reduce budgets. On Cassini-Huygens, technical managers were empowered to act as project managers for their particular subsystems. Personnel developing each element of a section of the spacecraft were to report directly to the technical manager. This additional autonomy given to line managers was meant to reduce Cassini-Huygens’ staffing levels by over 700 work-years.78 Goldin was particularly distressed that although CRAF/Cassini had been cut from “a double headed mission to rendezvous with a comet and make a detailed survey of Saturn”79 to a far smaller Saturn-only mission with fewer instruments, this had only slightly lowered the total price. Estimated development costs for CRAF/Cassini, as listed in a 1993 General

2.3

Change of Administrator

39

Accounting Office report, had been $1.85 billion, while for Cassini alone, they were estimated at $1.69 billion, a dip of only 8.8%. Even after launch and operations costs were factored in, the reduction attained by eliminating CRAF was projected to be only 19% of the dual mission total.80 Goldin railed at these disappointing figures, claiming there had been a study done for Cassini-Huygens estimating that it could be built for half the price JPL was telling him. The actual study Goldin was referring to is difficult to confirm, but Torrence Johnson of the Cassini-Huygens Imaging Science Team (and the project scientist of the Galileo mission) reckoned he knew which one it was. Johnson and John Casani, who served as project manager of both Cassini-Huygens and Galileo, had worked out a plan during the Galileo mission such that if NASA would provide funding to buy enough extra parts for the spacecraft, these could be used to fabricate another vessel “at cut rate that will go to Saturn.”81 Johnson and Casani estimated that this Saturn spacecraft would cost only 40% the price of building the Galileo spacecraft, as long as the Saturn spacecraft was an absolute clone of Galileo. Many engineers liked this idea because the capabilities of the Galileo spacecraft had been thoroughly studied. A similar approach, in fact, had been successful in the Mariner program. NASA had put a heat shield on a Mars Mariner spacecraft and sent it on a voyage to Mercury. Ron Draper, who was the Mariner Mark II project manager, explained why the Galileoclone idea was not pursued. Draper participated in the study examining the use of a second Galileo spacecraft for a Saturn voyage, but pointed out that this idea had to be abandoned during the study “because we had only one half of the needed subsystem hardware left over from Galileo as spares,”82 and the rest of the parts were no longer available because technology advances had rendered many of them obsolete. The decision was instead made to develop the Mark II spacecraft, which would “utilize Galileo design experience, but with ‘state-of-the-art’ parts.”83 If Dan Goldin was referring to the Galileo-clone study when he commented that Cassini-Huygens could be built for half-price, he may not have been aware that the required spare Galileo parts were no longer available. The Casani/Johnson strategy had a caveat: you would have to buy the extra parts when the Galileo spacecraft was being developed.84 A funding profile from the study envisioned starting work on the Galileo Saturn spacecraft at the beginning of 1983.85 During the early years of his tenure, Goldin continued to target Cassini-Huygens for cost and scope reductions, if not complete cancellation, and this raised the ire of many people. One of Goldin’s approaches, according to Caltech meteorologist Andy Ingersoll, was to respond to proposals from scientists and engineers involved in the mission by telling them to double their planned output, but at half the cost. They’d “cut to the bone and then he’d say, now do it again. At a certain point it was totally unrealistic.”86 In an interview with Science magazine, Goldin singled out Cassini-Huygens as an example of what was wrong with NASA’s traditional approach to space science, saying it was an overburdened “battleship Galactica.”87 He thought Cassini-Huygens presented too much risk as well as cost, and took too many years of preparation to return science data. NASA Chief Scientist at the time, France Cordova, explained the enormous risk when she said, “We can’t fail with that mission. It would be very, very damaging for the agency.”88 Too many of NASA’s resources were tied up in this one massive spacecraft. In Cordova’s

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Building an international partnership…

words, it was “truly putting all your eggs in one basket – your 18 instruments on one firecracker … you have to think of Mars Observer,”89 a “ten year, billion-dollar effort”90 that lost contact with Earth most probably on 21 August 1993, after an explosion of fuel and oxidizer elements as the spacecraft was preparing to maneuver for Martian orbital insertion.91 Putting such weighty importance on any one mission’s success thus was a perilous strategy. Part of the concern was with the Titan 4 launch vehicle that would lift the spacecraft off of Earth. It had suffered a number of failures, and Cordova feared a catastrophic loss if it crashed. It is interesting that Goldin was strongly risk averse when it came to putting so many resources into one large, expensive mission, but was quite the risk-taker for small-budget missions that pushed the envelope of what was achievable and had a significant chance of failure. Many space scientists spoke out vehemently against his objections to CassiniHuygens as well as his faster, better, cheaper approach to outer solar system exploration. “Cassini is a wonderful and exciting mission, and it can’t be done in a $100 million program,”92 said University of Michigan’s Andrew Nagy, a member of the Cassini-Huygens investigator team. University of Colorado’s Larry Esposito expanded on this theme, explaining that a planetary probe venturing as far from the Sun as Saturn needed a $100-million radioisotope thermoelectric generator instead of inexpensive solar cells to produce electric power, and that would use up a major part of a small mission’s budget.93 If a large mission was split up into several smaller efforts, they would require a greater overall amount of equipment and effort, including several launch vehicles as well as “more computers, more tracking, and more data transmittal time to achieve the same results.”94 This approach could end up being much more expensive.

2.3.1.1

SSB/COMPLEX’s position

Nagy’s and Esposito’s views overlapped with those of the Space Studies Board of the National Academy of Sciences, whose opinions were highly respected by both Congress and most of NASA. As Lanzerotti and Burns, respectively the chairs of the SSB and its Committee on Planetary and Lunar Exploration (COMPLEX), noted in a letter report to Fisk at NASA, they were concerned that current budget issues were “jeopardizing all of the planetary program’s large missions, including Cassini.”95 Furthermore, reconfiguring a large spacecraft into a series of small space vehicles “might be thought to provide a guide for the achievement of science goals outside the context of large missions. Such an analogy is inappropriate for Cassini [author’s italics].”96 Missions to the outer solar system required different designs than those to nearby targets. Long-lived components were needed to endure the long travel times and in situ mission durations. Specialized power systems (the expensive radioisotope thermoelectric generators noted above) were employed at distances far from the Sun that made the use of solar panels very difficult. And long-distance communication technologies and procedures were required that were fundamentally different from those for closer targets. These differences added considerable costs to outer planet missions. COMPLEX also believed that intermediate-sized or smaller spacecraft would be unable to achieve many of the objectives necessary for thorough exploration of the Saturnian

2.3

Change of Administrator

41

system. For example, studies of the interactions between the different parts of that system and concurrent coordinated observations of Titan’s atmosphere in situ by the Huygens Probe and remotely by the Orbiter would exceed the resources of a smaller spacecraft. COMPLEX just did not think that adequate analysis of Saturn, its fields, particles, rings, and moons could be accomplished by reconfiguring Cassini-Huygens into a number of small spacecraft.

2.3.1.2

Stable financial support

Another advantage of large missions was that they also helped provide more stable financial support to NASA centers than small missions could. According to Peter Westwick in his book Into the Black, JPL managers viewed the faster-better-cheaper strategy as “an insufficient business model to sustain the lab.”97 Cassini-Huygens, and big projects like it, provided fairly steady, extensive sources of support for JPL’s staff. This was hugely important, especially in times of shifting budgets. A project the size of Cassini-Huygens might directly fund 500 work-years of effort, roughly 10% of the laboratory’s total staff. During its peak, in fact, Cassini-Huygens funded almost 20% of JPL’s budget. The fiscal influence of big missions such as this rippled through the laboratory in many directions, supporting new infrastructure and funding the development of novel technologies with applications beyond the boundaries of one mission. Large flight projects also paid for test facilities, work stations, and computer capabilities that future missions could then employ. In fact, JPL’s executive council in 1994 questioned just how small projects with limited budgets could even survive at the laboratory “without a Cassini ‘cash cow’”98 to help support them. This point of view was apparently vindicated by what happened at JPL after Cassini’s launch. The programs supporting smaller missions, such as Discovery and Surveyor, could not offer the level of support that Cassini had, and JPL flight project staffing fell by about 300 work-years.99 Nevertheless, Goldin vehemently disagreed with the need for a cash cow project. He considered that approach an inappropriate, bureaucratic way of thinking. He held that there was “only one reason for big projects – if the laws of physics and biology demand that you have to make things bigger or heavier.”100 Goldin strongly believed that developing infrastructure such as test facilities and work stations was the job of NASA’s Administration and labs. It was their task to get the right equipment to their people. He advocated a dramatic cultural change at JPL and other NASA facilities – from the system where large projects drove the laboratory, to a system in which the laboratory found ways of providing sufficient infrastructure for its projects.101 Although large missions had long been vital to the way that NASA did business, the Agency’s new Administration meant to radically change things. France Cordova said quite candidly in 1994 to the Space Scientists Working Group, a subset of the Association of American Universities, that NASA would no longer develop missions the size of Cassini-Huygens.102 But canceling Cassini-Huygens several years in was another matter. Such an action would have been a huge blow to NASA’s European partners and to their

42

Building an international partnership…

trust in future agreements with the U.S. As expressed by Derek Davis, Director General of the British National Space Centre, in a 6 July 1994 letter to Goldin: “I would hope that every effort could be made by the U.S. Administration to avoid cancellation of international collaborative efforts such as these, which are under way and in which the partners have made substantial investment. Otherwise, the basis for future international collaboration in scientific ventures, and particularly in the case of Cassini, the value which the U.S. places on formal international agreements, will be called into question.”103 A week later, the matter was escalated by Jean-Marie Luton, the Director General of ESA. He sent strong words right to Vice President Al Gore that a U.S. withdrawal from Cassini would cast doubt on its reliability “as a partner in any future major scientific and technological cooperation.”104 Cassini-Huygens also received key support from inside of NASA. Wes Huntress, who had replaced Lennard Fisk as Associate Administrator for Space Science, and his colleagues stirred up the White House Office of Science and Technology Policy (OSTP), the Office of Management and the Budget (OMB), and allies in Congress, all of whom were against canceling a large investment mission that had already been approved and abandoning international partners.105 Goldin eventually came to terms with Cassini-Huygens and its powerful backers, acknowledging that it would not be feasible to divide the nearly developed mission into several less expensive parts. Years later he said, “I did not feel that at the time I had a way of breaking the mission up without wasting a lot of time and money. … There was no way to reconstitute a multiple spacecraft exploration of Saturn within a decade.” He also recognized the value of a successfully executed project, admitting that “I wanted to get that mission off because it was really important science.” But Goldin did ask project staff to streamline the way they were managing the program, to make it more cost effective. As noted above, the implemented changes included flatter management structures with more autonomy given to technical managers of various spacecraft components, resulting in significant reductions in work-years. The aggressive response of mission personnel to his requests greatly impressed Goldin. In an interview with the author, he commented that “I challenged them, they stepped up to the challenge with an incredibly positive attitude … and they did an incredible job.”106

REFERENCES 1. D. H. Kindt, “JPL/ESA Cassini/Huygens Project Implementation Plan (CHPIP),” JPL Interoffice Memorandum MMII-DHK-O1-89, 16 June 1989. 2. J.-P. Lebreton and D.L. Matson, “The Huygens Probe: Science, Payload and Mission Overview,” ESA Bulletin No. 92, November 1997 3. D. Kindt, “JPL/ESA Project Implementation Plan for the Cassini Mission: Saturn Orbiter/ Huygens Probe System,” JPL D-7546, PD 699–81, 1 June 1990. 4. ESA, “Huygens – The March to Titan,” press release no. 57, 19 Dec. 1990, NHRC 17908 Cassini Probe (81–97 August).

References 43 5. Debra J. Rahn and Paula Cleggett-Haleim, “NASA and ESA Sign Agreement for a Joint Saturn Mission,” NASA News release 91–1, 3 January 1991. 6. ESA, “Huygens – The March to Titan.” 7. R.D. Launius, “The View from Washington,” Proceedings of the International Symposium on “The History of ESA,” Science Museum, London, held 11–13 Nov. 1998. ESA SP-436, June 1999. 8. Launius, “The View.” 9. Both quotes in sentence are from Launius, “The View.” 10. Launius, “The View.” 11. Andrew J. Stofan, Space Station: The Next Logical Step, http://ntrs.nasa.gov/archive/nasa/casi. ntrs.nasa.gov/19910009810_1991009810.pdf, NASA Technical Memorandum NASA-TM103398, (1991). 12. John M. Logsdon, Together in Orbit: The Origins of International Participation in Space Station Freedom (Washington D.C.: Monographs in Aerospace History #11, NP-1998-10244-HQ, November 1998). 13. Michelangelo De Maria et al., “Italy in Space: 1946–1988 ,” ESA HSR-30 (March 2003):6, 13. 14. JPL, “International Participation in the Cassini Project,” in Spehalski memos, May 94, JPLCassini CASTL; B. Pernice, “The Cooperation Between NASA and ASI on the Cassini Mission,” Il Nuovo Cimento C 15(6) (Nov. 1992): 1133–1136. 15. JPL, “Spacecraft – Cassini Orbiter Instruments – VIMS,” http://saturn.jpl.nasa.gov/spacecraft/ instruments-cassini-vims.cfm, Cassini-Huygens Mission to Saturn and Titan Web site, accessed 20 May 2008. 16. Peter G. Smith letter to Luciano Guerriero, 15 August 1990, JPL-CASTL; F. Nirchio, B. Pernice, L. Borgarelli, and C. Dionisio, “The Italian Involvement in Cassini Radar,” in Radars and Lidars in Earth and Planetary Sciences, ESA, SEE N92-25668 16–32, December 1991, pp. 79–81; Lebreton and Matson, “The Huygens Probe.” 17. J. Zimmerman email to P. Smith, S. Ballard, “Cassini MOU Approved,” 5 Jun 1993, attachment to Gary Parker interoffice memo, “Italian MOU,” 8 June 1993, JPL-Cassini CASTL. 18. J. Zimmerman email. 19. NASA, “Memorandum of Understanding Between the United States National Aeronautics and Space Administration and the Italian Space Agency concerning the Cassini Mission,” signed by NASA 14 June 1993 and by ASI 22 June 1993, JPL-Cassini CASTL. 20. Memorandum of Agreement between the National Aeronautics and Space Administration and United States Air Force on Titan IV-Centaur Launch Support for the Cassini Mission,” Aug. 11, 1994. In particular, the Air Force was to provide a Titan IV expendable launch vehicle with a Centaur upper stage, as well as launch service from Cape Canaveral Air Force Station in Florida. 21. GAO, Cassini Mission: Estimated Launch Costs for NASA’s Mission to Saturn, GAO/NSIAD95-141BR (May 1995); Defense Daily, “GAO Examines Saturn Probe’s Launch Cost” (30 May 1995). 22. Aerospace Daily, “Killing SRMU Could Add ‘Hundreds of Millions’ to CRAF/Cassini,” 9 July 1991, p. 34, NHRC 17908 Cassini Probe (81–97 Aug.). 23. Aerospace Daily, “Killing SRMU.” 24. Aerospace Daily, “Killing SRMU.” 25. Aerospace Daily, “Cuts Push Cassini to Edge of Launch Window; Upset Development,” 31 Jan. 92, pp. 165–166, NHRC 17908 Cassini Probe (81–97 Aug.). 26. Eliot Marshall, “Space Station, NSF Funds Approved,” Science 254 (11 Oct. 91):191. 27. Marshall, “Space Station, NSF Funds Approved.” 28. Helen Gavaghan, “NASA Cuts Back to Win Over Congress,” New Scientist (30 May 1992).

44 Building an international partnership… 29. Colin Norman, “Civilian R&D: The Big Four Federal Spenders,” Science 255 (7 Feb. 92):674. 30. Louis J. Lanzerotti letter to Lennard A. Fisk, “On the CRAF/Cassini Mission,” 30 March 1992, Space Studies Board-National Academies Web site, accessed 22 May 2008. 31. Aerospace Daily, “Cuts Push NASA to Edge of Launch Window; Upset Development,” (31 Jan. 1992):165–166. 32. Richard M. Jones, “Lessons Learned: GAO Report on the CRAF/Cassini Missions,” http:// www.aip.org/enews/fyi/1994/fyi94.014.txt, American Institute of Physics, FYI No. 14, 4 Feb. 1994. 33. Frank Degnan and Allan Roberts, Space Science: Causes and Impacts of Cutbacks to NASA’s Outer Solar System Exploration Missions, U.S. General Accounting Office Report to Chairman, Subcommittee on Investigations and Oversight, Committee on Science, Space, and Technology, House of Representatives, Dec. 1993, pp. 3–5; Andrew Lawler, “White House to Cancel Two NASA Programs,” Space News 3(3) (27 Jan.-2 Feb. 1992): 1. 34. Andrew Lawler, “White House to Cancel Two NASA Programs,” Space News, 27 Jan.-2 Feb. 1992, NHRC 17908 Cassini Probe (81–97 Aug.):29. 35. Aerospace Daily, “OMB Official Warns Cassini on Thin Ice in Executive Branch, Congress,” (13 Feb. 1992):241–242. 36. Louis J. Lanzerotti, chair of the Space Studies Board, letter to Lennard A. Fisk, associate administrator for NASA’s Office of Space Science and Applications, “On the CRAF/Cassini Mission: Letter Report,” National Research Council (30 March 1992). 37. Aerospace Daily, “OMB Official Warns Cassini.” 38. Ibid. 39. Ibid. 40. Lanzerotti letter to Fisk, 30 March 1992. 41. Ibid. 42. Andrew Lawler, “White House to Cancel Two NASA Programs,” Space News, 27 Jan.-2 Feb. 1992, NHRC 17908 Cassini Probe (81–97 Aug.):29. 43. Lawler, “White House to Cancel;” Dave Nicponski, “Alliant Techsystems SRMU Boosters Launch Cassini/Huygens Mission to Saturn Aboard Titan IV B Vehicle,” Alliant Techsystems (ATK) Media Center news release (15 October 1997). 44. Peter J. Westwick, Into the Black (New Haven & London: Yale University Press, 2007), pp. 260–261. 45. Dennis L. Matson email to author, 1 Apr. 2010. 46. Space Science Board, Committee on Planetary and Lunar Exploration, A Strategy for Exploration of the Outer Planets: 1986–1996 (Washington, D.C.: National Academy Press, 1986). 47. Lanzerotti letter to Fisk, 30 March 1992. 48. Dennis L. Matson email to author, 1 Apr. 2010. 49. Douglas Isbell, “NASA Reduces Size, cost of Cassini Spacecraft,” Space News (2–8 March 1992):3, NHRC 17908 Cassini Probe (81–97 Aug.). 50. Linda Spilker review of manuscript, March 2011. 51. Douglas Isbell, “NASA Curtails Future Costs of Cassini Saturn Probe,” Space News (18–24 May 1992), NHRC 17908 Cassini Probe (81–97 Aug.); Bruce A. Smith, “JPL Redesigns Cassini to Cut Weight, Development Cost,” Aviation Week & Space Technology (1 June 1992), NHRC 17908 Cassini Probe (81–97 Aug.). 52. Mark Dahl interview, Washington D.C., by author, Sep. 2007. 53. Bob Mitchell review of manuscript, Feb. 2011. 54. Krishan Khurana interview with author, JPL, 26 October 2010.

References 45 55. Louis J. Lanzerotti, chair of the Space Studies Board, and Dr. Joseph A. Burns, chair of the Committee on Planetary and Lunar Exploration, letter to Dr. Lennard A. Fisk, Associate Administrator for NASA’s Office of Space Science and Applications, “On the Restructured Cassini Mission: Letter Report,” Committee on Planetary and Lunar Exploration, National Research Council (19 October 1992), pp. 3–4. 56. Space News, “Going the Way of the Dinosaurs” (23–29 March 1992), NHRC 17908 Cassini Probe (81–97 Aug.). 57. Space News, “Going the Way of the Dinosaurs.” 58. W. Henry Lambright, “Downsizing Big Science: Strategic Choices,” Public Administration Review 58(3) (May/June 1998):262. 59. W. Henry Lambright, Transforming Government: Dan Goldin and the Remaking of NASA,” PricewaterhouseCoopers Endowment for the Business of Government (March 2001). 60. Howard E. McCurdy, Faster, Better, Cheaper: Low-Cost Innovation in the U.S. Space Program (Baltimore: Johns Hopkins Univ. Press, 2001):47. 61. Stephanie A. Roy, “The Origin of the Smaller, Faster, Cheaper Approach in NASA’s Solar System Exploration Program,” Space Policy 14 (Aug. 1998):153–171, as reported in McCurdy, Faster, Better, Cheaper. 62. Lambright, “Downsizing Big Science,” p. 262. 63. Lambright, “Transforming Government.” 64. Lambright, “Downsizing Big Science,” p. 262. 65. Elvia Thompson and Jennifer Davis, “Daniel Saul Goldin,” http://history.nasa.gov/dan_goldin. html, NASA History Division, 12 March 2004. 66. Bill Livingstone, “Goldin Announces Initiatives to Improve NASA Performance,” NASA News release 92–154, 17 September 1992, NHRC 17908 Cassini Probe (81–97 Aug.). 67. Peter J. Westwick, Into the Black (New Haven & London: Yale University Press, 2007), p. 273. 68. Bram Groen and Charles Hampden-Turner, The Titans of Saturn (Singapore: Marshall Cavendish Business and London: Cyan Communications Limited, 2005), pp. 148–149. 69. Groen and Hampden-Turner, p. 149. 70. Dan Goldin telephone interview with author, 26 Apr. 2010. 71. Lambright, “Transforming Government.” 72. Dan Goldin telephone interview with author, 26 Apr. 2010. 73. MIT Club of Cape Cod, Newsletter, No. 49, http://alumweb.mit.edu/clubs/capecod/news/mit49. htm, Sept. 2001, accessed 10 May 2011. 74. Groen and Hampden-Turner, p. 150. 75. Quotes in the paragraph are from Lennard A. Fisk interview by Rebecca Wright, http://www. jsc.nasa.gov/history/oral_histories/NASA_HQ/Administrators/FiskLA/FiskLA_9-9-10.htm, NASA Headquarters Oral History Project (Ann Arbor, Michigan: 9 September 2010). 76. Fisk interview. 77. Science, “Space Scientists Get the Jitters,” Science 258 (20 Nov. 1992):1296. 78. Bill Livingstone, “Goldin Announces Initiatives to Improve NASA Performance,” NASA News release 92–154, 17 September 1992, NHRC 17908 Cassini Probe (81–97 Aug.). 79. Eliot Marshall, “Space Scientists Get the Jitters,” Science 258 (20 Nov. 92):1296. 80. U.S. General Accounting Office, Space Science: Causes and Impacts of Cutbacks to NASA’s Outer Solar System Exploration Mission, Report to the Chairman, Subcommittee on Investigations and Oversight, Committee on Science, Space, and Technology, House of Representatives (Dec. 1993):19–20. 81. Torrence Johnson interview, Rome, Italy, by author, 11 June 2008.

46 Building an international partnership… 82. Ronald F. Draper letter to Phillip de Aragon (5 Dec. 1989), JPL CASTL, in a set of Cassini documents beginning with: Jet Propulsion Laboratory Interoffice Memorandum, August 22, 1989, MMII – RFD-43–89. 83. Ibid. 84. Johnson interview, 11 June 2008. 85. “Galileo Comparative Funding Profile” (14 Dec. 1982), JPL Cassini CASTL, in a package of documents in the folder “DRAPER 83 Chron Files” and beginning with an October 28, 1983 letter to Geoffrey A. Briggs. 86. Andy Ingersoll interview with author, 18 Dec. 2009, AGU, San Francisco. 87. Eliot Marshall, “Space Scientists Get the Jitters,” Science 258 (20 November 1992):1296. 88. Liz Tucci, “Goldin Subjects Cassini to Cost, Risk Reductions,” Space News (14–20 March 1994):3, NHRC 17908 Cassini Probe (81–97 Aug.). 89. Ibid., p. 21. 90. Goldin interview, 26 Apr. 2010. 91. Goddard Space Flight Center, “Mars Observer,” http://heasarc.gsfc.nasa.gov/docs/heasarc/ missions/marsobs.html, accessed 9 March 2010, last modified 26 June 2003. 92. Ibid., p. 21. 93. Richard A. Kerr, “Scaling Down Planetary Science,” Science 264 (27 May 1994):1246. 94. McCurdy, Faster, Better, Cheaper, p. 107. 95. Louis J. Lanzerotti, chair of the Space Studies Board, and Dr. Joseph A. Burns, chair of the Committee on Planetary and Lunar Exploration, letter to Dr. Lennard A. Fisk, Associate Administrator for NASA’s Office of Space Science and Applications, “On the Restructured Cassini Mission: Letter Report,” Committee on Planetary and Lunar Exploration, National Research Council (19 October 1992). 96. Ibid. 97. Peter J. Westwick, Into the Black (New Haven & London: Yale University Press, 2007), p. 223. 98. Ibid. 99. Westwick, p. 272–273. 100. Goldin interview, 26 Apr. 2010. 101. Westwick, p. 223. 102. Liz Tucci, “Goldin”; Marshall, “Space Scientists.” 103. Derek Davis letter to Daniel S. Goldin, 6 July 1994, NHRC 17908 Cassini Probe (81–97 Aug.). 104. Charley Kohlhase, “Return to Saturn’s Realm,” Planetary Report (Mar./Apr. 2004):13. 105. Groen and Hampden-Turner, pp. 150–151. 106. All quotes in the paragraph are from Dan Goldin telephone interview with author, 26 Apr. 2010.

Part II

Designing, fabricating, and integrating the Cassini-Huygens space vessel Part II seeks to convey the sheer complexity of developing a spacecraft with the capabilities of Cassini-Huygens. To fully appreciate what the mission accomplished, the detail that I give to developing the spacecraft is very necessary. Engineering and constructing a reliable vehicle for the Saturnian environment involved multifaceted, cross-border coordination between and within the agencies NASA, ESA, and ASI. For instance JPL, the NASA laboratory that managed the mission, needed to oversee and synchronize the activities of other NASA centers, numerous contractors in the U.S. and those of ESA and ASI in Europe that were supplying vital components and services. In developing the Huygens Probe, ESA had to orchestrate the involvement of sixteen European nations in the mission, each of which had their own contractors and participating organizations to keep track of. Finally, all of the components had to be combined and integrated into a working spacecraft. That the spacecraft ultimately performed as well as it did is a glowing tribute to management capabilities, engineering efforts, and cooperation between the parties involved.

3 Constructing the Cassini Orbiter “In some ways, the Cassini spacecraft has senses better than our own. For example, Cassini can ‘see’ in wavelengths of light and energy that the human eye cannot. The instruments on the spacecraft can ‘feel’ things about magnetic fields and tiny dust particles that no human hand could detect.” – The Cassini Outreach Team1

The Cassini Orbiter should not be thought of as simply a sophisticated collection of propulsion mechanisms, sensors, and microprocessors. Scientists and engineers view such craft as extensions of our limited human perceptions; advanced receptors that allow us to detect phenomena that, unaided, we could never hope to study. Each of the optical instruments described later in this chapter expanded our abilities to see extremely faint occurrences as well as events that took place outside the very limited range of wavelengths that our eyes are equipped to pick up. Other instruments allowed us to perceive subtle variations in particle flows, electric and magnetic fields, and planetary chemistries that can tell us amazing things about the Saturnian system. All of these instruments were mounted on a vessel that far exceeded our frail bodies’ abilities to withstand tough conditions. To conduct a successful mission, the Orbiter’s engineers had to design a robust extension of ourselves that could operate comfortably for two decades in extremes of temperatures that would have quickly fried or frozen our delicate skin and organs. Furthermore, this Orbiter had to operate independent of any real-time guidance from us, sometimes for hours. It needed to make its own decisions to ensure its survival and ability to function. Every system described below played an important role in making the Orbiter what it was: a highly refined enlargement of our own limited human machinery. The Orbiter and its accompanying Probe enabled us to see farther, hear better, and feel sensations that we otherwise could not have. # On 16 November 1990, President Bush signed a NASA authorization bill that granted the CRAF/Cassini project a budget of $1.6 billion for development of the spacecraft and their scientific experiments, conducting the launch, and carrying out the first 30 days of © Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_3

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operations. But even before the presidential signing of the bill that became Public Law Number 101–611, NASA and ESA had begun developing concepts for the mission, hoping that it would receive Congressional approval. Their planning for the mission started early in the 1980s, as detailed in Chapter 1. During this pre-approval period, initial design work for both the Cassini Orbiter and the Huygens Probe took place. Efforts to develop the Orbiter are detailed here, while the development of the Probe is covered in the next chapter

3.1

FROM GALILEO TO CASSINI-HUYGENS

“Galileo was the nugget from which Cassini grew – ways of handling things were developed on Galileo.” – Torrence Johnson2 In the early 1980s, the space science community envisioned that the Cassini mission would be very analogous to the Galileo mission to Jupiter. Both spacecraft would travel through the outer solar system to a gas giant planet, exploring the body itself as well as its satellites, magnetic fields, and trapped particles. And both would continue the exploration that had begun with the Pioneer and Voyager flybys. But between the times when the Galileo and Cassini craft were designed and constructed, technologies arose that offered new options for Cassini’s operating systems. Driven by the need to prevent the nearly mission-ending problems that arose on Galileo, the Cassini engineering staff turned toward some of these new technologies, creating a spacecraft that was far more sophisticated and, in certain critical ways, far simpler. This book will repeatedly focus on the details of this evolution from Galileo to Cassini-Huygens, which built on lessons learned during the former mission, but implemented new technologies only available to the latter. According to Torrence Johnson, the Galileo mission’s project scientist, “CassiniHuygens [as it became known] started in vision … with program objectives and Orbiter objectives very similar to Galileo. But we knew that it would get more complex than Galileo, because there were more targets.”3 Unlike Galileo, the Cassini mission was to explore two primary heavenly bodies: Saturn and Titan. The moon Titan strongly resembles a planet. It is bigger than Mercury and has a thicker atmosphere than Earth. The Saturn system also has more satellites of interest than Jupiter. And while both Galileo and Cassini-Huygens would devote considerable attention to their planets’ magnetospheres, Cassini was also to investigate Saturn’s intricate ring system. Nevertheless, many “ways of handling things”4 were developed first on Galileo and later used on Cassini. For instance, according to Johnson, the Galileo team was the first to undertake electronic distribution of data and data products. On Voyager, scientific data got to investigators by shipping them magnetic tapes. But “by the time we got to Galileo, we had bought VAX computers for all the investigators and used the early version of the ARPANET – which became the Internet – across the Atlantic.”5 The U.S. Department of Defense’s Advanced Research Projects Agency Network, or ARPANET, was conceived to facilitate general communication among computer users at dispersed locations. By employing it, Galileo helped eliminate the need for so many of the mission’s scientific staff to be physically present at JPL.

3.2

Developing a mission to Saturn

51

The Galileo mission also helped make work schedules of teams in different parts of the world more efficient. Johnson noticed that on Galileo’s imaging team, for instance, data were processed at JPL during the day and evening, then sent electronically to investigators in Germany. After JPL staff went home for the night, German investigators nine time zones away put together mosaic images that were then ready for JPL scientists when they returned to work. Galileo, in other words, pioneered a greater degree of around-the-clock scientific analysis, and this approach continued on the Cassini-Huygens mission.6

3.2

DEVELOPING A MISSION TO SATURN

An important aspect of NASA mission development in the 1980s was that it was not done serially. Mission planners did not wait for complete results and full processing of the data from one mission before they began another. If they had waited, then exploring the solar system would have taken much longer.7 For instance, almost immediately after receiving Voyager transmissions from Saturn in 1980, the space science community began thinking seriously about a follow-up Saturn mission, this one using an orbiter. And earlier, as space scientists were receiving Voyager flyby communications from Jupiter in 1979, they were already involved in the design of the Galileo orbiter and atmospheric probe mission to that planet. From the early 1980s on, European as well as U.S. scientists and engineers were heavily involved in planning a Saturn mission. The Europeans were looking for a good followon to their Ulysses mission, which was originally going to be a two-spacecraft, joint venture with the U.S., but eventually became a one-ship, mostly European endeavor after the U.S. reduced its role (as discussed earlier in the book). The European space scientist community were considering building a vehicle that would orbit Saturn and conduct fields and particles research, while the U.S. supplied a probe to explore the veiled moon, Titan. This was actually the inverse of what happened; it was the U.S. that eventually built the orbiter and Europe the probe. 3.2.1

The Ulysses, Giotto and Galileo designs considered

European designs for deep space vehicles included one for Ulysses, a craft being developed to explore the Sun’s environment. This was a spin-stabilized vehicle in which attitude stabilization was achieved in the same way it was for a gyroscope – by giving the vehicle a rotation, as was done for the Pioneer 10 and 11 outer solar system missions. A spacecraft’s attitude must be stabilized in some way, to enable its high-gain antenna to accurately point at Earth for communications, its onboard experiments to point in the right direction, and heating and cooling effects of sunlight and shadow to be employed efficiently.8 Spin-stabilization is a good design for fields and particles research, because the sensors, often mounted on long booms, move through different locations and can measure fluxes coming from different directions. But a spinning vehicle is not an ideal platform from which to take high resolution images of planets and satellites. A stationary vehicle is better for that. ESA also considered basing a Saturn-bound spacecraft on the design of the spin-stabilized Giotto, its first deep space vehicle, built to study Halley’s Comet.9 But the U.S. wanted to

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conduct an aggressive, ambitious orbiter mission capable of both high resolution imaging and remote sensing – dual objectives that would satisfy the planetary science as well as the space physics communities. The existing European vehicle designs were too limited for an orbiter capable of such diverse science goals. European and U.S. scientists and engineers discussed other possibilities, including two orbiters: one for particles and fields that the Europeans would construct and one that would specialize in high resolution imaging that the U.S. would build. As the discussions continued, another idea emerged. The U.S. would fabricate a Galileo-class Saturn orbiter that had a stable platform for imaging and a rotating one for fields and particles. Our country was already building such a spacecraft for the Jupiter mission. Meanwhile, the Europeans would design and build a probe capable of taking key measurements while it descended through Titan’s thick atmosphere.10 Initially, the Europeans were quite leery of this idea. They didn’t think that their industries were capable of such a task. But upon further analysis, space scientists realized the entry conditions at Titan were not that severe. In fact, they resembled the conditions for medium-range ballistic missiles entering Earth’s atmosphere, and France had developed the knowledge base to build and fly such missiles. Eventually, Europe’s engineering consortium decided that a company such as Alcatel Space of France could fabricate such a craft. And that is what happened, although “to this day we [the U.S.] can’t get detailed specs about the materials on the Huygens Probe because it’s the same stuff France has on its warheads. That type of detailed information is held fairly close.”11 France insisted on remaining quite independent of the world science community in this sensitive area. As we will observe later in the book, keeping important information proprietary and not openly sharing it with other members of an international space exploration team led to a serious communication problem that nearly crippled the mission. Lack of transparency between international organizations on the mission was an ongoing cross-border issue. 3.2.2

Use a spare Galileo spacecraft for Saturn, or develop a new design?

In the early 1980s, the Galileo mission team was busy procuring parts and putting together its Jupiter-bound spacecraft. They sought to build one spacecraft and also put together a store of spare parts and subsystems. But the repeated launch date delays – especially one that pushed the launch back to May 1986 – created the need to hold the teams together longer than planned. The project managers realized that the teams now had the time to assemble and test a second Galileo spacecraft for a small percentage of the cost of the first. Galileo Project Manager John Casani and Project Scientist Torrence Johnson were also involved in planning for the Cassini mission to Saturn and had conducted some early needs assessments for this project. They, as well as Al Diaz, NASA’s Program Manager for Galileo, came up with an idea that could potentially save NASA considerable expense on Cassini: use a second Galileo spacecraft to explore Saturn, launching it as early as 1987.12 The Galileo project’s spare components already constituted half of what was needed for a second spacecraft, but NASA would have to fund the purchase of the remaining parts. This would allow JPL to fabricate a cut-rate clone of Galileo capable of journeying to Saturn. As discussed in Chapter 2, JPL staff estimated that this clone could be put together for only 40% of the cost of designing and building the first spacecraft.13 A 1982 NASA study

3.2

Developing a mission to Saturn

53

calculated that while the Galileo spacecraft bound for Jupiter would cost $245 million to develop, a second craft could be built for only $102 million (in real year dollars).14 Flying a Galileo-type spacecraft to Saturn interested many space scientists and engineers because they would not have to use some unknown or untried vessel – they knew well what the Galileo spacecraft capabilities were. Such a strategy had been used successfully several times during the Mariner program. As mentioned in Chapter 2, NASA had once taken a “Mars Mariner spacecraft, put a heat shield on it, and sent it to Mercury.”15 Using a Galileo clone for the Cassini mission ran afoul, however, of NASA and JPL plans at that time. The Agency was currently developing the Mariner Mark II space platform, with the intent of making it a product line of spacecraft that could be used on a variety of different deep space missions, including those to planets, comets, and asteroids. Many engineers thought that the best way to produce such a versatile space vehicle was to develop it from scratch, rather than use an existing craft with dated technology for such a key mission as Cassini. NASA hoped that a multi-application vehicle, which was going to employ dependable assembly designs from previous missions as well as new, highly efficient and improved equipment, would ultimately reduce mission costs to a 1982-equivalent cost of between $150 million and $300 million.16 This program was where the Agency planned to direct its funding, with the first Mark II being employed for the Comet Rendezvous Asteroid Flyby (CRAF) mission and the next for Cassini. There was “already a bow wave going that was pushing a new design of spacecraft, not this old Galileo thing. People were thinking they needed more space with these new instruments and SAR [synthetic aperture radar].”17 An additional factor in choosing the Mark II design was that the architecture of the Galileo Orbiter with Probe attached had a basic problem. The Orbiter carried the Jupiter Probe in front of its main engine nozzle. This worked fine for the Galileo mission because the main engine on the Orbiter was not needed until after release of the Probe. In fact the main engine’s first use was the deflection maneuver carried out shortly after releasing the Probe on a collision course with the planet, in order to set up the spacecraft for orbit insertion. Cassini, however, needed to use its main engine several times prior to Probe release, and so the Probe could not, while still mounted, block the engine nozzle. The Galileo craft would thus have had to be redesigned if it was to be made suitable for the mission to Saturn.18 Another concern about using a Galileo spacecraft clone was its dual spin design. Part of the Galileo Orbiter had rotated continuously at typically 3 revolutions per minute to accommodate the fields and particles experiments. The magnetometer, for instance, gathered information about the local magnetic field by sweeping through it. The other part of the Orbiter, the despun section, had maintained a fixed orientation for imaging equipment and other sensors.19 A dual-spin spacecraft was quite useful for conducting a variety of different types of measurements, but it required a delicate, tricky-to-manufacture spin bearing for carrying power and data between the spun and despun sections. This bearing caused considerable trouble in the design phase, but most of the problems were ironed out during the development of the spacecraft. The bearing performed quite dependably for the entire mission, even though there was a little bit of arcing here and there. Engineers had worried about build-up of debris on the bearing’s slip rings, but this never caused a major problem.20 Nevertheless, there was some concern that it might cause problems for Cassini.

54 Constructing the Cassini Orbiter There were also budget considerations that worked against using a spare Galileo craft, one of which was that NASA would have to come up with immediate funding, probably tens of millions of dollars, for new parts costs and for keeping the Galileo assembly and testing teams running. And the Agency would have had to do this in the fiscally conservative environment of the Reagan Administration, for a mission that might not launch until well into the 1990s.21 The narrow window of opportunity for building a cheap Galileo clone to fly to Saturn was a major factor in NASA abandoning this strategy. Mark II Project Manager Ron Draper explained that the Galileo-clone concept was dropped because advancements in technology were quickly making many parts obsolete and the components necessary to complete a duplicate Galileo spacecraft were becoming unavailable or difficult and expensive to obtain. So for all these reasons, NASA decided to develop the new Mark II design that would draw from the Galileo design but have the advantage of more advanced parts and components.22 Draper believed this would result in an efficient craft that was superior to Galileo in its capability to return science data. The Mark II did not employ a dual-spin design, but instead was meant to employ a large, stationary scan platform from which imaging would be performed and a separate spin table for conducting particle and field experiments. This, at least, was the original plan. In the end, Cassini did not possess either a scan platform or a spin table. As discussed earlier in the book, budget limitations forced Cassini to body-fix its instruments, or in other words mount them to the main body of the spacecraft.23 Later chapters will relate the enormous complications that this supposed cost-saving strategy caused. 3.2.3

The Mark II space platform

The flagship multibillion-dollar missions of the 1960s and 1970s such as Apollo and Viking were highly important space exploration ventures, but were very expensive. The Apollo project, for instance, required $1.2 million per pound (in 2000 dollars) to transport humans and hardware to the Moon.24 In the 1980s, NASA sought to reduce the massive expense of space missions. Such high costs slowed down the exploration of space as well as its commercial exploitation – an objective that was particularly appealing to President Reagan, who strongly supported “private sector investment in space.”25 To develop the more affordable spacecraft that would eventually carry out the solar system missions of the 1990s and beyond, NASA convened the Solar System Exploration Committee (SSEC) in the early 1980s. This was to “figure out a cheaper way to go exploring the solar system.”26 JPL’s Ron Draper expressed this mandate in even stronger terms when he wrote that: “For the United States to continue with a meaningful program of exploring the planets and other constituents of the solar system domain, it would be necessary above all [author’s italics] to cut the costs associated with each mission.”27 Whereas space exploration had in the past used largely custom-designed craft for each mission, the SSEC and JPL believed that both cost-cutting objectives and maximizing scientific goals for future expeditions could be fulfilled by developing two essentially generic spacecraft designs. In 1983, the SSEC conceptualized two programs: Planetary Observer and Mariner Mark II. The Observer program was to include a series of low-cost missions to the inner solar system, including expeditions to the Moon, near-Earth

3.2

Developing a mission to Saturn

55

asteroids, and the planets out to Mars. The Mark II program was to develop much larger but affordable spacecraft “for missions to comets, main-belt asteroids, and the giant outer planets and satellite systems.”28 NASA designed the Mark II to support Voyager- and Galileo-quality planetary observations in deep space, which translated into high data transmission rates, pointing accuracy, and other key spacecraft capabilities.29 The Mark II also had to be capable of delivering Galileo-type atmospheric probes to the outer planets. These objectives had to be achievable for a fraction of the cost of Voyager or Galileo. JPL employed several strategies for limiting costs, including (1) the use of newer, more efficient technologies; (2) designing the spacecraft to be applicable to a variety of different deep space missions; and (3) using wherever possible technology designs inherited from previous missions.

3.2.3.1

New technologies implemented into the Mark II

Since the time Galileo was designed, technologies had evolved which could greatly augment the reliability of a spacecraft, and the Mark II employed many of these. In place of conventional gyroscopes with spinning wheels and bearings that could wear out, the Mark II’s design team initially intended to make use of fiber optics rotation sensors (FORS), a technology available since the early 1980s that could determine the vehicle’s rotational motion without the need for moving parts.30 Other benefits were lower mass, lower power requirements, and higher reliability than conventional mechanical gyroscopes. FORS were kept in the Cassini design until November 1990, then dropped due to cost and schedule risk. Their expected lifetimes were also an issue because the Cassini mission was designed to be a long one, and hence required gyroscopes that could operate continuously for 100,000 hours, or approximately 12 years. Also, FORS exhibited some degradation in a radiation environment, and such conditions would be encountered at Saturn.

3.2.3.1.1

Hemispherical resonator gyros

In 1994, the Cassini project selected instead hemispherical resonator gyros (HRG), employing a technology developed by Delco. The HRG consumed slightly greater power but weighed only one-quarter what a FORS did, and eliminating mass meant reducing propulsion fuel requirements, or making room for other equipment aboard the spacecraft. Like FORS, the HRG had no moving parts. It also had a long lifetime and no susceptibility to radiation degradation. The HRG was a rotation sensor that employed a bowl-like resonating structure known as the hemispherical resonator to determine rate of rotation. A standing wave was induced and maintained on the rim of the resonator (as might be done on the rim of a wine glass). This standing wave moved around the rim of the resonator at a rate proportional to the rotation rate of the resonator axis (comparable to a wine glass stem). The standing wave movement was measured to determine the HRG axis rotation rate (and consequently that of the spacecraft).31 One of the themes of this book concerns the ways in which one mission is built on the scientific and engineering achievements of, or problems experienced by, its predecessors. Problems suffered by the Galileo spacecraft did indeed suggest the benefits of designing next-generation spacecraft with far fewer moving parts.

56

Constructing the Cassini Orbiter

The HRG principle is hardly new. In fact, the HRG can trace its lineage back to a time many decades before any spacecraft had been built. The technology on which the HRG is based was initially conceived in 1890 after physicist G. H. Bryan struck a wineglass. He observed that “if we select a wine-glass which when struck gives, under ordinary circumstances, a pure and continuous tone, we shall on twisting it round hear beats [rises and falls in amplitude].”32 This observation told him that such a simple apparatus as a wine glass could detect and quantify rotation. What he didn’t know was that his simple experiment would lead to a chain of events that ended up helping spacecraft carry out explorations of the planets.33

3.2.3.1.2

Bipropellant engines using hypergolics

The spacecraft designers selected a high-performance engine design that both saved mass and augmented reliability. They chose a bipropellant engine for monomethyl hydrazine (fuel) and nitrogen tetroxide (oxidizer), which are hypergolic chemicals that ignite spontaneously on coming into contact with each other. This eliminated the need for a separate ignition system and its attendant mass and complexities.34 The engines were designed to reduce mass requirements by 15%, allowing the spacecraft to carry additional science payload. They were built by Kaiser-Marquardt Inc., a subsidiary of Kaiser Aerospace & Electronics. Each of the two main engines, one of which was the prime engine and the other the backup, was gimbaled so that it could be adjusted during firing, under control of the attitude and articulation control computers, to ensure the thrust was in line with the spacecraft’s center of mass.35

3.2.3.2

The generic Mark II

NASA intended the Mark II design to be applicable to a range of missions, avoiding the need to develop a custom-made vehicle for each expedition. The main body had a modular design that could be reconfigured, at low cost, for the requirements of a variety of mission types. The antenna, electric power unit, electronic controls, data-processing systems, and other components were to remain the same from mission to mission. Even though they might not be optimally sized for a mission, this would be cheaper than, for instance, building different capacity fuel tanks for each expedition. Only the scientific experiments and deployable probes would be customized to meet the specific objectives of individual missions.

3.2.3.3

Ground support system

To support a variety of different mission configurations, including those of the Mark II, NASA designed and implemented a multi-mission ground system which provided the subsystems, tools, and infrastructure common to most missions. These included telemetry, spacecraft command sequencing, spacecraft and planetary ephemerides (values that specify the positions of astronomical objects), orbit determination, file management and storage, and the underlying networks which incorporated multiple data flow and command networks, both physical and virtual, that were employed in operations. Over three dozen

3.3

Final design of the Cassini Orbiter 57

planetary, heliophysics, astrophysics, and Earth/lunar science missions have benefitted from this multi-mission support.36

3.2.3.4

Inheritance of technologies from previous missions

A goal of the Mark II Project was to do as much as possible with already developed resources and technologies. Historical evidence supported this practice, showing that deep space vehicles that were based on the designs of earlier ones resulted in much lower-thanaverage mission costs. NASA’s bookkeeping data for the total costs of building and operating a spacecraft and analyzing its returned data showed that the Mariner 1 and 2 mission costs were less than those of the earlier Ranger craft from which they were derived; Mariner 5 costs were less than one-third those of Mariners 3 and 4 from which it was derived; and Mariners 8, 9, and 10 all had reduced costs compared to the Mariner 6 and 7 vehicles from which they evolved.37 A number of technologies from previous missions were considered at one time or another for reuse on Cassini, including hardware and designs from Voyager, Viking, and Galileo. Voyager-design thrusters were actually implemented on Cassini for its attitude control system. The Galileo tape recorder was also considered for Cassini, but ultimately the mission selected a solid-state recorder in order to avoid the use of moving parts. The Cassini project examined many different candidates for its flight computer, ultimately settling on one called the Generic Very High Speed Integrated Circuit Space Computer (GVSC). Cassini was the first civilian application of this technology. It was designed to meet the requirements of Military Standard MIL-STD-1750A, and hence was referred to as the GVSC-1750A.38

3.3

FINAL DESIGN OF THE CASSINI ORBITER

The Mariner Mark II spacecraft concept went through significant evolution from its early 1980s conceptualizations to the finished Cassini-Huygens spacecraft that took off for Saturn in 1997 (Figure 3.1). NASA completed the Cassini Orbiter’s design in December of 1994. Fabrication of individual assemblies was followed by extensive testing.39 Two major elements made up the vehicle: the Cassini Orbiter and the Huygens Probe. Built into the Orbiter were the “sensory” organs that allowed it to move in useful ways – its navigation, attitude control, propulsion, and power systems. All are described in detail below. The Huygens Probe architecture is examined in the next chapter. The technology for stabilizing the vehicle as it flew through space operated very differently than for Galileo, which, like a gyroscope, used its spin as a means of stabilization. Cassini-Huygens used a 3-axis stabilization strategy in which the desired attitude was maintained by sensor-controlled thrusters and reaction wheels (types of flywheels). Four main modules stacked together made up the Cassini-Huygens spacecraft.40 While NASA oversaw and was ultimately responsible for their development, various contractors, laboratories, and partners played key roles. At the top of the stack was the 4 meter (13 foot) diameter parabolic high-gain antenna (HGA) provided by the Italian Space Agency (ASI).

58

Constructing the Cassini Orbiter

Figure 3.1 Spacecraft parts.

This was affixed to the upper equipment module, which included the electronics bay. Beneath this was the propulsion module, fabricated by Lockheed Martin Astronautics Company, although its electronics were developed at JPL. It contained the main spacecraft engine. At launch, the liquids contained in this module’s propellant tanks constituted over half the mass of the entire vehicle, which was 5,574 kilograms (more than 12,000 pounds). The Cassini-Huygens’ propellant mass alone exceeded the mass of the Galileo and Voyager spacecraft combined. The Huygens Probe, designed and fabricated by ESA, was connected to the side, in the vicinity of the propulsion module. At the base was the lower equipment module with the reaction wheels and the small thruster cluster pods used for attitude control. The stack stood 6.8 meters (22 feet) tall, so was about the size of a school bus. Several extensions were deployed soon after launch, making it even larger. These included three rod-like plasma wave antennas and a 13 meter (42 foot) boom for the magnetometer, a sensitive instrument which had to be isolated from electromagnetic interference emitted by the spacecraft’s other electronics components. NASA constructed the space vehicle largely of aluminum, with titanium and beryllium pieces added to augment its structural integrity. Graphite epoxy materials that could withstand wide extremes of temperature made up much of the HGA. As a gauge of the spacecraft’s complexity, it contained 22,000 wire connections and more than 12 kilometers (7.5 miles) of cabling linking together its instruments, computers and mechanical devices.41 When the Mark II was under development, NASA thought that its use on multiple missions would result in significant savings. The Mark II was to be used for CRAF as well as the Cassini mission, and possibly for other deep space endeavors, such as an ESA-led follow-on to CRAF – the Comet Nucleus Sample Return mission – a flyby of Pluto, and a Neptune orbiter with an atmospheric probe.42 But this is not how things turned out. CRAF was canceled because of budgetary concerns, and the Mark II was used on one

3.3

Final design of the Cassini Orbiter 59

mission only: Cassini-Huygens.43 Therefore the potential economy of using the Mark II platform for multiple missions was never realized. If a Galileo-clone spacecraft had instead been used, as was suggested in 1984, Cassini-Huygens mission costs might have been considerably lower, but the mission would not have been able to benefit from a stateof-the-art spacecraft with refinements described later in this chapter that greatly improved its reliability. 3.3.1

The high-gain antenna

Among the many truly exceptional instruments on the Cassini-Huygens spacecraft was the HGA, “one of the most complex antennas that was ever launched for exploration of the planets.”44 The Italian Space Agency (ASI) team designed it radically different than Galileo’s collapsible antenna, which stuck partially open and nearly ended that mission. The many functions that the Cassini-Huygens HGA was to perform drove its design. It not only was required to communicate with Earth and the Huygens Probe, but also to provide radio science data about the Saturn system’s gravity fields, atmospheres, and surfaces, observe the ring system, and take radar images of Titan’s surface. And that wasn’t all – it needed to shield the spacecraft from the Sun while in the inner solar system. The HGA thus had to be extremely robust, able to survive high temperatures when it flew by Venus and low temperatures when it traveled in the outer solar system. According to Enrico Flamini, ASI’s Solar System Exploration Missions Manager, the temperature at the HGA’s focal point exceeded 180°C (356°F) at Venus and fell below −200°C (−328°F) at Saturn.45 ASI designed the HGA with a solid 4 meter (13 foot) parabolic dish composed of an aluminum honeycomb reinforced on its rear with a structure of ribs and rings. The dish did not fold up, and thus was not subject to jamming during deployment, as had happened with the Galileo HGA. Furthermore, since the Cassini HGA could shade the vehicle from the Sun, it eliminated the need for the sunshade structure developed for Galileo. ASI investigated the best materials capable of surviving not only temperature extremes, but also the intense ultraviolet and charged-particle fluxes. From a thermal point of view, the most critical component was a frequency-selective reflector of Kevlar and Kapton layers that was suspended over the main dish of the antenna. This assembly required a specialized coating which would not degrade, but remain transparent to radio-frequency (RF) signals. Identifying and testing such a coating and developing a highly reliable application process required “a remarkable technological effort to overcome problems due to the compatibility of an inorganic paint with the organic substrate.”46 ASI finally selected a white thermal control paint called PCBZ, designed for use on rigid aluminum alloy surfaces.47 ASI built the HGA to operate simultaneously in four different frequency ranges in the microwave region of the electromagnetic spectrum: the S-, X-, Ka- and Ku-bands. Frequency is a measure of the number of electromagnetic wavelengths (cycles) that arrive at a given location per second. The speed of electromagnetic waves is constant in the vacuum of space, so the higher the frequency, the shorter the wavelength. The S-band ranges from 2 to 4 gigahertz (GHz), where a gigahertz is equal to one billion cycles per second. The X-band ranges from 7 to 12.5 GHz and slightly overlaps the Ku-band of 12 to 18 GHz. The higher frequency Ka-band ranges from 18 to 40 GHz. The HGA used the

60 Constructing the Cassini Orbiter X-band to communicate with Earth, performing functions such as navigation, reception of commands from Earth, and telemetry from the spacecraft that included both science and spacecraft health data.48 The HGA used its S-band to receive science data from the Huygens Probe, while the Ku-band was used for radar mapping Titan’s haze-covered surface, since it penetrated the haze better than other microwave bands or visible light. The Ku-band also was used to measure blackbody radiation49 from targets.50 The antenna’s two-way Ka-band was used on the cruise to Saturn for a gravitational wave experiment (described later in the book), but ceased to function after an equipment failure occurred.51 The antenna could also operate in a synthetic aperture radar (SAR) mode through the use of sophisticated information processing techniques to produce high resolution images. Cassini’s SAR capabilities are discussed later in this chapter. During a flyby of Titan the instrument was able to map a swath equivalent to approximately 1% of the surface and resolve features less than 500 meters (1,600 feet) in size.52 Why did the Cassini-Huygens team build its HGA as a solid structure, whereas the Galileo team designed one that folded up and was supposed to reopen like an umbrella? The fact that Galileo’s HGA never fully opened certainly influenced the Cassini team, but there were also other reasons for this major difference between the two designs, in particular minimum antenna size and available storage space. The Galileo team chose a collapsible antenna because it wanted the spacecraft to send data to Earth at a greater data rate than the 112 to 114 kilobits/second that was possible for the largest fixed antenna that was compatible with the Space Shuttle that was to launch Galileo. To achieve the desired data transmission rate of 132 to 134 kilobits/second a larger antenna would be required. The solution was to use a folding antenna similar to those used by the geostationary communications satellites that the Shuttle was launching. Cassini-Huygens, on the other hand, was not going to be deployed from inside the Space Shuttle cargo bay. It was going to ride a Titan/Centaur launch vehicle, whose payload shroud was wide enough to accommodate a 4 meter fixed antenna. This size of antenna was fully large enough for all of the mission’s planned actions. There was therefore no reason to use a folding antenna, even if its reliability had not been under question. Regarding this issue, Program Manager Bob Mitchell commented that “independent of the Galileo experience, I suspect that Cassini would have gone with the rigid antenna anyway, simply because it fit in the shroud.”53 3.3.2

Power for the spacecraft

Three radioisotope thermoelectric generators (RTG) supplied the spacecraft with electric power. These used radioactive plutonium as their energy source. Chapter 6 discusses how the use of this material prompted widespread protests and attempts to block the spacecraft’s launch. At the time that Cassini-Huygens took off in 1997, the RTGs supplied 875 watts of electric power, but engineers knew that as the plutonium slowly decayed, this figure would go down. As expected, by the end of the Prime Mission in July 2008 it had dipped below 700 watts.54 Power from the RTGs flowed through 192 solid-state power switches (SSPS)

3.3

Final design of the Cassini Orbiter 61

that protected the sensitive electronics of the spacecraft by acting as circuit breakers in the event of overloads. One or more SSPS protected each electronic component of the craft. This included both science instruments and spacecraft operating systems. 3.3.3

Command and Data Subsystem

Like a human body, the Cassini Orbiter had to keep functioning even in the event of injury. NASA designed the craft to be one-fault tolerant wherever feasible, which according to Mark Dahl, the mission’s program executive, meant that “pretty much any one thing could fail and you would still be fully functional.”55 Such a design approach greatly reduced the risk of mission impairment or even curtailment due to trouble with a vital component. To achieve one-fault tolerance in the command and data subsystem, NASA implemented two command computers, two attitude control computers, two transmitters, two receivers, and a solid-state recorder that had two halves to it. If one computer failed, for instance, the spacecraft would switch to the other. Such redundant systems made the spacecraft far more adaptable to a certain amount of equipment failure and far more likely to continue operating throughout its long mission. The Cassini team implemented redundant IBM-1750A computers (compatible with MIL-STD-1750A) known as engineering flight computers (EFC) to manage command, data handling, telemetry, and timekeeping functions. When the EFCs received commands from the onboard telecommunication subsystem, they processed and routed them to the intended engineering subsystem or science instrument. This routing occurred through a part called the MIL-STD-1553B bus. For reliability and redundancy there were two 1553B buses, one prime and the other in standby. Each instrument or engineering subsystem was equipped with a bus interface unit (BIU) to receive the commands routed from the computer through the 1553B bus, as well as to send telemetry packets when requested.56 3.3.4

Solid-state recorders

The spacecraft’s two solid-state recorders (SSR) each stored up to 2.2 gigabits of science and engineering telemetry data until it was able to be downlinked to Earth. A second SSR function was to store copies of flight software for the attitude and articulation control subsystem (AACS), the command and data subsystem (CDS), and the scientific instruments. An SSR was able to store two different flight software packages for each of these components. This allowed an instrument, the CDS, or the AACS to test a new flight software load and still have an older copy to fall back on if necessary.57 Redundancy was only one of the strategies that NASA implemented in order to keep Cassini-Huygens functional. Another key strategy was radiation hardening. 3.3.5

Radiation hardening of spacecraft electronics

The Cassini-Huygens team and its component suppliers spent considerable effort hardening onboard computers and other electronic devices; that is, rendering them resistant to the charged particles and energetic rays of the space environment that can cause temporary as well as permanent damage to electronics. Sources for such radiation include the solar wind,

62

Constructing the Cassini Orbiter

belts of charged particles trapped in planetary magnetic fields, galactic cosmic rays, and particles issued by solar coronal mass ejections. A spacecraft’s shipboard electronics are particularly at risk when the Sun is at its peak activity. The Sun constantly emits radiation in the form of high-energy charged particles, but solar flares which can last up to several days can increase by a factor of 1,000 the radiation flows through interplanetary space.58 High-energy particles can penetrate deep into spacecraft components and cause damage if they hit vulnerable spots. In addition, when these particles penetrate the skin of a space vessel, X-rays may be emitted, and these can invade semiconductors in the electronic components, causing the material’s silicon and silicon dioxide layers to ionize. The damage from this may be temporary, such as corrupting the contents of a memory cell, or permanent.59

3.3.5.1

Cumulative versus single event effects

Spacecraft designers seek to mitigate two particular types of space radiation damage. The first impairment arises from the total ionizing dose received by the craft. This causes a cumulative effect due to large numbers of particles impacting a component throughout its life, slowly degrading it until it fails. One way that total ionizing dose degrades certain transistors is that it gradually causes the device’s insulating layers to develop a charge that then impedes the proper flow of current through the device. Newer space vehicle designs, however, are more resistant to this problem. The second type of radiation impairment is due to high-energy particles colliding with a component’s vulnerable areas, resulting in damage referred to as single-event effects. This problem also occurs when particles deposit charge into memory circuits, altering the data stored there. The trend toward smaller circuit and transistor volumes renders spacecraft more vulnerable to such upsets, because the total charge needed to trigger an event in a circuit element decreases. On the other hand, as circuitry used on spacecraft shrinks, the thicknesses of the insulating layers of transistors decrease, presenting less opportunity for charge build-up. To attain good radiation protection, the Cassini-Huygens team implemented strict requirements on electronic parts, materials, circuits, and shielding. Flight hardware had to employ space-qualified parts, including electronics that had been radiation hardened. Most of the requirements for qualifying space rated electronics are found in Department of Defense documents such as MIL-M-38510, MIL-I-38535 and MIL-STD-883.60 The latter, Test Method Standard - Microcircuits,61 for instance, has extensive environmental, mechanical, and electrical test procedures to determine resistance to deleterious impacts encountered during space operations. The specific methods for radiation hardening of electronic components are often kept confidential by manufacturers’ patents. Several general approaches used by the CassiniHuygens team for protecting spacecraft electronics, however, included implementing:62 • • • • •

Shielding materials Redundant hardware and software Materials that do not generate harmful byproducts Encapsulation Self-shielding spacecraft configurations.

3.3 3.3.5.2

Final design of the Cassini Orbiter 63

Shielding materials

After engineers developed an initial layout for the vehicle’s electronics, they carried out a radiation impact analysis to determine whether any modifications were needed. Brought into these studies were specialists in materials, specific types of parts, and electronics packaging, as well as experts in reliability and radiation transport. Their detailed analysis yielded requirements for the thicknesses of shielding material, and especially the need for spot shields to protect radiation-soft items.63 Installation of some shielding was inevitable, but the mission’s design philosophy was to make this “a highly restricted solution.”64 Shielding materials added considerable weight to the spacecraft. The intent of the designers was therefore to install simple shield solutions only where necessary and in a minimum of locations. Mission designers used various shielding materials to attenuate radiation hits before they could cause damage. Outer shear plates of the spacecraft’s electronic bays were made of various thicknesses of aluminum, or brass if higher shielding was required. Thermal blankets, primarily for temperature control and micrometeoroid protection, also served as radiation shielding; the number of layers varied depending on the protection needed. Any cables outside the thermal blanketing system had two or three layers of fabric overbraiding, although that was more for micrometeoroid protection than radiation shielding.65

3.3.5.3

Redundant hardware and software

As discussed above, the spacecraft design incorporated many redundancies to increase the probability that a system would continue to operate even if some of its components failed because of radiation hits or other influences. The design approach was to build extensive component redundancies into the spacecraft. These included redundant computers, gyros, Sun sensors, star trackers, transponders, and amplifiers, as well as many other components. In addition, critical electronic components were given dual power supplies controlled by separate solid-state switches. The Orbiter’s main computers were radiation hardened by the manufacturer. This strategy appeared to be successful, for according to JPL spacecraft team chief Julie Webster, “we have never had an SEU [single event upset] like some of the other newer spacecraft have.”66 Nevertheless, just in case the functioning of any of the computers were to be upset by radiation effects, there were multiple copies of flight software stored onboard. Computer resets were necessary during the mission, but to date, they were driven mainly by software glitches or updates, rather than radiation impacts.

3.3.5.4

Material selection

Radiation damages a spacecraft not only through hitting its sensitive components, but also by degrading its materials and causing them to emit harmful byproducts. Particles from a degraded material might, for instance, enter a sensitive instrument and impair its function. Some chlorine-bearing polymer materials such as polyvinyl insulation on wires generate corrosive byproducts such as chlorine and hydrogen chloride when degraded by radiation. Mission designers avoided using polyvinyl on the spacecraft. The wire insulation selected instead was Kapton67 (a polyimide film developed by DuPont) with metal overbraiding.68

64

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3.3.5.5

Encapsulation

Degradation of certain components and damage from non-corrosive byproducts can be minimized by encapsulating a part in a conformal coating, so-called because it conforms to the shape of the object. Such coatings were applied to circuit boards on the spacecraft.69

3.3.5.6

Self-shielding spacecraft configurations

The mission’s engineers employed the spacecraft structure and some of its systems to help protect other components from radiation damage. In designing the spacecraft, the team used a segmentation strategy in which equipment and components were divided into two distinct layers: outer and inner. Components less susceptible to radiation were allocated to the outside of the spacecraft, where they helped shield more sensitive components deeper inside the structure. In some cases, outer-layer equipment was located directly behind the structural elements of the spacecraft for additional radiation protection. And instead of locating all the inner components in a single module, the team distributed them throughout the vessel. This design strategy did not add significant complexity to the system, but managed to reduce the need for expensive and weighty radiation hardening materials in order to protect components that were at risk.70 3.3.6

Attitude and Articulation Control Subsystem

The AACS (Figure 3.2) provided dynamic control of spacecraft orientation, keeping the vehicle fixed for such tasks as HGA use and remote sensing, as well as managing repetitive motion patterns required during certain types of imaging such as scans and mosaics.71 Cassini engineers designed the AACS with similar redundancies to those of the command and data subsystem (CDS), with redundant IBM-1750A computers, called AACS flight computers, and redundant MIL-STD-1553B buses. The AACS flight computers received

Figure 3.2 The Attitude and Articulation Control Subsystem.

3.3

Final design of the Cassini Orbiter 65

commands from and sent telemetry to the CDS computers through a special bus interface unit (BIU) on the CDS bus. The AACS flight computers used their own internal 1553B bus to communicate with the spacecraft’s Sun sensors, stellar reference units, and inertial reference units (all of which helped determine the space vehicle’s attitude), the electronics for the propulsion system (which altered the attitude when necessary), and the spacecraft’s accelerometer. The AACS flight computers had their own flight software and fault protection routines.

3.3.6.1

Determining spacecraft attitude

The major component for providing attitude knowledge was the stellar reference unit (SRU), which essentially navigated by the stars. It accomplished this by focusing light from a star onto a charge coupled device, or CCD, a sensor that converted the incident light into digital data.72 To determine which stars to use as a reference, the SRU detected stars in its field of view and compared the view with its onboard, full-sky coverage catalog, which contained data for up to 4,000 stars, including position vectors, color, magnitude, and usability information – for instance, that the star had no similar neighbors within 0.5° and therefore was a good candidate to navigate by. These data helped in choosing and ordering the stars that would be used for tracking purposes. Only the stars in the catalog could be identified or selected for tracking, lessening the likelihood of tracking the wrong star.73 The catalog identified up to five of the brightest stars appearing in the field of view at any given moment. SRU observations were fed every 1 to 5 seconds into attitude estimation flight software. In between star updates, spacecraft attitude was estimated using an inertial reference unit (IRU) that used the hemispherical resonator gyros discussed earlier.74 If SRU star updates were suspended, either by a command from Earth or by a malfunction, the IRU could continue to estimate the spacecraft’s attitude for many hours. When SRU star updates resumed, a small attitude correction was made due to the imperfections in the IRU-only estimation. In the event of a fault (for example, hardware malfunction), attitude estimation had to be re-initialized. In this situation, the first task was to find the Sun. Until the Sun was detected, the spacecraft was commanded to continuously slew in a spiral Sun search mode, a maneuver that insured that in order to prevent damage, sensitive science instrument boresights or radiators were not continuously pointed at the Sun. Two redundant Sun sensors were mounted in cutout holes in the high-gain antenna. One was the prime sensor and was normally powered on at all times; the other was the backup and was typically powered off. When the prime Sun sensor detected the Sun, attitude estimation flight software combined with the rest of the attitude control subsystem damped out any residual spacecraft angular rotation and used SRU data to re-establish 3-axis attitude knowledge. The output from the Sun sensor was coarse, with a degree or two of error expected, but it was vital in the process of re-acquiring more accurate attitude knowledge.75 Finding the Sun in this manner determined spacecraft attitude to sufficient accuracy that star identification could then be carried out by the SRU when it began operating again. The Sun sensor also provided a Sun reference for spacecraft thermal safing purposes – in other words, for shutting down spacecraft systems to protect them if thermal overload was imminent.76

66 Constructing the Cassini Orbiter Officine Galileo of Florence, Italy carried out the detailed design, assembly, and testing of the SRU using flight qualified CCDs provided by JPL.77

3.3.6.2

Controlling attitude

The Cassini spacecraft had two methods for controlling its attitude. The first used the reaction control subsystem (RCS) which consisted of small, 1-newton thrusters that maintained proper attitude within a narrow range called a deadband. The AACS flight computers permitted the craft’s orientation to twist a slight amount, typically +/− 2 milliradians (mrad, about one-tenth of a degree) away from the ideal position. When the attitude reached the edge of this deadband, a thruster fired and returned the vehicle to the center of the deadband. These thrusters gave sufficient attitude control for downlinking to Earth, but not for more demanding applications. Although 2 mrad may not seem like a large discrepancy in optimum attitude, it was large enough to impair the acquisition of high quality images. The field of view of the narrow-angle camera was 6 mrad, so a 2 mrad pointing error was one-third of the field of view – not a good situation if you want to have some specific target fill the image frame.78 For finer control, the AACS flight computers needed to employ the spacecraft’s reaction wheels. These resembled flywheels, in that they were spinning devices that generated torque and stored angular momentum. In their simplest form, reaction wheels consisted of electric motors driving flywheels. They were called on to make small adjustments in attitude through an exchange of angular momentum between vehicle and wheel, and they achieved this in a smoother manner than the “chattering control exhibited by thruster techniques.”79 The conservation of angular momentum meant spinning up the wheel in one direction (i.e., adding angular momentum to it) caused the rest of the spacecraft to rotate in the other direction. Braking the wheel’s rotation (taking away angular moment from it) had the opposite effect. The name reaction wheel is taken from Newton’s Third Law of Motion,80 which states “For every action there is an equal and opposite reaction.”81 The Cassini spacecraft had three reaction wheel assemblies mounted on the lower equipment module, with each aligned roughly 120° from the others.82 This enabled it to turn in any direction and assume any attitude. A backup fourth reaction wheel on the upper equipment module was designed so that it could be rotated parallel to any one of the three other wheels, enabling it to take over for a malfunctioning unit.83 The reaction wheels were able to maintain space vehicle attitude stability to within 40 microradians per second. That is roughly to within two-thousandths of one degree (0.002°) per second.84 3.3.7

Thermal control system

The Cassini team implemented several means of controlling the temperature aboard the spacecraft. The most visible was the outer black-and-gold multi-layer insulation. In addition, automatically oriented reflective louvers covered the 12-bay electronics bus, while strategically placed heaters and radiators helped stabilize temperatures in numerous systems and instruments. Components also had to be designed that would dissipate waste heat from the plutonium-bearing RTGs, as well as from the various electronics subsystems, the majority of which had to be maintained between 5 and 50°C. The propulsive thruster

3.3

Final design of the Cassini Orbiter 67

clusters also had to be temperature controlled. This was achieved using electrical heaters and variable radioisotope heater units (RHU), the latter containing plutonium. Electrical heaters were used on the main engines. A heat shield protected parts of the engine from overheating caused by radiant thermal energy during and after main-engine firings.85 3.3.8

Propulsive maneuvers

Two independent technologies handled the spacecraft’s propulsive needs: its main engines and its thrusters. Large maneuvers used the 455-newton86 main engines. For the sake of redundancy, there were two of these engines, each with a separate feed system. They burned monomethyl hydrazine (N2H3CH3) fuel with nitrogen tetroxide (N2O4) oxidizer. This was a hypergolic combination in that the two chemicals would spontaneously ignite on coming into contact. The engines could be operated once the spacecraft was on its way to Saturn. Mounted below the main engines was a retractable cover, sometimes referred to due to its appearance as an “articulated baby buggy cover.”87 This was kept closed when the engines were not in use because the thin disilicide88 refractory ceramic coating on their insides was vulnerable to micrometeoroids, the damage from which could have led to burn-through and engine loss. The Cassini team designed the cover to be opened and closed many times during the mission, but there was a pyrotechnic ejection mechanism in case it became stuck and interfered with engine function.89 The largest maneuver ever performed by a Cassini main engine was Saturn orbit insertion (SOI). Thrust from one of the main engines altered the craft’s velocity by 626 meters/second, consuming 27% of the total of 3,132 kilograms of propellant that was loaded at launch.90 NASA developed the propulsion system’s plumbing with the 1993 loss of the Mars Observer spacecraft in mind. The independent board investigating that failure, which was headed by Timothy Coffey, Director of Research at the Naval Research Laboratory, determined that the most probable source of the accident that led to loss of communication with Mars Observer was a rupture of its monomethyl hydrazine fuel tank, resulting in a pressurized leak that caused the spacecraft to spin. This sent the craft into its contingency mode, which interrupted the stored command sequence and thus prevented the transmitter from switching on. Another possible cause of the accident was overpressurization and rupture of the nitrogen tetroxide tank. NASA was exceptionally careful to examine the plumbing of Cassini’s propulsion system to prevent such an accident from happening again.91 Small space vehicle maneuvers were made using the hydrazine monopropellant thrusters. These initially generated 1.0 newton, but their thrust decreased over time. The Cassini team designed the spacecraft with four thruster clusters (Figure 3.3) on the lower equipment module, each having two Z-axis-facing and two Y-axis-facing thrusters; a total of 16 thrusters. Only one thruster on each axis in each cluster was used at a time; the other thruster provided redundancy.92 3.3.9

Power system

The power module for Cassini-Huygens contained radioisotope thermoelectric generators (RTG), the plutonium-bearing energy source of choice for outer planet voyages, since the Sun was too far away from the spacecraft for it to efficiently use solar panels.93 Because an RTG contained an energy source, it had to be isolated from the scientific instruments to a

68 Constructing the Cassini Orbiter

Figure 3.3 Spacecraft propulsion system.

certain extent in order to minimize interference with their measurements. But it wasn’t the ionizing radiation the plutonium emitted that was of concern, it was the heat from the natural decay of this isotope. Thermal shields (also called shades) were placed between the RTGs and the instruments. Electrical output from RTGs declines if they overheat. Thus, they too require shading, particularly while cruising through the inner solar system. The cooling radiators of the VIMS and CIRS instruments were also shaded for more efficient operation. On the Galileo spacecraft, RTGs were mounted on the ends of long booms in order to isolate them from instrument packages. Based on data from Galileo, Cassini engineers realized that RTGs did not have to be so distant and, in fact, there was an advantage to mounting them closer. The Cassini RTGs were deliberately mounted on short booms in order to use their thermal energy. Their proximity facilitated heat conduction into the propellant tanks, to help keep the propellants at more optimal temperatures for high performance.94

3.4

SPACECRAFT PERFORMANCE PREDICTIONS

A variety of predictive analyses were employed to determine expected spacecraft performances under different environmental conditions and, where appropriate, the designs were modified. These analyses targeted many individual components, piece parts, materials, and assemblies, and were concerned with spacecraft specifications relating to:95 • • • •

Hardware magnetic fields Micrometeoroids Electrostatic discharge Contamination.

3.4 3.4.1

Spacecraft performance predictions 69

Hardware magnetic fields

The primary objective in establishing specifications on the magnetic fields from spacecraft hardware was to limit the total magnetic field generated by the vehicle. If this field was excessive, it could interfere with the sensitivity of certain instruments, especially as Saturn’s magnetic field magnitudes were expected to be low in certain regions.96 Limitations in the available magnetic testing facilities necessitated the analysis of individual components rather than large sections of the spacecraft in order to assure compliance with specifications. Magnetic fields from 56 hardware elements were measured and analyzed and their overall effect calculated and confirmed to be within specifications. 3.4.2

Micrometeoroid impacts

The Cassini-Huygens spacecraft passed through regions where potential risks from a variety of solid particles were present, including human-made debris orbiting Earth, interplanetary micrometeoroids, and Saturn ring material. Due to possible encounters with micrometeoroids during both the 7 year cruise to Saturn and the primary and extended mission periods in orbit around that planet, micrometeoroid impacts were of the most concern. As a result of analyses, the final designs of several spacecraft parts reflected these risks:97 • • • • 3.4.3

Plates housing spacecraft electronics were thickened Multi-layer insulation blankets were mounted farther from spacecraft surfaces in order to better dissipate energy More material was added to blankets covering critical hardware A shield was installed to protect the nozzles of the main engines. Electrostatic discharge

The mission team analyzed spacecraft hardware to verify that the vehicle was not prone to electrostatic discharge – the release of static electricity due to build-up of charge on its various parts. A familiar example of this is the shock we receive when we walk across a carpet and touch a metal doorknob. A lightning discharge is a more extreme example. The energy from an electrostatic discharge can damage sensitive electronics. To avoid such problems, the specifications included a requirement that the exterior of the spacecraft be designed to minimize build-up of charge on non-conducting surfaces. In addition, all conducting surfaces greater than half a square centimeter in area had to be grounded to prevent charge build-up.98 3.4.4

Contamination

To maintain cleanliness of spacecraft surfaces, tanks and optics, mission staff installed dust covers on scientific instruments, baked out hardware before integrating it with the spacecraft, applied gaseous nitrogen purges to various contamination-sensitive subsystems, and implemented a rigorous contamination control program. The design and composition of all external surfaces needed to be such that they could be adequately cleaned with isopropyl alcohol. In addition, shipping operations used only approved, clean containers.99

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Constructing the Cassini Orbiter

3.5

IMPROVEMENTS OVER GALILEO

Galileo returned amazing science results and was, overall, a very successful mission. But it ran into a series of mechanical problems that were nearly terminal. One of the objectives of Cassini design activities was to develop a spacecraft that would not fall victim to the same types of predicaments that Galileo did, such as the umbrella-like parabolic antenna that did not fully open and the data-storage tape recorder that had a tendency to jam. Lessons learned from these troubles definitely influenced Cassini design decisions, but other factors played a part as well. Spacecraft technology was rapidly improving, and it became possible to build a vehicle with far fewer moving parts. Recorders could now be digital, while switches and even gyroscopes could be made without the mechanical systems that were prone to aging and possible failure. Besides reducing failure risks, these new technologies had other advantages, some of which are described below. 3.5.1

Solid-state recorders

The solid-state digital recorders used on Cassini for data storage, besides not being prone to tape jamming as was the device aboard Galileo, also could record and play back data simultaneously,100 a capability that Galileo’s reel-to-reel tape recorder did not provide. A major function of Cassini’s recorders was to store engineering and scientific data until they could be sent down to Earth. A feature of Cassini’s solid-state recorders that made them very reliable for this task was that they were fail-soft. If one of their memory chips failed, they remained functional and the mission could continue. The failure of one memory chip due, for instance, to a radiation hit, would result in a loss of only about half a percent of the solid-state recorder’s total memory. Spacecraft design tolerances allowed for several such radiation hits with little impact to the mission.101 3.5.2

Switches

Power distribution was accomplished throughout the Cassini spacecraft utilizing 192 solid-state switches that also functioned as circuit breakers in the event of an overload condition.102 The Cassini engineering team developed these switches by combining two devices with switching capabilities: metal-oxide semiconductor field-effect transistors (MOS FET) and application-specific integrated circuits (ASIC). Each ASIC replaced one hundred or more traditional chips. The combination of MOS FETs and ASICs created an advanced solid-state power switch that had better performance characteristics than traditional mechanical ones. These switches eliminated transients – rapid fluctuations typically found in circuits employing conventional power switches. In addition, the lack of mechanical moving parts in the solid-state switches resulted in longer switch lifetimes and higher efficiencies. These switches were of interest beyond the space community, for a variety of industrial and consumer electric and electronic applications.103 While the solid-state switches were a powerful feature of the Cassini spacecraft, they did occasionally exhibit problems by tripping when they were not supposed to, possibly because of cosmic ray impacts. Such impacts did not damage the switches, but they needed to be reset, and this took time. Meanwhile, data from an experiment could be lost.

3.5 Improvements over Galileo 71 Like circuit breakers, Cassini’s power switches had three states – on, off, and tripped. In this last state the circuit was open, just as it is when a current overload trips a traditional circuit breaker in a building. The spacecraft experienced 21 such switch trips during its first 10 years of flight. A switch trip could interrupt power to a spacecraft component or engineering subsystem, and this could cause trouble. One switch trip resulted in the spacecraft entering its safe mode, during which time it functioned on a greatly reduced level and relayed limited data. The flight software automatically placed the vehicle into safe mode to render it as resistant to harm as possible from a power, thermal, and communications perspective until mission staff on Earth determined what happened and got the craft fully functional again without damaging it.104 The biggest concern about switch trips was that if a switch changed from on to tripped while it was powering some critical component, this might cause damage. For example, such switches controlled heaters in some instruments. If one were to trip, the instrument would get cold and in some cases this would be a major problem. One thing the Cassini team did to mitigate a switch trip was to develop software that gave autonomous protection to some of the more sensitive components of the spacecraft. This enabled the spacecraft to independently detect the problem and reset certain switches. Still, there were cases when the spacecraft-Earth downlink was lost due to a switch trip. In at least one instance, data could not be sent to Earth during a switch trip and were lost.105 3.5.3

ASIC benefits

The Cassini team devoted considerable effort to the development of ASIC chips for switches. Because one ASIC did the work of some one hundred traditional chips, the use of ASICs on the spacecraft resulted in significant parts-count reduction, and this was a major benefit because many fewer individual parts had to be acquired, stored, and tracked. Also, spacecraft mass was reduced.106 3.5.4

Hemispherical resonator gyro

Produced by the Delco Division of Hughes Aircraft Company and Litton Guidance and Control Systems, the hemispherical resonator gyro (HRG) was part of an inertial reference unit (IRU), a sensor to determine changes in the orientation of an aircraft or spacecraft over a period of time. IRUs are typically used for attitude control and navigation. The hemispherical resonator gyro was the first space version of an application that departed from the traditional large, mechanical devices whose many moving parts rendered them susceptible to failure. While the application was new, the theory of this device was not. Physicist G. H. Bryan of Cambridge envisioned something like this in 1890, when analyzing vibrations in a revolving cylinder or bell.107 An HRG looks sort of “like a mushroom sitting on a stem.”108 The mushroom-like hemispherical quartz shell mounted on top of a post is induced by electric fields to resonate at over 4,000 cycles per second. This resonating can be pictured as similar to the vibrations induced in a wine glass when one runs a finger along its rim at just the right rate. The vibration pattern is so tiny that it barely creates any stress or fatigue effects on the HRG, resulting in its long life and the reduction of failure risk. Electronics around the

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hemispherical shell measure these vibrations, which are standing (stationary) waves. When the space vehicle turned, the standing waves also moved. Measuring this change determined the spacecraft’s new orientation.109

3.6

THE SCIENCE INSTRUMENT SELECTION PROCESS

NASA officially kicked off development of science instrumentation on 10 October 1989 with its Announcement of Opportunity [AO] for the Cassini Mission: Saturn Orbiter, offering “the opportunity to propose scientific investigations for the Saturn Orbiter portion of the Cassini mission.” Simultaneously, ESA invited researchers to propose “scientific investigations for the Titan Huygens Probe.”110 Probe instrument selections are discussed in the next chapter. It is important to recognize that the AO was open to the international scientific community, not just to U.S. organizations. In fact, proposals from non-U.S. institutions were strongly encouraged. Mixed teams, with co-investigators from U.S. institutions participating in non-U.S. proposals as well as co-investigators from non-U.S. institutions on U.S. proposal teams, were also encouraged. The general scientific objectives for the Saturn system were defined by scientific advisory groups that included the Solar System Exploration Committee (SSEC) of NASA’s Advisory Council and the Committee on Planetary and Lunar Exploration (COMPLEX) of the Space Science Board (SSB) of the U.S. National Academy of Sciences. It was specifically noted that the concept of a combined Saturn orbiter and Titan probe mission that would be carried out as a joint NASA/ESA undertaking was first proposed by a consortium of European scientists.111 Specific objectives for the mission, including those for both the Saturn orbiter and the Titan probe, were developed by the Cassini Joint Science Working Group, which included members from Europe and the U.S.112 These objectives arose from guidance in key studies made during the conceptual development of the mission, including the Phase A Study,113 the SSB study – Strategy for Exploration of the Outer Planets: 1986–1996,114 and work by Dan Gautier, Wing Ip and Tony Owens (see Chapter 1). The Cassini-Huygens mission science objectives listed in Table 3.1 defined the capabilities needed in the instruments that would be selected. The Cassini Project Science Group (PSG) provided overall science management of Cassini-Huygens mission science. Principal investigators (PI), team leaders, and interdisciplinary scientists (IDS) for the Cassini-Huygens mission were members of the PSG. In addition, a Huygens Science Working Team (HSWT) was established to provide guidance to the Huygens project manager on issues specific to that aspect of the mission.116 The function of the spacecraft was to reliably transport its scientific instrument payload from Earth to the Saturn system and to support that payload’s investigations. These encompassed analyses of Saturn’s atmospheric structure and composition, the physical properties of ring particles, examination of the moonlets within the rings, and detailed studies of the satellites, particularly Titan. Other investigations included the characteristics of Saturn’s magnetosphere and its interactions with other bodies, as well as the relationships between plasma, dust, and radiation offering clues to the formation of the Saturn system.117 To thoroughly examine the range of properties of the Saturn system and meet the above objectives, the twelve science instruments on the Cassini Orbiter had to be of several

3.6

The science instrument selection process 73

Table 3.1. Scientific objectives for exploring the Saturnian system.115 Titan • • • • • • • • • • • • • •

Determine abundances of atmospheric constituents Establish isotope ratios for abundant elements Constrain scenarios of formation and evolution of Titan and its atmosphere Observe vertical and horizontal distributions of trace gases Search for more complex organic molecules Investigate energy sources for atmospheric chemistry Model photochemistry of stratosphere Study formation and composition of aerosols Measure winds and global temperatures Investigate cloud physics, general circulation, and seasonal effects in Titan’s atmosphere Search for lightning discharges Determine the physical state, topography, and composition of surface Infer the internal structure Investigate the upper atmosphere, its ionization and its role as a source of neutral and ionized material for the magnetosphere of Saturn Saturn • Determine the temperature field, cloud properties, and composition of atmosphere • Measure the global wind field, including wave and eddy components • Observe cloud features and processes • Study diurnal variations and magnetic control of ionosphere • Provide observational constraints, including gas composition, isotope ratios, and heat flux • Develop scenarios for the formation and evolution of Saturn • Investigate sources and morphology of Saturn lightning, including Saturn electrostatic discharges (SED) and lightning whistlers Rings • Study the configuration and dynamical processes (gravitational, viscous, erosional, and electromagnetic) responsible for ring structure • Map the composition and size distribution of ring material • Investigate the interrelation of rings and satellites, including those embedded in rings • Determine the dust and meteoroid distribution both in vicinity of the rings and in interplanetary space. • Study interactions between rings and Saturn’s magnetosphere, ionosphere, and atmosphere Icy satellites • Determine the general characteristics and geological histories • Define mechanisms of crustal and surface modifications, both external and internal • Investigate the compositions and distributions of surface materials, particularly dark, organic-rich materials and low melting point condensed volatiles • Constrain models of satellites’ bulk compositions and internal structures • Investigate interactions between the magnetosphere and ring system and possible gas injections into magnetosphere Magnetosphere of Saturn • Determine the configuration of the nearly axially symmetric magnetic field and its relation to modulation of Saturn kilometric radiation (SKR) • Determine current systems, composition, sources, and sinks of magnetosphere charged particles. • Investigate wave-particle interactions and dynamics of Saturn’s dayside magnetosphere and magnetotail and their interactions with solar wind, satellites, and rings • Study the effect of Titan’s interaction with solar wind and magnetospheric plasma • Investigate interactions of Titan’s atmosphere and exosphere with surrounding plasma Note: Scientific objectives also included observations during flybys of a main-belt asteroid and Jupiter, as well as various experiments undertaken during the cruise from Earth to Saturn, including searching for gravitational waves and investigating the behavior of the solar wind, interstellar ions, and interplanetary dust.

74

Constructing the Cassini Orbiter Table 3.2. Cassini Orbiter instruments.119 Optical remote sensing instruments • Composite Infrared Spectrometer (CIRS) • Imaging Science Subsystem (ISS) • Ultraviolet Imaging Spectrograph (UVIS) • Visual and Infrared Mapping Spectrometer (VIMS) Fields, particles, and waves instruments • Cassini Plasma Spectrometer (CAPS) • Cosmic Dust Analyzer (CDA) • Ion and Neutral Mass Spectrometer (INMS) • Magnetometer (MAG) • Magnetospheric Imaging Instrument (MIMI) • Radio and Plasma Wave Science (RPWS) Microwave remote sensing instruments • Radar (RADAR) • Radio Science (RSS)

types: optical remote sensing instruments, which conducted observations in the visible, infrared, and ultraviolet regions of the electromagnetic spectrum; fields, particles, and waves instruments that studied the dust, plasma, and magnetic fields around Saturn; and microwave remote sensing instruments that used radio waves to map atmospheres, determine masses of moons, measure ring particle sizes, and unveil Titan’s surface.118 The instruments chosen are listed in Table 3.2. Other key observations were taken by Huygens Probe instruments, which will be discussed in the next chapter.

3.7

OPTICAL REMOTE SENSING INSTRUMENTS: SEEING THE VISIBLE AND INVISIBLE

The Cassini team placed four instruments on the remote sensing pallet (Figure 3.4). These instruments were co-aligned and usually imaged the same target.120 As will be seen, developing each optical instrument was truly an international effort, involving technical personnel from multiple countries. The optical remote sensing instruments included: • • • • 3.7.1

Composite Infrared Spectrometer (CIRS) Imaging Science Subsystem (ISS) Ultraviolet Imaging Spectrograph (UVIS) Visual and Infrared Mapping Spectrometer (VIMS). Composite Infrared Spectrometer

This instrument examined objects the way a snake’s tongue does, by searching for heat emanations. It was able to “see” thermal emissions – infrared (IR) light, redder than our eyes can see. “Another way to look at it,” commented Glenn Orton, a CIRS co-investigator, “is that we’re looking at what our skin feels as heat, rather than what our eyes see as light.”

3.7 Optical remote sensing instruments

75

Figure 3.4 Remote sensing pallet.

It helped meet the science objective in the AO to “Investigate the compositions and distributions of surface materials” on Saturn’s icy moons. As explained by John Spencer, a CIRS team member from Southwest Research Institute in Boulder, Colorado, the temperature differences that the instrument detected were able to reveal subtle differences between one sector of a moon and another, such as variations in texture that might arise between “old, dense snow and freshly fallen powder.”121 CIRS was a spectrometer, which meant it split light into different colors, like a glass prism or raindrops creating a rainbow. Except all the light waves that CIRS separated were in the infrared portion of the electromagnetic spectrum. Besides helping to unlock the mysteries of a satellite’s surface, CIRS also helped meet the AO’s science objectives to analyze the atmospheres and deeper regions of Saturn and Titan. Studying a body’s infrared spectrum yields clues as to its composition. Measuring precisely how hot the inside, surface, and atmosphere of a planet is tells us useful things, and can help predict the weather on that body. Atmospheres, such as those around Saturn and Titan, are composed of a range of gases distributed “in layers upon layers of varying temperatures that increase and decrease from the surface up through the edge of space,”122 and data from CIRS aided scientists in figuring out these atmospheres’ compositions. Each gas in an atmosphere emits or absorbs heat rays in a characteristic way, some wavelengths more than others. The infrared spectrum of an atmosphere identifies the gases present and their relative percentages. NASA’s Goddard Space Flight Center constructed CIRS, aided by hardware contributions from England and France as well as support from several other ESA countries.123 3.7.2

Imaging Science Subsystem

“We are the eyes of Cassini,” said Carolyn Porco, Cassini Imaging Team leader, adding that a sense of adventure and of being there was captured by the spacecraft’s cameras that could otherwise only have been imagined.124

76 Constructing the Cassini Orbiter The ISS might also be thought of as our eyes, the eyes of the human race, sent to Saturn so that we on Earth can see far and better understand a different planetary system. The ISS examined a wide variety of targets within the Saturn system, including the planet, its ring system, its large satellite Titan, and its icy and rocky satellites. It did this over a range of observing distances in order to address numerous scientific objectives, including:125 •

Atmospheres of Saturn and Titan • • • •

•

Saturn’s satellite surfaces • • •

•

Three-dimensional structures and motions Composition, distribution, and physical properties of clouds and aerosols Scattering, absorption, and solar heating phenomena Lightning, aurorae, airglow, and planetary oscillations

Nature and composition of surface materials Geological histories Rotation states

Ring system characteristics • • •

Gravitational interactions with Saturn’s satellites Rate and nature of internal energy and momentum transfer Ring thicknesses and sizes, composition, and physical nature of ring particles.

The ISS consisted of narrow-angle and wide-angle cameras. The narrow-angle camera provided high resolution images and was sensitive enough to discern a quarter 4 kilometers (2.5 miles) away. The wide-angle camera provided more extended spatial coverage but a lower resolution. A charge coupled device (CCD) served as each camera’s light detector, converting visual images into digital information. CCDs are basically integrated circuits possessing arrays called wells, each representing one pixel, or picture element. Light falling on a well is absorbed by a photoconductive material such as silicon, which releases electrons in numbers proportional to the intensity of the light. A well thus accumulates an electric charge due to released electrons. A CCD detects these accumulated electrical charges, whose numbers are converted to digital data. This system facilitated many options for data collection, including various ways of compressing data.126 Since the basic operation of a CCD involves small amounts of electric current released by incident light, any residual currents within the system can potentially impair the accuracy of the data generated. On the spacecraft, these dark currents had to be suppressed as much as possible, and several strategies were implemented to accomplish this. Dark current is somewhat dependent on temperature, and thus the ISS team implemented radiators to chill the CCD to its optimum operating temperature of −90°C. Ionizing radiation can also cause problems, and so shielding was installed. Furthermore, the entire narrow-angle camera, which had more stringent operating requirements than the wide-angle one due to its high resolution specifications, was thermally isolated from the remote sensing pallet (RSP) on which it was mounted.127 The imaging team also used time-lapse photography to produce movies of various targets. Such movies were useful for studying time-variable phenomena and celestial motions. Examples include complex motions in Saturn’s atmosphere, movements of its satellites,

3.7 Optical remote sensing instruments

77

and the rippling of Saturn’s F ring. “Not only are these scientifically informative, they are mesmerizing,” Porco noted. “Some of my favorite images are those in black and white, showing the shadow-draped Saturn atmosphere, the paper thin rings, and one or two lonely little moons.”128 Several of the ISS’s many amazing images are included in Figure 3.5.

Figure 3.5 Imaging Science Subsystem (ISS) images. (a) Saturn’s rings are confined to a plane many times thinner, in proportion, than a razor blade. This image was taken as the spacecraft flew through the ring plane. (b) The satellite Rhea with irregularly shaped Janus a million kilometers in the distance. (c) Enhanced image of Enceladus ice jets.

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Figure 3.5 (continued)

Each camera had two filter wheels for taking images at specific wavelengths of light. The narrow-angle camera included 12 filters in each wheel for a total of 24, while the wide-angle had 9 in each for a total of 18. Some of these filters allowed only light of a certain color to reach the camera’s sensor. Such techniques helped in interpreting the surfaces of satellites and aspects of Saturn’s atmosphere.129 3.7.3

Ultraviolet Imaging Spectrograph: Seeing in the butterfly range

Although our human vision systems cannot directly observe ultraviolet radiation, the eyes of certain other creatures can. According to UVIS Principal Investigator Larry Esposito, while “no person has ever seen ultraviolet light … some butterflies can. Our pictures may thus represent a ‘butterfly’s-eye view’ of the Saturn system.”130 Just as CIRS allowed us to “see” light that was redder and of longer wavelength than what our eyes alone are capable of detecting, UVIS enabled us to peer into the shortwavelength, high-energy ultraviolet region of the spectrum. In order to display such

3.7 Optical remote sensing instruments

79

Figure 3.6 Ultraviolet Imaging Spectrograph (UVIS) images. (a) Ultraviolet auroral emissions, invisible to the human eye, at Saturn’s south pole. (b) Density waves in Saturn’s A ring, observed by UVIS when the spacecraft was 6.8 million kilometers (4.2 million miles) from Saturn.

observations, the UVIS team of scientists and engineers had to decide which visible colors to use to portray various ultraviolet wavelengths. Some of their “false color” choices are evident in the striking images in Figure 3.6. The image in Figure 3.6a was taken on 21 June 2005 and shows auroral emissions invisible to the human eye. This image was obtained by scanning the instrument across Saturn to capture the entire oval of auroral emissions at the south pole. Similar emissions have also been observed at Saturn’s north pole. In this picture, blue represents auroral emissions from hydrogen gas excited by electron bombardment, while red-orange represents reflected sunlight.

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Figure 3.6b depicts density waves in Saturn’s A ring, observed by UVIS when the spacecraft was 6.8 million kilometers (4.2 million miles) from Saturn. In this case, bright areas indicate the denser regions of the rings. UVIS saw gases in the Saturn system that other types of cameras were not able to sense. It was also able to see the dark night sides of moons, which stood out against the sky because they were bright at ultraviolet wavelengths. Ultraviolet light studies identified some interesting chemical elements and compounds by using the observed light patterns as fingerprints for particular materials. In this manner, UVIS detected hydrogen, oxygen, methane, water, acetylene, ethane, and other substances. These observations were valuable for analyzing different parts of the Saturn system, such as the chemical composition of Titan’s upper atmosphere. The instrument also found “battleship-sized clumps of particles in Saturn’s rings that come together and then disperse on a faster-than-daily basis,” according to Esposito.131 UVIS had four different capabilities built into it. Two separate optical channels provided images and spectra for wavelengths in the range 56 to 118 nanometers132 (nm) and in the range 110 to 190 nm. A third optical path was designed to observe stellar occultations by rings and atmospheres. The term stellar occultation refers to the dimming of starlight when an object such as a planet, moon, atmosphere, or ring passes directly in front of a star. The Cassini-Huygens mission used many stellar occultation opportunities to examine Saturn’s ring system as well as conduct various atmospheric studies.133 How an object blocked out or interacted with the light gave clues as to the object’s structure, composition, and temperature. Stellar occultations allowed this instrument to study aspects of Titan’s atmosphere and map features in Saturn’s rings 10 times smaller than those visible to other Cassini cameras.134 These phenomena, which yielded data on the history of Saturn’s ring system, are discussed in Chapter 12. A separate hydrogen/deuterium absorption cell measured the relative abundances of deuterium and hydrogen (deuterium is an isotope of hydrogen that is chemically identical except that its nucleus contains a neutron whereas that of hydrogen does not). Ratios of these two isotopes can tell us important things about the origin and evolution of the Saturn system and other planets. For example, the D/H ratio offers insight into the role of cometary ices in the formation of Titan’s atmosphere.135 The UVIS instrument was built by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado in Boulder. The UVIS team included scientists and engineers from the U.S., France, Belgium, Germany and Japan.136 The University of Colorado also supplied ultraviolet spectrometers for previous Mariner, Pioneer, and Galileo missions.137 3.7.4

Visual and Infrared Mapping Spectrometer

VIMS is another instrument that extends the range of our human senses. It can detect 352 different wavelengths of light, most of which are beyond the capability of our eyes to see. VIMS consists of two cameras, one to measure visible wavelengths and the other for the infrared range. Italy developed the visible-range capability and the U.S. supplied the infrared channel. The instruments provided a significant amount of data on the natures of

3.8 Fields, particles, and waves instruments 81 satellite surfaces, the rings, and the atmospheres of Saturn and Titan. Its data was key in determining the compositions, temperatures, and structures of these objects, as well as other, surprising features. For instance, VIMS data led to the discovery of fresh ice on Enceladus and a possible ice volcano (also known as a cryovolcano) on Titan.138 Several VIMS images are included in Figure 3.7. Taken in July 2009, Figure 3.7a shows the first observation of a specular (mirror-like) reflection from a site near the north pole of Titan. This indicates an extremely smooth surface that is either a body of liquid or a surface frozen from a liquid. The infrared light at a wavelength of 5 microns (seven times the longest wavelength that is visible to the human eye) was colorized to match visible-light pictures of Titan. This and other data supported the existence of hydrocarbon lakes on that moon.139 Figure 3.7b shows an unusual hexagonal-shaped storm that has persisted at Saturn’s north pole for at least several decades. This picture was captured using the planet’s thermal glow (infrared emissions) as the light source. Using the planet’s own radiance as illumination allowed the storm to be viewed during the nighttime conditions of Saturn’s north polar winter.140 VIMS was designed to measure contiguous visible and infrared wavelengths from approximately 350 to 5,100 nm, corresponding to light reflected or emitted from atmospheres, rings, and satellite surfaces. To put this in perspective, the visible light spectrum – wavelengths that we can see – falls between 400 nm and 700 nm, a small portion of the wavelengths that the instrument gathered.141 The VIMS team included members from the University of Arizona, University of Washington, Cornell University, University of Hawaii, NASA’s Ames Research Center, JPL, and the U.S. Geological Survey. It also included personnel from Italy, Germany, and France.

3.8

FIELDS, PARTICLES, AND WAVES INSTRUMENTS

Fields, particles, and wave instruments examined the environment in the immediate vicinity of the vehicle, measuring magnetic fields and the masses, electrical charges and densities of atomic particles. They also analyzed the quantities and compositions of dust, the strengths of plasma (electrically charged gas), and the characteristics of radio waves.142 These instruments included: • • • • • • 3.8.1

Cassini Plasma Spectrometer (CAPS) Cosmic Dust Analyzer (CDA) Ion and Neutral Mass Spectrometer (INMS) Magnetometer (MAG) Magnetospheric Imaging Instrument (MIMI) Radio and Plasma Wave Science (RPWS). Cassini Plasma Spectrometer

The charged-particle gas called plasma is often referred to as the fourth state of matter, or as CAPS Principal Investigator David Young put it, “electromagnetic Jell-O.”143

82 Constructing the Cassini Orbiter

Figure 3.7 Visual and Infrared Mapping Spectrometer (VIMS) images. (a) Glint of sunlight reflecting off a possible Titan hydrocarbon lake. This image was taken by VIMS in infrared light. (b) The bizarre, six-sided storm at Saturn’s north pole. This VIMS image was the first picture to capture the entire hexagonal feature and north polar region in one shot. It employed Saturn’s thermal (infrared) glow at 5 microns (seven times the wavelength visible to the human eye) as the light source.

3.8 Fields, particles, and waves instruments 83 Built to investigate the composition, density, flow, velocity, and temperature of charged particles in Saturn’s magnetosphere (which engulfs the planet, its rings, and the orbits of Titan and most of the icy moons),144 CAPS addressed the requirement of the AO to determine plasma current systems, composition, sources, and sinks, and to analyze:145 • • • • • •

Composition of ionized molecules originating from Saturn’s ionosphere Composition of ionized molecules originating from Titan’s atmosphere Features of Saturn’s magnetic field Interaction of Saturn’s magnetosphere with solar wind Interaction of rings with magnetosphere Plasma input to magnetosphere from icy satellites.

The principal investigator institution for CAPS was Southwest Research Institute of San Antonio, Texas. The team included NASA’s Goddard Space Flight Center, Los Alamos National Laboratory, University of Montana, University of Virginia, Johns Hopkins University, and Rice University, as well as institutes from Hungary, Norway, Finland, France, the U.K., the Netherlands, and Israel.146 Almost 170 people helped develop the 50-pound, microwave oven-sized package.147 3.8.2

Cosmic Dust Analyzer

CDA’s objective was to provide, as mandated in the AO, observations of particulate matter in the Saturn system, as well as in interplanetary space en route, and to study the physical, chemical, and dynamical properties of these particles. The Saturn system’s dust particles are typically quite small. The broad, diffuse E ring, for instance, is composed primarily of particles one-thousandth of a millimeter in size, much smaller than the width of a human hair and even smaller than red blood cells. They are analogous to the particles in cigar smoke. Nevertheless, they were easily detectable by the CDA on Cassini, which measured their miniscule impacts on its sensors. Under ideal conditions, the instrument (Figure 3.8) was even capable of detecting nano-dust, with particles only one-millionth of a millimeter in size. These particles are even smaller than a single influenza virus, and detecting them was sort of like finding a single raindrop falling into the Gulf of Mexico.148 CDA was a versatile instrument that could reliably measure dust particle impacts occurring as infrequently as 1 per month and as rapidly as 104 times per second.149 Saturn’s dust particles are worthy of study because they carry within them evidence of their origin in some distant place, possibly billions of years ago. From their states and compositions, it was possible to infer the processes involved in their formation and the environmental conditions they were subject to.150 Like other scientific instruments designed to address a variety of objectives, CDA consisted of several components, each optimized for a different task. The High Rate Detector (HRD) used two redundant, independent polyvinylidene fluoride sensors to count the frequent dust impacts as the spacecraft crossed Saturn’s ring plane. The objective was to make quantitative measurements of particle flux and mass distribution throughout the Saturn ring system. As with much of the technology aboard CassiniHuygens, the HRD owed its design in large part to work done earlier for other missions. In this case it was “significant inheritance”151 from the University of Chicago Dust Counter and Mass Analyzer instrument on spacecraft that examined Halley’s Comet.

84 Constructing the Cassini Orbiter

Figure 3.8 The Cosmic Dust Analyzer (CDA).

The Dust Analyzer (DA) determined the electric charges carried by dust particles, their impact directions and speeds, as well as their masses and compositions in terms of chemical elements. To achieve all this, it needed various subsystems. The charge detector consisted of stainless steel grids at the front end of the instrument. When an electrically charged particle passed through the grids it induced charges on them in direct proportion to the charge of the dust particle, enabling a direct determination of the charge on the particle. Also measured was the duration of the charge signal on the grids, which equated to the particle’s time-of-flight through them and was used to calculate the particle’s speed.152 When dust particles slammed into the DA’s hemispherical target, both particle and target fragments were generated, as well as neutral atoms, ions, and electrons. An electric field separated electrons and ions, after which they were collected and analyzed. The ion collector had a grid that was negatively biased in order to collect positively charged ions produced by impacts with the target. Key in the analysis of the ions was a time-of-flight mass spectrometer – an instrument that employed the duration of an ion’s flight between two electrodes to determine its mass-to-charge ratio and chemical composition.153 CDA was on an articulation mechanism that enabled the entire instrument to rotate and reposition with respect to the spacecraft. This helped CDA to measure particle fluxes in a wide range of directions, without the need to reorient the entire spacecraft. As seen in Figure 3.8, the CDA’s mounting base is at the bottom; the electronics box sits on that, and the instrument detectors are above that. The main detectors are in the cylinder at the top of this stack. The CDA base is fixed, but everything above it rotates 270° about an axis that is vertical.154

3.8 Fields, particles, and waves instruments 85 3.8.2.1

Articulation mechanisms

CDA was not the only instrument having the ability to reposition independent of the spacecraft. CAPS and MIMI could also articulate the positions of their detectors by rotating about a single axis. MIMI’s articulation mechanism eventually failed. The four remote sensing instruments, on the other hand, needed to have two degrees of freedom (rather than one as did the CDA, CAPS and MIMI) for articulation to be of any use, and this would have been equivalent to putting them on a scan platform, a complex and expensive undertaking that had been canceled for financial reasons. It was generally not critical which direction the MAG and RPWS pointed in order to take useful measurements, so there was little reason to provide them articulation mechanisms. INMS might have benefited from a single axis articulation capability, but it was a late addition to the suite of instruments and may have been selected too late for a mechanism to be incorporated, or perhaps the utility of articulating it was not recognized in time.155 3.8.3

Ion and Neutral Mass Spectrometer

Mass spectrometry is a powerful technology for determining a sample’s chemical composition. INMS analyzed atoms and molecules in a neutral or charged state. If the sample was not yet ionized but was instead made up of neutral particles, the first thing that INMS did was use an electron beam to ionize the particles. Electric and magnetic fields directed ionized particles through the instrument, where the fields deflected various species by different amounts and separated them based upon their mass/charge ratios.156 The INMS carried by Cassini analyzed the composition and structure of Titan’s upper atmosphere in terms of positive ions and neutral gases, the magnetosphere of Saturn, and the icy satellites and rings.157 Its major components included:158 •

•

•

•

•

Open ion source This produced ions by ionizing neutral gases, that then were analyzed by the instrument’s sensor. Closed ion source This was especially useful for ionizing the more inert atomic and molecular species, such as nitrogen (N2) and methane (CH4). Due to the speed of the vehicle, instrument sensitivity was greatest in the closed ion source mode because the ram pressure of inflowing gas enhanced the density of the non-reactive neutral gases in the source’s antechamber. Quadrupole deflector Ions generated by the ion sources were directed by the voltages on the quadrupole deflector to the mass analyzer. Quadrupole mass analyzer This used specific voltages between its poles to separate ions according to their mass-to-charge ratio, allowing only ions with a specific ratio to pass through at a given time and strike the detector. Dual detector system This used two electron multipliers to read input from the quadrupole mass analyzer. When ions from the mass analyzer hit an electron multiplier, the device initiated and then amplified a current of electrons until a measurable current was attained.159

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INMS was built by a team of engineers and scientists at NASA’s Goddard Space Flight Center (which also developed the Neutral Mass Spectrometer for the Galileo mission) and University of Michigan. Hasso B. Niemann of Goddard directed the development and fabrication.160 Hunter Waite of the Southwest Research Institute was the principal investigator for the instrument team.161 3.8.4

Dual-Technique Magnetometer

In describing the wide-ranging capabilities of the magnetometer, team-member Marcia Burton commented that the neatest thing about it was it allowed you to see inside of planets. Magnetometers have extended our sensory range beyond the limits of what optical or even radio wave instruments are able to tell us about a heavenly body, because magnetic fields that originate deep within it are able to travel out into space. For many decades, scientists studied the core of our Earth using information gleaned from its magnetic field. And on the Galileo mission, it was a magnetometer that provided the key data supporting the existence of a warm-water ocean inside the Jovian moon Europa. On the CassiniHuygens mission, its MAG gave us profiles of Saturn’s magnetic field and told us about the part of the planet’s interior (most likely a metallic hydrogen region) which can carry electric currents and generate that field. The instrument has supplied information on Saturn’s interior rotation rate and has helped infer the existence of a rocky and possibly icy core of some 15 to 20 Earth masses (ME) at the planet’s center.162 Another coup for MAG is that it was the first to detect the geysers at Enceladus.163

3.8.4.1

Saturn’s rotation rate

Determining Saturn’s rotational speed has proved to be a difficult task compared with other planets. On Earth, magnetic north (the direction that a compass points) does not point in the same direction as geographic north, which is the direction of the axis around which our planet rotates. The two axes differ in orientation by about 11°. Similar situations occur on Jupiter, Neptune, and Uranus, where offsets between rotational and magnetic axes range from 10° to 60°.164 Where the two axes are offset, a planetary rotation rate can be measured by observing the rotation of the magnetic around the geographic axis. Saturn is the only planet on which these axes align, and the planetary rotation rate cannot be determined just by measuring how quickly the magnetic axis revolves around the geographic axis. If we could watch the movement of solid features of the planet, a rotation rate could be determined from that. But no solid surface can be observed by our instruments, only clouds and other atmospheric features. Attempts to measure the rotation rate of the planet as a whole by observing fairly persistent features in its atmosphere have not been very successful; the rotation rate determined by tracking such features appeared to vary with latitude. Measuring the rotation of the radio signal emitted by Saturn was also attempted, but the period of that signal slowed noticeably from the early 1980s to the present. If the signal was locked into the actual rotation of the entire planet, this would indicate that Saturn had slowed down dramatically in just a few decades, an occurrence that made no sense to scientists.165 While the situation is

3.8 Fields, particles, and waves instruments 87 confusing, MAG data is one of several sources helping to make better estimates of the planet’s rotational characteristics. The problem of Saturn’s rotation rate is discussed in more detail in Chapter 11.

3.8.4.2

Satellite observations

MAG data also helped shed light on characteristics of the moons orbiting Saturn. Nick Achilleos, an engineer on the team, commented on how magnetic field data sometimes became a key determinant in understanding a process: “The Enceladus flybys showed us that very often, an important physical process such as mass loss from Enceladus can be detected magnetically before other instruments pick up any signatures. The distortion of the magnetic field often seen near the icy satellites of Saturn can be used to infer what physical processes are operating at and beneath the atmospheres of these moons.”166

3.8.4.3

MAG components

MAG had two major parts – a vector/scalar helium magnetometer (V/SHM) and a flux gate magnetometer (FGM). The V/SHM sensor developed by JPL made both vector (magnitude and direction) and scalar (magnitude only) measurements of a magnetic field. The FGM developed by Imperial College London made only vector field measurements. Principal Investigator Michele Dougherty was a space physicist at Imperial College.167 Other members of the team came from South Africa, Canada, the Netherlands, Argentina and Australia.168 The Cassini MAG was quite similar to the dual magnetometer on the NASA-ESA Ulysses spacecraft, which performed very well on its trajectory around Jupiter and the Sun. The main difference was the scalar capability of the helium magnetometer, which could very accurately measure the magnitude of a magnetic field. Having two types of magnetometer gave redundancy and improved measurements of the residual spacecraft field, which was essential for proper calibration. Both components of the MAG provided high quality data, but using the V/SHM in combination with FGM’s simultaneous vector measurements allowed even more accurate measurements of the magnetic field. And the broad range over which the FGM could take measurements was complemented by the low noise and sensitivity of the V/SHM.169

3.8.4.4

Preventing interference from other spacecraft instruments

Because magnetometer readings can easily be influenced by nearby electric currents and ferrous metal components, spacecraft designs typically locate a magnetometer’s sensors on extended booms, as far as possible from vehicle electronics. The Cassini team placed the FGM sensor midway out on the 11 meter (36 foot) boom while the very sensitive sensor of the V/SHM, which was tasked with measuring the absolute magnitudes of Saturn’s field, was placed at the end of the boom, farthest away from the spacecraft and any of its stray fields,170 as well as some distance from the FGM sensor. Possible sources of interference also included the electronic systems of the magnetometer itself, such as its data

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processing unit and power supplies, so these circuitries were placed in a bay of the spacecraft’s upper equipment module.171 The boom was composed of thin, non-metallic rods. Electric currents flow freely in metals, and the fields that those currents generate can interfere with magnetometer operations. Although the distance the boom provided between magnetometer sensors and various electronics greatly reduced interference with the instrument’s function, vehicle components such as electric motors, amplifiers, relays, and other devices still had the potential to magnetically contaminate the data being collected. Calculations showed that permanent magnets within these subsystems could collectively impair the MAG science experiments. In addition, the RTGs presented potential problems owing to the fields that their circuitries generated. The Cassini team initiated a magnetics control program. A control review board of science team members, including the principal investigators and co-investigators, and personnel from JPL and ESA, was established to oversee progress. To minimize interference, engineering subsystems and science instruments were required to limit the fields that they generated. The Board implemented specific restrictions on certain vehicle components, such as minimizing current loop areas in printed circuit boards, choosing nonmagnetic materials, and demagnetizing wherever possible. The radio frequency subsystem components were packaged in their housing in a side-by-side manner, but with their respective magnetic field polarities in opposite directions so that they would largely cancel out. Similarly, the RTGs were orientated so that their magnetic fields tended to cancel out.172 In cases where there was an odd number of components (such as with magnetic latch relays), small compensation magnets were used to cancel the fields of these unneutralized components. Compensation magnets were also used for neutralizing the particularly intense fields of latch valves (whose functions included switching a system to a redundant system, should the primary one fail).173 To minimize magnetic flux leakage from the many electric motors onboard the spacecraft, instrument designers sought to enclose them inside special shielding materials. Also, non-magnetic materials such as titanium, beryllium copper, plastics and composites were substituted for ferrous (iron-containing) metals.174 3.8.5

Magnetospheric Imaging Instrument

Contained in MIMI was a unique sensor, the Ion and Neutral Camera (INCA), able to remotely “generate a picture of what [a] magnetic field looks like.”175 INCA used a technology called energetic neutral atom imaging to accomplish this. A planetary magnetosphere consists of a variety of elements, including magnetic fields, plasmas (ionized gases), magnetically trapped energetic charged particles (electrons and ions), neutral gas (from planetary and satellite atmospheres), and particulates. The coexistence of trapped energetic ions and neutral gases leads to an interaction called charge exchange in which a very energetic, charged ion steals an electron from a very cool neutral gas atom, becoming an energetic neutral atom (ENA). Because an ENA has no charge, the magnetic field can no longer restrain it. Some of the ENAs entered MIMI’s INCA sensor, which determined their arrival directions, energies, and mass species. From this data, images were constructed, remotely, of the shapes and structures of Saturn’s magnetic field.176

3.8 Fields, particles, and waves instruments 89

Figure 3.9 The shape and structure of Saturn’s magnetosphere, imaged by MIMI’s Ion and Neutral Camera (INCA).

MIMI was the first spaceborne instrument capable of imaging an entire planetary magnetosphere (Figure 3.9), a magnetic envelope of charged particles that surrounds some planets, including Saturn and Earth, that is totally invisible to the human eye. Analyzing the overall configuration and dynamics of the Saturnian magnetosphere helped determine its interactions with the planet’s atmosphere, rings, icy moons, the satellite Titan, and the solar wind.177 MIMI also made in situ measurements inside the magnetosphere, building up a three-dimensional data set of particle distributions and properties, including ion compositions and charge states. Altogether, the global imaging and in situ analyses produced an impressive insight into the characteristics of Saturn’s magnetic environment. Besides INCA, the team fitted the MIMI instrument with two additional sensors: the low-energy magnetospheric measurement system (LEMMS) and the charge-energy-mass spectrometer (CHEMS). LEMMS studied the energies and angular distributions (the number of particles approaching from each direction) of low-energy and high-energy protons, ions, and electrons. The instrument’s head was mounted on a small platform capable of 180° rotation. CHEMS measured charge state, composition, and energy of ions in the most energetically important plasma region of the magnetosphere.178 S. M. Krimigis of the Applied Physics Laboratory of Johns Hopkins University served as principal investigator of the MIMI team, which included people from the Max-PlanckInstitut für Aeronomie in Lindau, Germany, University of Maryland, University of Kansas, University of Arizona, Bell Laboratories in New Jersey, the National Central University in Taiwan, and the Centre d’Etude Spatiale des Rayonnements in Toulouse, France.179 3.8.6

Radio and Plasma Wave Science

Saturn has various mechanisms for generating radio signals, one is by interacting with the charged particles of the solar wind. The RPWS experiment received and measured Saturn’s

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Constructing the Cassini Orbiter

radio signals, investigated electric and magnetic waves in its magnetosphere and in the surrounding interplanetary plasma,180 and mapped its ionosphere and lightning emissions. RPWS also determined dust and meteoroid distributions throughout the Saturn system.181 Three nearly orthogonal electric field antennas (in other words, nearly at right angles to each other) were used to detect electric fields over a frequency range from 1 hertz (Hz, or cycle per second) to 16 megahertz (million cycles per second, or MHz), and three orthogonal search coil magnetic antennas detected magnetic fields over a frequency range from 1 hertz to 12 kilohertz (thousand cycles per second, or kHz). A sensor called a Langmuir probe,182 which utilized a 5 centimeter (2 inch) diameter sphere carried at the end of a 1 meter rod, measured electron density and temperature. From the measured currents and voltages induced in the probe system by conditions in the plasma, physical properties of the plasma were calculated.183 Cassini’s RPWS was a great improvement over the instruments of the Voyagers, the only previous spacecraft to make radio and plasma wave measurements in the vicinity of Saturn. It featured superior sensitivity and dynamic range, the ability to make directionfinding measurements of remotely generated radio emissions, active and passive measurements of plasma resonances that gave precise information on electron densities, and Langmuir probe measurements of the electron densities and temperatures. Cassini was thus able to undertake a broad range of radio emission, wave-particle interaction, thermal plasma, and dust studies.184 Understanding the variety of wave phenomena that the RPWS measured is not an easy task, but Donald Gurnett of the University of Iowa, the instrument’s principal investigator, wanted to reach out to a wider audience and give a subjective feel for these cosmic vibrations. He converted them to audible sound waves. Over a decade ago, the Croons Quartet took these strange sounds and blended them with choral and string music. The resulting 90 minute piece called Sun Rings has been performed around the world, combined with images from outer space.185 Don Gurnett’s career has mirrored the development of the entire space age. While he still lives not far from the farm in eastern Iowa where he was raised, The Iowan magazine has said of him that “If space research earned frequent flyer miles, Gurnett would have racked up more credits than almost anyone on this planet.”186 His body may have stayed on Earth, but his curiosity and research activities have reached out to the surface of Mars, the emanations of Jupiter, the characteristics of the solar wind and, of course, the wave phenomena of Saturn. Over his half-century-plus career he has participated in more than 35 space missions. His biggest pleasure, he says, has been solving scientific puzzles, “a passion that was ignited six decades ago when he helped his father tinker with engines on the family farm.”187 Like many researchers, discovering something truly new has always been a great source of joy to him. One of his early achievements, before he started designing sensors for spacecraft, was winning the U.S. national championship for model airplanes, the designs of which had kept him fascinated since he was eight. The RPWS team is one of the more international ones of the mission, drawing its membership from University of Minnesota, NASA’s Goddard Space Flight Center, the Observatoire de Paris, Centre d’Etude Spatiale des Rayonnements in Toulouse, France, the Centre d’Etudes des Environnements Terrestre et Planétaires in Vélizy-Villacoublay, France, the University of Sheffield in the U.K., the Austrian Academy of Science Space Research Institute, the Swedish Institute of Space Physics, and University of Oslo in Norway.188

3.9 Microwave remote sensing instruments 3.9

91

MICROWAVE REMOTE SENSING INSTRUMENTS

Using radio waves, two instruments mapped atmospheres, pierced Titan’s opaque atmosphere to observe its surface, determined the masses of moons, and collected data on ring particle sizes:189 • • 3.9.1

Radar (RADAR) Radio Science Subsystem (RSS). RADAR

The Cassini RADAR system took pictures like a camera, but in the microwave rather than visible part of the electromagnetic spectrum. It used the spacecraft’s high-gain antenna (HGA) and its associated five-beam Ku-band antenna feed assembly to send radar transmissions toward targets and capture reflected radar signals and blackbody radiation.190 The RADAR delineated objects not able to be seen by instruments operating in the visible light or infrared bands. A vital aspect of the instrument was that it could “pierce through an atmosphere, even one as thick and murky as the one engulfing Titan.”191 By bouncing radio signals off Titan’s surface and timing their return, the instrument produced maps and measured the heights of objects such as mountains and canyons. The commonly used term radar is an acronym for “radio detection and ranging,” and it was used on the Cassini-Huygens mission in a variety of different ways. For instance, the RADAR system investigated the satellite Titan by taking four different types of observations: imaging, altimetry, backscatter, and radiometry.192 Using its imaging mode, the RADAR bounced pulses of microwave energy off Titan’s surface from different directions and recorded the time the pulses needed to return to the spacecraft. These measurements were converted to distances, and used to construct visual images of the surface topography.193 In this mode, the RADAR relied upon synthetic aperture radar (SAR) technology for generating the highest resolution pictures possible. SAR employed a modest-sized radar dish antenna to cleverly simulate the performance of a much larger antenna. A large radar antenna can receive more electromagnetic radiation per second, and thus can provide higher image resolution. But a spacecraft is limited in how big an antenna it can carry, and so SAR technology is invaluable as a means of increasing resolution. By collecting many observations of a surface feature from numerous points along the spacecraft’s flight path and then combining them with the aid of sophisticated software, the SAR system gave synthesized images similar in quality to those otherwise attainable only by a much larger antenna. In effect, SAR “substitutes software for hardware – that is, data processing for a physically large antenna.”194 SAR techniques, which measure both the intensity and phase of reflected microwaves, are highly sensitive to surface textures and effective in a penetrating cloudy, hazy atmosphere such as on Titan.195 In the altimetry mode, the RADAR also bounced microwave pulses off Titan’s surface and measured the time the echo took to return to the spacecraft. But in this mode, the objective was not to create visual images; it was to obtain numerical data on the precise altitudes of various surface features. According to Ralph Lorenz of the RADAR team, “The distance from Cassini to the surface could be measured with an impressive precision of around 50 m[eters],”196 which meant the heights of various features could be determined

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Constructing the Cassini Orbiter

to within this amount.197 It would have been useful to obtain surface height estimates from the RADAR altimetry mode coincidentally with analysis of unique surface features using SAR techniques and the RADAR imaging mode. There was an inherent problem in doing this, however. Radar altimetry is optimally performed using nadir observations (with the target directly beneath the observer), while SAR techniques require off-nadir scrutiny in order to obtain the best image.198 In the backscatter mode of operation, the RADAR acted as a scatterometer. It bounced pulses off Titan’s surface and then measured the intensity of the energy that came back. This backscatter was always less than the original pulse, because surface features inevitably reflected (scattered) the pulse in more than one direction. From the backscatter measurements, scientists were able to determine the radar reflectivity of the surface and infer something about the composition and physical state of that material.199 The backscatter mode was used, for instance, to analyze the Huygens Probe landing site to help determine whether that vehicle “was in for a splash or a crash.”200 In its radiometry mode, the RADAR operated passively, not sending out signals but simply recording the energy emanating from the surface of Titan. In other words, the RADAR observed “the faint natural radio glow from surfaces rather than radar echoes,”201 and thus helped determine its surface composition. Organic materials, for instance, were good emitters of microwave energy. Radiometery also obtained data on the amount of latent heat (such as due to moisture) in the atmosphere, which had an impact on the precision of the other measurements taken by the instrument.202

3.9.1.1

Italian involvement in the Cassini radar

The Cassini RADAR instrument consisted of the digital subsystem (DSS) that JPL developed and which included the flight computer and the RF electronic subsystem (RFES) provided by the Italian Space Agency (ASI). The DSS exchanged timing information, telemetry data, and commands with the RFES, which interfaced with the high-gain antenna (HGA), which was also made by ASI, to transmit and receive signals.203 RADAR was designed to operate at altitudes between 950 and 5,000 kilometers above Titan’s surface. The objectives of the instrument at Titan were to:204 • • •

• 3.9.2

Acquire low resolution imagery (0.6 to 2.5 kilometer resolution205) of at least 30% of Titan’s surface Obtain high resolution imagery (400 to 600 meter resolution) at altitudes of less than 1,500 kilometers above the surface Obtain a surface profile (a depiction of the topography viewed from the side) with vertical resolutions between 225 and 450 meters and horizontal resolutions between 52 and 75 kilometers, depending on spacecraft altitude Acquire radiometer data of Titan’s entire surface. Radio Science Subsystem

RSS involved a clever application of the spacecraft’s communications equipment. Imagine an experiment which measured how well sound traveled through various materials, such

3.10

Building flight instruments at JPL 93

Figure 3.10 Radio science.

as a thick curtain or a wooden door. By studying changes in the sound as it went through various materials, we would learn something about the composition and characteristics of those materials. This was similar to the RSS experiment concept, except instead of sound, radio waves were transmitted and instead of a curtain or door that was studied, the radio waves passed through, and provided information about, the rings and atmosphere of Saturn as well as the atmosphere of Titan (Figure 3.10).206 The RSS was the largest instrument on the mission, because only one rather compact part of it sat on the spacecraft. The other components resided at each of the three Deep Space Network antenna complexes on Earth. In addition to measuring how radio signals were affected by whatever material came between the spacecraft and Earth, allowing scientists to identify the composition and properties of Saturn’s rings and atmosphere, the RSS measured forces acting on the spacecraft by detecting slight changes in the frequency of radio signals sent from it to Earth. The Cassini team used the RSS at Saturn to conduct atmospheric, ionospheric, and ring occultations207 as well as mass and gravity field determinations.208 The team also used the instrument during the interplanetary cruise, to undertake experiments pertaining to gravitational waves209 and general relativity. Details of these studies are described in later chapters.

3.10

BUILDING FLIGHT INSTRUMENTS AT JPL: INAPPROPRIATE COMPETITION WITH OUTSIDE ORGANIZATIONS?

When Lew Allen gained the directorship of JPL in 1982, he noted the laboratory’s increasing focus on developing sophisticated research instruments for spacecraft. Over the next few years, in fact, he saw instrument development agreements become “a thriving flow of new business.”210 The new activities included development of spectrometers, radar, and scatterometers (see the above section on Cassini’s RADAR instrument for a discussion of what scatterometers do). This led to concern that JPL would displace other developers of such instruments, including both NASA centers and universities.

94

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Charles Elachi, who helped grow this new business line, noted that developing the increasingly complex science instruments that would meet the requirements of new space missions provided “a technical and managerial challenge now comparable to that of entire spacecraft.”211 Designing and building such experiments could cost well over $100 million, employ dozens of people, and produce instruments weighing hundreds of kilograms. The scale, cost, and intricacy of such endeavors resembled those of small flight projects, and this justified, at least in many managers’ views, committing JPL’s extensive engineering experience to producing such instruments. The danger in such a course, however, was that JPL instrument development activities might undermine the laboratory’s solid relationships with the NASA and university shops which, for more than two decades, had supplied these instruments. Traditionally, JPL provided the platforms for instruments from outside researchers. In other words, JPL designed and built the spacecraft that university, institute, and other NASA scientists “hitched rides on”212 into deep space. The scientists relied on JPL creating these opportunities. But as JPL ramped up its instrument development focus, outside scientists complained it was unfair competition. JPL personnel could access inside information, talk informally to engineers involved in designing new spacecraft and tailor their proposals to the technological needs that were emerging. JPL answered these accusations with the argument that NASA had agreed “to support a cadre of researchers at the lab.”213 If the researchers could not compete for flight instrument contracts, they would be forced to leave JPL. A 1988 NASA Center Science Assessment Team evaluating JPL held that the laboratory should not try to compete with scientists from universities, but instead, focus on large research efforts that universities did not have the resources to carry out.214 JPL’s executive council eventually decided that laboratory scientists should restrict their development of scientific payload instruments to what was appropriate. But JPL never spelled out a longterm policy regarding competition for instrument contracts. As a result, the laboratory’s staff have remained free to prepare instrument proposals in response to any Announcement of Opportunity that NASA produces.215 JPL competes on an equal footing with universities and other NASA centers.216 The evaluation criteria for proposals are outlined in the Announcements of Opportunity, and are usually limited to technical, cost, schedule, and management factors. Every proposal is evaluated against the same criteria.

3.11

PRINCIPAL INVESTIGATOR VERSUS FACILITY INSTRUMENTS

Science instruments developed for space missions are classified as either principal investigator (PI) instruments or facility instruments, with the differences relating to the types of competition used in allocating time on, and control over the instrument to given researchers. 3.11.1

PI instruments

Individual researchers write proposals and enter NASA-run competitions to design, develop, and operate what are called PI instruments. A scientist or engineer who has won

3.11

Principal Investigator versus facility instruments 95

such a competition receives NASA funding to create and deliver the instrument. The proposal also identifies a set of co-investigators who help conduct the specified experiments. PIs tend to like this arrangement since they have the final say on how the instrument is used, the observations that get made, and what gets published. PIs, as discussed above, can come from organizations outside NASA, or from the Agency or one of its laboratories. For example, the Magnetometer (MAG) team was set up by PI David Southwood (of ESA and formerly of Imperial College London). Later Michele Dougherty of Imperial College London took over in this role. It has members from institutions that included the Technical University of Braunschweig (Germany), the Central Institute for Physics Budapest (Hungary), and JPL.217 The Composite Infrared Spectrometer (CIRS) team, on the other hand, was led by PIs Virgil Kinder and then Mike Falser, both of NASA’s Goddard Space Flight Center (GSFC), where the device was also developed. The team included personnel from a range of U.S. and European universities as well as NASA’s Marshall Space Flight Center, the Observatoire de Paris-Meudon, and other organizations.218 The value of PI instrument development to JPL staff in particular was made apparent after the 1997 completion and launch of the Cassini spacecraft, when its flight project workforce dropped severely. Instrument development activities soon picked up some of the slack, employing many personnel who had completed their flight project assignments. In 1998, instrument development surpassed flight projects as JPL’s largest endeavor, supporting over 500 work-years.219 3.11.2

Facility instruments

Sometimes NASA decides to designate a certain technology not as a PI instrument, but as a facility instrument, and assigns its development to an Agency institution such as JPL or GSFC without that facility having to compete with other institutions. NASA makes this choice in cases where it requires a specific capability for a given mission, or where the instrument is to be used for a variety of purposes. For instance, Cassini’s Imaging Science Subsystem (ISS) facility instrument was also used for the optical navigation that helps keep the spacecraft on course.220 What is competed on facility instruments are spots on the science team that will operate it. Individual scientists submit proposals to NASA to use the instrument for their own investigations. However, NASA still maintains ultimate responsibility for the instrument’s operations, health, and safety throughout the mission.221 JPL developed the Imaging Science Subsystem (ISS), the Visual and Infrared Mapping Spectrometer (VIMS), the RADAR, and the Radio Science Subsystem (RSS) as facility instruments for the Cassini Orbiter. Many researchers submitted proposals to use these instruments for their research. In the case of the ISS, NASA selected 14 proposals for investigations using its cameras.222 The Orbiter’s Ion and Neutral Mass Spectrometer (INMS) was a special case. It was developed by GSFC as a facility instrument but NASA later directed JPL to operate it as a PI instrument because “the responsibility for instrument operations, health, and safety clearly lay with the PI and not the institution that provided it.”223

96 3.12

Constructing the Cassini Orbiter THE SPACECRAFT: OUR EYES, HANDS, LEGS, AND BRAINS AT SATURN

As expressed in the opening section of this chapter, the Cassini spacecraft can be thought of as a concentration of human-like senses and features, but far more acute and durable than we humans could ever hope to be. This concept of the spacecraft is well expressed in Figure 3.11, which recognizes the cameras and magnetospheric imagers as akin to its eyes, the cosmic dust analyzer as its hands, the ship’s computer as its brain, the main engine and thrusters as its “walking” and “dancing” legs, the RTGs’ electrical output as its food, the Huygens Probe as the baby it will birth, and so on. But envisioning the vehicle in this way is more than simply entertaining. It’s a recognition that Cassini really did become our eyes and hands at Saturn, the means by which humans now visit distant locations that our corporeal bodies cannot reach.

Figure 3.11 The Cassini spacecraft – our eyes, hands, brains, and legs at Saturn.

References 97 REFERENCES 1. JPL, “Spacecraft: Introduction,” http://saturn.jpl.nasa.gov/spacecraft/index.cfm, last updated 12 June 2008, accessed 24 Sep. 2008. 2. Torrence Johnson interview, Rome, Italy, by author, 11 June 2008. 3. Johnson 2008. 4. Johnson, June 2008. 5. Johnson, June 2008. 6. Johnson 2008. 7. Johnson 2008. 8. “Attitude and Articulation Control Subsystems (AACS),” http://www2.jpl.nasa.gov/basics/ bsf11-2.html, in chapter 11 of Basics of Space Flight, 2008, accessed 22 July 08. 9. JPL, “Over the Poles of the Sun,” http://ulysses-ops.jpl.esa.int/, last updated 12 July 2008, accessed 21 July 2008; ESA and JPL, “Giotto: European Space Agency Probe to Halley’s Comet,” http://www.solarviews.com/eng/giotto.htm, accessed 21 July 2008; ESA, “Giotto: Summary,” http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31716, last updated 6 June 05, accessed 22 July 08. 10. Johnson 2008. 11. Johnson 2008. 12. Al Diaz and John R. Casani, “Galileo 1986 on Centaur,” Astronautics and Aeronautics 21 (Feb. 1983):33. 13. Johnson 2008. 14. NASA, “Galileo Comparative Funding Profile,” 14 Dec. 1982 presentation, in a package of documents entitled “Draper 83 Chron Files” and beginning with Ronald F. Draper letter to Geoffrey A. Briggs, 28 Oct. 1983, JPL-Cassini CASTL. 15. Johnson 2008. 16. Marcia Neugebauer, “Mariner Mark II and the Exploration of the Solar System,” Science 219 (4 February 1983):443–449. 17. Johnson 2008. 18. Robert (Bob) Mitchell email to author, 20 August 2008. 19. Michael Meltzer, Mission to Jupiter: A History of the Galileo Project, NASA SP-2007-4231 (Washington D.C.: NASA, 2007), p. 112. 20. Johnson 2008. 21. Johnson 2008. 22. Ronald F. Draper letter to Phillip de Aragon, 5 December 1989, in a set of JPL Cassini CASTL documents beginning with R.F. Draper JPL interoffice memo to Mariner II Project Staff, MMII RFD-43-89, 22 Aug. 1989. 23. All of Cassini’s science instruments were body-fixed. While most were immovable relative to the spacecraft, three of them had detectors that could be articulated. Bob Mitchell review of manuscript, Feb. 2011. 24. Howard E. McCurdy, Faster, Better, Cheaper: Low-Cost Innovation in the U.S. Space Program (Baltimore: Johns Hopkins University Press, 2001). 25. President Ronald Reagan, State of the Union Address (25 January 1984). 26. Toby Owen telephone interview, 31 July 08, by author. 27. Ronald F. Draper, “Aerospace America – Comet Rendezvous Asteroid Flyby,” undated manuscript, in a group of documents, EFC PDF CAS908 DOC23925-2, beginning with R.F. Draper letter to Achim v . Heynitz, 17 July 1986, JPL Cassini CASTL. 28. Draper, “Aerospace America.” 29. Draper, “Aerospace America.”

98 Constructing the Cassini Orbiter 30. R. P. De Paula et al., “Fiber Optic Rotation Sensor for Space Missions,” NASA Technical Reports Server (NTRS), Document ID 19890022982, Accession ID 89A10353, publication date 1 Jan. 1988, updated/added to NTRS: 3 Nov. 2004. 31. Edward C. Litty email to Robert T. (Bob) Mitchell, JPL, 20 Oct. 2011; Robert T. (Bob) Mitchell email to author, 20 Oct. 2011; David M. Rozelle, “The Hemispherical Resonator Gyro: From Wineglass to the Planets,” http://www.es.northropgrumman.com/solutions/hrg/assets/hrg.pdf, Northrop Grumman 25126 (9 Sep. 2010); Mark R. Dahl email to author, 18 Mar. 2010; Ron Draper, “New Technologies for New Missions: Opportunities and Obstacles – Panel Discussion,” in a set of documents, JPL no. EFC PDF CAS908 DOC23932-1, beginning with: Ronald F. Draper, MMII Project Manager interoffice memo to Senior Staff, JPL MMII-RFD-17-89, 8 March 1989, https://cassini.jpl.nasa.gov/cel/cedr/nlf/inv/efc/pdf/cas908/doc23932-1.pdf, JPL Cassini CASTL. 32. G. H. Bryan, “On the Beats in the Vibrations of a Revolving Cylinder or Bell,” Proceedings of the Cambridge Philosophical Society VII (24 Nov. 1890):101. 33. David M. Rozelle, “The Hemispherical Resonator Gyro: From Wineglass to the Planets,” http://www. es.northropgrumman.com/solutions/hrg/assets/hrg.pdf, Northrop Grumman 25126 (9 Sep. 2010). 34. Linda J. Spilker (ed.), “Vehicle of Discovery,” chapter 8 in Passage to a Ringed World, NASA SP-533 (Wash. D.C.: NASA SP-33, Oct. 1997):92. 35. NASA-JPL, “Cassini’s Main Rocket Engines,” http://www2.jpl.nasa.gov/basics/cassini/ me.html, accessed 24 March 2010. 36. Marcia Neugebauer, “Mariner Mark II and the Exploration of the Solar System,” Science 219 (4 February 1983):443–449; EconomicExpert.com, “Mariner Mark II,” http://www.economicexpert.com/a/Mariner:Mark:II.html, accessed 23 July 2008; John Noble Wilford, “NASA Turns From Custom Design to Standard Models,” NY Times (7 March 1989); Yasuharu Inugi, “Characterization of the Comet Rendezvous Asteroid Flyby Mission,” http://www.tsgc.utexas. edu/archive/fulltext/craf2.pdf, Univ. of Texas Space Grant Consortium, Spacecraft Design Archive, 26 Feb. 1993, accessed 29 July 2008; Mark R. Dahl emails to author, 18 Mar. 2010 and 24 Mar. 2010. 37. Neugebauer, pp. 443–444. What the deep-space mission costs in this paper did not include were those of the launch vehicle and the operation of the deep-space data acquisition facilities. 38. D. Rapp JPL interoffice memo to T. Cole/R. Heacock/K. Watkins, “MM II Review,” 28 March 1984, in a group of attached memos, EFC PDF CAS908 DOC23944-1, beginning with R.F. Draper telecom to W. Piotrowski, 3 Jan. 1984, JPL CASTL archive; Mark R. Dahl email to author, 18 Mar. 2010. 39. NASA-JPL, “Startup, Design and Assembly,” http://cassini-huygens.jpl.nasa.gov/cassini/english/ops/devopsyellow.shtml, Cassini-Huygens Web site, accessed 24 Nov. 2008. 40. Julie Webster interview with author, JPL, 26 October 2010. 41. Julie L. Webster, “The Cassini Spacecraft Design and Operations,” Space Technology & Application International Forum, Albuquerque NM, 13–17 Feb. 2005 (Pasadena, CA: JPL, 2004); NASA, Cassini Launch, http://saturn.jpl.nasa.gov/news/press-kits/cassini.pdf, press kit, Oct. 1997, accessed 20 Aug. 2008. 42. “CRAF/Cassini Status Report,” ftp://ftp.seds.org/pub/spacecraft/CRAF/craf12.17.91.2, Univ. of Arizona FTP Directory, 17 December 1991, accessed 24 July 2008. 43. Mark R. Dahl email to author, 18 Mar. 2010. 44. Enrico Flamini interview, Rome, Italy, by author, 10 June 2008. 45. Flamini interview, June 2008; ESA, “Dramatic Shoreline Seen on Titan,” http://www.esa.int/ esaMI/Cassini-Huygens/SEMIGZ7X9DE_0.html, 21 Sept. 2005, accessed 28 Aug. 2008. 46. Gp. Fabiani and V. Costabile, “Cassini HGA/LGA1 Antenna: When Thermal Design is a Project Driver,” Proceedings of the Sixth European Symposium on Space Environmental Control Systems, Noordwijk, The Netherlands, May 1997 (ESA SP-400, Aug. 1997):101.

References 99 47. TPUB.com, “10.3.8 PCBZ White Paint,” http://www.tpub.com/content/nasa1995/NASA-95cr4661pt2/NASA-95-cr4661pt20119.htm, NASA 1995 Technical Documentation, accessed 28 Aug. 2008. 48. Dave Doody and Diane Fisher, “The High-Gain Antenna,” http://www2.jpl.nasa.gov/basics/ cassini/hga.html, in Basics of Space Flight (JPL D-20120, CL-03-0371, last modified 15 Feb. 2008). 49. Blackbody radiation refers to emissions from a surface, the intensity of which is determined by the temperature of the surface. 50. Charles Elachi, “RADAR Engineering Technical Write-up,” http://saturn.jpl.nasa.gov/ spacecraft/inst-cassini-radar-details.cfm, in SPACECRAFT - Cassini Orbiter Instruments RADAR, on the JPL Cassini-Huygens Mission to Saturn and Titan Web site, accessed 2 Sept. 2008. 51. Robert T. (Bob) Mitchell email to author on 8 Mar. 2010 and review of manuscript in Feb. 2011. 52. C. Kohlhase and C.E. Peterson, “The Cassini Mission to Saturn and Titan,” http://www.esa.int/ esapub/bulletin/bullet92/b92kohlh.htm, ESA Bulletin no. 92, Nov. 1997; NASA, “Cassini HGA/LGA1 Back-up Plan,” June 1994, in a package beginning with a Pete Ulrich handwritten memo date 30 June 1994, JPL Cassini CASTL. 53. Mitchell interview, Feb. 2008. 54. Webster. 55. Mark Dahl interview, Washington D.C., by author, September 2007. 56. Webster. 57. Webster. 58. Andrew S. Keys and Michael D. Watson, “Radiation Hardened Electronics for Extreme Environments,” NASA Marshall Space Flight Center, 2007; Robert Hendricks, “Radiation Hardening in Space,” http://www.mse.vt.edu/faculty/hendricks/mse4206/projects97/group02/ space.htm, Virginia Tech Materials Science and Engineering Web site, accessed 20 Aug. 2008; Mayer and Lacoe. 59. Andrew S. Keys and Michael D. Watson, “Radiation Hardened Electronics for Extreme Environments,” NASA Marshall Space Flight Center, 2007; Robert Hendricks, “Radiation Hardening in Space,” http://www.mse.vt.edu/faculty/hendricks/mse4206/projects97/group02/ space.htm, Virginia Tech Materials Science and Engineering Web site, accessed 20 Aug. 2008; Mayer and Lacoe. 60. Robert T. (Bob) Mitchell email to author, 9 Mar. 2010. 61. U.S. Department of Defense, Test Method Standard – Microcircuits, http://scipp.ucsc.edu/ groups/glast/electronics/mil-std-883.pdf, MIL-STD 883E (31 Dec. 1996). 62. Robert Hendricks, “Radiation Hardening in Space,” http://www.mse.vt.edu/faculty/hendricks/ mse4206/projects97/group02/space.htm, Virginia Tech Materials Science and Engineering Web site, accessed 20 Aug. 2008. 63. Alan R. Hoffman and John C. Forgrave, “Cassini Environmental Test and Analysis Program Summary,” Beacon eSpace – JPL Technical Report Server 1992+, file 99–0116, URI http://hdl. handle.net/2014/16721, preprint of paper presented at 18th Aerospace Testing Seminar (Manhattan Beach CA, 16–18 March 1999):18. 64. T.M. Langley, Cassini Radiation Control Plan, https://cassini.jpl.nasa.gov/cel/cedr/nlf/inv/efc/ pdf/jpl69901/doc69.pdf, JPL D-8813, PD 699–229 (Feb. 1993):1–3, JPL Cassini CASTL. 65. Julie Webster email to author, 30 Apr. 2010. 66. Julie Webster email to author, 30 Apr. 2010. 67. Kapton is made from polyimide—a hydrocarbon also containing oxygen and nitrogen. DuPont developed Kapton to remain stable over a very wide range of temperatures, from −273 to +400 °C. Kapton is thus a good material to use in spacecraft.

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68. Julie Webster email to author, 30 Apr. 2010; T.M. Langley, Cassini Radiation Control Plan, https://cassini.jpl.nasa.gov/cel/cedr/nlf/inv/efc/pdf/jpl69901/doc69.pdf, JPL D-8813, PD 699– 229 (Feb. 1993):2–4, JPL Cassini CASTL. 69. Webster email, 30 Apr. 2010. 70. T.M. Langley, Cassini Radiation Control Plan, https://cassini.jpl.nasa.gov/cel/cedr/nlf/inv/efc/ pdf/jpl69901/doc69.pdf, JPL D-8813, PD 699–229 (Feb. 1993), JPL Cassini CASTL; Michael S. Slocum, “TRIZ Meets Cassini,” http://www.realinnovation.com/commentary/archive/triz_ meets_cassini.html, realinnovation.com, 6 Sept. 2007, accessed 25 Aug. 08. 71. Kohlhase and Peterson. 72. S. Lisman, Guidance Analysis Book, https://cassini.jpl.nasa.gov/cel/cedr/nlf/inv/pdl/ spacecraft/699-409/html/document.html, 699–409, JPL D-7841, Sept. 1994, JPL Cassini CASTL; NASA-JPL, “Cassini Orbiter Engineering Subsystems,” http://saturn.jpl.nasa.gov/spacecraft/cass iniorbiterengineeringsubsystems/, accessed 9 Apr. 2010. 73. James W. Alexander and Daniel H. Chang, “Cassini Star Tracking and Identification Algorithms, Scene Simulation, and Testing,” in Cassini/Huygens: A Mission to the Saturnian Systems, Proceedings of SPIE 2803 (7 Oct.1996):311. 74. Julie L. Webster and Thomas A Burk email to author, 29 Nov. 2010. 75. Webster and Burk. 76. C. Kohlhase and C.E. Peterson, “The Cassini Mission to Saturn and Titan,” http://www.esa.int/ esapub/bulletin/bullet92/b92kohlh.htm, ESA Bulletin no. 92, Nov. 1997. 77. Valerie C. Thomas and Randel C. Blue, “Cassini Stellar Reference Unit: Performance Test Approach and Results,” in Cassini/Huygens: A Mission to the Saturnian Systems, Proceedings of SPIE 2803 (7 Oct.1996):288. 78. Bob Mitchell review of manuscript, Feb. 2011. 79. M. Omer, Q. Saeed, and M.R. Suddle, “A Simulation Framework for a Reaction Wheel Based AOCS,” Multi Topic Conference, 2003; INMIC 2003, 7th International (8–9 Dec. 2003):298–302. 80. Bob Mitchell review of manuscript, Feb. 2011. 81. The Physics Classroom, “Newton’s Laws,” http://www.physicsclassroom.com/class/newtlaws/, accessed 14 Jan. 2012. 82. Bob Mitchell review of manuscript, Feb. 2011. 83. Linda J. Spilker (ed.), Passage to a Ringed World, (NASA SP-533), p. 92. 84. Bob Mitchell review of manuscript, Feb. 2011. 85. Kohlhase and Peterson. 86. A newton is the unit of force necessary to increase the speed of one kilogram by one meter per second, every second. 87. Julie Webster, Cassini Spacecraft Operations Manager, 28 March 2006 NASA CHARM telecom, FTS-NASA-VOICE, moderated by Trina Ray, http://saturn.jpl.nasa.gov/multimedia/ products/docs/20060328_CHARM_transcript.doc, accessed 24 Sep. 08. 88. Disilicide is a refractory compound containing two atoms of silicon and another element, typically a metal. Refractory materials, such as silicon ceramics, are notably resistant to heat and have high melting points. 89. Kohlhase and Peterson. 90. C. Kohlhase and C.E. Peterson, “The Cassini Mission to Saturn and Titan,” http://www.esa.int/ esapub/bulletin/bullet92/b92kohlh.htm, ESA Bulletin no. 92, Nov. 1997 91. Donald L. Savage and James Gately, “Mars Observer Investigation Report Released,” NASA news release 94–1, 5 Jan. 1994; JPL press release on a JPL investigation of the loss of the Mars Observer spacecraft, 5 Jan. 1994; Malin Space Science Systems, “The Loss of the Mars Observer,” http://www.msss.com/mars/observer/project/mo_loss/moloss.html, and “HQ94-1/ Mars Observer Report Released,” http://www.msss.com/mars/observer/project/mo_loss/nasa_ mo_loss.txt, both accessed 29 Aug. 2008.

References 101 92. Marcia Neugebauer, “Mariner Mark II and the Exploration of the Solar System,” Science 219 (4 Feb. 1983):447–448; Jesse W. Moore, “Effective Planetary Exploration at Low Cost,” Astronautics & Aeronautics (Oct. 1982):37; Mark Dahl interview; Webster; Bob Mitchell review of manuscript, Feb. 2011; Philip M. Parker, http://www.websters-dictionary-online. com/definition/BIPROPELLANT+ROCKET, Webster’s Online Dictionary, 2008, accessed 14 Aug. 2008; Kohlhase and Peterson. 93. The Mark II power module was capable, however, of accommodating solar panels for missions closer in to the Sun. 94. Robert T. Mitchell emails to author, 25 Aug. 2011 and 29 Aug. 2011; Neugebauer; Moore. 95. Alan R. Hoffman and John C. Forgrave, “Cassini Environmental Test and Analysis Program Summary,” Beacon eSpace – JPL Technical Report Server 1992+, file 99–0116, URI http://hdl. handle.net/2014/16721, preprint of paper presented at 18th Aerospace Testing Seminar (Manhattan Beach CA, 16–18 March 1999):16. 96. Hoffman and Forgrave, “Cassini Environmental Test and Analysis Program Summary,” p. 16. 97. Hoffman and Forgrave, “Cassini Environmental Test and Analysis Program Summary,” p. 18; Bob Mitchell review of manuscript, Feb. 2011. 98. Hoffman and Forgrave, “Cassini Environmental Test and Analysis Program Summary,” p. 19. 99. Hoffman and Forgrave, “Cassini Environmental Test and Analysis Program Summary,” p. 20. 100. Bob Mitchell interview, by author, JPL, 5 Feb. 2008. 101. Mark Dahl interview, by author, NASA Headquarters, Sept. 2007. 102. Julie L. Webster, “Cassini Spacecraft Engineering Tutorial,” http://saturn.jpl.nasa.gov/multimedia/products/pdfs/20060328_CHARM_Webster.pdf, JPL, 28 March 2006, accessed 4 Sept. 2008. 103. JPL, “Cassini’s Earthly Benefits,” http://saturn.jpl.nasa.gov/multimedia/products/pdfs/earthly. pdf, May 1995, accessed 4 Sept. 2008. 104. Jefferson Morris, “NASA Considering Extending Cassini Mission Through 2010,” http://www. aviationweek.com/aw/generic/story_generic.jsp?channel=aerospacedaily&id=news/ CASS12275.xml&headline=NASA%20Considering%20Extending%20Cassini%20 Mission%20Through%202010, AviationWeek.com, 27 Dec. 2005, accessed 4 Sept. 2008; Alicia Chang, “Cassini in Safe Mode After Saturn Flight,” http://www.bookrags.com/news/ cassini-in-safe-mode-after-saturn-moc/, bookrags.com, AP News, 13 Sept. 2007, accessed 4 Sept. 2008; Mitchell interview, Feb. 2008; Michael Meltzer, Mission to Jupiter: A History of the Galileo Project (Washington D.C.: NASA SP-2007-4231, 2007), p. 248; Bob Mitchell review of manuscript, Feb. 2011. 105. Mitchell interview, Feb. 2008. 106. Dahl interview, Sept. 2007. 107. T. Gavin, V. Thomas, S. Spitz, L. Gresham, C. Kahn, and K. Clark, “The Hemispherical Resonator Gyro for Cassini,” http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/21810/1/ 97-0210.pdf, JPL TRS 1992+, file 97–0210.pdf, URI http://hdl.handle.net/2014/21810, 19 Feb. 1997, accessed 4 Sept. 2008; Edward C. Litty, Lennor L. Gresham, Patrick A. Toole, and Debra A. Beisecker, “Hemispherical Resonator Gyro: an IRU for Cassini,” Proc. SPIE 2803 (Oct. 1996):299–310; Emily L. Burrough and Allan Y. Lee, “In-flight Characterization of Cassini Inertial Reference Units,” http://pdf.aiaa.org/preview/CDReadyMGNC07_1500/ PV2007_6340.pdf, AIAA 2007–6340, AIAA Guidance, Navigation and Control Conference and Exhibit, August 2007, Hilton Head, South Carolina. 108. Dahl interview. 109. Dave Doody and Diane Fisher, “Typical Onboard Systems,” http://www2.jpl.nasa.gov/basics/ bsf11-2.html, chapter 11 in Basics of Space Flight (JPL, 2008); Don Barteld, “Cassini-Huygens Relies on Northrop Grumman Navigation Systems,” http://www.irconnect.com/noc/press/ pages/news_releases.html?d=65846, Northrop Grumman - News Releases (20 Oct. 2004).

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110. Both quotes are from NASA, Announcement of Opportunity for the Cassini Mission: Saturn Orbiter, https://cassini.jpl.nasa.gov/cel/cedr/nlf/inv/document/pdl_document/project/NASA_ AO_OSSA-1-89/AO_Saturn_Orbiter/pdf/document.pdf, A0 No. OSSA-1-89, October 10, 1989, accessed 18 July 2008. 111. Announcement of Opportunity for the Cassini Mission. 112. Ibid. 113. ESA/NASA, Cassini: Report on the Phase A Study, doc. no. SCI(88)5 (Oct. 88):i, JPL/Cassini CASTL. 114. SSB, Strategy for Exploration of the Outer Planets: 1986–1996, Interim Report, National Academy of Sciences - National Research Council, Space Science Board (Washington, D.C.: Dec. 1986). 115. Announcement of Opportunity for the Cassini Mission. 116. M. Coradini, “Cassini Investigations Evaluation and Selection Procedure,” ESA briefing, (9 Nov. 1989), p. 31. 117. Paula Cleggett-Haleim, “NASA Selects Investigations for Cassini Saturn Orbiter Mission,” NASA News, release 90–150, 13 Nov. 1990. 118. JPL, “Spacecraft: Cassini Orbiter Instruments,” http://saturn.jpl.nasa.gov/spacecraft/instrumentscassini-intro.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 5 Sept. 2008. 119. C. Kohlhase and C.E. Peterson, “The Cassini Mission to Saturn and Titan,” http://www.esa.int/ esapub/bulletin/bullet92/b92kohlh.htm, ESA Bulletin no. 92, Nov. 1997; Announcement of Opportunity. 120. Julie L. Webster, “The Cassini Spacecraft Design and Operations,” Space Technology & Application International Forum, Albuquerque NM, 13–17 Feb. 2005 (Pasadena, CA: JPL, 2004). 121. Jia-Rui C. Cook, “1980s Video Icon Glows on Saturn Moon,” http://saturn.jpl.nasa.gov/news/ newsreleases/newsrelease20100329/, NASA/JPL press release 2010–103 (29 Mar. 2010). 122. All quotes in this section are from JPL, “Spacecraft: Cassini Orbiter Instruments – CIRS,” http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-cirs.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 6 Sept. 2008. 123. Goddard Space Flight Center, “Cassini Composite Infrared Spectrometer (CIRS) Instrument,” http://cirs.gsfc.nasa.gov/, accessed 8 Sept. 2008. 124. JPL, “Spacecraft: Cassini Orbiter Instruments - ISS,” http://saturn.jpl.nasa.gov/spacecraft/ instruments-cassini-iss.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 6 Sept. 2008. 125. Carolyn C. Porco, “ISS General Description,” http://saturn.jpl.nasa.gov/spacecraft/instcassini-iss-details.cfm, JPL Cassini-Huygens Mission to Saturn & Titan Web site, accessed 8 Sept. 2008. 126. Carolyn C. Porco, “ISS General Description,” http://saturn.jpl.nasa.gov/spacecraft/inst-cassiniiss-details.cfm, JPL Cassini-Huygens Mission to Saturn & Titan Web site, accessed 8 Sept. 2008; JPL, “ISS.” 127. Porco, “ISS General Description.” 128. JPL, “ISS.” 129. JPL, “ISS.” 130. JPL, “Spacecraft: Cassini Orbiter Instruments - UVIS,” http://saturn.jpl.nasa.gov/spacecraft/ instruments-cassini-uvis.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 9 Sept. 2008. 131. JPL, “UVIS,” http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/instrumentscassiniuvis/, Cassini Solstice Mission Web site, accessed 19 June 2011. 132. A nanometer is a unit of length equal to one billionth of a meter (10−9 meters). Nanometers are commonly used as units of wavelength for light.

References 103 133. J.E. Colwell, L.W. Esposito, J.J. Lissauer, R.G. Jerousek, M. Sremčević, “Three-dimensional Structure of Saturn’s Rings from Cassini UVIS Stellar Occultations,” EPSC Abstracts 3, EPSC2008-A-00 135 (European Planetary Science Congress: 2008); G.R. Smith and D.M. Hunten, “Study of Planetary Atmospheres by Absorptive Occultations,” Rev. of Geophys. 28 (1990):117. 134. JPL, “Spacecraft: Cassini Orbiter Instruments - UVIS.” 135. Larry W. Esposito, Charles A. Barth, Joshua E. Colwell, George M. Lawrence, William E. McClintock, Ian F. Stewart, H. Uwe Keller, Axel Korth, Hans Lauche, Michel C. Festou, Arthur L. Lane, Candice J. Hansen, Justin N. Maki, Robert A. West, Herbert Jahn, Ralf Reulke, Kerstin Warlich, Donald E. Shemansky, and Yuk L. Yung, “The Cassini Ultaviolet Imaging Spectrograph Investigation,” Space Science Reviews 115 (Jan. 2004). 136. JPL, “Spacecraft: Cassini Orbiter Instruments - UVIS.” 137. Esposito et al. 138. IFSI-Roma, “VIMS Channels,” http://www.ifsi-roma.inaf.it/vims/index.php?categoryid=14, Cassini VIMS Instrument Web site, Institute of Physics of Interplanetary Space (IFSI), Rome, accessed 10 Sept. 2008; JPL, “Spacecraft: Cassini Orbiter Instruments - VIMS,” http://saturn. jpl.nasa.gov/spacecraft/instruments-cassini-uvis.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 9 Sept. 2008; “Visual and Infrared Mapping Spectrometer,” http:// wwwvims.lpl.arizona.edu/Howitworks.html, Cassini VIMS Web site, University of Arizona, accessed 21 June 2011. 139. Emily Lakdawalla, “Cassini VIMS Sees the Long-Awaited Glint Off a Titan Lake,” http:// www.planetary.org/blog/article/00002267, Planetary Society Blog (17 Dec. 2009). 140. NASA/JPL/University of Arizona, “Saturn’s Active North Pole,” http://www.ciclops.org/view. php?id=4967, NASA image PIA 09188, released 27 March 2007. 141. Lunar and Planetary Laboratory, “Cassini VIMS Science Investigations,” http://wwwvims.lpl. arizona.edu/, Univ. of Arizona, accessed 10 Sept. 2008. 142. NASA-JPL, “Inside the Spacecraft,” http://saturn.jpl.nasa.gov/spacecraft/overview/, Cassini Equinox Mission Web site, accessed 9 Aug. 2010. 143. Todd J. Barber, “Insider’s Cassini – Dr. David Young and the Cassini Plasma Spectrometer,” http://saturn.jpl.nasa.gov/news/cassiniinsider/insider20101030/, JPL Cassini Insider (30 Oct. 2010). 144. Southwest Research Institute, “Cassini Plasma Spectrometer,” http://caps.space.swri.edu/ caps/index.shtml, accessed 10 Sept. 2008. 145. David T. Young, “CAPS Engineering Technical Write-up,” http://saturn.jpl.nasa.gov/spacecraft/ inst-cassini-caps-details.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 10 Sept. 2008; NASA, Announcement of Opportunity for the Cassini Mission. 146. Southwest Research Institute, “Cassini Plasma Science (CAPS) Investigation Operations Team List,” http://caps.space.swri.edu/caps/teamMembers/teamMembers.shtml#SWRI, accessed 10 Sept. 2008. 147. Barber, “Insider’s Cassini – Dr. David Young and the Cassini Plasma Spectrometer.” 148. JPL, “Spacecraft: Cassini Orbiter Instruments - CDA,” http://saturn.jpl.nasa.gov/spacecraft/ instruments-cassini-cda.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 10 Sept. 2008. 149. R. Srama et al., “The Cassini Cosmic Dust Analyzer,” http://www.mpi-hd.mpg.de/cassini/paper/ ssr_2002_special/cda.SpaceSciRev_4.1-33.PDF, Space Science Reviews 114 (September 2004). 150. Heidelberg Dust Research Group, “Dusty Messengers,” http://www.mpi-hd.mpg.de/dustgroup/, Max Planck Institute Web site, last modified 10 March 2008, accessed 10 Sept. 2008. 151. Srama et al., 2004. 152. Srama et al., 2004.

104 Constructing the Cassini Orbiter 153. 154. 155. 156. 157.

158.

159.

160. 161.

162.

163. 164.

165. 166. 167. 168.

169. 170. 171.

172.

Srama et al., 2004. Bob Mitchell email to author, 10 Aug. 2010. Ibid. Hunter Waite interview with author, JPL, 28 October 2010. J. Hunter Waite, “INMS Engineering Technical Write-up,” http://saturn.jpl.nasa.gov/ spacecraft/inst-cassini-inms-details.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, last updated 6 April 2005, accessed 18 Sept. 2008. J.H. Waite et al., “The Cassini Ion and Neutral Mass Spectrometer (INMS) Investigation,” http://cat.inist.fr/?aModele=afficheN&cpsidt=16549250, Space Science Reviews 114 (Sept. 2004):113–231; Meltzer, Mission to Jupiter, p. 122. W. Kasprzak, H. Niemann, D. Harpold, J. Richards, H. Manning, E. Patrick, and P. Mahaffy, “Cassini Orbiter Ion and Neutral Mass Spectrometer Instrument,” http://sprg.ssl.berkeley.edu/ inms/INMS.html, Cassini INMS: Investigations at UC Berkeley Web site, accessed 18 Sept. 2008; Goddard, “Mass Spectrometer: Detector,” http://huygensgcms.gsfc.nasa.gov/MS_Detector_1. htm, accessed 18 Sept. 2008. Waite et al., “The Cassini Ion and Neutral Mass Spectrometer”; Meltzer, Mission to Jupiter, p. 121. Carolina Martinez and Dwayne Brown, “Cassini Tastes Organic Material at Saturn’s Geyser Moon,” http://www.jpl.nasa.gov/news/news.cfm?release=2008-050, JPL press release 2008– 050 (26 Mar. 2008). William B. Hubbard et al., “The Interior of Saturn,” chapter 4 in Michele K. Dougherty et al. (eds.), Saturn from Cassini-Huygens (Springer, 2009):80; JPL, “Spacecraft: Cassini Orbiter Instruments - MAG,” http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-mag.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 12 Sept. 2008; Michael Meltzer, Mission to Jupiter: A History of the Galileo Project, NASA SP-2007-4231 (Washington D.C.: NASA, 2007); Linda J. Spilker email to author, 6 March 2010. Bob Mitchell review of manuscript, Feb. 2011. Geological Survey of Canada, “Geomagnetism: Earth’s Magnetic Field,” http://gsc.nrcan. gc.ca/geomag/field/index_e.php, last modified 16 Jan. 2008, accessed 12 Sept. 2008; Cambridge University Press, Cambridge University Press, “Book Resources,” http://www.cambridge.org/ resources/0521546206/678_s283b1f6_04.pdf, accessed 12 Sept. 08. JPL, “Spacecraft: Cassini Orbiter Instruments - MAG.” JPL, “Spacecraft: Cassini Orbiter Instruments - MAG.” Michelle Dougherty, interview with author, 25 June 2009, London; NASA, “Solar System Exploration: People,” http://sse.jpl.nasa.gov/people/profile.cfm?Code=DoughertyM. David J. Southwood, “MAG Engineering Technical Write-up,” http://saturn.jpl.nasa.gov/ spacecraft/inst-cassini-mag-details.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, last updated: 04.06.2005, accessed 15 Sept. 2008; JPL, “Spacecraft: Cassini Orbiter Instruments - MAG.” Imperial College London, “The MAG Instrument,” http://www3.imperial.ac.uk/spat/research/ missions/space_missions/cassini/mag_instrument, 2008, accessed 15 Sept. 2008. Michele Dougherty email to author, 1 Sep. 2010. Imperial College London, “The MAG Instrument,” http://www3.imperial.ac.uk/spat/research/ missions/space_missions/cassini/mag_instrument, 2008, accessed 15 Sept. 2008; Southwood, “MAG Engineering Technical Write-up”; JPL, “Spacecraft: Cassini Orbiter Instruments - MAG.” P. Narvaez, “The Magnetostatic Cleanliness Program for the Cassini Spacecraft,” Space Science Reviews 114 (2004):385–394; K. Mehlem and P. Narvaez, “Magnetostatic Cleanliness of the Radioisotope Thermoelectric Generators (RTGs) of Cassini,” http://www.emcs.org/ pdf/99symp/00353.pdf, Electromagnetic Compatibility, 1999 IEEE International Symposium, accessed 16 Sept. 2008.

References 105 173. Elwin Ong and Nancy Leveson, “Fault Protection in a Component-Based Spacecraft Architecture,” http://sunnyday.mit.edu/papers/smcit.doc, Proceedings of the International Conference on Space Mission Challenges for Information Technology (Pasadena: July 2003). 174. Narvaez. 175. Bob Mitchell interview, 5 February 2008, JPL, by author. 176. S.M. Krimigis et al., “Magnetospheric Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan,” http://sd-www.jhuapl.edu/CASSINI/documents/SSR_MIMI_Paper.pdf, Space Science Reviews 114 (Sept. 2004):233–329; Johns Hopkins University Applied Physics Laboratory, “INCA,” http://sd-www.jhuapl.edu/CASSINI/incabase.html, accessed 18 Sept. 2008. 177. JPL, “Spacecraft: Cassini Orbiter Instruments - MIMI,” http://saturn.jpl.nasa.gov/spacecraft/ instruments-cassini-mimi.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 17 Sept. 2008. 178. Stamatios M. Krimigis, “MIMI Engineering Technical Write-up,” http://saturn.jpl.nasa.gov/ spacecraft/inst-cassini-mimi-details.cfm#INCA, JPL Cassini-Huygens Mission to Saturn & Titan Web site, last updated 6 April 2005, accessed 17 Sept. 2008; Krimigis et al., “Magnetospheric Imaging Instrument”; JPL, “Spacecraft: Cassini Orbiter Instruments - MIMI.” 179. Johns Hopkins University Applied Physics Laboratory,” Cassini MIMI Magnetospheric Imaging Instrument,” http://sdwww.jhuapl.edu/CASSINI/mimioverview2.html, accessed 18 Sept. 2008. 180. A plasma is a gas of free electrons and positively charged ions, the latter being atoms or molecules that have lost one or more electrons. 181. ESA, “RPWS: Radio and Plasma Wave Science,” http://sci.esa.int/science-e/www/object/ index.cfm?fobjectid=34954&fbodylongid=1622, last update 25 Feb. 2005, accessed 21 Sept. 2008; Donald A. Gurnett, “RPWS Engineering Technical Write-up,” http://saturn.jpl.nasa.gov/ spacecraft/inst-cassini-rpws-details.cfm, last updated 6 Apr. 2005, accessed 21 Sept. 2008. 182. This instrument was named after the Nobel Prize-winning physicist Irving Langmuir. 183. D.A. Gurnett et al., “The Cassini Radio and Plasma Wave Investigation,” Space Science Reviews 114 (2004):395–463. 184. Gurnett, “The Cassini Radio and Plasma Wave Investigation.” 185. Lori Erikson, “Space Pioneer: Don Gurnett’s Enduring Journey,” The Iowan (September/ October 2010). 186. Ibid. 187. Ibid. 188. Univ. of Iowa, “Cassini RPWS: Radio and Plasma Wave Science,” http://www-pw.physics. uiowa.edu/plasma-wave/cassini/home.html, last modified 20 Sept. 2007, accessed 21 Sept 2008. 189. NASA-JPL, “Cassini Orbiter Instruments,” http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/, Cassini Equinox Mission Web site, accessed 9 Aug. 2010. 190. Charles Elachi, “RADAR Engineering Technical Write-up,” http://saturn.jpl.nasa.gov/spacecraft/inst-cassini-radar-details.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 21 Sept. 2008. 191. JPL, “Spacecraft: Cassini Orbiter Instruments - RADAR,” http://saturn.jpl.nasa.gov/spacecraft/ instruments-cassini-radar.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 21 Sept. 2008. 192. Elachi, “RADAR Engineering Technical Write-up.” 193. Elachi, “RADAR Engineering Technical Write-up.” 194. Peter J. Westwick, Into the Black (New Haven & London: Yale University Press, 2007), p. 109. 195. Jian Bing, “Synthetic Aperture Radar Satellite,” http://www.sinodefence.com/strategic/spacecraft/ jianbing5.asp, last updated 12 Nov. 2007, accessed 22 Sep. 2008.

106 Constructing the Cassini Orbiter 196. Ralph Lorenz and Jacqueline Mitton, Titan Unveiled (Princeton and Oxford: Princeton University Press, 2008), p. 115. 197. Elachi, “RADAR Engineering Technical Write-up.” 198. Brian W. Stiles et al., “Obtaining Titan Surface Heights Using Cassini Synthetic Aperture RADAR Echo Data,” American Astronomical Society, DPS meeting #38, #56.07, Bulletin of the American Astronomical Society 38 (Sep. 2006):586. 199. Elachi, “RADAR Engineering Technical Write-up.” 200. Lorenz and Mitton, Titan Unveiled, p. 115. 201. Lorenz and Mitton, Titan Unveiled, p. 114. 202. Elachi, “RADAR Engineering Technical Write-up.” 203. F. Nirchio, B. Pernice, L. Borgarelli, and C. Dionisio, “The Italian Involvement in Cassini Radar,” International Symposium on Radars and Lidars in Earth and Planetary Sciences (ESA: Dec. 1991):79–81. 204. Nirchio et al. 205. “Resolution” refers to the ability of the RADAR to produce separate images of closely located objects. The low-resolution function of the RADAR, for instance, can generate images of objects that are as close as 0.6 kilometers. 206. JPL, “Spacecraft: Cassini Orbiter Instruments - RSS,” http://saturn.jpl.nasa.gov/spacecraft/ instruments-cassini-rss.cfm, Cassini-Huygens Mission to Saturn & Titan Web site, accessed 22 Sept. 2008. 207. An occultation occurs when one astronomical body (in this case the spacecraft) passes behind another astronomical body (such as Saturn), which obscures the first body. 208. A.J. Kliore et al., “Cassini Radio Science,” Space Science Reviews 115 (2004):1–70. 209. “MAPSview - Cassini/Huygens Spacecraft Overview,” http://mapsview.engin.umich.edu/ data_descriptions/spacecraft_overview.php, University of Michigan, accessed 22 Nov. 2010, last modified 13 Sep. 2006. 210. Peter J. Westwick, Into the Black (New Haven & London: Yale University Press, 2007), p. 162. 211. Westwick, Into the Black, p. 162. 212. NASA Center Science Assessment Team, “Science at NASA Field Centers,” May 1988, as reported in Westwick, Into the Black, p. 163. 213. Westwick, Into the Black, p. 163. 214. Westwick, Into the Black, p. 163–164, 354. 215. Robert T. (Bob) Mitchell email to author, 8 Mar. 2010. 216. Mark R. Dahl email to author, 24 Mar. 2010. 217. The original successful proposal and instrument development and delivery was led by David Southwood. Michele Dougherty took over the team when Southwood left to take a key science management position with ESA. Bob Mitchell review of manuscript, Feb. 2011. 218. NASA-GSFC, “Cassini/CIRS Team Membership,” http://cirs.gsfc.nasa.gov/team.html, accessed 26 Mar. 2010. 219. Westwick, Into the Black, p. 273. 220. NASA-JPL, “ISS,” http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/instruments cassiniiss/, accessed 26 Mar. 2010. 221. Robert T. (Bob) Mitchell email to author, 9 Mar. 2010. 222. Robert T. (Bob) Mitchell email to author, 8 Mar. 2010. 223. Robert T. (Bob) Mitchell email to author, 8 Mar. 2010.

4 The Titan Huygens Probe

“Something hidden. Go and find it. … Lost and waiting for you. Go!” – from The Explorer by Rudyard Kipling, 1898

The Huygens Probe, the European Space Agency’s (ESA) element of the Cassini-Huygens mission, was meant to discover hidden things. It was to examine Saturn’s largest moon, Titan, analyzing its thick, opaque atmosphere and the surface hidden beneath it. Developing the Huygens mission involved years of complex relationships and cooperation between NASA, which launched the spacecraft and had responsibility for the entire Cassini-Huygens mission, and ESA, which oversaw the design and fabrication of the Probe. As shown in Table 4.1, the consortium that manufactured the systems and structure involved organizations across Europe as well as several in the U.S.1

4.1

PHASES OF THE HUYGENS PROBE MISSION

The Huygens Probe mission consisted of various phases, all of which had different objectives: • • • • • • •

Design and development Testing, integration with the Orbiter, and release to NASA Cruise through interplanetary space Arrival at Saturn and separation from the Orbiter Coast to Titan Atmospheric entry, deceleration, and descent Surface operations.

This chapter examines the first phase, the design and development of the Probe. The next chapter discusses its testing, integration with the Orbiter, and delivery to NASA for the launch. The phases pertaining to the operational part of the mission are discussed in later chapters. © Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_4

107

108 The Titan Huygens Probe Table 4.1. Industrial consortium for the Huygens Probe’s development phase.2 Country

Organization

Instrument or Function

Austria

ORS

Belgium Denmark

Schrack Etca Terma

MGSE Thermal blanket Swivel PDRS EGSE Power Subsystem DC/DC converters Mission timer unit Aerothermodynamics Thermal protection Pyro devices Umbilical connectors Radar Altmeter System Integration and Test Thermal control Independent S/W validation Data handling subsystem Communication subsystem Probe Data Relay Subsystem (PDRS) Balloon drop test model System EGSE Reliability Analysis Cost Analysis Documentation Internal Structure Harness Parts procurement Mass dummies Project control Probe Transmit Antennae Receiver Front End Balloon drop test Front shield structure Back cover structure Mechanisms P/L Interface Simulator Flight Software Descent Subsystem Parts Procurement Parachutes Accelorometers Batteries, Inertia & Other Switches

France

Finland Germany

Aerospatiale (Prime Contractor) Dassault/Pyrospace Framatom Ylinen Dasa

Ireland Italy

Captec Laben Alenia Spazio

Netherlands Norway

Fokker NFT DNV

Spain

Casa

Sweden

Tecnologica Crisa Eyser Saab/Ericsson

Switzerland

Esrange Contraves/Vevey

United Kingdom

United States

CIR Logica MBA IGG Irvin Systron Donner Allian

4.2 Huygens Probe development 109 4.2

HUYGENS PROBE DEVELOPMENT

“The design, the testing, the building of the [Probe] by the Europeans was … a very remarkable accomplishment. To have that thing work after seven years of flight in a pretty much unknown environment and do everything it was supposed to do … very impressive. They deserve a lot of credit for that.” – Bob Mitchell, Cassini-Huygens Program Manager3 In November 1986, ESA’s Science Programme Committee (SPC) approved the Phase A study for a Titan probe. An industrial consortium led by Marconi Space Systems conducted the study from November 1987 to September 1988 and issued scientific objectives and a probe technical design (the Phase A study is described more fully in Chapter 1). The Phase A study products, as well as ESA’s design guidelines,4 steered the Huygens Probe’s development. For instance, the suite of analytical instruments (Table 4.2) envisioned by the Phase A study were a template for the instruments that were selected.5 As will be shown, the science instruments that flew aboard the Probe closely resembled the Phase A vision, although not all of them could be included. Two industrial consortia submitted bids for the Huygens Probe development, one headed by British Aerospace and the other by Aerospatiale, a French aerospace corporation which manufactured both civilian and military aircraft and rockets. ESA selected the Aerospatiale consortium as the industrial prime contractor in November 1990. Aerospatiale had built up an impressive track record for prime contracting on ESA’s science programs and had a broad range of in-house engineering, scientific, and management capabilities. Aerospatiale’s victory was also aided by the diversity of its project partners, including CASA of Spain, Deutsche Aerospace, Logica from the U.K., Alenia Spazio of Italy, and players from many other countries (Table 4.1). Table 4.2. Model payload envisioned by the Phase A study.

Envisioned Instrument Science objectives Atmospheric Structure Instrument (ASI) Atmosphere temperature and pressure profile, winds and turbulence Probe Infra-Red Laser Spectrometer Vertical profile of trace species, nephelometry (PIRLS) (light-scattering analyses) Gas Chromatograph/Neutral Mass Atmosphere composition profile, aerosol analysis Spectrometer (GCMS) Aerosol Collector and Pyrolyzer (ACP) Aerosol composition profile with GCIMS used as detector Descent Imager/Spectral Radiometer Atmospheric composition and cloud structure, (DI/SR) surface imaging Lightning and Radio-Emission Detector Titan lightning characteristics (LRD) Surface Science Package (SSP) Titan surface state and composition Doppler Wind Experiment (DWE) Probe Doppler tracking from the orbiter for zonal wind-profile measurement Radar Altimeter Science (RAS) Surface roughness and reflectivity, subsurface sounding Total mass: 39.9 kilograms

110

The Titan Huygens Probe Table 4.3. Subcontractors responsible for Huygens’ operational components.11

Operational component Descent module Front heat shield Separation system Aft cover Data management Descent control RF data relay Power supply Probe harness Flight software Thermal control

Company responsible EADS12-CASA EADS Space Transport. Contraves Space Contraves Space Laben Martin Baker Alenia Spazio Alcatel ETCA EADS-CASA Logica CMG EADS-Astrium

Host country Spain France, Germany Switzerland Switzerland Italy U.K. Italy Belgium Spain U.K. France, Germany, U.K., Spain, Netherlands

The overall Probe system consisted of the 318 kilogram (700 pound) Huygens Probe and 30 kilograms (66 pounds) of support equipment that remained attached to the Orbiter after the Probe was sent toward Titan. Manufacturing operations began in January 1991. During construction of the Huygens Probe, Aerospatiale served as the focal point for contractual, technical, cost, and schedule interfaces with ESA, and from its Cannes facility, directed its contractors. The subcontractors responsible for particular Huygens operational components are listed in Table 4.3, which along with Table 4.1 demonstrates the wide range of European companies (as well as two from the U.S.) that supplied parts for the Probe.6 4.2.1

ESA’s juste retour policy for dispersing development contracts

The diversity of the Huygens Probe development team was not just advisable, it was necessary to meet ESA’s longstanding requirement for juste retour (fair return). This is the principle that the overall value of contracts awarded to corporations in a given country under a particular program must be in proportion to the funding that country has contributed to the program.7 This policy can be quite restrictive. A program’s managers are not free to select the perceived best bids, nor the cheapest, for needed components and services without making certain that each country gets its fair share of the total pot. Ensuring juste retour is a tremendously complex task, and one that the U.S. does not have to carry out. Juste retour was originally introduced to encourage the growth of pan-European industrial capability for space missions, and has accomplished this goal to a certain extent.8 Spain provides an example of the potential benefits. Its industries were at first incapable of handling the work it was entitled to for its level of contribution to ESA programs, and French companies initially absorbed the bulk of this work. But the balance shifted back to Spain as it grew its space industry. Although juste retour has been a sacred cow of ESA policy, it does not have universal support among the Agency’s member states. The U.K., for instance, has long opposed it, even though the U.K. has had one of the highest financial returns under juste retour of all ESA member states. The U.K. has argued that a more competitive system would enhance value for money. Therefore, contracts ought to be awarded to the organizations best able to do the work, regardless of which country they represent, “rather than trying endlessly to balance exactly the amount coming back in

4.2 Huygens Probe development 111 work versus how much you put in.”9 Some ESA member nations fear, however, that a move toward full, open competition would lead to certain smaller nations, which can perhaps do the work at lower cost, taking “the ball away”10 from other member states. 4.2.2

Jean-Pierre Lebreton

Jean-Pierre Lebreton, the Huygens project scientist as well as its mission manager, has been almost a “lifer” on the mission since he joined ESA in 1978 after earning his doctorate in math and physics at the Université d’Orléans in France. Much of his career was spent developing a space probe which would explore one heavenly body: Titan, Saturn’s planet-sized moon. One of his biggest challenges was helping to keep ESA and NASA working smoothly together, even during budget scarcities when NASA, in Lebreton’s words, “threatened to stop the project several times.”13 Many of the scientists and engineers that Lebreton managed were world class in their areas of expertise. He needed to manage them in ways that would not restrict their creativity nor block the vital contributions they could make. Lebreton understood them, for he was himself a high powered planetary scientist with a focus on plasma physics. He implemented what he referred to as a Parliament of Scientists that included a senior group called the Senate, made up of Huygens Probe principal investigators. Lebreton and this parliament respected the abilities of technical staff on all levels to make their own decisions about a wide range of issues, and employed an operating principle called subsidiarity that allowed them to do so. Subsidiarity had been used in the European Union for many years to regulate the exercise of powers, and is in fact defined in Article 5 of the Treaty on European Union. Simply put, subsidiarity as applied to the Huygens mission enabled management decisions to be made at the lowest possible level for which full information was available. Upper management did not take action unless doing so would be more effective than action taken at the local level. This helped ensure that decisions were made on the basis of scientific needs, rather than imposed because of politics or a manager’s position.14 4.2.3

Features of the Huygens Probe

The Probe itself was 2.7 meters (nearly 9 feet) in diameter and shaped sort of like a Frisbee with a pod of instruments attached, as shown in Figure 4.1. Its construction resembled that of a shellfish, with a hard external layer, the entry assembly, cocooning and protecting the descent module and delicate payload instruments within from the temperatures which would be experienced during Titan atmospheric entry and deceleration (Figures 4.2 and 4.3).

4.2.3.1

Separation system

A special system was designed to separate Huygens Probe from Cassini Orbiter, while giving the Probe a predetermined amount of velocity and spin. The Probe separation system provided both mechanical and electrical attachment to the Orbiter. Three separation mechanisms were connected on one end to the Probe and on the other end to the Orbiter.

112 The Titan Huygens Probe

Figure 4.1 Artist’s conception of the Huygens Probe approaching Titan.

4.2 Huygens Probe development 113

Figure 4.2 The Probe’s hard external layer protected its Descent Module and delicate payload during Titan atmospheric entry and deceleration.

Figure 4.3 The “shellfish” construction of the Probe, with the hard external layer – the entry assembly – cocooning and protecting the Descent Module and the delicate payload instruments within.

114

The Titan Huygens Probe

Each mechanism incorporated a pyrotechnic explosive bolt detonated at the time the Probe was released from the Orbiter. To facilitate this release, each mechanism also contained a stainless steel spring to push Probe and Orbiter apart with a force of 500 newtons (112 pounds). Guide devices, each with axial rollers running along a helical track, ensured that (1) the Probe spun up to approximately 7 revolutions per minute to provide stability; and (2) it left the Orbiter with a relative velocity of 0.3 meters per second (1 foot per second or 0.6 miles per hour).15 In addition, this operation disconnected the three 19-pin connectors that had provided Orbiter-Probe electrical links.16

4.2.3.2

Entry assembly

The entry assembly, consisting of a cone-shaped front heat shield and an aft cover (also called the back cover, as in Figure 4.2), was aerodynamically configured to provide a controlled deceleration once the Probe entered Titan’s atmosphere at 6.1 kilometers per second (13,700 miles per hour), “speeding like a meteorite.”17 In fact, nearly one-third of the Probe’s 320 kilogram (705 pound) mass went into the entry assembly and its thermal defense capabilities.18

4.2.3.2.1

Front heat shield

The rotation imparted to the Huygens Probe on its release from the Orbiter stabilized it and made sure that it entered Titan’s atmosphere with the front heat shield facing the direction of travel. In addition to acting as a powerful brake, this heat shield was also subjected to the lion’s share of thermal and mechanical stresses of atmospheric entry. The shock wave that formed in advance of the front heat shield reached an amazing 12,000°C (22,000°F),19 hotter than the surface of the Sun, but the sensitive components, including the scientific experiments, stayed comfortably sheltered from this inferno. They were protected from thermal damage by tiles of ablative material, various insulation layers, and other features described below. Titan’s upper atmosphere decelerated the Probe over a mere 3 minutes by a factor of fifteen, from over 6 kilometers per second to 400 meters per second (900 miles per hour). By the end of this deceleration, the Probe had descended to an altitude of 160 kilometers (100 miles, slightly less than in ESA’s design guidelines discussed above).20

4.2.3.2.2

Aft cover

The Probe’s aft cover provided several functions, one of which was that it furnished some thermal protection for the equipment within during the plunge through Titan’s atmosphere, although the aft cover was not heated nearly as much as the front heat shield. A layer of small silica spheres sprayed on the stiffened aluminum sheets of the aft cover was expected to give it sufficient heat protection during this phase.

4.2 Huygens Probe development 115 A secondary function of the aft cover was to ensure that the interior of the Probe depressurized as the spacecraft rose from sea level pressure on Earth into the vacuum of space. The aft cover also carried multi-layer insulation (as did the front heat shield) for protection during the spacecraft’s cruise phase through interplanetary space and the Probe’s coast from the Orbiter to Titan.21 Included in the aft cover was an access door for late inspections or repairs during spacecraft integration operations and for forced-air ground cooling of the interior of the Probe. In addition, a break-out patch provided an opening through which the first parachute was ejected at the end of the 3 minute atmospheric entry phase at Titan. A special sealing joint between the aft cover and front heat shield constituted a thermal and particulate barrier.22

4.2.3.3

4.2.3.3.1

Descent module

Parachutes

When onboard accelerometers noted that the Probe had slowed to a speed of Mach 1.5 (1.5 times the ambient speed of sound), a mortar ejected the 2.6 meter (8.5 foot) diameter pilot parachute (Figure 4.4) through the aft cover’s break-out patch, the attachment pins of which sheared under the impact. The pilot chute inflated and trailed 27 meters (89 feet) behind the Probe, pulling free the entire aft cover. As it broke away, the cover pulled the main parachute (8.3 meters, or 27 feet, in diameter) from its container in the descent module. The main chute stabilized and quickly decelerated the Probe, decreasing its speed to Mach 0.6 after approximately 30 seconds. At this point the front heat shield was released and fell away, and the Probe’s transmitters powered on. The science instrument components were deployed and inlet ports for collecting atmospheric samples were opened. Initial scientific measurements were made. When the Probe descended to an altitude of about 145 kilometers it started to transmit data to the Orbiter passing overhead, where it was stored and later relayed to Earth. A copy was retained onboard the Orbiter as backup.23 The Probe was designed to transmit data throughout its descent and hopefully a little longer if it survived impact with the surface. To achieve these objectives, the main chute could not be retained. If it was, then the Probe would be slowed so much that long before it reached the surface the Orbiter would either pass beyond the beam width of the Probe’s antenna or below the horizon and be unable to receive data. The main parachute was thus separated from the Probe after about 15 minutes and a 3 meter (10 foot) stabilizing chute was deployed in its place. The stabilizing chute had been sized to yield a descent time within the duration of the data link to the Orbiter.24 All of the Probe’s parachutes were made of Kevlar lines and nylon fabric. They were connected to swivels using redundant low-friction bearings to ensure that as the Probe spun, the lines of its parachutes would not get tangled.25 All of the parachutes were of the disk-gap-band (DGB) type,26 a design whose reliability had been proven on earlier missions, including the Viking Landers and Mars Pathfinder. In particular, the reliability of DGB design had been established at supersonic deployment speeds, and this capability was needed for the Probe mission.27

116

The Titan Huygens Probe

Figure 4.4 The Probe’s three parachutes and how they were employed in the descent through Titan’s atmosphere to its surface.

4.2.3.3.2

Estimating descent times

Key to the success of the Huygens mission was to have the best possible estimate of its descent time through an unknown atmospheric environment. Various approaches were used to obtain these estimates, such as using Titan atmospheric data collected by Voyager and Earth-based instruments and combining it with previous NASA and ESA parachute

4.2 Huygens Probe development 117 drag data. Another approach was to install sensor packages into scale models of the Huygens descent module and conduct parachute drop tests in Earth’s atmosphere, then attempt to scale the results to Titan atmospheric conditions.28

4.2.3.3.3

Equipment platforms, after cone, and fore dome

The descent module included two aluminum honeycomb equipment platforms: the top platform and the experiment platform. The top platform supported the descent control subsystem as well as the antenna for communicating with the Orbiter. The experiment platform carried science experiments. The descent module also included the after cone and the fore dome which, with the top platform, formed a protective aluminum envelope around the experiment platform (Figure 4.2). Thirty-six vanes on the periphery of the fore dome established a controlled rate of spin on the descent module as it dropped through Titan’s atmosphere. The descent module was linked to the surrounding entry assembly, in particular its front heat shield and the aft (back) cover, by fiberglass struts and pyrotechnically operated release mechanisms.29

4.2.3.4

4.2.3.4.1

Probe subsystems

Thermal control

Sensitive electronics aboard the Probe could have been damaged if permitted to get either too cold or too hot. The thermal control subsystem was to maintain all science instruments and operational components within strictly specified temperature ranges during all mission phases. To achieve this, project engineers drew upon experience gained during the Galileo mission, which like Cassini-Huygens had to deal with high temperatures in the inner solar system, low temperatures in the outer system, the vacuum of space, and the plunge into another world’s atmosphere. As depicted in Figure 4.5, the thermal control subsystem included components which performed insulating, radiative, and heating functions.30 Tiles of ablative material consisting of a silica fiber felt reinforced by phenolic resin, mounted on the front heat shield, provided thermal protection during Titan atmospheric entry.31 One unknown factor was the extra thermal load that might be generated if the entry elicited chemical reactions involving atmospheric gases, so engineers left a wide safety margin in their tile design.32 Prosial, a suspension of hollow silica spheres in silicon elastomer, sprayed onto the aluminum structure of the front heat shield inner surfaces and the aft cover. Multi-layer insulation (MLI) covering all external surfaces apart from the thermal window of the front heat shield (described below). This insulation was expected to burn and tear away during atmospheric entry, leaving protection against internal overheating largely to the high-temperature ablative tiles and the Prosial. Thermal window, a small white-painted, thin aluminum sheet on the forward face of the front heat shield that acted as a controlled, low-level heat leak during the cruise phase, emitting about 8 watts.

118

The Titan Huygens Probe

Figure 4.5 Thermal Control System components. Abbreviations used: MLI, Multi-layer insulation, HTP, Huygens Thermal Protection, RHU, Radioisotope Heater Unit, Prosial, A suspension of hollow silica spheres in silicon elastomer, sprayed as a protective coat onto some of the Probe’s aluminum structures, SED, Spin Eject Device, CFRP, Carbon fibers reinforced plastic, AQ60, A silica fiber felt material reinforced with phenolic resin.

Radioisotope heater units. Each of the 35 plutonium-powered RHUs continuously provided 1 watt of thermal energy, even when the Probe was dormant.33 Open-cell foam insulation. During the descent phase, additional thermal control was provided by lightweight, open-cell foam insulation covering the internal walls of the descent module’s aluminum shell and one of its platforms. The task was to decouple sensitive electronics from the aluminum shell, chilled by Titan’s atmosphere, which is −200°C at 45 kilometers altitude. Gas-tight seals around all components protruding through the descent module’s shell. This minimized gas influx. The entire descent module was gas tight, except for a single 6 square centimeter (0.9 square inch) vent hole on the top which equalized internal and ambient pressures while leaving Earth and while descending to Titan’s surface.34 In space, the thermal control subsystem partially insulated the Probe from heat emitted by the Orbiter and ensured only small variations in the Probe’s internal temperatures, despite solar radiation being intense near Venus and very minimal in the outer solar system.

4.2.3.4.2

Electrical power

During the cruise to Saturn, an umbilical connection with the Orbiter allowed Probe functionality to be monitored and power supplied to it. After separation, five lithium sulfurdioxide (LiSO2) batteries aboard the Probe furnished its power. Some of this power ran the timer during its 20 day coast phase to Titan.

4.3

Selection procedures for the Probe’s science package 119

The Probe had no propulsion unit and no communication capability with either the Orbiter or Earth until it had entered Titan’s atmosphere and its transmitters were turned on. A greater power draw on its batteries was required after this point and especially during its 2.5 hour descent phase, when its instruments took numerous samples and made many observations. The battery system had redundancy built into it, and the Probe could survive, continue to function, and support a complete mission even if it lost one of its batteries.35

4.2.3.4.3

Command and data management

This subsystem provided monitoring and control of Probe and payload activities. At the end of the 20 day coast to Titan and just before atmospheric entry, the command and data management subsystem activated the Probe functions. It sent commands to the operating systems and science instruments, and furnished these instruments with a descent data broadcast providing a timeline of conditions on which the instruments based the scheduling of their operations. This subsystem also collected scientific and housekeeping data and forwarded them to the Orbiter via the umbilical cable prior to separation and by way of the data relay subsystem while descending through Titan’s atmosphere.36

4.2.3.4.4

Data transmission

The data relay subsystem employed equipment on both Probe and Orbiter to provide a one-way communications link from the Probe to the Orbiter. The Probe carried a pair of redundant S-band transmitters, each with its own antenna. Signals from one transmitter were delayed about six seconds in order to avoid data loss if there was a brief transmission outage. Reacquisition of the probe signal would normally occur within this time interval.37

4.3

SELECTION PROCEDURES FOR THE PROBE’S SCIENCE PACKAGE

The instruments to undertake scientific investigations during the Probe’s encounter with Titan were chosen in a year-long ESA-led competitive process that began in October 1989 with an Announcement of Opportunity. This ran parallel to NASA’s selection of the Orbiter experiments. Americans as well as Europeans competed to install experiments on the two vehicles. Two classes of instruments were envisioned for the Probe: • •

Self-contained instruments Instruments requiring access to engineering subsystems, such as the radio relay subsystem, in order to operate.

The proposals received by February 1990 were subjected to rigorous technical and scientific evaluations. European and U.S. judges participated in selecting the instruments for the Probe, just as in choosing the instruments for the Orbiter.38 The selected Probe instruments were announced in October 1990.

120

The Titan Huygens Probe

One of the constraints placed on the Huygens Probe science instrument package was that its mass must not exceed 48 kilograms. The battery power storage system was required to supply at least 1,400 watt-hours39 of energy to run the instruments and other Huygens functions (the final design ended up having a nominal battery capacity of 1,059 watt-hours).40 The Probe’s instrument package needed to conform with the scientific intent of the mission, which was to conduct measurements and gather data primarily during the atmospheric entry and descent phases of the Huygens project. ESA hoped, but was uncertain, that the Probe would function after impact with Titan’s surface, and selected its instruments accordingly. In particular, the investigations were to address specific science objectives defined by the Joint Science Working Group (JSWG), which supported the mission’s Phase A planning activities (see Chapter 1 for a description of the JSWG and its work on the Phase A study).41 Titan atmospheric study objectives were: • • • • • • • • • •

Determine abundances of atmospheric chemical constituents, including any noble gases Establish isotope ratios for abundant elements Observe vertical and horizontal distributions of trace gases Determine the energy source driving atmospheric chemical processes Model photochemical processes in Titan’s stratosphere Analyze the formation and composition of aerosols Measure winds and global temperatures Analyze cloud physics, general circulation patterns, and seasonal effects Search for lightning discharges Analyze the upper atmosphere, its ionization characteristics, and its role as a source of neutral and ionized material for Saturn’s magnetosphere.

Titan surface science objectives were to investigate: • •

4.4

Physical state, topography, and composition of the surface Internal structure underlying the surface.

THE HUYGENS PROBE’S SUITE OF INSTRUMENTS

ESA and NASA selected six instruments to conduct key analyses:42 • • • • • •

Aerosol Collector Pyrolyzer (ACP): In-situ observations of clouds and aerosols in Titan’s atmosphere Descent Imager and Spectral Radiometer (DISR): Temperatures and images of Titan’s atmospheric aerosols and of its surface Doppler Wind Experiment (DWE): Analyses of winds from their effect on the Probe during its parachute descent Gas Chromatograph and Mass Spectrometer (GCMS): Measurements of the chemical compositions of atmospheric gases and aerosols Huygens Atmospheric Structure Instrument (HASI): Determination of the atmosphere’s physical and electrical properties Surface Science Package (SSP): Measurements of the physical properties of Titan’s surface.

4.4 The Huygens Probe’s suite of instruments 121

Figure 4.6 Mass comparison of the Probe’s science instruments, in kilograms and percentage of the total science package mass.

This suite of instruments met the scientific objectives of the mission’s planners, in that five of the six were focused on characteristics of Titan’s atmosphere and were able to analyze the important parameters identified by the JSWG. Even the sixth instrument, the Surface Science Package, offered a limited capacity to perform atmospheric measurements. Figure 4.6 gives a mass breakdown of these instruments. Because the Titan study was such an important part of the overall mission, Titan-related staff included three interdisciplinary scientists (IDS) – one to oversee Titan aeronomy observations, another for atmosphere-surface interactions, and a third for chemistry and exobiology matters. The aeronomy IDS, Daniel Gautier of the Observatoire de ParisMeudon, watched over the study of Titan’s atmospheric aerosols, photochemistry, general circulation, and origins.43 François Raulin of the Laboratoire de Physique et Chimie de l’Environnement, Paris specialized in Titan’s chemistry and exobiology. Jonathan Lunine of the University of Arizona, Tucson and the University of Rome Tor Vergata specialized in interactions between Titan’s atmosphere and surface.44 4.4.1

Aerosol Collector and Pyrolyzer

Guy Israel of the Service d’Aeronomie in Paris led the development work for the ACP and served as its principal investigator. He already had experience building such an instrument, having fabricated a simpler version of it for the USSR’s VeGa mission to Venus.45 Also collaborating on the ACP was the Space Research Institute of the Austrian Academy of Sciences.46 The ACP gathered aerosols – typically minute mineral particles suspended in the atmosphere, onto which liquid droplets or crystals and other chemical compounds could adhere – and subjected them to chemical analysis. The ACP accumulated its samples through a tiny stainless steel filter that extended in front of the Probe as it descended through Titan’s atmosphere. A pump drew the atmosphere in through the filter so as to trap the aerosols. The filter then retracted into an oven in the ACP, where the sample was heated

122 The Titan Huygens Probe in two stages: first to 250°C (about 480°F) to vaporize its volatile part, then to 600°C (about 1,100°F) to thermally decompose (pyrolyze) its solid part into gaseous species. After each step, the gases evolved were transferred by a pipe to the Gas Chromatograph Mass Spectrometer (GCMS) for analysis.47 Two sample groups were taken: one ranging from the top of the descent trajectory down to the tropopause, and the second within the cloud layer.48 The tropopause is the boundary between the stratosphere and the troposphere, the lowest region of the atmosphere. On Earth most weather phenomena occur in the troposphere, which is characterized by declining temperature with increasing altitude. The stratosphere, on the other hand, is a very stable layer in which the temperature gradually increases with height. ACP scientific objectives were to study:49 • • • •

Chemical composition of the photochemical aerosols: hydrogen (H), carbon (C), nitrogen (N), and oxygen (O) Relative concentrations of organic condensates inside the lower stratosphere, including carbon, hydrogen, and cyanide (CN) Relative concentrations of organic condensates in the troposphere, mainly methane (CH4) and ethane (C2H6) Non-condensable constituents, such as carbon dioxide (CO2), trapped in the collected particles.

Owing to its organic-rich environment, planetary scientists considered Titan to be “a reference laboratory for studying prebiotic chemistry on a planetary scale.”50 The analyses performed by ACP and GCMS were expected to provide key insights into these organics. 4.4.2

Descent Imager/Spectral Radiometer

Although Huygens was an ESA mission, the DISR experiment was U.S.-led, with Marty Tomasko, a specialist in planetary atmospheres and radiative transfer of energy and a research professor at the University of Arizona, serving as the principal investigator. DISR generated images and made spectral measurements using wide spectral-range sensors. It took pictures looking down toward the surface as well as sideways, and measured the spectra of sunlight filtering down through Titan’s haze and also reflecting up from its surface. When the probe descended to an altitude of about 700 meters, its surface science lamp was switched on to illuminate terrain features near the landing site and enable DISR to analyze their spectra.51 Actual construction of the DISR was carried out by a collaboration of Martin-Marietta and local Tucson, Arizona companies. Its charge coupled device (CCD), a sensor that converted visual images into digital data (refer to Chapter 3 for a more detailed description), was furnished by Uwe Keller of the Max-Planck Institute for Aeronomie in Germany. Keller also supplied a component for the Cassini Orbiter’s Ultraviolet Imaging Spectrometer (UVIS) and had built the camera for ESA’s Giotto spacecraft that studied Halley’s Comet and Comet Grigg-Skjellerup; indeed he was one of the initiators of that mission.52 DISR’s infrared spectrometer was developed by Michel Combes, who served as president of the Paris Observatory from 1992 to 1998,53 in collaboration with other participants from France, Germany, and the U.S.

4.4 The Huygens Probe’s suite of instruments 123 The instrument addressed a number of science objectives:54 •

•

•

•

4.4.3

Atmospheric composition of Titan. Atmospheric absorption as a function of wavelength and the mixing ratio of methane as a function of altitude were to be determined. Measuring the vertical profile of methane was analogous to measuring relative humidity on the Earth. Titan’s methane was potentially in liquid, solid, or gaseous form, and possibly formed atmospheric clouds. Nature and distribution of photochemical haze in Titan’s atmosphere. This involved measuring the size, shape, vertical distribution, and absorption of haze particles as a function of wavelength to study their production and life cycle in Titan’s atmosphere. Heat balance of the atmosphere. In order to understand how much solar energy is absorbed by the atmosphere versus by the ground, DISR measured absorption of sunlight as a function of altitude. One objective was to better understand the small greenhouse effect observed in Titan’s atmosphere and the driving force for Titan’s winds. DISR also measured wind speed directly from the horizontal drift of the Probe relative to features seen on the surface. Characteristics of the surface and its interaction with the atmosphere. Some 700 images of the surface at resolutions varying from 150 meters to less than 1 meter were planned, taken from altitudes of 150 kilometers (93 miles) to a few hundred meters. Reflection spectra of the surface were also measured in several thousand locations, thereby supplying a data base of reflectivity and composition. Doppler Wind Experiment

The primary scientific objective of DWE was to determine the direction and strength of zonal winds in Titan’s atmosphere. As the Probe descended on its parachute the winds would impose Doppler shifts55 on its radio link to the Orbiter. After they were corrected for all known orbit and propagation effects these shifts would provide a vertical profile of wind velocity that would aid understanding of Titan’s atmospheric turbulence, the atmospheric surface layer, and methane convection currents.56 DWE also had the capability to examine the Probe’s swinging motion beneath its chute and other radio-signal-perturbing effects such as atmospheric attenuation, as well as its location, orientation, spin rate and spin phase during the descent.57 DWE was fairly simple in concept. The basic hardware it required was an ultra-stable oscillator (USO) on the Probe and another reference USO on the Orbiter. To minimize costs, these two oscillators were constructed as identical units in the same program. From the difference between the transmitted frequency of the radio waves generated using the Probe’s oscillator and the received frequency of those waves by the Orbiter, the Doppler shift caused by the Probe’s motion was to be determined. Most of this shift in frequency would be due to the Orbiter’s high velocity approach toward Titan, while some would be due to the Probe’s descent. The frequency shifts caused by these two effects would be calculated and subtracted from the observed Doppler shift. Any remaining frequency shift would for the most part represent wind induced motions of the Probe,58 thereby enabling a wind profile to be calculated for Titan’s atmosphere.

124

The Titan Huygens Probe

The DWE experiment was headquartered in the University of Bonn (Germany) Radioastronomy Department, with Michael K. Bird as principal investigator. The project included co-investigators from the U.S., Germany, and Italy. Two USOs for the transmitter on the Huygens Probe and the receiver on the Cassini Orbiter were constructed by DaimlerBenz Aerospace, Satellite Systems Division, in Ottobrunn, Germany.59 4.4.4

Gas Chromatograph/Mass Spectrometer

The Huygens GCMS was a versatile gas chemical analyzer designed to identify and quantify various atmospheric constituents. It was also equipped with gas samplers that were filled at high altitude for analysis later in the descent, when more time was available.60 A GCMS instrument couples two powerful analysis techniques – the chemical separation potential of gas chromatography with the ability of mass spectroscopy to characterize substances. A mass spectrometer produces ions (charged particles) from chemical substances, then employs electric and/or magnetic fields to determine the masses of the ions. The masses and relative abundances of a collection of ions can characterize the various chemicals present. There are several different types of mass spectrometer, based on their method of separating substances by their masses. The instrument onboard the Huygens Probe was a quadrupole mass spectrometer. The term “quadrupole” derives from the instrument’s four-pole mass filter which used specific voltages between its poles to allow only ions having a given mass-to-charge ratio to strike the detector.61 A quadrupole mass spectrometer was chosen because it could withstand the g-forces of launch and also the extreme temperatures of space. The Huygens GCMS analyzed both Titan’s atmospheric gas compositions and the gases derived from the evaporation and pyrolysis operations of the Aerosol Collector and Pyrolyzer (as described above). Because Titan’s atmospheric chemistry is very complex, a simple mass spectrometer such as the one on the Galileo mission would not have been capable of distinguishing the many substances that might have had the same molecular mass. However, a GCMS distinguishes compounds not only by their molecular masses but also by their different retention times as they travel through a narrow tube known as the gas chromatograph (GC) column.62 The Huygens GCMS had three GC columns, each one a coil 10 to 20 meters (33 to 66 feet) in length and coated with special chemicals on its inside. The coating was chosen so that when gas was injected into the system, certain compounds from the gas were adsorbed onto the column walls. Each of the three columns was designed to separate compounds of interest, as follows: • • •

Column 1: carbon monoxide, nitrogen, and other stable gases Column 2: nitriles (compounds containing nitrogen) and organic materials (carbonbearing compounds) containing up to three carbon atoms Column 3: organic materials with larger molecules, containing more than three carbon atoms.

The instrument was also designed to detect small quantities of atmospheric argon as well as neon and other noble gases. The GCMS effort was another U.S.-directed experiment on the Huygens Probe. Principal Investigator Hasso Niemann of NASA’s Goddard Space Flight Center led the

4.4 The Huygens Probe’s suite of instruments 125 international team that developed it.63 From Figure 4.6 it can be seen that the GCMS dominated all other science instruments in size, accounting for 40% of the total mass of the science package.64 4.4.5

Huygens Atmosphere Structure Instrument

HASI was a multi-sensor package to measure the physical and electrical properties of Titan’s atmosphere during Huygens’ descent, and at the surface if the Probe survived its landing. In addition, it included a microphone to receive the sounds of Titan, such as turbulence or storms in its atmosphere. This audio component on the Probe, called the acoustic sensor unit, was designed to return enough information for the science team to detect nearby thunder.65 As events transpired, it also recorded the signature of the Probe impacting on the surface. The objectives were to determine atmospheric density, pressure, and temperature profiles, to detect the presence of turbulence, winds, and waves, and to analyze atmospheric electrical features such as electron and ion conductivities. Investigating Titan’s electrical parameters provided an opportunity to detect electric wave activity and possibly lightning, a phenomenon that might be necessary for the development of organic molecules. Also of interest were types of quasi-static electric fields which lead to storm formation. HASI instrument sensors included the following: • • • •

Accelerometer, monitoring atmospheric deceleration, descent of the Probe, and its impact with Titan’s surface Pressure profile instrument, measuring atmospheric pressure Temperature sensors, determining atmospheric temperature Permittivity,66 wave, and altimetry instrument, analyzing atmospheric: • • • • • •

Electrical conductivity Electric waves DC electric fields Lightning Acoustic noise due to turbulence or storms Radar echoes below 60 kilometers.

HASI provided data from the very beginning of the Probe’s atmospheric entry at an altitude of 1,500 kilometers (930 miles). Data pertaining to density, pressure, and temperatures were derived indirectly from deceleration measurements by the 3-axis accelerometer. After the Probe released its front heat shield and entered its descent phase from 162 kilometers (101 miles) down to the surface (in the process passing through the stratosphere and troposphere), HASI performed direct measurements of pressure, temperature and electrical properties. These measurements were continued after impact. This data contributed to knowledge of Titan’s atmospheric composition and structure and the nature of its surface, including its roughness, mechanical and electrical properties, and liquid/solid phase.67 HASI epitomized the concept of international collaboration in space (Table 4.4) by involving 17 institutions from 11 countries, with Marcello Fulchignoni from the University of Paris/Observatoire de Paris-Meudon serving as principal investigator. Note that the

126

The Titan Huygens Probe Table 4.4. HASI principal investigator and co-investigators.

Principal Investigator: M. Fulchignoni Universite de Paris VII / Dept. de Recherche Spatiale, Observatoire de Paris-Meudon, France Co-Investigators: F. Angrilli Center of Studies and Activities for Space, University of Padova, Italy G. Bianchini F. Ferri A. Bar-Nun Dept. of Geophysics and Planetary Sciences, Univ. of Tel Aviv, Israel M.A. Barucci Dept. de Recherche Spatiale, Observatoire de Paris-Meudon, France A. Coustenis W. Borucki NASA/AMES, Moffet Field CA 94035, USA C. McKay M. Coradini ESA Headquarter, Science Directorate, Paris, France P. Falkner ESTEC, Solar System Division, Noordwijk, The Netherlands R. Grard H. Svedhem E. Flamini Agenzia Spaziale Italiana (ASI), Roma, Italy M. Hamelin Laboratoire de Physique et Chimie de l’Environnement, Orleans, France A.M. Harri Finnish Meteorological Institute, Helsinki, Finland G.W. Leppelmeier J.J. Lopez-Moreno Instituto de Astrofisica de Andalucia (IAA-CSIC), Granada, Spain R. Rodrigo J. Zarnecki Planetary and Space Sciences Research Institute, The Open Univ., Milton Keynes, U.K. J.A.M. McDonnell University of Kent, Canterbury Kent, U.K. F.H. Neubauer Institut fuer Geophysik und Meteorologie, University of Cologne (UKK), Koeln, Germany A. Pedersen University of Oslo, Norway G. Picardi University “La Sapienza” Roma, Italy V. Pirronello Osservatorio Astrofisico di Catania, Italy K. Schwingenschuh Space Research Institute, Austrian Academy of Sciences, Graz, Austria

participating organizations included not only those from Europe and the U.S., but also the University of Tel Aviv in Israel. The industrial contractor was Officine Galileo of Italy. In addition to collecting its own scientific data, HASI provided information for reconstructing the Probe’s attitude and trajectory. It also helped to calibrate other Huygens instruments as well as remote sensing observations taken from the Cassini Orbiter.68 4.4.6

Surface Science Package

The main purpose of the SSP was to ascertain the surface properties at the Probe’s impact site, yielding key information about the moon’s composition. This was the instrument that would provide ground truth data about what was actually happening on Titan’s surface. For many years after the Huygens mission, instruments on the Orbiter would make all sorts of observations of the moon. But these would be taken from far away, and possessing a set of in situ data for comparison would prove to be invaluable. A particularly vital task

4.4 The Huygens Probe’s suite of instruments 127

Figure 4.7 Surface Science Package (SSP) location within the Huygens Probe.

was to provide data that could be compared with the imagery from the Orbiter’s radar, which could view the terrain beneath the haze. Specific tasks that the SSP was designed to carry out on Titan’s surface included the following: • • • •

Determine the physical nature and condition of the surface around the landing site Determine the abundances of major chemical constituents Assess thermal, optical, acoustic, and electrical properties, providing data to validate physical and chemical models Analyze, in the event the Probe landed in a body of liquid (which it didn’t), wave properties and ocean/atmosphere interaction.

Although SSP was primarily designed to investigate Titan’s surface, several of its sensors were also to take measurements during the Probe’s descent to help determine the atmospheric properties. The results yielded by all of these measurements will be discussed in Chapter 14. Figure 4.7 shows the location of SSP within the Probe. The team created a suite of nine independent sensor subsystems,69 seven of which were mounted inside or on the lower rim of a cavity in the fore dome in order to be exposed to the atmosphere and surface. Two sensors which did not require direct exposure to the atmosphere or surface (the tilt sensor subsystem) were mounted on the SSP electronics box inside the descent module.70 The characteristics of the SSP’s different subsystems are summarized below: •

•

Impact penetrometer consisted of a piezoelectric71 sensor mounted on a spear that projected below the fore dome to provide data on the landing impact and the response of the Probe’s structure. Impact accelerometer was mounted inside the instrument on the SSP electronics box and measured atmospheric and surface accelerations.

128 •

•

•

•

•

•

The Titan Huygens Probe Tilt sensors were constructed of sealed glass tubes containing a methanol-based liquid and platinum electrodes. Two elements were used in order to give the tilt angle in any plane, as well as pendulum motion of the Probe during descent. Both elements were mounted on the SSP electronics box. Thermal properties sensor was constructed of small platinum wires. An electric current passed through the wires heated them and a small volume of the surrounding atmosphere or surface material (if it had been a fluid). A series of resistance measurements taken approximately every tenth of a second measured the rate of heating of the wires and determined temperature and thermal conductivity of Titan’s lower atmosphere. If the surface had been fluid, the sensor would have measured surface conductivity and heat capacity. Two acoustic properties sensors consisted of small piezoelectric ceramic transducers72 similar to those used on Earth in marine applications. An electrical pulse fed into a transducer caused it to deform, initiating a sound pulse in the surrounding medium. A sound pulse hitting a transducer deformed it slightly, generating a measurable electric signal. The Probe’s velocity sensor was constructed of two transducers mounted facing each other. Each alternated between transmitting and receiving an acoustic signal at 1 megahertz (MHz, million cycles per second). The propagation time between the transducers, either in the atmosphere or in a liquid, was related to the local velocity of sound. The third transducer was the acoustic sounder. This pointed vertically downward and emitted a 15 kilohertz (kHz, thousand cycles per second) signal meant to bounce off the surface of Titan as the Probe descended, or the bottom of a lake or ocean, should the Probe have landed in one. The return signal from the bounce was picked up by the acoustic sounder. Travel time between the transmitted and returned signal would give a measure of the distance to Titan’s surface at a particular time, or depth of a liquid body. Fluid permittivity sensor consisted of electrodes that, in the event of a liquid landing, could have measured electrical properties of the ambient fluid. This sensor also had some ability to detect condensates in the atmosphere.73 Fluid density sensor was meant to determine the density of liquid entering the so-called top hat (if the Probe had landed in a liquid body), which would have displaced a float and produced readings on four sensitive strain gauges. Refractive index sensor was composed of a specially shaped prism with a curved surface, two LED light sources illuminating the curved surface via light guides, and a group of light detectors called a linear photodiode array. Light passing through the top surface of the prism was fed into the array. The refractive index of the material surrounding the sensor was determined from the position of the light/dark transition on the photodiode array. This sensor was primarily for taking measurements in a body of liquid, although it could also measure the refractive index of the gas of the atmosphere.

Table 4.5 shows which SSP sensors could be used in Titan’s atmosphere during the descent and which could be used following landing to measure properties in the liquid, muddy, or solid surface beneath.

4.4 The Huygens Probe’s suite of instruments 129 Table 4.5. Operating environments of the science instruments. SENSOR

SURFACE ENVIRONMENT Liquid Mud Solid

ATMOSPHERE

Impact penetromete--xternal sensor Impact penetromete--nternal sensor Tilt sensor Thermal properties sensor Acoustic propertie--elocity sensor Acoustic propertie--ounding sensor Fluid permittivity sensor Fluid density sensor Refractive index sensor

no yes yes yes yes yes yes yes yes

no reduced yes yes yes reduced reduced no reduced

yes yes yes reduced no yes reduced no no

yes yes yes no no yes no no no

Note: “Yes” indicates full function; “No” indicates no function, and “Reduced” denotes reduced function.

What Table 4.5 indicates is that all but one of the sensors would operate at full capacity in a liquid, while only about half of them would do so on a muddy or solid surface. Seven out of nine would operate in the atmosphere, but four of these would do so in a reduced capacity.

4.4.6.1

The SSP principal investigator

SSP development was headed by Principal Investigator John Zarnecki, professor of space science at the Open University at Milton Keynes in the U.K. From the time he was appointed PI for the instrument in 1990 until the launch seven years later, the team painstakingly assembled, tested, and retested the SSP, and it was at times a rocky road to design and fabricate a reliable package of sensors. His lean budget, less than the amount that seemed to him necessary for the project, was one of the problems. Zarnecki had to be creative in order to successfully finish the instrument, and he sometimes got help from unusual sources. At one point in the development process, he was able to convince a group of Polish scientists to furnish his team with an important part of the instrument for no charge. Then in January 1996 during the final stages of testing, with the end seemingly in sight, the SSP team subjected its instrument to its final vibration test, only to have its structure crack, a clear sign of a major design flaw. With less than two years remaining to the launch date, the group undertook a major redesign effort. But the problems were ironed out and a working SSP was delivered to ESA and launched aboard the Cassini-Huygens spacecraft.74 # Like many scientists on the Cassini-Huygens mission, Zarnecki’s fascination with outer space began at an early age. He remembers very clearly how he was drawn to the space program, and the details of his conversion are interesting, for they give us clues as to how great careers can begin.

130 The Titan Huygens Probe When he was a schoolboy, Zarnecki was deeply moved by accounts of the early Soviet and U.S. attempts to reach outer space. In a 2007 lecture, he talked about them: “The triumphs, the disasters, the impossible achieved. It was a truly incredible time, when one event could pull together entire nations in a breath-holding moment. I have to confess that for me, it was the drama of the whole thing that grabbed me as much as the science and the technology of it.”75 But Zarnecki really became captivated by space exploration in 1961 during a chance encounter with Yuri Gagarin, the first man in space. In the summer of that year, Gagarin embarked on a world tour and his first stop was the U.K. He met the Queen, he met the Prime Minister, and then he traveled to Highgate in North London to visit the grave of Karl Marx. Local schoolchildren went to see Gagarin, including young John Zarnecki, who stood just a few feet from the cosmonaut as he saluted the grave. Zarnecki remembers that Gagarin was much smaller than he expected. The cosmonaut seemed dwarfed by the big military hat he was wearing. But the man had been in space for 93 minutes. Seeing Gagarin and realizing what he’d done was a critical moment for Zarnecki. He decided he would do something himself in space. He didn’t know what, but he wanted to somehow play a part in its exploration.76

4.5

PROBE TESTING, INTEGRATION, AND RELEASE TO NASA

Many testing protocols were applied to the Huygens Probe flight model (FM) or to one of the partial replicas that lacked such parts as the suite of scientific instruments. These tests served to verify that the FM was ready to be integrated with the Orbiter and launched into space. A drop test and a lightning susceptibility test are described below. 4.5.1

Drop test

In May 1995, roughly two-and-a-half years before the Cassini-Huygens mission was launched, a full-sized replica of the Probe, designated SM2, was dropped through Earth’s atmosphere above the European Space Range balloon launch site at Kiruna, Sweden.77 This trial was designed to simulate the Probe’s journey through Titan’s atmosphere. The external shape of SM2 was almost identical to that of the version that went to Titan. The aft cover, front heat shield, and descent module were built to flight-standard, as were its release mechanisms and descent control subsystem. To conduct the test, Huygens engineers attached SM2 to a gondola and used a balloon to hoist it to a stratospheric altitude of about 38 kilometers (24 miles). On command from the ground, the Probe separated from the gondola for the parachute descent test, the specific objectives of which were to demonstrate: • • • •

The descent sequence under dynamic conditions Functioning of the descent subsystem, including deployment, inflation, drag coefficient, structural strength, and stability of the parachutes Operation of the parachute deployment device Separation of the aft cover

4.5 Probe testing, integration, and release to NASA 131 • • •

Separation of the front heat shield Operation of the parachute jettisoning mechanism Functioning of the spin vanes under the stabilizing chute that followed release of the main chute.

This test also yielded critical data to correlate models and predictions with actual flight results.78 Martin Baker Ltd. of the U.K. conducted the post-flight parachute analyses and Aerospatiale the Probe system analyses. The pilot, main and stabilizer parachutes all deployed cleanly and inflated correctly in the times predicted. Video confirmed that, as desired, there was no post-separation contact between the Probe and either its aft cover or front heat shield. The main parachute provided effective damping of Probe oscillations, as desired. But the stabilizer chute that followed the main chute did not provide damping, in contradiction with expectation in the test and was predicted for the actual descent through Titan’s atmosphere. Additional helicopter drop tests were conducted. They demonstrated that this undamped motion was not due to poor design, related for instance to interaction between the descent module’s wake and the stabilizer chute. Further studies of wind conditions during the original drop test revealed high wind speeds and wind shear during the descent through the lower stratosphere and troposphere. These conditions caused turbulence that continuously excited the Probe and stabilizer chute, resulting in undamped motion. But conditions on Titan were believed to be such that there would indeed be sufficient damping to stabilize the Probe within specified limits.79 Analysis of the Probe’s spin, deliberately induced by vanes for stability, showed good agreement between predictions and test results on the main parachute. Lower than expected spin rates occurred on the stabilizer chute, but even under worst-case assumptions the Probe was expected to meet all of its scientific objectives.80 Although individual parachutes and mechanisms had been tested extensively, the drop test demonstrated expected performance under realistic dynamic conditions of the entire SM2 model, and thus provided a high degree of confidence that the actual flight model would make a successful descent through Titan’s atmosphere.81 4.5.2

Lightning susceptibility testing

The Huygens Probe had to descend through Titan’s atmosphere, where it faced the possibility of a lightning strike, particularly in the lower atmospheric regions where clouds are known to exist. Sensitive electronics could be damaged by such electrical discharges, and so all of the Probe’s instruments were encased in a metal cocoon to provide an adequate shield. The effectiveness of the metal layer had to be tested and verified, however. For this, an engineering model (EM) of the Probe was employed. The EM simulated all the physical and electrical characteristics of the flight model, except that it did not contain the suite of scientific instruments.82 This was the first time that an ESA spacecraft had to be tested for susceptibility to lightning, as none of its vehicles had yet undertaken an entry or re-entry through an atmosphere. The test was performed at the Universitat der Bundeswehr in Munich, Germany, not far

132 The Titan Huygens Probe from Daimler-Benz Aerospace, the company responsible for the assembly, integration, and testing of the Probe. A lightning current simulator in the university’s high-voltage test facilities was used to simulate direct lightning strikes on the Probe. It “proved to be virtually impervious to any strength of strike that was directed at it and continued to operate normally throughout the test.”83 A post-test checkout confirmed that all of the systems survived. 4.5.3

Other testing

Environmental testing was carried out on another replica of the flight model called the structure, thermal, and pyrotechnic model (STPM) in order to verify the Probe’s structural and thermal integrity. The STPM was delivered to JPL on 5 July 1995 for combined testing with the Cassini Orbiter. For electrical system analysis, a special engineering model was prepared and subjected to a battery of functional tests.84 4.5.4

Internal integration and flight acceptance review

In 1996 and early 1997, elements of the Huygens Probe were integrated with each other and extensively tested at Daimler-Benz Aerospace Dornier Satellitensysteme in Ottobrunn, near Munich, Germany. By March 1997, ESA reported that the Probe had passed its Flight Acceptance Review and was ready to be delivered to Kennedy Space Center. On arrival, it was to undergo additional tests and then, if it passed, be mated to and integrated with the Cassini Orbiter, aiming for launch in October 1997 atop a Titan 4B/Centaur rocket.85

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References 135 43. NASA-JPL, “Cassini Interdisciplinary Investigation: Titan Aeronomy (IDS),”http://cassinihuygens.jpl.nasa.gov/cassini/Science/MAPS/IDSGautier.shtml, accessed 8 Match 2010. 44. ESA, “Huygens Investigators,” http://www.esa.int/esaMI/Cassini-Huygens/SEM3E5XJD1E_2. html, updated 23 December 2004; American Astronomical Society, “Cassini Radar Observes Seasonal Change in Titan’s South Pole,” http://www.astronomy.com/asy/default.aspx?c=a&id= 8695, Astronomy (6 Oct. 2009). 45. Ralph Lorenz and Jacqueline Mitton, Lifting Titan’s Veil (Cambridge University Press: Cambridge, U.K., 2002), p. 175. 46. Das Institut für Weltraumforschung (IWF) der Österreichischen Akademie der Wissenschaften (ÖAW), “ACP,” http://www.iwf.oeaw.ac.at/index.php?id=246&L=1, last updated 16 Nov. 2007, accessed 11 Nov. 2008. 47. G. Israel et al., supplementary information associated with the proposed letter to Nature: “Evidence for the Presence of Complex Organic Matter in Titan’s Aerosols by In Situ Analysis,” http://www.nature.com/nature/journal/v438/n7069/extref/nature04349-s1.doc, reference article: 2005-05-06080, version: 10 May 2005, accessed 11 Nov. 2008. ESA, “Huygens Instruments,” http://www.esa.int/SPECIALS/Cassini-Huygens/SEM9W82VQUD_0.html, accessed 4 Nov. 2008; Ralph Lorenz and Jacqueline Mitton, Lifting Titan’s Veil (Cambridge University Press: Cambridge, U.K., 2002), pp. 174–175; ESA, “Huygens Investigators,” http://www.esa.int/ esaMI/Cassini-Huygens/SEM3E5XJD1E_2.html, updated 23 December 2004. 48. ESA, “ACP: Aerosol Collector and Pyrolyser,” http://sci.esa.int/science-e/www/object/index. cfm?fobjectid=31193&fbodylongid=1605, accessed 4 Nov. 2008. 49. ESA, “ACP: Aerosol Collector and Pyrolyser,” http://sci.esa.int/science-e/www/object/index. cfm?fobjectid=31193&fbodylongid=1605, accessed 4 Nov. 2008. 50. F. Raulin, P. Coll, D. Coscia, M. C. Gazeau, R. Sternberg, P. Bruston, G. Israel, and D. Gautier, “An Exobiological View of Titan and the Cassini-Huygens Mission,” Advances in Space Research 22(3) (1998):353–362. 51. ESA, “Huygens Instruments,” http://www.esa.int/SPECIALS/Cassini-Huygens/ SEM9W82VQUD_0.html, accessed 4 Nov. 2008. 52. “Uwe Keller,” http://phoenix.lpl.arizona.edu/kellerUwe.php, Phoenix Mars Mission Web site, accessed 25 May 2011; Lorenz and Mitton, p. 174; ESA, “Giotto Overview,” http://www.esa. int/science/giotto, 21 April 2004, accessed 25 May 2011. 53. Lutz D. Schmadel, Dictionary of Minor Planet Names (Springer, 2003), p. 594. 54. M. Tomasko, “DISR Scientific Objectives,” http://www.lpl.arizona.edu/~kholso/objectives. htm, Lunar and Planetary Laboratory, University of Arizona, last updated 26 Apr. 2006, accessed 12 Nov. 2008; Lunar and Planetary Laboratory, University of Arizona, “DISR at a Glance,” http://www.lpl.arizona.edu/~kholso/overview.htm, last updated 26 Apr. 2006, accessed 12 Nov. 2008. 55. Doppler shift refers to the apparent change in the frequency of a radio signal caused by the relative motion of the transmitter and receiver. If the transmitter (aboard the Probe) is moving away from the receiver (on the Orbiter), the apparent frequency of the radio waves will be less than if the transmitter was approaching the receiver. 56. M.K. Bird, M. Heyl, M. Allison, S.W. Asmar, D.H. Atkinson, P. Edenhofer, D. Plettmeier, R. Wohlmuth, L. Iess, and G.L. Tyler, “The Huygens Doppler Wind Experiment,” in Huygens: Science, Payload and Mission, ESA SP-1177, E. Wilson (ed.), ESA Publications Division, ESTEC (1997):139–162. 57. M.K. Bird et al., “Doppler Wind Experiment DWE Instrument Description,” http://www.astro. uni-bonn.de/~dwe/dweid/id.html, Argelander-Institut für Astronomie, Bonn, Germany, last updated 4 Oct. 2005, accessed 12 Nov. 2008; ESA, “Huygens Instruments,” http://www.esa.int/ SPECIALS/Cassini-Huygens/SEM9W82VQUD_0.html, accessed 4 Nov. 2008.

136 The Titan Huygens Probe 58. Lorenz and Mitton, p. 174. 59. Argelander-Institut für Astronomie, “DWE Personnel” and “The DWE Hardware,” http://www. astro.uni-bonn.de/~dwe/dwe_pers.html and http://www.astro.uni-bonn.de/~dwe/dwe_hard. html, respectively, last updated 4 Oct. 2005, accessed 12 Nov. 2008. 60. ESA, “Huygens Instruments,” http://www.esa.int/SPECIALS/Cassini-Huygens/SEM9W82 VQUD_0.html, accessed 4 Nov. 2008. 61. Michael Meltzer, Mission to Jupiter: A History of the Galileo Project (Washington D.C.: NASA SP-2007-4231, 2007):123. 62. Atmospheric Experiments Laboratory at NASA Goddard Space Flight Center, “Huygens Probe Gas Chromatograph Mass Spectrometer,” http://huygensgcms.gsfc.nasa.gov/, excerpt from Nature article (8 December 2005); Lorenz and Mitton, pp. 173–174. 63. ESA, “GCMS: Gas Chromatograph and Mass Spectrometer,” http://sci.esa.int/science-e/www/ object/index.cfm?fobjectid=31193&fbodylongid=736, last updated 07 Dec 2005, accessed 13 Nov. 2008; Astrobiology Magazine, “The Methane Mystery,” http://www.astrobio.net/news/index. php?name=News&file=article&sid=1800&theme=Printer, 10 Dec. 2005, accessed 13 Nov. 2008. 64. Zarnecki, “The Huygens Surface Science Package.” 65. Francesca Ferri and Piero Lion Stoppato, “Huygens Entry Into Titan’s Atmosphere,” http:// cisas.unipd.it/project/hasi/welcome.html, CISAS-ASI, last updated 10 Jan. 2005, accessed 13 Nov. 2008; ESA, “Huygens Instruments,” http://www.esa.int/SPECIALS/Cassini-Huygens/ SEM9W82VQUD_0.html, accessed 4 Nov. 2008; Planetary Society, “The Huygens ‘Microphone,’” http://www.planetary.org/explore/topics/cassini_huygens/huygens_microphone.html, accessed 13 Nov. 2008; M. Fulchignoni, “HASA Huygens Atmospheric Structure Instrument,” poster presentation at ESA Titan Conference, ESTEC, 13–17 Apr. 2004, accessed 14 Nov. 2008. 66. Permittivity is the preferred term for what used to be termed “dielectric constant,” a parameter related to the quantity of electrostatic energy that can be stored within a unit volume of a material. In this case, the “material” is Titan’s atmosphere. 67. F. Ferri, M. Fulchignoni, and the HASI Team, “Expected Results of HASI Experiment During Huygens Probe Descent on Titan in Cassini Mission,” Bulletin of the American Astronomical Society 29 (July 1997):1038; M. Fulchignoni, F. Ferri, G. Colombatti, and the HASI team, “The Huygens Atmospheric Structure Instrument (HASI) Results at Titan,” http://ippw.jpl.nasa.gov/ 20070607_doc/2_2FULCH.pdf, Fourth Annual International Planetary Probe Workshop (Pasadena, California: JPL/NASA, 27–30 June 2006); Space Research Institute (IWF) of the Austrian Academy of Sciences (AAS), “HASI,” http://www.iwf.oeaw.ac.at/index. php?id=245&L=1, last updated 16 Nov. 2007, accessed 14 Nov. 2008. 68. M. Fulchignoni et al., “The Huygens Atmospheric Structure Instrument (HASI) Results at Titan.” 69. ESA, “Huygens Instruments,” http://www.esa.int/SPECIALS/Cassini-Huygens/SEM9W82 VQUD_0.html, accessed 4 Nov. 2008. 70. ESA, “SSP: Surface Science Package,” http://sci.esa.int/science-e/www/object/index.cfm?fobj ectid=31193&fbodylongid=740, accessed 17 Nov. 2008. 71. A piezoelectric material is one having the ability to generate a voltage when mechanically deformed, or conversely, to deform when a voltage is applied. 72. The transducers converted mechanical energy in a sound wave into a measurable electrical signal. 73. J.C. Zarnecki et al., “Huygens’ Surface Science Package,” http://www.mrc.uidaho.edu/ ~atkinson/Huygens/Documents/SpSciRev/Zarnecki%20SSP.pdf, Space Science Reviews 104 (2002):593–611. 74. “About Me,” http://www.johnzarnecki.com/829696/About-me, from johnzarnecki.com/ Web site, accessed 25 May 2011.

References 137 75. John Zarnecki lecture, “Yuri and Me,” http://open2.net/oulecture2007/yuri.html, The Open University Lecture 2007, BBC open2.net Web site, last updated 6 Dec. 2007, accessed 25 May 2011. 76. Ibid. 77. A. Sarlette, M. Perez-Ayucar, O. Witasse, J.P. Lebreton, “Comparison of the Huygens Mission and the SM2 Test Flight for Huygens Attitude Reconstruction,” www.montefiore.ulg.ac. be/~sarlette/Data/Huygens1.pdf, accessed 22 Nov. 08; StratoCat, “Details of the Balloon and Launch Operations,” http://stratocat.com.ar/fichas-e/1995/KRN-19950514.htm, accessed 22 Nov. 08, 78. E. Jäkel, P. Rideau, P.R. Nugteren, and J. Underwood, “Drop Testing the Huygens Probe,” ESA Bulletin No. 85, Feb. 1996. 79. Jäkel et al. 80. Jäkel et al. 81. Jäkel et al. 82. C. McCarthy and H. Hassan, “Lightning Susceptibility of the Huygens Probe,” http://www.esa. int/esapub/bulletin/bullet85/mcca85.htm, ESA Bulletin No. 85, February 1996; NASA-JPL, “Startup, Design and Assembly,” http://cassini-huygens.jpl.nasa.gov/cassini/english/ops/ devopsyellow.shtml, Cassini-Huygens Web site, accessed 24 Nov. 2008. 83. C. McCarthy and H. Hassan, “Lightning Susceptibility of the Huygens Probe,” http://www.esa. int/esapub/bulletin/bullet85/mcca85.htm, ESA Bulletin No. 85, February 1996. 84. ESA, “Huygens probe on target,” http://esamultimedia.esa.int/esaCP/Pr_17_1995_i_EN.html, reference no. 17–1995, 31 July 1995. 85. ESA, “Huygens Space Probe Ready to Leave Europe,” http://www.esa.int/esaCP/Pr_8_1997_p_ EN.html, press release no. 8–1997, 3 March 1997

5 Integrating the Cassini Orbiter, Huygens Probe, and Titan/Centaur launch vehicle Before the Cassini-Huygens spacecraft could be launched, it had to be made flight ready, which meant that all of its subsystems, developed by many different U.S. contractors as well as by the project’s international partners, had to be integrated to create a working space vehicle and thoroughly analyzed in a systems environment. No matter how beautifully each component operated by itself, it also had to be tested in relation to the rest of the spacecraft. These activities had to take place in an organized, well-documented manner so as to give as much assurance as possible that the vessel’s hardware and software would operate properly during and after launch. The process to achieve this for the Cassini-Huygens mission was termed the Assembly, Test, and Launch Operations (ATLO). The culmination of these efforts included launch preparations at Cape Canaveral Air Force Station managed by Kennedy Space Center (KSC).1 The ATLO program officially kicked off on 22 November 1995, although some management and planning efforts had been initiated earlier, beginning in January 1991 when NASA appointed Warren Moore the ATLO manager and Hank Doupe his deputy. At that time, there were still two similar spacecraft under consideration, the Comet Rendezvous Asteroid Flyby (CRAF) and the Cassini-Huygens vehicles, and the program set out to plan the ATLO actions for both of them. The effort was disrupted in 1992 when NASA chose, owing to the budget constraints imposed on it, to request the cancellation of CRAF. This request was approved by President Bush and eventually Congress (details of CRAF and its cancellation were discussed in Chapters 1 and 2). This left only Cassini-Huygens in the ATLO program. Preparing it for launch involved detailed coordination between NASA, ESA, ASI, and their contractors.2 The first steps in this complex process were to fully integrate and test the Cassini Orbiter and Huygens Probe separately, then connect them and integrate them together.

5.1

ORBITER INTEGRATION

As engineers, scientists, and contractors on the Cassini Orbiter team finished the design, fabrication, and analysis of individual subassemblies, they sent them on to the Spacecraft Assembly Facility (SAF) at JPL, where mission staff integrated them, although the RTGs © Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_5

139

140

Integrating the Cassini Orbiter, Huygens Probe…

Figure 5.1 Cassini Orbiter subsystems (“plus-Y side”).

were sent directly to KSC.3 Components were delivered as and when they became available. The Orbiter structural elements, for instance, arrived at SAF on 20 November 1995. A few days later, the bus cable harness4 was delivered and installed, followed by integration of the power and pyrotechnic subsystem. The delivery and integration of subsystems continued through 1996. By April 1996, all Orbiter flight engineering subsystems (i.e., those that controlled the spacecraft) and some science instruments (those pieces of equipment that carried out experiments and investigations) were integrated. By early August, the remaining Orbiter science instruments were integrated.5 Figures 5.1 and 5.2 depict many of the subsystems that were integrated to form the Cassini Orbiter. Several critical subsystems required particular attention. These included the attitude and articulation control subsystem (AACS), the command and data subsystem (CDS), the solid-state recorder (SSR), and the test telemetry and command subsystem

5.2

Probe integration

141

Figure 5.2 Cassini Orbiter subsystems (other side of Orbiter).

(TTACS) that was a portion of the Cassini ground system. A special JPL facility, the Integration and Testing Laboratory (ITL), was used for these subsystems6 because it offered a high quality hardware-in-the-loop testbed, which was a powerful tool for complex, real time simulations.

5.2

PROBE INTEGRATION

The final version of ESA’s Huygens Probe, called the flight model (FM), required somewhat longer than was scheduled for ESA to complete integration and testing activities. These activities had to be finished before the FM could be sent to the U.S. for integration with the rest of the spacecraft. In order not to hold up the mission’s schedule, ESA

142

Integrating the Cassini Orbiter, Huygens Probe…

fabricated, tested, and sent JPL an engineering model (EM) and a structural, thermal, and pyrotechnic model (STPM) of the Probe while completing the FM. The EM and STPM simulated all the physical and electrical characteristics of the FM, but did not contain its suite of scientific instruments. By temporarily connecting the EM and STPM at appropriate times to the Cassini Orbiter, JPL was able to test most parts of the Cassini-Huygens spacecraft as a whole. Meanwhile, work continued at ESA to complete internal integration and testing of the FM. In April 1997 the FM was finally delivered to KSC.7

5.3

THE ASSEMBLY AND TESTING PROCESS

ATLO was run by JPL staff specialized in assembling and testing interplanetary spacecraft. Mission staff transported all the individual subsystems to the SAF, where they were assembled into a complete Cassini-Huygens spacecraft. The integration of each subsystem with the entire spacecraft proceeded roughly as follows: • • • • • • •

Spacecraft power was turned off Subsystem was installed Electrical connections were inspected with the power off Spacecraft power was then turned on Electrical testing of the subsystem was conducted Subsystem was analyzed to verify that it operated as expected while connected to the entire spacecraft If any problems surfaced, they were resolved when time permitted.

This process continued until all subsystems were integrated into the spacecraft and analyzed to verify performance. Afterwards, several types of environmental tests were performed.8

5.4

ENVIRONMENTAL TESTING

By the middle of August 1996, spacecraft integration was completed to the point where preparations for the environmental testing phase of the ATLO process could begin (although the FM of the Probe had not yet been delivered). These procedures included: •

Electromagnetic interference testing • •

•

Dynamic environmental testing • • • •

• •

Radiated susceptibility Radiated emissions

Acoustical impacts Science aliveness Vibrational impacts Pyro shock

Solar/thermal/vacuum (STV) testing Post-environmental system testing.

5.4 Environmental testing 143

Figure 5.3 Stacked spacecraft, ready for environmental testing.

Each of these tests is described below. The environmental testing phase of the ATLO activities was initiated by stacking the spacecraft subsystems into their flight configuration. This began with the mission team mounting the propulsion module subsystem onto the lower equipment module. Then it installed the upper equipment module (with the highgain antenna and one of the low-gain antennas in place) onto the propulsion module subsystem (Figure 5.3).9 5.4.1

Electromagnetic interference testing

Environmental tests in the SAF began in November 1996 with an electromagnetic interference (EMI) trial, but the Probe FM did not arrive until April 1997. JPL staff conducted a series of measurements to determine radiated susceptibility, which meant the potential effects on the spacecraft of radar and ground transmitters in use at the launch site. With the spacecraft in as near to the launch configuration as possible, JPL staff placed antennas and transmitters around it at various levels and distances. Staff switched on the spacecraft power and the transmitters, applied radio frequency (RF) energy to the vehicle, then analyzed telemetry from the vehicle. One concern had been that the RF energy might ignite one of the pyrotechnic devices, but fortunately this did not happen, and no other effects were observed. While the radiated susceptibility test focused on effects from sources external to the space vehicle, the radiated emissions test investigated the effects of onboard interference between subsystems. In particular, the spacecraft contained an X-band receiver for commands sent from Earth during flight. One concern in designing the spacecraft was that some subsystem on the vessel might emit an RF signal which the X-Band receiver would receive, causing problems with the uplink from Earth. The radiated emissions test involved

144

Integrating the Cassini Orbiter, Huygens Probe…

a search for such RF signals. JPL staff powered up the spacecraft, and each subsystem on it was turned on while sensitive antennas and receivers placed around the spacecraft listened for any interfering signals radiating from it. Some emissions were detected, but all the test objectives and environmental requirements were satisfied.10 5.4.2

Dynamic environmental test

The dynamic environmental test analyzed effects of acoustics, random vibrations, and pyrotechnic shock on the spacecraft. Also included was a science aliveness test. The structural, thermal and pyrotechnic model (STPM) substituted for the Probe FM in these tests. To develop the criteria for a suitable acoustic test, Cassini mission staff employed acoustic measurements recorded during eight missions that had flown the Titan 4 launch vehicle with the same payload fairing as was to be installed for the Cassini-Huygens mission. They also drew on test data from NASA’s Lewis Research Center for a Cassini spacecraft simulator inside a full-scale Titan 4 payload fairing with various acoustic blanket configurations. These data revealed that the most intense acoustic environment occurred during liftoff. Of special concern were vibrations at the mounting locations for the RTGs elicited by high levels of acoustic energy.11 The spacecraft received its acoustic test on 5–6 November 1996. Models with the same mass and stiffness characteristics were used in place of the plutonium-bearing RTGs. One of the main objectives of the test was to demonstrate that the spacecraft could withstand the Titan 4 acoustic environment during launch. This appeared to be the case, because all spacecraft instrumentation performed well during the test. The motor nozzle continued to move as expected and no anomalies were detected in any of the controls or electronics. The only mechanical failures were science instrument fasteners that backed out of position during the acoustic test. Actions were taken to prevent this from occurring during launch.12 The science aliveness test addressed the concern that the conditions to which the science instruments would be subjected at launch might impair their operation. The test verified that the instruments were still functioning and appeared able to perform their experiments after electromagnetic and acoustic environmental testing had been performed. Further science instrument analysis was done later, including during the solar/thermal/vacuum and post-environmental systems tests (see below) and launch preparations at KSC.13 The mission team performed random vibration tests in JPL’s Environmental Test Laboratory using a shaker device that could simulate accelerations during liftoff with a Titan vehicle.14 In tests in November 1996, the shaker applied a series of different vibrations to the spacecraft; as with the acoustic test, models with the same mass and stiffness characteristics substituted for the RTGs. This test was to demonstrate the mechanical integrity of various spacecraft instruments. After the testing, the mission team carefully analyzed the spacecraft to determine whether any part of its structure was damaged and found that the RTG case was no longer electrically isolated from the rest of the vehicle. To correct this, engineers redesigned the insulation that was supposed to isolate the RTG from the Orbiter’s lower equipment module.

5.4 Environmental testing 145 The pyro shock test consisted of firing three pyrotechnic devices which would be used during the mission and that could produce undesirable shock levels, potentially causing damage. Devices included: • • •

Probe deployment pyro device Launch vehicle separation pyro device, meant to break the spacecraft away from the Centaur stage at the end of launch operations Main engine assembly cover ejection device.

During two periods in November 1996, JPL staff fired the devices and measured the imparted shocks. This test was meant to demonstrate that the pyro devices could perform their functions correctly and that spacecraft hardware in the vicinity of the devices would not be damaged. During the Probe deployment test, the pyro device successfully cut the cable connecting Probe to Orbiter, but the Probe failed to fully deploy. JPL staff determined that in the absence of gravity, as would be the case during the actual mission, the Probe would indeed have fully deployed. A minor problem occurred when attempting to jettison the cover of the main engine nozzle. The bolts holding the cover in place were redesigned to eliminate this problem. After the three pyro tests had been completed, JPL carefully checked all electrical systems aboard the spacecraft and found that they operated normally.15 5.4.3

Solar/thermal/vacuum test

The solar/thermal/vacuum (STV) test performed in January and February 1997 used the 25 foot diameter Space Simulator at JPL to subject Cassini-Huygens to a space-like environment.16 The test was to determine the spacecraft’s thermal integrity and functionality in conditions of vacuum, frigid temperatures, and the widely different solar intensities at Venus and Saturn. The spacecraft was placed in the test chamber so that its high-gain antenna pointed toward the simulated solar radiation source, to shade the rest of the vehicle as it would for much of the cruise phase. The test was to demonstrate that spacecraft temperatures would remain within specifications during both extremes of the mission, and verify the accuracy of the thermal model used in its design.17 JPL staff conducted the STV test in two phases. The first phase, called the tour configuration, allowed the vehicle’s subsystems to approach the lower temperature limit that would occur when the spacecraft was orbiting Saturn and at its maximum distance from the Sun. The vacuum chamber pumpdown for this phase began on 17 January 1997. The spacecraft then spent two days at high temperatures to drive off as much volatile contamination from its surfaces as possible. After sensors indicated that the rate of contaminant boil-off had decreased to an acceptable level, engineers cooled the spacecraft and tested functionalities of its operating systems and some of its science instruments. This phase was carried out first so that if a low temperature problem were discovered, this would allow the most time to make corrections. Data from the tour phase indicated that no changes to the spacecraft’s thermal design were necessary. By the time the tour phase testing was finished, the craft had spent around ten days under vacuum.18 The second, cruise phase of the STV test subjected the spacecraft to a wide range of simulated solar intensities representative of what it would experience on its long journey

146

Integrating the Cassini Orbiter, Huygens Probe…

from the inner solar system out to Saturn. During this time, the vehicle was under vacuum for over four days.19 The spacecraft passed the STV test with no hardware failures observed. 5.4.4

Post-environmental system testing

The purpose of this round of system testing was to determine, prior to shipping to the launch site, if any subsystems had suffered degradation in environmental testing. Instruments were examined to determine whether the tests had knocked them out of alignment. No anomalies were found. The organizations that had developed several of the Orbiter’s instruments asked for them to be sent back for detailed analysis, refurbishment if necessary, and calibration prior to flight. These included the Ion and Neutral Mass Spectrometer (INMS), Magnetospheric Imaging Instrument (MIMI), Cosmic Dust Analyzer (CDA), Cassini Plasma Spectrometer (CAPS), Ultraviolet Imaging Spectrograph (UVIS), and Composite Infrared Spectrometer (CIRS). The mission team extensively tested the fault protection software meant to detect and mitigate errors and problems in spacecraft operations before they could impair the mission.20 For example, if a sensor detected a problem in the propulsion system, such as with a valve in one branch of the piping that fed its propellants, then the fault protection software could shift to a backup branch. Similarly, if sensors detected the overpressurization of a propellant tank then the software would invoke a mitigating response, sending predetermined commands to latch valves and pyrotechnic valves to halt the flow of gas into the tank.21 The testing showed that the fault protection software had been greatly improved since a previous test several months before, but still needed more work. The mission team scheduled the work to be carried out both at ITL and at the launch site. A fair amount of fault protection software development was also done after launch.22

5.5

RISK ISSUES IN SHIPPING THE SPACECRAFT TO KENNEDY SPACE CENTER

During the early years of the Cassini-Huygens project, NASA intended to transport the Orbiter and all of its parts to Kennedy Space Center (KSC) by truck and trailer, as it had for many other missions including Galileo. But by 1996, as the launch date approached, citizens’ groups in some parts of the country had become agitated about the plutoniumbearing RTGs that would be installed aboard the spacecraft. These concerns are examined in detail in Chapter 6. The words and plans of these protest groups seemed increasingly militant, and NASA grew concerned about the safety of its valuable spacecraft on a road trip between California and Florida that would last between three and five days. After considerable debate, NASA and JPL decided that the risks of trucking the Orbiter across country were excessive, and opted for the more expensive option of flying it to KSC. This was done in stages. In March 1997, after environmental and subsequent system testing, the propulsion module subsystem (PMS) and all of its support equipment, as well

5.5 Risk issues in shipping the spacecraft to Kennedy Space Center 147 as the flight spare high-gain/low-gain antenna assembly (HGA/LGA1), were transported by truck to Edwards Air Force Base in California’s Mojave Desert, about 90 miles from Los Angeles. This apparatus was loaded into an Air Force C-17 transport, which flew nonstop to KSC’s Shuttle Landing Facility. It was then transported by truck to the Spacecraft Assembly and Encapsulation Facility (SAEF-2) to undergo launch processing.23 5.5.1

Processing spacecraft components at KSC

At SAEF-2, mission staff conducted functional testing of the PMS. These trials included cycling each of the module’s valves, energizing its heaters, exercising the main engine parts, and examining telemetry equipment. After these trials, mission personnel installed pyrotechnic devices in the PMS, pressurized its helium tanks, and loaded its propellants. SAEF-2 was a quarter of a mile from the Payload Handling and Servicing Facility (PHSF) where the Probe and Orbiter would undergo further processing. This separation was important because of the hazardous nature of certain PMS processing tasks, such as propellant loading. Processing the PMS in SAEF-2 enabled increased safety during the parallel delivery, testing, and processing of other spacecraft subsystems.24 Near the end of May 1997, the mission team finished processing the PMS and transported it from SAEF-2 to the PHSF, where they connected it to the rest of the Orbiter and performed a functional test. It passed, but this trial was necessarily less exhaustive than an examination at JPL because the valves could not be operated after the PMS had been fueled.25 5.5.2

Arrival of the Huygens Probe

In March 1997, the Huygens Probe FM passed ESA’s Flight Acceptance Review and was prepared for transport on a U.S. Air Force C-17. After its arrival in early April 1997, KSC staff took the Probe to the PHSF for functional testing.26 There, the ESA team verified that it was still operating within specifications. By early May, ESA and NASA staff were ready to integrate the Probe with the Orbiter. 5.5.3

Arrival of the rest of the Cassini Orbiter

After confirming that it was still operating within its specifications, NASA flew the rest of the Orbiter to KSC. This shipment did not include the RTGs; they had been sent directly to the Cape by the supplier. Before dawn on Saturday, 19th April 1997, trucks arrived at JPL and the Orbiter was loaded aboard. JPL security personnel and California Highway Patrol escorted the caravan to Edwards Air Force Base. The apparatus was transferred to a secure hangar to await the arrival of the C-17. Due to crew rest requirements, the equipment could not be loaded aboard the aircraft until Sunday evening. The plane took off late in the evening for a non-stop flight to KSC, where it landed just after dawn on Monday. Upon arrival, the Orbiter was removed from the transport plane and taken to the PHSF for functional testing.27 These trials included baseline testing of the Orbiter’s engineering subsystems, and lasted until 5 May.

148 Integrating the Cassini Orbiter, Huygens Probe… 5.6

PREPARING THE SPACECRAFT FOR FLIGHT

Integration of Orbiter and Probe took NASA and ESA staff until October 1997. These operations began with electrically connecting the two vehicles, followed by some preliminary power-off measurements. After this, mission personnel turned Orbiter power on and sent commands through it to power up the Probe as well. The interface between the vehicles was checked out and end-to-end testing of the mated combination was conducted.28 5.6.1

Probe preparation

After successful completion of these trials, the Probe was disconnected from the Orbiter and wheeled away to another location in the PHSF, where it underwent preparations for flight that included further functional tests and the installation of:29 • • • • • •

Flight batteries Radioisotope heater units (RHU) Internal insulating material Parachute deployment device (PDD) Pyrotechnic devices External covers.

Some of the many other actions in preparation for launch included installing the highgain/low-gain antenna assembly on the Orbiter and performing various software-related testing such as: • • •

Testing of new software controlling such actions as launch sequence, engine functions, and instrument maintenance Testing of updated fault protection software Verifying the ability of mission staff to load software updates into spacecraft computers during flight.

In addition, proper RTG operation had to be verified, thermal blankets had to be attached to the two vehicles, pyrotechnic devices had to be tested, and the Probe had to be remated to the Orbiter. 5.6.2

RTG temporary installation and test

After retrieving the three plutonium-fueled RTG electrical power sources from the RTG Storage Facility, where they had been kept secure and separate from the rest of the spacecraft, mission staff installed them on the Orbiter using the same procedures as would be employed on the launch pad. Mission staff brought the RTGs online electrically and let them power the Orbiter for 12 hours while they tested the spacecraft’s functionality. Upon completion, they shorted out the RTGs one by one, thereby turning off the Orbiter, then mechanically demated the RTGs and returned them to storage. The RTGs would be reinstalled on the launch pad shortly before launch.30

5.7 The launch vehicle 149 5.6.3

Thermal blanket installation and pyro testing

Once the team safely stored the RTGs, they applied thermal blanketing to the Orbiter. Approximately 250 blankets of various sizes and shapes were installed. Testing to verify that the vehicle’s pyrotechnic control circuits delivered the correct amount of energy to each device to set it off (the actual pyros were not yet installed). This test ensured there were no unwanted high electrical resistance areas in the circuit that could interfere with pyro device functionality. Once the team successfully finished this test, they connected live pyrotechnic devices. Extreme care had to be taken to ensure there were no “stray voltages”31 present which might set off the explosive charge as soon as the team connected a device. 5.6.4

Probe re-installation and test

When the ESA personnel and contractors who had accompanied the Probe to KSC were satisfied that it was “fully ready for launch,”32 it was installed on the Orbiter. First the mechanical connections were made, then the electrical interfaces. The team powered up the Orbiter and put both it and the Probe through another series of tests. 5.6.5

Preparing for transport

The mission team mated the Cassini-Huygens spacecraft with its launch vehicle adapter (LVA) and placed the entire stack of flight components on the transporter that would carry it to Space Launch Complex 40, where NASA would mate it with the Titan 4B/Centaur.33

5.7

THE LAUNCH VEHICLE

A Lockheed Martin-built Titan 4B booster rocket with a Centaur upper stage was to provide the thrust to lift Cassini-Huygens from its Cape Canaveral Air Force Station launch pad and place it on an interplanetary trajectory bound for Saturn.34 The Titan 4B was a heavy-lift rocket consisting of a two-stage, liquid-propellant vehicle with two attached solid-fuel rocket motors. The Titan’s liquid propellant consisted of its fuel, which was a hydrazine/dimethylhydrazine mixture known as Aerozine 50, and an oxidizer in the form of nitrogen tetroxide, each of which was stored in structurally independent tanks designed to minimize the hazard of mixing in the event of a leak. The propellant chemicals were storable for extended periods at normal temperatures and pressures. This reduced delays in launch operations and made the vehicle better able to meet critical launch windows.35 The solid-fueled rockets were the first engines to fire during the Titan 4B launch. When they had nearly depleted their propellant, a little over two minutes into flight, the main stage of the liquid-propellant vehicle was ignited and the spent solid-fueled rockets were jettisoned. The second liquid stage fired as the previous stage depleted its fuel and separated, and the Centaur upper stage took over when the second stage had done its job.36 The Titan 4B/Centaur heavy-lift expendable launch vehicle was tall as a 20-story building and,

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including its liquid-fueled engines and its solid-fueled booster engines, weighed about 1 million kilograms (2.2 million pounds). The propellant accounted for 90% of the total launch vehicle weight.37 As a result of the Solid Rocket Motor Upgrade (SRMU) program, the Titan 4B delivered an increase in payload mass of 25%, as compared to the Titan 4A, and this was essential for the Cassini-Huygens mission. The SRMU reduced the cost per pound of payload. The new solid rockets also provided improved safety, reliability, and launch site operability. The SRMU featured a three-segment design that was an important safety feature. During the 1986 Challenger Space Shuttle disaster, jets of burning fuel were able to escape around the O-ring within a solid-rocket motor field joint, the place where segments of the motor were joined together. Such an accident was much less likely to occur on a Titan 4B versus a Titan 4A, because the number of critical field joints was reduced from eight to only two.38 5.7.1

The Centaur stage

The Centaur upper stage has been called “America’s Workhorse in Space”39 for its long years of reliable service. Developing it was one of NASA’s Lewis (now Glenn) Research Center’s most important achievements. The Centaur was vital to Cassini-Huygens, providing sufficient thrust to send the large, heavy spacecraft on its way to Saturn. The U.S. developed the Centaur in the early years of the Space Age. The vehicle was key in satisfying NASA’s quest to “tame liquid hydrogen,”40 or in other words, determine how to use the volatile and potentially dangerous material as a dependable rocket fuel. The high energy density of a liquid-hydrogen/liquid-oxygen propellant enabled NASA to send massive payloads into space.41 The vision for Centaur was conceived back in 1957, almost one year before Congress created NASA. The Air Force was studying a proposal from General Dynamics/Astronautics Corporation to develop a rocket that would enable the U.S. to orbit heavy payloads. The vehicle that emerged was Centaur. The U.S. sorely needed this “new muscle in space,”42 and the Lewis engineers rose to the challenge, initiating a complex and effective research and development program that assured the new rocket’s reliability. Important to the program were the special facilities for ground testing at Lewis’ Plum Brook Station in Sandusky, Ohio. Before the end of 1963, NASA accomplished its first successful launch of an Atlas/Centaur rocket combination, scoring a significant milestone with the first in-flight start of a liquid-hydrogen/liquid-oxygen engine. The Centaur engine had some interesting characteristics. In order to prevent its liquid hydrogen and oxygen from boiling, the tank that contained them had to be of special construction to maintain the fuel under pressure and very cold. The liquid oxygen was kept at −297°F (−147°C), but this was too hot for the liquid hydrogen in the other compartment of the tank, which had to be stored at no warmer than −423°F (−217°C). To prevent the liquid hydrogen from boiling off, General Dynamics (the contractor that assembled the Centaur) designed a special double-walled bulkhead as a heat barrier between the compartments. Choosing materials for the tank presented additional challenges, because many metals grew brittle when exposed to the frigid temperature of liquid hydrogen. The fuel tank also had to be carefully insulated from all external sources of heat such as rocket engine exhaust, air friction from the flight through Earth’s atmosphere during launch, and radiant energy from the Sun once in space. Nevertheless, the liquid hydrogen would absorb

5.8

Final launch preparation 151

some heat and expand, so a venting system was needed to prevent the tank from exploding. Finally, hydrogen is extremely difficult to contain. Its tiny molecules are able to leak out through minute cracks and pores in the tank’s seams. Containment was thus another challenge that had to be met.43 The fuel tank had surprisingly light stainless steel walls that were thinner than a dime. This was possible because the pressure of the fuel it carried helped keep the sides of the tank rigid. The minimum of heavy structural elements in the tank added to the energy per unit mass available from the Centaur rocket.

5.8

FINAL LAUNCH PREPARATION

The Lewis Research Center (whose name was changed in 1999 to honor astronaut John H. Glenn) of Cleveland, Ohio was responsible for managing the integration of the CassiniHuygens spacecraft with its Titan/Centaur launch vehicle. Lewis’ duties included overseeing analytical as well as operational aspects of integration. Working closely with Lewis on these tasks was the Air Force Space Division Titan 4/Centaur Program Office.44 Although this was a NASA science mission, the Titan rocket had been developed by the Air Force for military payloads. Lewis oversaw activities that included ignition of the Titan, the firing of its engine stages, and Centaur upper stage firing operations. Lewis was a logical choice for launch-related management. Since 1962, it had been responsible for well over 100 NASA launches, including the deep space Ranger, Mariner, Surveyor, and Voyager missions. 5.8.1

Transport of spacecraft to launching pad and mating with launch vehicle

To protect the Cassini-Huygens spacecraft on its short journey to the launch pad, the mission team first covered it with a newly constructed “inner bag”45 secured at the bottom (Figure 5.4), followed by a tubular aluminum transport cover with an “outer bag” already installed (Figure 5.5). The spacecraft was then towed from the Payload Hazardous Spacecraft Facility at KSC to Launch Complex 40 at the adjoining Cape Canaveral Air Force Station for mating with its launch vehicle.46 On the evening of 28 August 1997, the tow vehicle, security police, and other vehicles arrived for Cassini-Huygens’ trip to Launch Complex 40. The convoy left for the launch pad at 12:10 a.m. on 29 August, reaching its destination by 1:00 a.m. The outer cover and support frame was removed and the spacecraft was made ready for lifting. Mating the spacecraft to the Centaur was accomplished by 8:15 a.m. The scheduled date of launch was 6 October, 1997.47 5.8.2

Discovery of an air conditioning problem

The 6 October launch date was not met due to an issue with the Huygens Probe. It contained 35 radioisotope heater units (RHU) to prevent its vital subsystems from freezing in space. Prior to launch and to delivery into space, the thermal energy from these heaters was sufficient to raise the temperatures of the Probe’s five batteries above their specifications. ESA required JPL to conduct the launch preparations in such a manner that the

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Figure 5.4 Spacecraft with protective “inner bag” installed, standing on transporter.

Figure 5.5 Installation of transport cover, which included a tubular aluminum frame and outer bag.

5.8

Final launch preparation 153

amount of time the Probe spent without cooling air did not exceed six hours during any one period. Shortly after the spacecraft arrived at the launch pad, the mission team connected an air conditioning source to the Probe and adjusted the flow rate. This air conditioning was supposed to operate while lifting the spacecraft and mating it with the Centaur. But Probe engineers found indications that the air flow rate might be too high and, using an inspection device called a bore scope, peered inside the Probe. They found that insulation had been damaged. After further analysis, it was decided to demate the spacecraft from the Centaur and repair the Probe. Umbilical cables were disconnected and the inner protective bag rolled down to cover the spacecraft. Bolts connecting JPL’s launch vehicle adapter to the forward adapter of the Centaur were removed, and the spacecraft was hoisted clear. On 7 September, with its protective covering in place, the spacecraft was transferred back to the PHSF. The Probe was removed from the Orbiter and a round-the-clock cycle of work began that lasted 4-1/2 days. Only several square inches of insulation turned out to be damaged; it was removed and replaced. An ensuing investigation decided that inadequate system engineering and unclear documentation about how the Probe should be cooled were likely responsible for this incident.48 5.8.3

Back to the launch pad

Approximately 111 hours after the Probe was removed from the Orbiter for repairs, mission personnel were able to reinstall it. Thermal blankets were put back into their proper locations around the Probe mounting points. The spacecraft was again placed on its transporter and the inner cover installed and secured, followed by the outer frame and cover. At 8:25 p.m. on 15 September, a convoy left the PHSF for another trip to the launch pad. Once again, after positioning the transporter on the pad deck, the outer frame and cover were removed and set aside. The lift fixture was attached and the spacecraft hoisted up. By 1:30 a.m. on 16 September, mission personnel had lowered the spacecraft onto the Centaur. This time they had difficulty installing the mating bolts. Lockheed Martin engineers began working the problem in conjunction with colleagues at the company’s Denver facility. Application of dry lubricant to the threads of the bolts allowed them to be properly installed. After another battery of verification tests and checkouts, Lockheed Martin staff installed the payload fairing, which served as the nose cone of the launch vehicle, with JPL personnel in a support role. Further testing followed, and rehearsals of the launch procedure. On 10 October mission staff were ready to reinstall the RTGs. It took over 11 hours for the engineers to transport and connect the RTGs, after which they were turned on and began supplying power to the Orbiter. The conditioning for the launch sequence began. Memories were loaded, heaters and the other subsystems required for launch were activated and adjusted. By that evening, the spacecraft had been conditioned for a 13 October 1997 launch date.49 The next chapter, which focuses on the use of a plutonium power system aboard the spacecraft, will discuss attempts by anti-nuclear activists to prevent the launch, and what ultimately happened.

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REFERENCES 1. NASA-JPL, “Launch Phase,” http://www2.jpl.nasa.gov/basics/bsf14-1.php, Chap. 14 in Basics of Space Flight, Section III (2011) stv; Bob Mitchell review of manuscript, Feb. 2011. 2. Warren Moore, Cassini Spacecraft Assembly, Test and Launch Operations (ATLO) Final Report I, https://cassini.jpl.nasa.gov/cel/cedr/nlf/inv/pdl/spacecraft/699-209_VolI/html/document. html (NASA-JPL 699–209, JPL D-15701, 15 May 1998), p. 1, NHRC 19543, Cassini/ Huygens – Sources Used in Writing Cassini/Huygens Book by Michael Meltzer; NASA, “The International Team,” http://www.nasa.gov/mission_pages/cassini/team/index.html, last updated 22 Nov. 2007, accessed 1 Jan. 2009. 3. Q-Metrics Engineering Analysis And Design, “Radioisotope Thermoelectric Generator (RTG),” http://www.qmetrics.com/radioisotope_thermoelectric_generato.htm (2002), accessed 19 July 2011. 4. A cable harness, also called a wire harness, is a bundle of wires and cables, each cut to a specified length with specific connectors on either end. A cable harness greatly simplifies assembly, and replaces the need to individually wire each pair of connections. Such harnesses are commonly used in automobiles, machinery, and aircraft, as well as spacecraft. 5. Moore, Cassini Spacecraft. 6. Edberg, “Spacecraft Assembly”; D. Cervantes, L. Montafiez, L. Tatge, “Low Cost Test Bed Tool Development for Validation of Mission Critical Events,” 2004 IEEE Aerospace Conference Proceedings, vol. 1, March 2004. 7. NASA-JPL, “Startup, Design and Assembly,” http://cassini-huygens.jpl.nasa.gov/cassini/english/ops/devopsyellow.shtml, Cassini-Huygens Web site, accessed 24 Nov. 2008; ESA, “Huygens Probe on Target,” http://esamultimedia.esa.int/esaCP/Pr_17_1995_i_EN.html, reference no. 17–1995, 31 July 1995; Warren Moore, Cassini Spacecraft Assembly, Test and Launch Operations (ATLO) Final Report, Volume I,https://cassini.jpl.nasa.gov/cel/cedr/nlf/inv/pdl/ spacecraft/699-209_VolI/html/chap2.html#SEC2.1.9, 699–209, JPL D-15701 (15 May 1998), section 2.1.9. 8. NASA-JPL, “Startup, Design and Assembly,” http://cassini-huygens.jpl.nasa.gov/cassini/english/ ops/devopsyellow.shtml, Cassini-Huygens Web site, accessed 24 Nov. 2008. 9. Moore, Cassini Spacecraft. 10. Moore, Cassini Spacecraft; Alan R. Hoffman and John C. Forgrave, “Cassini Environmental Test and Analysis Program Summary,” http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/ 20175/1/98-1052.pdf, Institute of Environmental Sciences and Technology, 18th Aerospace Testing Seminar Proceedings (USA) (Mar. 1999):99–123. 11. Thomas F. Bergen, Harry Himelblau, and Dennis L. Kern, “Development of Acoustic Test Criteria for the Cassini Spacecraft,” Journal of the IEST (Jan.-Feb. 1998):26–38; William O. Hughes and Anne M. McNelis, Acoustic Testing of the Cassini Spacecraft and Titan IV Payload Fairing, Part 2 – Results, NASA Technical Memorandum 107475, prepared for 67th Shock and Vibration Conference (Monterey CA, 18–22 Nov. 1996). 12. Alan R. Hoffman and John C. Forgrave, “Cassini Environmental Test and Analysis Program Summary,” Beacon eSpace – JPL Technical Report Server 1992+, file 99–0116, URI http://hdl. handle.net/2014/16721, preprint of paper presented at 18th Aerospace Testing Seminar (Manhattan Beach CA, 16–18 March 1999):12. 13. Moore, Cassini Spacecraft Assembly. 14. Terry D. Scharton and Kurng Y. Chang, “Force Limited Vibration Testing of the Cassini Spacecraft and Instruments,” trs new.jpl.nasa.gov/dspace/bitstream/2014/22355/1/97-0836.pdf, JPL 97-0836[1], issued 25 June 1997, submitted to 17th Aerospace Testing Seminar, Institute of Environmental Sciences, Manhattan Beach, CA, 14–16 Oct. 1997, p. 11, accessed 2 Jan. 2009.

References 155 15. Hoffman and Forgrave. 16. NASA-JPL, “Cassini Program Status Report,” http://saturn.jpl.nasa.gov/news/press-release-details. cfm?newsID=110, NEWS-News Releases-1996, 1 Aug. 1996. 17. Hoffman and Forgrave, pp. 11, 13–14. 18. Moore, Cassini Spacecraft Assembly. 19. Moore, Cassini Spacecraft Assembly. 20. NASA, “Glossary of Astronomical Terms,” http://www.ocregister.com/articles/span-styleweight-1931952-bold-font, 29 Nov. 2007, accessed 3 Jan. 2009. 21. Elwin Ong and Nancy Leveson, “Fault Protection in a Component-Based Spacecraft Architecture,” http://sunnyday.mit.edu/papers/smcit.doc, Proceedings of the International Conference on Space Mission Challenges for Information Technology (Pasadena: July 2003): accessed 3 Jan. 2009. 22. Bob Mitchell review of manuscript, Feb. 2011. 23. Moore, Cassini Spacecraft. 24. Moore, Cassini Spacecraft, pp. 58–59. 25. Moore, Cassini Spacecraft, p. 64. 26. ESA, “Huygens Space Probe Ready to Leave Europe,” http://www.esa.int/esaCP/Pr_8_1997_p_ EN.html, press release no. 8–1997, 3 March 1997; NASA-JPL, “Spacecraft Arrives at Kennedy Space Center,” NEWS - Press Releases - 1997, 21 April 1997; Moore, Cassini Spacecraft. 27. Mary Beth Murrill, http://www.jpl.nasa.gov/news/releases/97/casarriv.html, JPL Public Relations Office, release no. 9738, 21 Apr. 1997; Moore, Cassini Spacecraft. 28. Moore, Cassini Spacecraft, pp. 63–64. 29. Moore, Cassini Spacecraft, p. 64. 30. Moore, Cassini Spacecraft, pp. 66–67. 31. Moore, Cassini Spacecraft, p. 67. 32. Moore, Cassini Spacecraft, p. 67. 33. Moore, Cassini Spacecraft. 34. Julie Andrews, “Lockheed Martin-Built Titan IV Rocket Launches from Cape Canaveral,” http://www.lockheedmartin.com/news/press_releases/2003/LockheedMartinBuilt TitanIVRocketLau.html, 35. GlobalSecurity.org, “Titan IV,” http://www.globalsecurity.org/space/systems/t4.htm, accessed 10 Dec. 2008. 36. GlobalSecurity.org, “Titan IVB,” http://www.globalsecurity.org/space/systems/t4b.htm, accessed 10 Dec. 2008. 37. NASA-JPL, “Cassini Mission Overview,” http://saturn.jpl.nasa.gov/multimedia/products/pdfs/ cassini_msn_overview.pdf, Cassini Equinox Mission Web site, accessed 31 May 2010; Glenn Research Center, “A Big Boost for Cassini,” http://www.nasa.gov/centers/glenn/about/fs04grc. html, ref. no. FS-1999-06-004-GRC, accessed 9 Dec. 2008; JPL, “Cassini Mission Overview,” http://saturn.jpl.nasa.gov/multimedia/products/pdfs/cassini_msn_overview.pdf, Cassini Equinox Mission Web site, accessed 27 May 2010. 38. GlobalSecurity.org, “Titan IVB,” http://www.globalsecurity.org/space/systems/t4b.htm, accessed 10 Dec. 2008. 39. NASA, “Centaur: America’s Workhorse in Space,” http://www.nasa.gov/centers/glenn/about/ history/centaur.html, last updated 2 May 2008. 40. Virginia P. Dawson and Mark D. Bowles, Taming Liquid Hydrogen: The Centaur Upper Stage Rocket, 1958–2002 (Washington D.C.: NASA SP-2004-4230), p. iii. 41. Ibid., p. 105. 42. NASA, “Centaur: America’s Workhorse in Space.”

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43. NASA, “Centaur: America’s Workhorse in Space”; Dawson and Bowles, Taming Liquid Hydrogen, p. v. 44. NASA, Memorandum of Agreement Among the Lewis Research Center and The Jet Propulsion Laboratory and the John F. Kennedy Space Center Concerning Planetary Missions on a Titan IV/Centaur Launch Vehicle (draft), 21 Feb. 1989, in a package of documents beginning with Ronald F. Draper letter to Senior Staff, 8 Mar. 1989, JPL-Cassini CASTL. 45. Moore, Cassini Spacecraft, pp. 67–68. 46. Hoffman and Forgrave, p. 20. 47. Moore, Cassini Spacecraft. 48. Doug Isbell, Don Savage, George H. Diller, and Mary Beth Murrill, “Repair Work on Cassini Huygens Probe Completed Successfully,” http://www.nasa.gov/home/hqnews/1997/97-198.txt, NASA news release 97–198, 12 Sep. 1997; Moore, Cassini Spacecraft, secs. 4.2.6.3 to 4.2.8.1; B.F. Boyd. J.W. Weems, and W.P. Roeder, “Weather Support for the Cassini Mission to Saturn,” http://ams.confex.com/ams/pdfpapers/76617.pdf, American Meteorological Society, accessed 10 Dec. 2008; Robert T. Mitchell email to author, 11 Oct. 2011; 49. Moore, Cassini Spacecraft, sec. 4.2.6.

6 Using plutonium to run a spacecraft NASA and its space programs have usually maintained excellent relations with the public, both in the U.S. and abroad. But one practice on Cassini-Huygens, Galileo, and other missions that triggered vehement controversy was the use of plutonium to generate shipboard electricity. This chapter discusses the reasons for the choice of plutonium as a power source on certain NASA missions, the safety features built into its use, and the mistrust and strong opposition that arose to such a potentially dangerous material. # A spacecraft needs electric power in order to operate, sometimes for many years, and there are few options available. Space vehicles flying within the inner solar system, not more distant from the Sun than Mars, can rely on an array of solar panels that convert sunlight into electricity. But as will be explained below, at the distance of Saturn, solar power use during the years that Cassini-Huygens was being developed was very problematic for technical as well as economic reasons. Thus for faraway missions, NASA turned instead to an extremely reliable, safe and technically more feasible source of electric power: radioisotope thermoelectric generators (RTG) that were fueled mainly with plutonium dioxide.

6.1

WHY NASA USES RADIOISOTOPE THERMOELECTRIC GENERATORS (RTG) FOR SHIPBOARD POWER

RTGs have provided electrical power for some of the U.S.’s most well-known and successful space expeditions, including the Apollo lunar surface experiments, the Viking Mars landers, and the Pioneer, Voyager, and Galileo journeys to the outer solar system. It is important to understand that an RTG is not a nuclear reactor. It uses neither fission nor fusion processes to produce energy. RTGs generate their power through the heat that is liberated by the natural radioactive decay of plutonium. Solid-state thermoelectric converters turn this heat into electricity. What makes radioactive materials so suitable for supplying the power needs of a space vessel is that they constantly emit thermal energy, regardless of the direction the craft is © Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_6

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pointing, and they remain reliable even in hostile environments, such as inside a planet’s radiation belts. Plutonium dioxide, the fuel in Cassini-Huygens’ RTGs, consists predominantly of the non-weapons-grade isotope plutonium 238, which decays at – and thus supplies thermal energy at – a precisely known and predictable rate. NASA can be confident that the power supplied by plutonium dioxide will not suddenly cease, an occurrence that would end a mission. Nor do RTGs have moving parts that can jam and interfere with the generation of power. As Cassini-Huygens Program Manager Bob Mitchell observes, “As a power source, [an RTG is] awfully hard to beat. It puts out a nearly constant power … just super reliability.”1 An RTG is an inherently simple device consisting of two basic components: (1) a thermal energy source made up of the fuel and its containment material; and (2) the thermoelectric generator that transforms heat into electricity. This transformation is not new science. It occurs by means of the thermoelectric effect, discovered in 1821. In that year, Russian-German scientist Thomas Seebeck noted that if two dissimilar metals were joined at two locations maintained at different temperatures, an electric current would flow. Devices that generate currents in this manner and that consist of two junctions of two different metals are known as thermocouples. Other substances also serve as thermocouples. For example, certain semiconductors such as silicon-germanium doped with trace impurities such as boron or phosphorus can be used in the place of metals, and can actually be more efficient power converters.2 In an RTG, the decay of a radioisotope fuel emits thermal energy that is used to heat one of the thermocouple’s junctions. The other junction is kept cold simply by radiating its thermal energy into outer space. In this way, a temperature difference of typically 400°C (800°F) is maintained and electricity is generated without employing moving parts that might wear and fail.3

6.2

WHY DIDN’T NASA USE SOLAR POWER ON CASSINI-HUYGENS?

Many critics of the Cassini-Huygens mission design asked why the program didn’t draw on solar energy for its electrical needs, instead of toxic plutonium 238? In fact, NASA supports solar energy use on missions orbiting Earth or operating within the inner system. But journeying to the outer planets made a solar energy solution quite difficult for the Cassini-Huygens mission. The reasons that NASA did not use solar arrays for outer planet missions such as Cassini-Huygens or Galileo included the following: • • • • •

Minimal solar energy per unit area available at great distances from the Sun Prohibitive costs of building a large enough solar collector array Insufficient lift capacity to launch a spacecraft with such a large solar collector array Questionable reliability of a huge array requiring extremely complex deployment operations The requirement to always face a solar collector array toward the Sun would overconstrain mission operations (although it has been done on other missions).

Saturn is 1.4 billion kilometers (890 million miles) away from the Sun, nearly ten times farther than Earth. And the Sun’s intensity declines in proportion to the square of the

6.3

Radioisotope heater units (RHU) 159

distance that its light has to travel. What this means is that the solar intensity at Saturn is only 1/102, or one hundredth (1%) what it is at Earth. Thus an array of solar panels would have to be 100 times larger at Saturn than at Earth to collect the same amount of energy.4 To generate the 600-700 watts of power needed by the Cassini-Huygens spacecraft, a solar panel array over 200 feet long and 30 feet wide would have been required, and it would have had to be deployable. In other words, NASA would have had to build it so that it could be rolled or folded up into a small package and stowed along with the spacecraft inside the fairing, then unrolled once the vessel was on its way to Saturn. This would have so many parts that the risk of mechanical or electronic failure would be very significant. Even if NASA had been able to build such a structure that could reliably deploy and operate for the required 11 or more years, the panels, support structure, batteries, electronics, electric power distribution equipment, and mechanisms to unfurl the huge array would have been heavy, complex, and very challenging to launch along with the spacecraft.5 Fulfilment of the Cassini-Huygens mission’s science objectives required a large spacecraft carrying 18 different science instruments and enough propellant to travel to Saturn, insert itself in orbit around the planet, and then operate for years, during which time the vehicle would repeatedly use its propellant reserves to visit many moons. The mass of propellant required to do this – approximately 6,000 pounds – meant that the remainder of the spacecraft had to be as light as possible. Mission engineers therefore decided against trying to design a craft that could carry all the equipment for an enormous storable solar array. NASA calculated that even using the latest, most efficient solar panel technologies developed by the European Space Agency (ESA), solar energy would be a problematic electric power source. Even if a suitably large solar array could have been constructed in orbit around Earth and then connected to the spacecraft just after it launched, there was another difficulty. NASA would have had to ensure that the unwieldy array was maintained facing the Sun at all times in order to generate power. This alone might have been a “showstopping constraint”6 for the Cassini-Huygens space vessel. Because it had no scan platform and instead had its science instruments bolted onto its body, the entire spacecraft needed to be turned in order to point different instruments at their targets. Doing this while at the same time keeping a massive solar array facing the Sun, and ensuring that it did not block some of the instruments from making their observations, would have severely overconstrained the mission. By using plutonium-fueled RTGs, on the other hand, mission control was able to “turn the spacecraft any way we want. … No matter where you turn it, you have power.”7 Nevertheless, it is important to state that it was not impossible to develop a solar power system for the mission, even using the solar panel technology available in the early 1990s. The Juno mission to Jupiter, which launched in 2011 and had access to more efficient solar technology, did use a solar panel system for onboard power.8

6.3

RADIOISOTOPE HEATER UNITS (RHU): A SECOND PLUTONIUM APPLICATION

Plutonium had a second function aboard the spacecraft: to keep instruments warm. Although missions in the inner solar system employ solar energy for heating, this is difficult on journeys to the outer solar system, where the Sun’s intensity is low due to the great distances. By using radioisotope heater units (RHU), a spacecraft does not have to draw

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Figure 6.1 Radioisotope Heater Unit, with its eraser-sized fuel pellet.

energy for heating its equipment from limited onboard electrical power sources intended to run engineering systems and scientific instruments. RHUs have the additional benefit of avoiding the electromagnetic interference generated by electrical heaters.9 Like RTGs, RHUs draw heat from the natural radioactive decay of plutonium dioxide. Each RHU uses a rather small amount of plutonium: a pellet the size and shape of a pencil eraser, weighing just 3 grams (0.1 ounce). In fact, the entire RHU aboard Cassini-Huygens was 3.2 centimeters (1.3 inches) long and 2.6 centimeters (1 inch) in diameter, weighing 40 grams (1.4 ounces). Spacecraft designers installed 117 RHUs10 in various locations inside the vehicle, each of which issued about one watt of heat that was directly transferred to the structures, systems and instruments without moving parts or intervening electronic components.11 As it did with the RTGs, the U.S. Department of Energy designed the RHUs with a robust, multi-layer containment system to prevent or minimize the release of fuel – even under severe accident conditions. An external graphite aeroshell and graphite insulator protected against impacts, fires, and atmospheric re-entry. Internally, the fuel was further protected by being encapsulated in high-strength, platinum-rhodium metal cladding (Figure 6.1). In addition to this containment, the plutonium dioxide was manufactured in a ceramic form meant to break into large pieces upon impact, rather than dispersing as fine particles, minimizing the chances of human inhalation or interaction with the environment in the unlikely event of the multiple containment barriers being breached. And since each pellet was individually encapsulated in its own aeroshell and fuel cladding, the risk of a single event affecting more than one pellet was greatly reduced.12

6.4

THE POLITICS OF OBTAINING PLUTONIUM 238

An issue with the use of plutonium 238-powered RTGs and RHUs on spacecraft is the dwindling supply of the fuel. This shrinking fuel supply is especially impactful for missions to the outer solar system. Solar radiation, an alternative energy source in the inner solar system, is problematic for powering space vessels journeying so far from the Sun.

6.4 The politics of obtaining plutonium 238 161 At the time Cassini-Huygens was being developed, U.S. stocks of plutonium 238 were running low and the Department of Energy (DOE) had ceased manufacturing it, in part owing to safety and environmental issues associated with the aging nuclear reactors configured to manufacture plutonium 238.13 Neither plutonium 238 nor the isotope that is used to create bombs, plutonium 239, occurs in nature. Both require nuclear reactors for their manufacture. DOE’s Savannah River Site (SRS) in South Carolina produced plutonium 238 until 1984, then it shut down for much-needed repairs. When the Cassini-Huygens space vessel was being developed in the early 1990s, the U.S. stockpile had only 130 pounds of this isotope. NASA clearly needed more if it was to continue using the isotope on future missions.14 Cassini-Huygens’ RTGs alone required 72 pounds of plutonium dioxide fuel, 71% of which was plutonium 238.15 The 117 RHUs, each of which used 0.1 ounce of plutonium 238, slightly increased this total. According to DOE documents, the U.S. required approximately 300 pounds of plutonium 238 to meet its needs through 1999. RTGs and RHUs could be developed that used other radioactive materials,16 but such a fuel must satisfy several criteria: • • • •

Half-lives need to be short enough for the fuel to decay quickly and generate usable amounts of heat, and a half-life of several decades is best for an RTG fuel. Half-lives have to be long enough to produce energy at fairly constant rates for the length of the mission. For use on spacecraft, the ratio of energy generated per fuel density has to be high, since spacecraft are very constrained by the size and weight of the materials they carry. Radiation has to be of high energy, but have low penetration, in order not to damage onboard equipment. Alpha radiation, whose particles consist of two protons and two neutrons, are best because more penetrating beta and gamma radiation would generally require heavy shielding aboard the vessel.

Of the feasible candidates, plutonium 238 has the lowest shielding requirements (thereby freeing more of the spacecraft mass for scientific instruments), the longest halflife (87.7 years), and a high power density (0.57 watts/gram) that is adequate for spacecraft needs.17 While other fuels such as curium 244 and strontium 90 could be employed in an RTG or RHU, they would require a major technology development effort. So in 1989, in order to replenish its dwindling stocks of plutonium 238, DOE asked Russia, its arch rival in space endeavors since before NASA was even created, to sell it plutonium 238. The two countries met in Moscow in May 1990 at Russia’s Institute of Atomic Power and Industry. To kick off a possible trade relationship, the U.S. asked for a sample of what Russia could produce. Russia agreed, delivering 20 grams of high grade plutonium 238, valued at $24,000, to DOE’s Los Alamos lab in New Mexico in 1991. This facility analyzed the sample and confirmed it was indeed of high quality. Russia reported that it could produce as much as 44 pounds of the material every year. U.S. negotiations for buying 11 pounds of plutonium 238 were almost completed in 1991 when the Bush Administration raised serious objections to helping Russia – our once and possibly future enemy – become a vendor of such a vital material. Not only is plutonium 238 needed in our space program, especially for missions far from the Sun, it also has applications in certain surveillance and espionage technologies. For instance,

162 Using plutonium to run a spacecraft intelligence agencies have employed nuclear-powered batteries in spy technologies. In the 1960s, nuclear batteries ran electronic listening devices in the Himalayas directed at China, and the U.S. Navy may currently be employing them for undersea eavesdropping. Organizations studying the issue of purchasing Russian plutonium 238 included the National Security Agency, the Central Intelligence Agency, and the National Security Council at the White House. In addition, the Departments of Defense and State both warned that “American purchases of Russian high-technology goods could strengthen the military muscle of a former adversary.”18 John P. Boright, then Deputy Assistant Secretary of State for Scientific Affairs, expressed Bush’s fears at a 21 February 1992 congressional hearing when he pointed out that the U.S. had to be “cautious not to inadvertently support organizations and capabilities that could represent a future threat.”19 The plutonium sale was one of several deals that were blocked, at least temporarily, by the Administration in an attempt to prevent Russia’s military-industrial complex from becoming too powerful and, in this case, possibly starving the U.S. of revenues. Other obstructed sales involved rocket engines and space reactors. The Administration’s view was not universally accepted. For instance, Ned T. Rasor, an industry consultant involved in the U.S.’s plutonium negotiations with Russia, was of the opinion that the deal would be fiscally advantageous to the U.S. and lobbied to enlist the support of Senator Sam Nunn, chair of the Senate Armed Services Committee. Supporters of trading with Russia identified other important advantages. In particular, a general easing of import restrictions on Russian goods would not only aid in stabilizing that country’s economy, it might also help prevent a relapse of totalitarianism and provide incentives for Russian weapons scientists and engineers to stay at home instead of leaving for lucrative jobs in the Third World.20 Despite the reluctance of the Bush Administration and a number of powerful government organizations to purchase plutonium 238 from Russia, NASA’s need for it to power outer solar system missions did not go away. The U.S. did eventually buy some of the material from its former enemy. This was a temporary fix, however, and plans were made to restart U.S. production of plutonium 238. The issues involved in funding this effort are discussed later in the chapter.21

6.5

DANGERS AND SAFETY FEATURES OF RTGS

RTGs generate electricity from the heat produced by the radioactive decay of their plutonium fuel. As mentioned earlier, it is important to understand that RTGs use a different process of heat generation than employed by nuclear power plants. Those facilities rely on chain reactions in which nuclear fission (breaking apart) of atoms releases neutrons, which hit other atoms and cause them to undergo fission. This leads to extremely rapid interactions of large numbers of atoms, generating massive quantities of heat. Chain reactions do not take place inside RTGs, and thus an uncontrolled nuclear reaction (which would be extremely dangerous) cannot happen. Since nuclear fission does not occur inside an RTG, the device cannot turn into a fission bomb. Instead, the plutonium fuel slowly decays. The fuel in an RTG is thus consumed much more gradually, producing far less power than in a nuclear reactor. And this is just what a spacecraft needs: a lowlevel, constant, reliable source of power.

6.5

Dangers and safety features of RTGs 163

Nevertheless, RTGs are a potential source of radioactive contamination. If their fuel containers leak, the radioactive material can contaminate the environment. To minimize this risk, the fuel in the Cassini-Huygens RTGs was distributed between individual modular units, each of which had its own robust shielding.22 In fact, the fuel for a single RTG was distributed between 18 separate modules, each 2 × 4 × 4 inches and containing four cylindrical plutonium dioxide fuel pellets of 1 × 1 inch (2.5 × 2.5 centimeters) in size,23 manufactured as a heat-resistant ceramic to reduce the danger of it vaporizing and releasing radioactive material in a fire or during re-entry. The ceramic form of the fuel was also highly insoluble, had a low chemical reactivity, and typically broke into large chunks and non-respirable particles. This was important, as plutonium 238 is extremely dangerous to human health if inhaled into the lungs.24 Each fuel pellet was encased in a shell of iridium, chosen because it had a very high melting point and was strong, corrosion-resistant, and chemically compatible with plutonium dioxide. The shells were fitted into impact protectors of graphite,25 selected for its low weight and high heat resistance. Thus, each fuel module had its own heat and impact-resistant enclosure. And the modules were designed to survive during a range of postulated accidents including launch vehicle explosion or fire, re-entry into the atmosphere followed by either land or water impact, and post-impact situations. The RTGs were designed to release their 18 modules individually in the event of an inadvertent re-entry. The graphite outer covering of a module provided protection against the structural, thermal, and ablative environments of a potential re-entry and impact. The iridium cladding was for post-impact containment.26 This design greatly diminished the likelihood of fuel release in an accident. The general purpose heat sources (GPHS) containing the plutonium dioxide fuel formed the core of the RTG, which also included layers of insulation and an aluminum outer shell (Figure 6.2). Each 55 kilogram (121 pound) RTG contained about 11 kilograms (24 pounds) of plutonium dioxide fuel, and there were three such RTGs on the Cassini-Huygens spacecraft. In testing RTGs for the Galileo Jupiter mission, which used the same design for the devices as was used on Cassini-Huygens, JPL conducted shock analyses which subjected

Figure 6.2 Cutaway view of Cassini-Huygens RTG. The General Purpose Heat Source (GPHS) is the part of the RTG that contains the plutonium dioxide

164 Using plutonium to run a spacecraft the containment structure of the RTGs to pressures of 2,000 pounds per square inch (psi). These were pressures three times higher than those to which the RTGs might be subjected during an explosion on the launch pad. The pressure tests caused no failure in containment. This boosted NASA’s confidence that the devices were safe to use on a mission.27 In the wake of the 1986 Challenger tragedy, NASA scrutinized its Space Shuttle in painstaking detail before allowed it to fly again. During this time, more analyses were conducted of RTG safety. Representative Edward Markey (D-Massachusetts) personally requested such a study, asking DOE to estimate the impacts of a launch accident involving plutonium releases. Analysts calculated that such an occurrence might result in over 200 cancer deaths and contaminate almost 370 square miles of land. But the cancer estimate was based on the unlikely case of people remaining in the contaminated launch area for one year, without carrying out decontamination or other protective actions.28 Unlike radioactive materials that issue intense gamma radiation that is capable of penetrating our bodies and causing severe damage, plutonium 238 is predominantly an emitter of alpha radiation, whose particles, as mentioned above, consist of two protons and two neutrons – they are the nuclei of helium atoms. These particles are generally stopped by our skin. But if even tiny particles of plutonium 238 enter our bodies through such routes as inhalation into our lungs or ingestion of contaminated food or water they can do intense, irreparable damage and cause a variety of cancers. The danger of a plutonium 238 release by a crash of the Cassini-Huygens craft was that small fragments of RTG fuel would enter Earth’s environment and, eventually, find their way into living organisms.29 Thus, if RTGs are to be relatively safe, they must be designed with a very minimal probability of plutonium release, even in the event of a spacecraft accident. Several such accidents have occurred on spacecraft carrying RTGs. In 1968, the launch vehicle of a Nimbus weather satellite failed and two RTGs fell into the Santa Barbara Channel off California. But they were recovered intact from the ocean floor five months later without loss of fuel. In 1970, the Apollo 13 spacecraft aborted its mission and did not land on the Moon, where it was to install an RTG powering a lunar surface experiments package. The Lunar Module eventually re-entered Earth’s atmosphere, plunging into the South Pacific Ocean near the Tonga Trench, where it still remains. Extensive air and water sampling of the vicinity found no evidence of plutonium release. However, in an accident in 1964, before NASA implemented its multiple robust layers of containment for the fuel, radioactive material did disperse. A U.S. Navy Transit 5BN-3 navigational satellite released radioactive fuel over a wide area of the upper atmosphere. Air-sampling and soilsampling analyses by the Atomic Energy Commission resulted in RTG designers switching to more reliable containment technologies that could prevent fuel release in an aborted launch or in the orbital phase of a mission.30 Nevertheless, activist movements against the use of RTGs sprang up around the country and internationally as well.

6.6

PUBLIC OPPOSITION TO THE USE OF PLUTONIUM

Bruce Gagnon, co-coordinator of the Global Network Against Weapons and Nuclear Power in Space, considered the use of radioactive material in space “sheer and utter madness,”31 adding that the danger of a plutonium-bearing space mission disaster was now “real and imaginable to the people of the world.”32 He was dedicated to stopping CassiniHuygens and all future missions carrying radioactive substances from launching.

6.6 Public opposition to the use of plutonium 165 The difficulty of defending against such well-meaning attacks as those from Gagnon’s organization was that the general public’s confidence in what nuclear engineers and government organizations said had been sorely depleted, especially after such disasters as Three Mile Island and Chernobyl. In the 1950s, an expert speaking about the promise and relative safety of nuclear power had considerably more credibility than he would today. The amount of misinformation that has been released to the public, not only regarding nuclear energy, but also in the wider area of general environmental and health impacts resulting from industrial activities, has caused tremendous skepticism concerning what so-called experts say. I know from personal experience the intractability of many people in anti-nuclear movements. A very widespread belief in the movements I’ve been associated with is, if something contains radioactive materials, it is bad. End of discussion. NASA went to great lengths to communicate the difference between RTG sources of electric power and the much more risky technologies used in nuclear power plants and weapons. Nevertheless, as Cassini-Huygens’ launch date approached, grassroots opposition to the mission increased dramatically. The predominant fear was that highly carcinogenic plutonium would be released either during a Cassini-Huygens launch accident, or when the spacecraft swung by Earth nearly two years later for a gravity assist. Adding weight to the protests were statements by professional people such as Horst Poehler, a retired Cape Canaveral scientist who made environmental preservation and public safety his top priority,33 and Helen Caldicott, founder of Physicians for Social Responsibility, who said that “less than one-millionth of a gram is carcinogenic.”34 By October 1997, the month of the launch, the anti-Cassini-Huygens movement had carried out highly visible rallies, protests near the launch site, and Internet and telephone campaigns that were gaining national attention. Alan Kohn, an emergency preparedness operations officer during two previous NASA plutonium-bearing missions, asked the question, “Can man build something that’s indestructible, in the fury and fire of a launch explosion?”35 In a speech at a rally sponsored by the Florida Coalition for Peace and Justice, he said, “Rocketry, believe me, is still in its infancy. There is no safe rocket. … Rockets blow up very easily.”36 If that were to happen to Cassini-Huygens in the Earth’s atmosphere, the assumption was that particles of plutonium 238 would likely rain down across our planet. The critical question was, would they significantly impact our health or that of our biosphere? NASA did not accept that its plutonium power sources presented the risks that the antinuclear movement was claiming. The Agency expressed this sentiment strongly. For instance, Cassini-Huygens Program Manager Richard Spehalski accused Kohn and others of “spreading fear, not facts”37 in their alarmist statements and speeches. Spehalski was supported by a risk study carried out by Jeff Cuzzi, a prominent Ames Research Center planetary scientist on the mission. Cuzzi applied basic principles of plutonium 238 decay by alpha-particle emissions and data supplied by the EPA and the Agency for Toxic Substances and Disease Registry (ATSDR), which was part of the Centers for Disease Control (CDC), to show that “the Cassini health threat is negligibly small even in the extremely small chance that anything does go wrong with the mission (either at launch or at flyby).”38 Cuzzi’s analysis was reviewed by the president of the Health Physics Society, a national organization that considered NASA to have, if anything, overestimated the health risks posed by its missions. In his analysis, Cuzzi assumed the worst-case scenario involving, through some kind of accident, the vaporization into the atmosphere of all 72 pounds of fuel in the RTGs.

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The main danger of plutonium 238, remember, is through inhalation, such as of vaporized RTG fuel. However, this fuel is actually about 30% oxygen and the less active isotopes of plutonium, so only 50 pounds of plutonium 238 (23 kilograms) would be involved. Cuzzi reasoned that if all the vaporized plutonium were to be dispersed evenly throughout Earth’s northern atmosphere, it would be of such a low concentration that a person would require to breathe this atmosphere for 10 years to inhale the equivalent of one year of naturally occurring background radiation. This assumed that the plutonium would remain in the atmosphere, whereas in fact many mechanisms would remove it from the atmosphere over time and hence the amount inhaled would be far smaller. Cuzzi’s analysis presumed all of the plutonium fuel vaporized, but that probably would not happen. The RTG housing itself would come apart if the space vehicle went out of control and accidentally re-entered and burned up in the atmosphere, but the tripleprotected plutonium modules, which were encased in two layers of carbon composite and iridium cladding, were extremely durable, and designed to withstand atmospheric deceleration and heating. They would strike the ground at a terminal velocity of 100 to 300 feet/ second, or one-tenth the speed of a rifle bullet. And rifle bullets do not vaporize in their flight through the air, or on impact. Cuzzi concluded that a person’s risk of dying from a Cassini-Huygens accident was one million times smaller than their risk of suffering a fatal auto accident in driving a distance of one mile.39 NASA performed many analyses that were far more detailed than Cuzzi’s which also indicated the extremely low probability that anyone would be harmed by the plutonium in Cassini-Huygens’ RTGs. Nevertheless, thousands of people around the world continued to believe that RTG fuel posed a grave danger. Critics of NASA’s risk analyses held that 5 billion people could suffer ill effects if Cassini-Huygens exploded on the launch pad or crashed into Earth on its 1999 flyby. Some Florida residents said they would leave town, or even the country, before Cassini-Huygens lifted off, and others tried in various ways to prevent the launch. Activists threatened to send boats or parachutists to Cape Canaveral to make launching impossible.40 Philip Morrison of Massachusetts Institute of Technology strove for a middle ground, recognizing the validity of both sides’ opinions when he said, “In general, plutonium’s danger is enormously exaggerated. On the other hand, I think it’s fair to keep people [at NASA and Department of Defense] honest and concerned. There is a thinkable catastrophe.” If Cassini-Huygens did not hold out the prospect of being a scientifically spectacular mission, then it would have been wrong for NASA to take the risk of flying plutonium fuel on it, Morrison believed. At the heart of the RTG debate were subjective issues that simply cannot be measured with probabilities or estimated numbers of deaths. While many people disagree on the specifics, there was some health risk associated with handling and manufacturing plutonium into RTG fuel pellets and putting them into a spacecraft that had a finite probability of crashing and dispersing its radioactive material. The essential question was whether it was worth such risks to journey to perhaps the most beautiful and surprising planet, satellite, and ring system that we know of, in order to shed light on its mysteries. There is no scientific answer to this question.41

6.7 Attempts to stop the launch 167 6.7

ATTEMPTS TO STOP THE LAUNCH

As the Cassini-Huygens launch date approached, efforts to prevent it intensified. A protest movement growing in Central Florida, not far from Cape Canaveral, received increasing international support in the form of demonstrations, petitions, postcard campaigns, a documentary video, and an Internet site. This support was elicited in part by anti-nuclear activists touring Europe and appearing at rallies against the use of plutonium aboard the spacecraft. Protesters feared that a launch accident would especially endanger Central Florida, covering it with radioactive contamination. And even if the launch was successful, the protesters were concerned that the spacecraft would veer off course in its 1999 flyby, re-enter the atmosphere, burn up, and “rain radioactivity on Earth.”42 During July 1997, the Gainesville-based Florida Coalition for Peace and Justice conducted a “Cancel Cassini”43 camp that carried out daily protests. In September, an aroundthe-clock vigil was begun at Cape Canaveral. The Coalition expressed its sentiments at various film festivals with a documentary criticizing NASA’s plans. In addition, activists distributed thousands of postcards around the world to be mailed to President Bill Clinton, imploring him to cancel the Cassini-Huygens mission. The retail store chain Body Shop also participated heavily in a postcard campaign against the mission. John Pike, space policy director for Federation of American Scientists, a liberal Washington D.C. think tank, believed that two recent launch accidents had fueled the protesters’ fears and were partly responsible for increases in the numbers of protesters compared to previous movements. He cited a Delta rocket that blew up at Cape Canaveral in January 1997 and the previous year’s Russian Mars 96 probe that fell back to Earth, crashing in South America with nuclear fuel aboard. Dwayne A. Day of George Washington University’s Space Policy Institute, however, thought the main reason for the anti-CassiniHuygens movement having more people was because they were better organized than previous movements. NASA spokesperson George Diller also commented on how well the movement was being organized.44 Dwayne Day belittled the movement as “your standard group of anti-technology peaceniks,”45 but Bruce Gagnon of the Florida Coalition for Peace and Justice said the protestors were not against our country’s space program. People simply wanted it to be conducted safely. He accused NASA of losing touch with what people don’t trust. Similar views were expressed by other groups, some of them far removed from Florida. The Marin County, California Board of Supervisors, for instance, believed the mission could directly threaten the health of their residents. This council issued a resolution stating, “The Marin County Board of Supervisors does not wish to oppose the Space Program and its mission but is charged with a responsibility for balancing Marin’s public safety [author’s italics] with scientific zeal and exploration.”46 Anti-Cassini activists threatened to use “all nonviolent means possible to stop or at least disrupt”47 the scheduled 6 October 1997 launch of Cassini-Huygens. Their plans included attempts to access the launch area and conduct a sit-in on the launch pad. Because of such threats, NASA took heightened precautions for protecting the spacecraft. When it was delivered to Kennedy Space Center (KSC) in April 1997, its arrival was not announced

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until after it was safely in secure storage. In August 1997, again quite discreetly, KSC personnel moved the spacecraft from a guarded NASA building to a heavily secured launch pad at the adjoining Cape Canaveral Air Force Station, then mounted it on the launch vehicle that would carry it aloft. During this time, the plutonium dioxide fuel remained under lock and key in another building, to be installed in the space vessel only days before its launch. The launch preparation procedures are described in greater detail in the previous chapter. Cape Canaveral Air Force Station went into “Threatcon Alpha” mode, a highly unusual course of action for a civilian launch. This implemented intensified security precautions against possible terrorist actions. All unofficial tours of the Air Force station were canceled and personnel, their badges, and automobiles were inspected more thoroughly. Numerous surveillance cameras were employed around the station, and the launch area, which was fairly isolated at the northern end of the station, was ringed by two chain fences that were topped by barbed wire, with motion sensors in between. Many armed guards patrolled the area, some in civilian clothing. The Air Force was taking seriously the protesters who said they would do anything in their power to stop the launch, including parachuting onto the site.48 The actions of protestors aimed at blocking or delaying the launch intensified as the scheduled liftoff drew near. In late August 1997 groups opposed to the mission requested that Florida Governor Lawton Chiles urge the U.S. government, possibly President Clinton himself, to push back the launch date. The activist groups included the Sierra Club and Pax Christi USA, a Catholic organization which was opposed to the militarization of space. A spokesperson for the governor, press secretary Karen Pankowsky, said her boss had no plans to seek a mission delay.49 Other actions to increase awareness of the plutonium issue included a 13 mile swim in Palm Beach County, a series of public debates, coverage by the television show 60 Minutes, and a news conference at the National Press Club in Washington D.C. that was followed by a protest march to the White House.50 Launch approval was required by presidential directive for Cassini-Huygens, due to the radioactive power sources aboard the space vessel, as well as the radioisotope heater units heater units carried to protect instruments and electronics from the cold temperatures in deep space. But before NASA sought such approval, it completed various analyses of the mission’s environmental and safety aspects, including two required by the National Environmental Policy Act, these being an Environmental Impact Statement (June 1995) and a supplement (June 1997). Activists around the country submitted requests to President Clinton, urging him to cancel the mission. But in August 1997, NASA Administrator Dan Goldin sent a letter to John H. Gibbons, Assistant to the President with responsibility for the White House Office of Science and Technology Policy (OSTP), requesting “nuclear launch safety approval”51 for Cassini-Huygens. The President consented, and on 3 October 1997 NASA received formal notice from OSTP to proceed with the launch. Goldin said he was confident of the mission’s safety, and fully expected that it would return spectacular images and scientific data about Saturn “in the same safe and successful manner as the Voyager, Galileo and Ulysses missions.”52 The day after President Clinton’s approval, demonstrators estimated by police to be 1,200 strong converged outside the gates of Cape Canaveral Air Station and took part in a

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peaceful demonstration. Air Force personnel warned that they would arrest any protesters who trespassed, and some of those assembled chose to do so. Brevard County Jail spokesperson Joan Heller reported that a total of 27 activists got arrested and charged with trespassing on a government facility. Some protesters climbed over the station’s 8 foot tall fence, but representatives from the group Grandmothers for Peace found another way in.53 Since the grandmothers were unable to actually climb the fence, they made a prior agreement with the guards to let them pass through the gate, thereby avoiding the possibility of injury, and then be arrested.54 Opponents of the mission made a last ditch effort to block its launch by using an environmental attorney in Hawaii, Lanny Sinkin, to ask U.S. District Court Judge David Ezra for an injunction, claiming that Cassini-Huygens would endanger people around the world. But these final hopes to stay the launch were dashed when Judge Ezra refused to issue an injunction, ruling that such an action would not be in the public interest.55

6.8

THE LAUNCH

In the days before the launch occurred, 70 protesters held a candlelight vigil outside the White House, with signs asking someone to “Halt nuclearization of space: Stop Cassini.”56 However, after a series of delays due to technical issues (as detailed in Chapter 4) and dangerously strong winds, NASA finally sent the Cassini-Huygens spacecraft skyward at 4:43 a.m. on 15 October 1997. The launch was “picture-perfect.”57 The mammoth Titan 4B launch vehicle lit up the predawn heavens58 as it and its precious cargo “blasted through clouds into a moonlit sky,” during which time “Saturn appeared as a yellowish speck above the nearly full moon.”59 The spacecraft set off on its long voyage with absolutely no indication of any malfunction that would lead to a plutonium release. Spacecraft development manager Chris Jones said, “I can’t recall a launch as perfect as this. Everything we see is within predictions, with no failures.”60

6.9

TWENTY-FIRST CENTURY RTG ISSUES

While current RTG issues do not directly impact the Cassini-Huygens mission, they are important for continued exploration of the outer solar system, including future missions to Saturn and its moons to more deeply analyze phenomena discovered by Cassini-Huygens. Such follow-on exploration may be delayed, however, because of current problems in developing reliable sources of RTG fuel, plutonium 238. There is presently no production facility for the material operating in the U.S., and Russia has proved to be a most unpredictable and unreliable source. In 2005 the Department of Energy (DOE) announced its intention “to consolidate the nation’s radioisotope power system activities at Idaho National Laboratory and start producing plutonium 238 there by 2011.”61 A restart of production capability was estimated at the time to cost $250 million and require five years, later upped to seven years. Plans for this restart were, however, “quietly shelved”62 by DOE in the winter of 2007-08.

170 Using plutonium to run a spacecraft In March 2008, NASA Administrator Mike Griffin told a House of Representatives subcommittee during the first days of hearings on the Agency’s fiscal year 2009 budget request, that the U.S. store of plutonium 238 was low and rapidly running out. He was asked about the specifics of plutonium 238 supplies by Representative Adam Schiff (D-California), whose district is home to JPL, the lab constructing the next spacecraft to employ nuclear power for its onboard electricity, the Mars Science Laboratory (MSL). Griffin’s response was that after MSL, NASA was “pretty much out of plutonium.”63 DOE spokesperson Angela Hill said plans for her agency to manufacture more of the fuel were on hold. Indeed, NASA did not ask for any funding in its 2009 budget request to re-establish DOE’s dormant production capability. According to Hill, the U.S. had been receiving some plutonium 238 from Russia since 2005 and had more on order.64 Alan Stern, NASA Associate Administrator for Science, testifying to the House subcommittee along with Griffin, said the U.S. had sufficient plutonium 238 in its stores or on order for MSL, an outer planets flagship mission, and a smaller mission. Griffin confirmed this. Nevertheless, in order to ensure there was indeed sufficient fuel for those missions, NASA had notified scientists earlier in the year that for its next New Frontiers mission it would not consider projects that required a nuclear power source. That decision put those scientists wanting to develop further outer planet explorations at a serious disadvantage. Griffin clarified how serious the situation was when he said, “In the future, in some future year not too far from now, we will have used the last U.S. kilogram of plutonium 238. And if we want more … we will have to buy it from Russia.”65 Absent a national decision to restart production, NASA’s outer planet exploration program could become severely constrained. John Logsdon, executive director of the Space Policy Institute at George Washington University, cautioned that not restarting plutonium 238 production would put our space program in “an undesirable position of vulnerability.”66 Under various plausible future political scenarios, we would not be able to count on Russia dependably supplying us with sufficient plutonium 238. And if it did, we might find ourselves paying dearly for it, since Russia would have a monopoly on the material. Griffin also announced during the congressional hearings that even the Russian supply of plutonium 238 might not last much longer. Russia had recently advised the U.S. that it had only about 10 kilograms (22 pounds) left.67 The Obama Administration staunchly supported a restart of domestic plutonium 238 production. In 2009 and 2010 it requested $30 million to enable DOE to initiate implementation of a process for “a national production capability of 5 kilograms of Pu-238 per year to support space exploration and national security applications.”68 Both the Senate and House Appropriations Committees had issues with the request, however. The Senate Appropriations Committee called DOE’s proposed program poorly defined and lacking an overall mission justification as well as a credible cost estimate. The House Appropriations Committee criticized DOE for not providing “a clear plan for how the $30,000,000 request will be utilized,”69 and wanted to know specifically how NASA was going to partner with DOE and contribute to the effort. No funding for the program was issued in fiscal year 2010. DOE’s fiscal year 2011 budget request, submitted to Congress in February 2010, outlined a 50/50 cost sharing with NASA. The Conference Report to Accompany H.R. 3183, Energy and Water Development and Related Agencies Appropriations Act, 2010, Report

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111‐278, asked DOE to provide a start‐up plan that included the role and contribution of major users of plutonium 238, such as NASA. The plan that DOE submitted in June 2010 outlined the technology needed for production, which would take place at existing facilities, and also discussed NASA’s intent to transfer its portion of infrastructure funding to DOE, since the production capability was to be entirely DOE-owned.70 Again, however, the Senate Appropriations Committee provided no funding for plutonium 238 production in its version of the fiscal year 2011 Energy and Water Development Appropriations Bill, explaining that because NASA was projected to be the only user of the material, then NASA should pay for the entire production service through such mechanisms as an interagency work-for-others arrangement71 negotiated between DOE and the space agency. The U.S. planetary science community has conducted lobbying efforts on behalf of restarting fuel production for outer solar system missions. For instance, Candy Hansen, chair of the Division of Planetary Sciences of the American Astronomical Society, as well as a longstanding scientist and manager on the Cassini mission, has been involved in extensive efforts to reach key members of Congress, in particular members of the Subcommittee on Energy and Water Development of the Senate Committee on Appropriations with the message that “there is a critical need right now”72 for reestablishing domestic production of plutonium 238. The American Astronomical Society continued its lobbying efforts in 2011. On March 11, its president, Debra Elmegreen, testified before the House Commerce, Justice, Science Appropriations Subcommittee, urging rejection of efforts that would “shortchange our future by cutting funding on our most talented students, essential research, and entrepreneurial potential.”73 Another major scientific organization that strongly lobbied for a resumption of domestic production was the American Geophysical Union (AGU). In June 2011, Kaitlin Chell, its public affairs coordinator, sent a letter urging the organization’s members to contact their congress people and call for renewed production of this material. In particular, the lobbying effort was directed toward an amendment to the House Appropriations Subcommittee on Energy and Water, which oversaw the DOE budget. Earlier, the President’s fiscal year 2012 (FY 2012) budget request asked for $10 million for NASA and $10 million for DOE to restart production of plutonium 238. The Congressional debate that followed contained much discussion of which agency – NASA or DOE – should fund such an effort. Plutonium 238 production funding was not allocated to DOE in a June 2011 House of Representatives draft of the FY 2012 Energy and Water Development Appropriations Bill. In fact, the House Committee on Appropriations was critical of the request, echoing its previous year’s sentiment that NASA would derive the primary benefit from a restart in production because it would use the vast majority of plutonium 238 produced or procured by the federal government. Thus the Obama Administration should “devise a plan for this project that more closely aligns the costs paid by federal agencies with the benefits they receive.”74 The AGU requested its membership to support the amendment to the spending bill by Congressman Adam Schiff (D-California). On 15 June 2011, the full House Appropriations Committee met to mark up its subcommittee’s draft bill and Schiff proposed an amendment allocating $10 million75 for DOE to restart production. He backed up this request with a series of salient points:76

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•

• •

Using plutonium to run a spacecraft Plutonium 238 cannot be made commercially but must be manufactured by DOE The U.S. was currently 100% dependent on Russia for the material so desperately needed by our space program, but there is no current agreement with Russia to continue to purchase the material Many of the flagship planetary missions called out in the National Academy of Sciences’ decadal survey, Vision and Voyages for Planetary Science in the Decade 2013-2022, were in jeopardy over this issue If plutonium 238 production were to be resumed immediately, there would still be a 5 year lag time before enough was produced to power a spacecraft Further delays could cause missions to reach prohibitively high costs which, in turn, could cause job losses and diminish the U.S. leadership role in planetary science.

Although no one suggested it was a bad idea to restart plutonium 238 production, Rodney Frelinghuysen (R-NJ), chair of the House Energy and Water Development Appropriations Subcommittee, complained that the Obama Administration had still not addressed or even acknowledged the previously stated concerns, and called for the development of a more equitable plan. In a heated tone, Schiff argued it should not be a battle between the Administration and Congress; the House ought simply to approve a program that needed to be implemented. Nevertheless, his amendment was not adopted.77 Schiff again moved to amend the bill when it was considered on the House floor in July 2011, and was again defeated, with Subcommittee Chairman Frelinghuysen citing similar reasons as previously. The bill then went to the Senate Energy and Water Development Appropriations Subcommittee, chaired by Dianne Feinstein (D-CA). Senator Feinstein and her subcommittee developed funding and policy recommendations for the Senate Appropriations Committee. However, these recommendations did not include funding to restart plutonium 238 production.78 The battle for plutonium 238 funding was by no means lost. The House FY 2012 Commerce, Justice, Science, and Related Agencies Appropriations Bill, in its section pertaining to NASA funding, did support a restart of plutonium 238 production, and made available $10 million for this purpose. In its report, the House Committee on Appropriations urged the space agency “to work expeditiously with the Department of Energy to bring Pu-238 production back online as quickly as possible.”79 The bill was passed by the House on 13 July 2011.80 In September 2011, the Senate Appropriations Committee approved its version of the FY 2012 Commerce, Justice, and Science Appropriations Bill, S. 1572. This bill, which provided funding for NASA as well as other agencies, allowed the transfer of up to $10 million from NASA to DOE to re-establish facilities capable of producing plutonium 238 fuel in support of future missions. The Committee also noted that the most recent decadal survey in planetary science urged NASA to reformulate flagship missions to live within the projected budget which, “like the Federal budget overall, is shrinking, not growing.”81 On 17 November 2011, the House and Senate passed H.R. 2112, an appropriations bill that provided funding for NASA. This was signed into law by President Obama the following day.82 In the context of future outer solar system exploration, “The bill makes available $10,000,000 … to restart production of Plutonium-238 (Pu-238), a radioisotope that is an essential source of electrical power for long-range planetary science missions.”83

References 173 As of January 2014, NASA envisioned that plutonium 238 production would be restarted and efforts to accomplish this were progressing as planned, with the DOE and NASA both playing essential roles in the process.84 Regarding this relationship, however, the House Appropriations Committee asked NASA to analyze DOE’s cost projections, establish a management structure that would give NASA “the necessary degree of control over the DOE facilities and personnel that would be operated using NASA’s funds,” and determine whether alternative facilities or processes could be used to produce plutonium 238 more economically.85

REFERENCES 1. Bob Mitchell interview, JPL, by author, 5 February 2008. 2. JPL, “Radioisotope Thermal Generators (RTGs),” http://www2.jpl.nasa.gov/galileo/messenger/oldmess/RTGs.html, Galileo Messenger 10, April 1984, accessed 10 Feb. 2009. 3. Joseph A. Angelo, Jr., Nuclear Technology (Westport CT: Greenwood Press, 30 November 2004), pp. 256-257. 4. NASA/JPL, Can a Spacecraft Use Solar Panels at Saturn? Educational Brief EB-2001-12-022-JPL, 5. JPL, Why the Cassini Mission Cannot Use Solar Arrays (Pasadena CA: JPL Cassini Public Information), Nov. 1996. 6. Bob Mitchell interview, 5 February 2008, JPL, by author. 7. Ibid. 8. Rebecca Regan, “Juno,” http://www.nasa.gov/mission_pages/juno/launch/Juno_solarpower. html, NASA missions (1 Aug. 2011). 9. Internet Encyclopedia of Science, “Radioisotope Heater Units (RHUs),” http://www.daviddarling.info/encyclopedia/R/RHU.html, Internet Encyclopedia of Science, accessed 1 Mar. 2009. 10. Daniel S. Goldin letter to John H. Gibbons, 13 Oct. 1997, NASA NHRC 1790 Cassini Probe (81-97 Aug.). 11. NASA, “Final Supplemental Environmental Impact Statement for the Cassini Mission,” (Washington D.C.: NASA, 1997); U.S. Department of Energy, “Radioisotope Power Systems: Radioisotope Heater Units,” http://www.ne.doe.gov/space/neSpace2f.html, accessed 1 Mar. 2009. 12. U.S. Department of Energy, “Radioisotope Power Systems: Radioisotope Heater Units.” 13. William J. Broad, “Sale of Plutonium by Russia to U.S. Faces Unseen Snag,” NY Times, 23 March 1992, p. A1, NHRC 13824 Safety of Nuclear (RTG) Systems (1988- ). 14. DOE, “SRS Completes Neptunium Work, Continues to Support NASA,” http://www.srs.gov/ general/srs-home.html, DOE Savannah River Site (SRS) Web site, last updated 23 January 2009; William J. Broad, “U.S. Has Plans to Again Make Own Plutonium,” NY Times, 27 June 2005. 15. G.L. Kulcinski, “Lecture 21 – Nuclear Power in Space,” Rawlings, SAIC, 22 Oct. 2001, NASA NHRC, #19543, CASSINI/HUYGENS - SOURCES USED IN WRITING CASSINI/ HUYGENS BOOK BY MICHAEL MELTZER. 16. Rockwell International, Rocketdyne Division, “Basic Elements of Static RTGs,” T90d-29-121, 883, NHRC, #19543, CASSINI/HUYGENS - SOURCES USED IN WRITING CASSINI/ HUYGENS BOOK BY MICHAEL MELTZER. 17. Dennis Miotla, “Assessment of Plutonium-238 Production Alternatives,” U.S. Department of Energy, Briefing for Nuclear Energy Advisory Committee (NEAC) Meeting, Arlington VA. (21 Apr. 2008).

174 Using plutonium to run a spacecraft 18. William J. Broad, “Sale of Plutonium by Russia to U.S. Faces Unseen Snag,” NY Times (23 March 1992):A1, NHRC 13824 Safety of Nuclear (RTG) Systems (1988- ). 19. John P. Boright statement to Congress, 21 February 1992, as reported by William J. Broad in “Sale of Plutonium by Russia to U.S. Faces Unseen Snag,” NY Times, 23 March 1992, p. A1, NHRC 13824 Safety of Nuclear (RTG) Systems (1988- ). 20. Broad, “Sale of Plutonium by Russia.” 21. U.S. DOE Office of Nuclear Energy, “Plutonium-238 Production Project,” http://www.ne.doe. gov/pdfFiles/factSheets/2012_Pu-238_Factsheet_final.pdf (15 Feb. 2011). 22. International Atomic Energy Agency, “What is an RTG?” http://www.iaea.org/Publications/ Magazines/Bulletin/Bull481/htmls/what_is_rtg.html, IAEA Bulletin 48(1) (Vienna, Austria: IAEA, 2006), accessed 9 Feb. 2009. 23. JPL, “Radioisotope Thermal Generators (RTGs),” http://www2.jpl.nasa.gov/galileo/messenger/ oldmess/RTGs.html, Galileo Messenger 10, April 1984, accessed 10 Feb. 2009. 24. JPL, “Spacecraft Power for Cassini,” NASA Fact Sheet, JPL Cassini Public Information, July 1999. 25. R.D. Cockfield, Design Review Presentation Data Package for the CRAF/Cassini RTG Program, CDRL No. B.2, section 3.1.1 in “RTG Description and Interfaces” (Philadelphia PA: General Electric Company, Astro-Space Division, 6 Nov. 1991), JPL CASTL archives, document 4562-1. 26. “What Independent Experts are Saying About the Cassini-Huygens Mission to Saturn,” http:// georgenet.net/misc/rtginfo.html, in Information about Cassini’s RTG’s (Radioisotope Thermoelectric Generators), accessed 10 Feb. 2009. 27. Michael Meltzer, Mission to Jupiter: A History of the Galileo Project (Washington D.C.: NASA SP-2007-4231), p. 77. 28. Washington Post, “Tentative Flight Plan is Drafted for Shuttle” (19 May 1986):A11. 29. Duncan C. Thomas and K. G. McNeill, “Risk Estimates for the Health Effects of Alpha Radiation,” http://www.ccnr.org/thomas_report.html, Atomic Energy Control Board Research Report INFO-0081, September 1982, accessed 10 Feb. 2009; Meltzer, p. 98. 30. Theresa Foley, “NASA Prepares for Protests over Nuclear System Launch in October,” Space Technology (26 June 1989): 23, 25; “RTG Fact Sheet: Past Releases of Radioactive Materials from U.S. Nuclear Power Sources,” NASA Spacelink, 17 June 1991; 31. Karl Grossman, “Despite the Risks, Push for Nuclear Technology in Space Steps Up,” The Sun (8 Dec. 1996):9-11, NASA NHRC 17908 Cassini Probe (81-97 Aug.). 32. Ibid. 33. “Horst Poehler,” http://www.goflight.org/HORST%20POEHLER%20-%20Died%20on%20 Oct%2003,%202003.htm, Memorials/Obituaries of KSC-Area Space Pioneers, accessed 15 Feb. 2009; Wayne Tompkins, “Group Holds Protest Against Cassini Mission,” Florida Today Space Online, 27 July 1997, NASA NHRC 17908 Cassini Probe (81-97 Aug.). 34. Helen Caldicott, Physicians for Social Responsibility, as reported in Karl Grossman, “Despite the Risks, Push for Nuclear Technology in Space Steps Up,” The Sun (8 Dec. 1996):10, NASA NHRC 17908 Cassini Probe (81-97 Aug.). 35. Marcia Dunn, Associated Press writer, “Plutonium on Spacecraft Safe, NASA Officials Say,” Birmingham News (6 Oct. 1997):18, NASA NHRC 5132 Cassini (Sep. 1997-Oct. 14, 1997). 36. Alan Kohn, “Speech at Cape Canaveral Air Force Station Main Gates (excerpts),” http://www. animatedsoftware.com/cassini/ak9707fl.htm, 26 July 1997, accessed 15 Feb. 2009. 37. Ibid. 38. Jeff Cuzzi, “Cassini Plutonium for the Technically Minded,” 16 Sep 1997, as reported in [email protected] email to [email protected] (Bill Higgins), 18 Sep. 1997,

References 175

39. 40. 41. 42. 43. 44. 45. 46.

47. 48. 49.

50. 51. 52.

53.

54. 55. 56. 57.

58. 59.

60.

NASA NHRC, #19543, CASSINI/HUYGENS - SOURCES USED IN WRITING CASSINI/ HUYGENS BOOK BY MICHAEL MELTZER. Cuzzi, “Cassini Plutonium.” K.C. Cole, “Saturn Mission Ignites Safety Debate,” Los Angeles Times (22 Sept. 1997). K.C. Cole, “Saturn Mission Ignites Safety Debate,” Los Angeles Times (22 Sept. 1997). Seth Borenstein, “Plutonium Worries Cassini Opponents,” Orlando Sentinel (9 June 1997):12, NASA NHRC 17908 Cassini Probe (81-97 Aug.). Robyn Suriano, “Anti-Nuke Activists Plot Peaceful War,” Florida Today (9 June 1997):10, NASA NHRC 17908 Cassini Probe (81-97 Aug.). Borenstein, “Plutonium Worries.” Ibid. Marin County Board of Supervisors, “RESOLUTION EXPRESSING MARIN COUNTY’S SERIOUS CONCERN TO THE INCLUSION OF PLUTONIUM AS PART OF THE CASSINI SPACECRAFT PROJECT, Resolution 97-26 (1997); Richard J. Spehalski letter to Marin County Board of Supervisors, 30 May 1997, NASA NHRC 17908 Cassini Probe (81-97 Aug.).h Marcia Dunn, “NASA Preparing for Cassini Launch to Saturn Amid Security, Secrecy,” Fox News, 31 Aug. 97, NASANHRC 17908 Cassini Probe (81-Aug. 97). Dunn, “NASA Preparing for Cassini Launch.” John Kennedy and Seth Borenstein, “Nuclear Launch Draws Protests,” Orlando Sentinel (30 Aug. 1997):23, NASA NHRC 17908 Cassini Probe (81- Aug. 97); Jim Ash, “Activists Seek Chiles’ Clout to Cancel Cassini,” Florida Today—Space Online (30 Aug. 1997), NASA NHRC 17908 Cassini Probe (81- Aug. 97). Kennedy and Borenstein, “Nuclear Launch.” Daniel S. Goldin letter to John H. Gibbons, 13 Oct. 1997, NASA NHRC 1790 Cassini Probe (81-97 Aug.). Douglas Isbell, Don Savage, and Matthew Donoghue, “NASA Receives Approval to Launch Cassini Mission,” C:\Documents and Settings\Michael\My Documents\Cassini\Chapters\Ch5 Pu\5.4 launch\Approval to Launch.HTM, NASA news release 97-225, 3 October 1997. E.J. Gong Jr., “Rally Against Cassini,” ABCNews.com (4 Oct. 1997), NASA NHRC 5132 Cassini (Sep. 97-14 Oct. 97); Brad Liston, “Police Arrest 27 at Florida Spacecraft Protest,” Yahoo! News (5 Oct. 1997), NASA NHRC 5132 Cassini (Sep. 97-14 Oct. 97). Patricia Altenburg, “Cassini was Launched-But Not Without Protest,” Grandmothers for Peace International (Dec. 1997):1. CNN.com, “Federal Judge Turns Down Bid to Stop Cassini,” 11 Oct. 1997. CNN.com, “Cassini Launch Postponed,” http://www.cnn.com/TECH/9710/13/cassini.delay/, 13 Oct. 1997, accessed 24 Feb. 09. NASA Glenn Research Center, “A Big Boost for Cassini: NASA Glenn Efforts Launch Cassini Toward Saturn,” Information and Publications Office, NASA Glenn Research Center, Document FS-1999-06-004-GRC, 1999, http://www.nasa.gov/centers/glenn/about/fs04grc.html, accessed 24 Feb. 09. ESA, “Successful Launch of Cassini-Huygens Mission,” ESA press release no. 32-97, 15 Oct. 1997, NASA NHRC 17903 Cassini (launch 10/15/97). This and then previous quote are from Marcia Dunn (Associated Press), “Saturn Bound: Cassini Begins its Seven-Year, 2 Billion-Mile Journey,” foxnews.com, 15 Oct. 1997, NASA NHRC 17903 Cassini (launch 10/15/97). NASA-JPL, “Cassini Status Update (Launch +1),” JPL Media Relations Office, news releases—1997, 16 Oct. 1997; NASA-JPL, “Cassini Status Update (Launch +2),” JPL Media Relations Office, news releases—1997, 17 Oct. 1997.

176

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61. Brian Berger, “Plutonium Shortage May Thwart Future NASA Missions to Outer Planets,” space.com (6 March 2008). 62. Ibid. 63. Ibid. 64. Berger, “Plutonium Shortage.” 65. Berger, “Plutonium Shortage.” 66. Berger, “Plutonium Shortage.” 67. Berger, “Plutonium Shortage.” 68. Richard M. Jones, “The Day of Reckoning Has Arrived: Appropriators to Decide on Key Isotope For Space Probes,” http://www.aip.org/fyi/2010/107.html, FYI: The AIP Bulletin of Science Policy News Number 107 (22 October 2010). 69. Ibid. 70. U.S. DOE, Startup Plan for Plutonium 238 Production for Radioisotope Power Systems, http:// www.ne.doe.gov/pdfFiles/Final_Startup_Plan_for_Plutonium238.pdf, Report to Congress (June 2010). 71. Jones, “The Day of Reckoning.” 72. Candy Hansen letter request to members of the Division of Planetary Sciences (DPS) of the American Astronomical Society, as reported in Emily Lakdawalla, “U.S. Citizens: Please write your elected representatives about restarting plutonium-238 production!” http://www.planetary. org/blog/article/00002538/, Planetary Society Blog (9 June 2010). 73. Richard M. Jones, “The American Astronomical Society Testifies Before House Appropriators,” http://www.aip.org/fyi/2011/037.html, FYI: The AIP Bulletin of Science Policy News, Number 37 (21 Mar. 2011). 74. Rodney P. Frelinghuysen, from the Committee on Appropriations, House of Representatives, “Energy and Water Development Appropriations Bill, 2012: Report together with Additional Views,” 112th Congress, 1st Session (June 2011):97-98. 75. Richard M. Jones, “No Go: House Appropriators Reject FY 2012 DOE Funding Request for Pu-238 Production,” http://www.aip.org/fyi/2011/076.html#, FYI: The AIP Bulletin of Science Policy News Number 76 (27 June 2011). 76. Kaitlin Chell letter to AGU membership, “Pu-238 & AGU Need Your Action TODAY on Capitol Hill,” American Geophysical Union, Washington D.C. (14 June 2011). 77. Committee on Appropriations, House of Representatives, “Full Committee Markup - Energy and Water Development Bill,” http://appropriations.house.gov/Calendar/EventSingle. aspx?EventID=245695 (15 June 2011). 78. Committee Reports – 112th Congress (2011-2012):Senate Report 112-075 on the Energy and Water Development Appropriations Bill, 2012, Library of Congress, http://thomas.loc.gov/cgibin/cpquery/?&dbname=cp112&sid=cp112BRzkB&refer=&r_n=sr075.112&item=&&&sel= TOC_0#; Richard Jones, “House Rejects Amendment to Allocate $10 Million in DOE Funding Bill for Pu-238 Production,” FYI: The AIP Bulletin of Science Policy News Number 86 (13 July 2011). 79. House Committee on Appropriations Report on the FY 2012 Commerce, Justice, Science, and Related Agencies Appropriations Bill, p. 72. 80. Richard Jones, “FY 2012 House Funding Bill: NASA,” FYI: The AIP Bulletin of Science Policy News Number 90 (15 July 2011). 81. Richard Jones, “FY 2012 Senate Appropriations Bill: NASA,” in FYI: The AIP Bulletin of Science Policy News 117 (22 Sep. 2011). 82. Richard Jones, “FY 2012 NASA Appropriations Bill Signed Into Law,” FYI: The AIP Bulletin of Science Policy News Number 138 (18 Nov. 2011).

References 177 83. House of Representatives, Commerce, Justice, Science, and Related Agencies Appropriations Bill, 2012, 112th Congress Report, 1st Session, http://appropriations.house.gov/UploadedFiles/ CJS_REPORT.pdf, p. 72. 84. James L. Green, 13 Jan. 2014 presentation, “NASA’s Planetary Science Program Support of Radioisotope Power Capability,” Outer Planets Assessment Group (OPAG) Mtg. (Tucson, Arizona: Jan. 2014). http://www.lpi.usra.edu/opag/jan2014/presentations/6_Green.pdf. 85. Committee on Appropriations, House of Representatives, “Report Together with Minority Views [to accompany H.R. 2787]” http://www.gpo.gov/fdsys/pkg/CRPT-113hrpt171/pdf/ CRPT-113hrpt171.pdf (23 July 2013) p. 63.

Part III

From Earth to Saturn Part III focuses on the flybys, gravity assists, and scientific observations that CassiniHuygens’ carried out on its voyage from Earth to Saturn, the Probe mission which followed its arrival at Saturn, and the features and dynamics of the Orbiter’s Saturn tour, including the organizational structure needed to make it successful.

7 The interplanetary journey This chapter examines a very powerful tool used to propel the spacecraft to Saturn: the gravity assist. It is a precisely designed maneuver in which a spacecraft closely approaches a planet and draws momentum from it. Pulled along by the planet’s gravity, the CassiniHuygens spacecraft’s speed was dramatically increased. Each gravity assist offered other advantages as well. It gave Cassini-Huygens the chance to carry out interesting scientific observations of the planet that provided the velocity boost, and also allowed instruments to be calibrated and various subsystems to be tested in a real space environment.

7.1

THE JOURNEY BEGINS

In order for the Cassini-Huygens space vessel to leap into the sky from Cape Canaveral and begin a trajectory to Saturn, its multi-stage launch vehicle had to provide sufficient thrust during a 20 minute burn to overcome Earth’s gravitational force as well as atmospheric friction. To accomplish this, the launch vehicle needed to ignite a huge amount of propellant. The Titan 4B/Centaur heavy-lift expendable launch vehicle, including its solidrocket booster engines and fuel, weighed about 1 million kilograms (2.2 million pounds). Of this, the propellant alone weighed 90% of the total launch vehicle weight.1 This was how much fuel was required to launch a spacecraft weighing roughly 6,000 kilograms (13,000 pounds), or about 1/150th of the total mass. The solid-rocket motors, the Titan stages, and some of the fuel from the Centaur upper stage burned for a total of about 12 minutes to propel the Cassini-Huygens spacecraft into orbit around Earth. After a 15 minute coast, the Centaur reignited and provided the final push in an 8 minute burn that enabled the spacecraft to escape Earth’s gravity and begin the first leg of its gravity-assisted trajectory to Saturn. Mission control then readied the Centaur, whose work was now done, to break away from the space vessel. Before the command could be given for separation, however, various subsystems in the spacecraft needed to be activated to enable it to operate on its own. Also before separation, the Centaur turned the spacecraft to point its high-gain antenna at the Sun, shading and thermally protecting sensitive equipment from overheating.2 © Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_7

181

182 The interplanetary journey After separation from the Centaur, the spacecraft did a curious thing. It entered onto a path that would initially take it not outward toward Saturn, but inward toward the Sun. The spacecraft pointed its bow toward a sector of space with intense solar radiation, the region where Venus was, because this planet would give the craft its first gravity assist. Mission personnel established communication with the space vessel after its separation from the Centaur by utilizing the 34 meter (112 foot) diameter antenna near Canberra in Australia. This antenna was part of NASA’s Deep Space Network (DSN) and enabled mission control at JPL to monitor spacecraft health, transmit commands, and receive data. Within an hour of launch, the DSN started receiving good, strong signals from the spacecraft.3 Also after separation, NASA needed to make sure that the Centaur carried out a maneuver to deflect itself off the spacecraft’s path toward Venus. It simply could not be allowed to collide with the planet at some future time,4 because the Centaur had not gone through the same cleaning regime as the Cassini-Huygens spacecraft, and thus could potentially contaminate the planet with terrestrial microorganisms.

7.2

THE CRUISE PHASE

The Cassini-Huygens mission was split into two major phases, known as the cruise and the tour. The cruise phase involved all activities getting from Earth to Saturn, including gravity assists, trajectory corrections, instrument testing, and various observations along the way. The tour phase began with the spacecraft’s arrival at Saturn, and involved a myriad of observations of the parent planet and its rings, fields and particles, and satellites, which will be discussed in subsequent chapters. The mission’s cruise phase began after launch, when the launch system put the spacecraft on its 2 billion mile (3.2 billion kilometer) trajectory to Saturn.5 This event heralded a major change in operations. Until initiation of the cruise phase, mission activities focused on building and integrating the spacecraft and setting it on its journey to the outer solar system. Once this was accomplished, the mission team went through heavy restructuring, both at the managerial and the technical levels, because a new set of skills were now required to get the space vessel to its target and commence its scientific activities. 7.2.1

What the team had to expect after launch

The launch of Cassini-Huygens, the endpoint of many years of work and struggle, was an exuberant event for the mission team. But it was also sad in a way for the hundreds of men and women whose jobs were now over. Many team members’ transitions to new projects were not easy, because they had given years of dedicated effort toward actualizing the Cassini-Huygens mission and suddenly they were no longer needed. As spacecraft manager Tom Gavin warned his younger staff, “You’re about to experience a feeling of separation. … You’ve been working now for five or six years with all of these people. You’re part of this great Cassini team here at JPL. We’re going to launch it, and then all of this is going to go away. You’re going to have a sense of loss.”6 Gavin underlined this prediction with accounts from his own experiences, such as a post-launch incident when he returned from Cape Canaveral, where he had been a star, to

7.2

The cruise phase

183

his JPL office. He found that it was filled with storage boxes. His manager of operations briefly welcomed him back, then asked how quickly he could get out of his office. On another return to JPL after months of high level interactions at the Cape, he found that his badge no longer got him into the mission support area, and the new guard didn’t know who he was. Gavin was confronted with the brusque question, “Why are you here?”7 7.2.2

Flybys and correction maneuvers

Cassini-Huygens took nearly seven years after its 15 October 1997 launch to reach Saturn. During this period, the craft carried out several major mission maneuvers, including the following planetary flybys in which the vessel received gravity assists (Figure 7.1):8 • • • •

Venus, 26 April 1998 Venus, 24 June 1999 Earth, 18 August 1999 Jupiter, 30 December 2000.

Mission staff also planned 21 trajectory correction maneuvers (TCM) on the way to Saturn.9 The project’s navigation team called for these adjustments in order to maintain the space vessel on its intended path. The team monitored the spacecraft’s position using both Doppler and ranging navigation methods. Doppler measured the speed at which the craft was racing away from or approaching Earth, while ranging used the speed of light to calculate the distance to the vehicle. In addition, optical techniques – those that employed the ship’s cameras to take pictures of solar system objects against a star background – were used on initiating the approach to Saturn’s moon Phoebe, just prior to Saturn Orbit Insertion. Images of Saturn’s moons were utilized for “op nav”10 applications. All this data allowed the navigation team to calculate the ship’s current and future positions.11 During the cruise, instruments and systems had to be carefully protected so that they remained operable for their tasks at Saturn. For instance, the Huygens Probe’s battery pack had to be well preserved to ensure that it would supply reliable power during Titan atmospheric entry and landing. Key observations would be taken by instruments needing to draw electrical power from the batteries, which also had to supply electricity for the transmission of data to the Orbiter. To protect the batteries during the 7 year cruise, it was vital to keep their temperature in the desired range. Special care was taken in the Probe’s thermal design to ensure that the battery pack storage temperature remained in the range 10–15°C (50–59°F).12 7.2.3 7.2.3.1

The early cruise phase TCM-1: Fine tuning the spacecraft path after launch

One day after launch, the spacecraft was already speeding at over 4 kilometers per second (9,000 miles per hour) away from Earth.13 The Cassini-Huygens operations team executed the first trajectory correction maneuver, TCM-1, on 10 November 1997, 25 days after launch. As precise as the launch vehicle thrusting accuracy was, it was still necessary to fine tune the spacecraft’s path.14 One source of trajectory error was due to the time of

184

The interplanetary journey

Figure 7.1 Spacecraft trajectory including planetary flybys.

7.2

The cruise phase

185

launch. In order to simplify launch operations, the specifications for a liftoff were calculated as if it was going to take place 40 minutes into the day’s launch window, which was approximately 110 minutes in length. But if liftoff actually occurred at a different time during this window, then a trajectory correction would be required. Since Cassini-Huygens lifted off at the opening of the 15 October launch window rather than 40 minutes into it, a correction was needed. TCM-1 had to adjust the spacecraft’s velocity by 2.7 meters per second (8.9 feet per second) to set it on the correct flight path; this required a main engine burn of 35 seconds15 that caused the spacecraft to curve in toward a Venus flyby at just the desired distance and time. And since TCM-1 was the first of the mission’s trajectory corrections, it gave the flight team useful data on operating the space vessel and its propulsion system.16

7.2.3.2

Activation and checkout operations

One of the main activities during the early cruise phase was activating and checking out the operation of the craft’s various subsystems and instruments. Five days after launch, for instance, mission engineers activated and analyzed the ultra-stable oscillator that provided an extremely steady downlink radio frequency source. The onboard radio receiver was also vetted to make sure it could receive commands from Earth. Scientific instruments activated during this time included the Cassini Plasma Spectrometer, Composite Infrared Spectrometer, and Magnetospheric Imaging Instrument.17 The three delicate antennas designed to sense plasma waves, which had been stowed for launch, were now deployed.18 Huygens Probe operations were analyzed on 23 October 1997, or launch + 8 days.19 By December 1997, the temporary covers meant to protect various of Cassini-Huygens’ sensitive detectors and optical devices during launch had been jettisoned and spacecraft operations entered a relatively quiet period, in which periodic system health checks and routine maintenance activities were scheduled. These regular “housekeeping” duties would continue throughout the mission.20 By January 1998, the mission flight team had completed most of its configuration activities. Various autonomous features of the spacecraft had been activated. Onboard software, for instance, was directing spacecraft activities as scheduled, while the vessel’s attitude in space was being maintained by small hydrazine thrusters. The 4 meter (13 foot) diameter high-gain antenna would remain facing the Sun for the first 14 months of the mission, in order to shade and thermally protect spacecraft systems from the intense solar energy of the inner solar system. Hence, communications with Earth were carried out employing one of the craft’s two low-gain antennas. The particular low-gain antenna selected for any given communication depended on the relative positions at the time of Earth, spacecraft, and Sun.21

7.2.3.3

TCM-2: Adjusting the first Venus flyby

During February 1998, the DSN allotted more time for communication with CassiniHuygens, in order to meet the data needs of navigators who were working on TCM-2 and TCM-3 scheduled in preparation for the 26 April 1998 Venus flyby (Venus-1).

186

The interplanetary journey

The detailed data collected during extra telecommunication periods furnished very accurate knowledge of the space vessel’s location, and this helped in setting precise parameters for the TCMs, in particular the durations of thruster burns.22 Mission engineers designed TCM-2, which took place on 25 February 1998, to “clean up”23 after the TCM-1 operation performed on 10 November 1997. Small discrepancies in the spacecraft center-of-gravity location had resulted in the TCM-1 burn giving a slightly incorrect thrust direction. Also, calibration of the spacecraft’s accelerometer had not yet been as precise as it would be when the characteristics of the instrument were better known under flight conditions. Since onboard software would end each TCM burn based on data from the accelerometer, this led to a small error in the thrust given to the vessel. The two errors were sufficiently important that if not corrected, would have resulted in the spacecraft either hitting Venus or flying by it at a wrong location.24 Thus, it was critical to perform another TCM before the craft reached Venus. TCM-2 was so successful that the navigation team deemed TCM-3, scheduled to occur 18 days before the first Venus flyby, to be unnecessary. The team also found that TCM-4, set for 18 days after the flyby, would not be required either.

7.2.3.4

A safing event

The spacecraft’s fault protection system had a chance to come into play during the approach to Venus. On 24 March 1998, the system detected a very small difference in measurements made of the vessel’s orientation by its two stellar reference units. To ensure that the spacecraft was operating correctly, it was important to resolve such discrepancies, even if they were small. The second stellar reference unit, part of Cassini-Huygens’ attitude and articulation control subsystem, was in the process of taking over while the first unit was given a routine maintenance bake-out to remove normal post-launch contaminants from its aperture. The attitude control software noticed a slight discrepancy in the way the two units were operating and as a result, the computer overseeing the attitude control subsystem brought the spacecraft into a low-activity “safe” state to await instruction from ground controllers. Analysis indicated that the discrepancy between the two stellar reference units was within specifications, and that control limits had been set unnecessarily tight, triggering the preprogrammed commands that initiated the safing event. When it was determined that the spacecraft was not at risk, ground controllers sent commands to return the vessel to a normal operational state. This event did not negatively impact the overall mission nor the impending Venus flyby.25 7.2.4 7.2.4.1

The first Venus flyby (Venus-1) The search for lightning

NASA’s DSN antennas in California’s Mojave Desert and near Madrid in Spain tracked Cassini-Huygens while it swung by Venus for the first time, making its closest approach at

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6:52 a.m. Pacific Daylight Time on 26 April 1998 and skimming 284 kilometers (176 miles) above the planet’s surface. One-way light time (the time of travel of radio waves or other forms of light) between spacecraft and Earth at that time was about 7 minutes. During the flyby, science instruments on the spacecraft searched for lightning in Venus’ atmosphere.26 The Orbiter’s Radio Detection and Ranging (RADAR) instrument conducted a test that attempted to transmit a radio signal at the fairly high frequency of 13.8 billion cycles per second (13.8 gigahertz, GHz) through Venus’ thick atmosphere and receive the reflection from its surface. Similar RADAR techniques would prove invaluable for mapping and analyzing the surface of Saturn’s largest moon Titan, which also was masked by an atmosphere that was opaque to visible light but not to radio waves. The purpose of the Venus-1 RADAR exercise, however, was predominantly “to obtain calibration data and rehearse instrument operations.”27 No reflection was actually detected. This was not unexpected, since the thick Venusian atmosphere strongly absorbs electromagnetic radiation of frequencies over 10 gigahertz. RADAR could have transmitted longer-wavelength (lower frequency) signals that would have penetrated the atmosphere (such as the Magellan and the Soviet Venera spacecraft had), but the main intent was to conduct an engineering exercise.

7.2.4.2

The gravity assist

During Venus-1, the tug of the planet’s gravity augmented the craft’s velocity by 26,280 kilometers per hour (16,330 miles per hour), with the result that it left Venus at a speed, relative to the Sun, of more than 141,000 kilometers per hour (87,000 miles per hour).28 Speed boosts from planetary flybys were critical features of the mission’s design for minimizing the propulsive energy that the launch vehicle was required to supply for the spacecraft to reach its ultimate goal: the Saturn system.29 Without gravity assists, the launch vehicle would have had to have a much greater propulsive capability (and carry dramatically more propellant) to get the job done. Days after the encounter with Venus, the navigation team reported that the gravity assist had been performed on time and right on target. Cassini Program Manager Dick Spehalski praised the maneuver’s precision, commenting that “the accuracy achieved by our navigators is roughly equivalent to shooting a basketball from Los Angeles to London and making a swish shot.”30 7.2.5 7.2.5.1

Preparing for the second Venus flyby (Venus-2) TCM-5: Maximizing the second gravity assist

At the beginning of December 1998, Cassini-Huygens successfully completed a longplanned, hour-and-a-half rocket engine burn (TCM-5) that slowed down the spacecraft by 450 meters per second (about 1,000 miles per hour) in order to adjust its trajectory for the second planned gravity assist (Venus-2), which would occur six months later.31 The TCM-5 engine firing was one of only two long burns planned for the entire mission;

188 The interplanetary journey the second occurred when Cassini reached Saturn in July 2004 and braked in order to achieve an orbit around the planet. The flight team continued to conduct checkouts of the spacecraft’s instruments as it sped toward its second Venus encounter. The team radioed commands to operate its optical instruments, including its imaging system, Visual and Infrared Mapping Spectrometer, and Ultraviolet Imaging Spectrograph. The instruments sighted on the bright star Spica, using it as a calibration and pointing target. The team then used data returned from the test to generate a photomosaic of Spica and the surrounding sky. From this, the pointing accuracy of the spacecraft and its optical instruments was determined.32 The portions of the cruise phase that occurred away from planetary gravitational fields provided ideal opportunities to calibrate accelerometers and prepare them for use at Saturn. In particular, checkouts of the Probe’s Huygens Atmospheric Structure Instrument (HASI) in the zero-gravity environment of the cruise phase allowed an extremely accurate assessment of its accelerometer measurements.33

7.2.5.2

Another safing incident

On 11 January 1999, the spacecraft placed itself in a “safe” mode, in which it turned off all nonessential functions in order to render itself as protected as possible from a power, thermal, and communications perspective until such time as mission staff on Earth could figure out what had happened and how to get the craft functioning again without damaging it.34 Mission engineers eventually determined that the safing was triggered by a particular geometry the spacecraft experienced during a slow roll maneuver. When its attitude and articulation control system measured a larger-than-expected change in the craft’s orientation, or attitude, the fault protection software was alerted. The measured attitude of the spacecraft was, as a matter of procedure, constantly compared to its predicted attitude, and even a slight difference between the two values resulted in a safing incident. In this particular case, one star was very near the edge of the field of view of the stellar reference unit for an extended period of time, and this contributed to a larger-than-normal estimated difference between the measured and the predicted attitudes. Because of the craft’s geometry and roll rate, this situation persisted for longer than the 50 second threshold required to trigger a safing incident. In response, the craft placed itself in a thermally protected orientation (so that it would not overheat) and awaited further commands from Earth. These were eventually received and normal spacecraft operations resumed.35

7.2.5.3

TCM-6: Fine tuning the second gravity assist

Where TCM-5 involved a major hour-and-a-half burn, altering the craft’s speed by 450 meters per second, the TCM-6 burn performed in February 1999 was for fine tuning, only lasting 2 minutes and modifying spacecraft’s speed by 12 meters per second. TCM-6 also provided data which would allow quite accurate main engine maneuvers during the cruise between the second Venus gravity assist and the Earth gravity assist.36

7.2 7.2.5.4

The cruise phase

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TCM-7: More fine tuning, performed without the spacecraft’s main engine

Three months later on 18 May 1999, the spacecraft executed TCM-7, a maneuver resulting in a tiny, 0.24 meter per second velocity change designed to very slightly move the vehicle’s trajectory closer to Venus for the second gravity assist.37 The velocity change was small enough that the reaction control subsystem (RCS) rather than the main engine could perform it. The RCS had four monopropellant hydrazine thruster clusters that could make velocity changes of up to 1 meter per second. See Chapter 3 for a discussion of the propulsion system.38 A further opportunity to refine the trajectory for Venus-2, TCM-8, was canceled.39 7.2.6

The second Venus flyby (Venus-2)

The spacecraft made its Venus-2 flyby on 24 June 1999, with the point of closest approach 600 kilometers (370 miles) above the surface. The gravity assist gave a velocity increment of 6.7 kilometers per second (15,000 miles per hour).40 Most of the Orbiter’s scientific instruments made observations during the flyby. Their data was transmitted to Earth in the ensuing days using one of the low-gain antennas.41 Of course, the high-gain antenna would have transmitted the data far more quickly, but it could not be pointed toward Earth because its 4 meter diameter dish was still shading the vessel’s instruments, so that they would not overheat. The spacecraft’s scientific observations included visible images of the planet, visible and ultraviolet spectrographic observations, ion and electron measurements, and magnetic field and plasma wave analyses. Cassini-Huygens scientists hoped to gain more insight during this flyby into the interaction between Venus and the solar wind, a stream of charged particles emitted by the Sun.42 At the time NASA launched the Cassini-Huygens spacecraft, little effort had been devoted to detailed planning of the scientific observations for the cruise phase. This situation rapidly changed. By the time of the Venus-2 encounter, major elements of the cruise science observation schedule had been put in place. Table 7.1 summarizes the types of observations taken during this flyby by different Orbiter instruments.43

Table 7.1. Cassini science instrument observations during Venus-2. Instrument

Abbreviation Activity

Cassini Plasma Spectrometer Cosmic Dust Analyzer

CAPS CDA

Imaging Subsystem Dual Technique Magnetometer Magnetospheric Imaging Instrument Radio and Plasma Wave Science Ultraviolet Imaging Visible and Infrared Mapping Spectrometer

ISS MAG MIMI RPWS UVIS VIMS

Ion composition Instrument calibration and characterization of inner solar system meteor streams and dust environments Calibration and UV measurements Support of other particle and field measurements Bow shock accelerated ions electrons observations Plasma wave measurements Airglow measurements Atmospheric sounding

190 The interplanetary journey 7.2.7

The search for Venusian lightning

The Cassini-Huygens spacecraft searched for evidence of lightning in the Venusian atmosphere during Venus-2, as it had done during Venus-1 (and as it would do at Saturn, looking for lightning in the atmosphere of that planet). The existence of lightning on Venus was a subject of controversy in space science circles. Questions of whether it existed began in 1978 when a Russian lander detected low-frequency emissions. Then in 1979 NASA’s Pioneer Venus spacecraft, in orbit of the planet, observed possible evidence for lightning. And in 1990, the Galileo spacecraft, using instrumentation similar to that on CassiniHuygens, detected small impulses that could have been from lightning. But Galileo was 60 times more distant from Venus than Cassini-Huygens, and the results were ambiguous. During the flybys, Cassini-Huygens’ Radio and Plasma Wave Science (RPWS) instrument, with its triplet of 30 foot long antennas, searched for impulsive high-frequency (0.125 to 16 megahertz) radio signals. Physicist Donald Gurnett of the University of Iowa and principal investigator on Cassini-Huygens’ RPWS said that such signals, known as “spherics,” were always produced by lightning on Earth and commonly heard as static on AM radios. But careful analysis of the Cassini-Huygens observational data did not detect any of the high-frequency radio waves associated with such lightning. Despite the Cassini-Huygens results, Gurnett did not rule out the possibility that some types of low-frequency electrical activity may exist on Venus. Because radio signals are not able to penetrate the planet’s ionosphere at frequencies below about 1 megahertz, no definitive statement was able to be made about the lightning spectrum at those frequencies.44 There are other possible explanations as well for the non-detection of lightning signals. According to Gurnett, any Venusian lightning “is either extremely rare, or very different from terrestrial lightning.”45 If terrestrial-like lightning events were occurring in the region of Venus studied by Cassini-Huygens, then it should have been easily detectable. But Venus’ atmosphere differs quite markedly from that of Earth, and so it might be expected that atmospheric electrical activity would be quite different on Venus. Earth lightning flashes are generally of two types – those that leap from cloud to ground, and those that travel from cloud to cloud (or from one part of a cloud to another part). Venusian clouds float at altitudes of 40 kilometers (25 miles) or more, far higher than the clouds in Earth’s troposphere or stratosphere. Thus it would be more difficult and possibly unlikely for lightning, which involves a current of charged particles, to make the leap from a Venusian cloud all the way to ground. If lightning does exist on Venus, this argues for it being the cloud-to-cloud type. Cloud-to-cloud lightning on Earth is generally less intense than the cloud-to-ground type. If the same is true for Venus, its non-detection may be because it is a rather weak phenomenon that emits correspondingly weak radio signals. Lightning on Venus could also resemble the cloud-to-ionosphere discharges, termed sprites, that have recently been discovered on Earth.46 7.2.8

Solar wind interaction

Unlike Earth, Venus has a very weak magnetic field. This is of interest to fields and particles scientists, in part because Venus interacts quite differently with the charged particles of the solar wind. Because Venus’ magnetic field is so weak, these particles are able to

7.2

The cruise phase

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Figure 7.2 Magnetosphere interactions with the solar wind.

approach far closer to its surface than they can in the case of Earth, whose stronger field deflects most of them while they are much farther away. The magnetic field of a planet divides the space in that neighborhood into the magnetosphere, where a planet’s magnetic field predominates, and interplanetary space, where the interplanetary magnetic field predominates and the solar wind flows unimpeded. The interface where the solar wind is pushed aside is called the bow shock, because it is similar to the wave which appears in front of a boat as it passes through the water, or in front of a plane traveling at supersonic speeds. As illustrated in Figure 7.2, planetary magnetic fields in the bow shock area slow the solar wind and force it to go around the planet’s magnetosphere. Our planet’s strong magnetic field results in a bow shock 10 Earth radii (65,000 kilometers or 40,000 miles) away from the center of the Earth. But Venus’ weak field cannot keep the solar wind at such a distance, and this results in Venus’ bow shock occurring nearly on the planet’s surface. This means the solar wind is able to approach so closely to Venus that it directly interacts with the planet’s ionosphere and dense atmosphere.47 When the Cassini-Huygens spacecraft flew by Venus at low altitude, it was well positioned to study this interaction.48 Several instruments played a part. The Orbiter’s Plasma Spectrometer (CAPS) and the Magnetospheric Imaging Instrument (MIMI, which sensed energetic particles) could measure ions and electrons over a wide range of energies, from several electron-volts (eV) to the million eV (MeV) range, and could also determine ion composition. MIMI in fact did sense both heated solar wind helium ions as well as oxygen, helium, and carbon ions escaping from Venus’s atmosphere. Unfortunately CAPS suffered a software problem and could not operate at Venus, although the mission team fixed it before the Earth flyby.49

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The Radio and Plasma Wave Science (RPWS) instrument picked up a broad spectrum of plasma waves around Venus, although as mentioned above, it was not successful in detecting any evidence of lightning. The Magnetometer (MAG) helped define Venus’ bow shock boundary. It noted the spacecraft’s inbound and outbound bow shock crossings by way of the significant jumps in magnetic field strength that accompanied them. It did this despite being stowed in its launch canister for thermal protection so close to the Sun, rather than extended out on a boom. The instrument’s ability to make these measurements at Venus under such compromised conditions was an indication of both its high sensitivity and the lack of magnetic interference emitted by the spacecraft.50 7.2.9

Surface and atmospheric structures and processes

The Visual and Infrared Mapping Spectrometer (VIMS) was able to detect thermal emissions from the planet’s surface. Such measurements were relatively new. Prior to 1984, Venus’ deep atmosphere and surface had been thought to be undetectable from above at all wavelengths shorter than 1 centimeter microwaves. The planet’s thick, carbon dioxidedominated atmosphere blocked the penetration and reflection of visible, ultraviolet, and near-infrared light through all except its highest altitude hazes. Near-infrared, for instance, could not be used to make observations below an altitude of 70 kilometers (43 miles). But then in 1984 space scientists D. A. Allen and J. W. Crawford employed the thermal radiation generated by Venus’ hot lower atmosphere to backlight overlying clouds around the 50 kilometer (31 mile) level, thereby revealing their spatially inhomogeneous nature. The Galileo spacecraft as well as Earth-based instruments made further such observations, demonstrating the utility of this approach for studying mid-altitude level (48–57 kilometer, or 30–35 mile) cloud structures. In 1990, Galileo and ground-based observations through a number of spectral windows, narrow wavelength ranges that could penetrate the atmosphere, picked up radiation emitted by Venus’ surface and lower atmosphere. This revealed, for the first time at optical wavelengths (rather than, for instance, at radar wavelengths), many features of the Venusian topography.51 The Cassini Orbiter’s Visual and Infrared Mapping Spectrometer (VIMS) was designed for making observations of Saturn’s atmosphere, rings, and satellites, but was also well suited for studying the Venusian atmosphere and surface. On 24 June 1999, during Venus-2, it obtained observations of the planet’s nightside, untainted by scattered light from the dayside. The VIMS observations consisted of 64 separate spectra that were taken from various perspectives as the vessel made its pass over the surface. These data included firsttime observations through spectral windows predicted by previous researchers. The data were integrated to produce spectra with which to identify possible surface materials. Knowledge of surface chemistries is contributing to our better understanding of the planet’s evolution and climate over very long time periods and its chemical, geological, and thermodynamic processes. As an example, one of the minerals identified – wollastonite – interacts with carbon dioxide and may be a clue that it is involved in regulating the gas’s pressure at the planet surface.52 The spacecraft’s Ultraviolet Imaging Spectrograph (UVIS) measured Venus’ dayside airglow, a faint light emitted by a variety of atmospheric photochemical reactions involving carbon dioxide and carbon monoxide compounds, as well as atom and ion emissions from hydrogen, oxygen, carbon, nitrogen, and helium.53

7.3

On to Earth

193

The Imaging Science Subsystem (ISS) had a valuable opportunity during the flyby to calibrate both its wide-angle camera and narrow-angle camera.54 Without the opportunities for instrument calibration provided by the planetary flybys, extra time and effort would have had to be spent on this once the spacecraft reached the Saturn system.55

7.3 7.3.1

ON TO EARTH TCM-9 through 12

Mission planners scheduled the flyby of Earth only 54 days after Venus-2. One of the first operations performed by the mission team in preparation for the flight back to Earth was to make another maneuver on 6 July 1999, TCM-9, that used the main engine to alter the velocity by 44 meters per second (97 miles per hour) and move the ship’s trajectory closer to Earth. TCM-9 also constituted a “clean-up”56 action meant to correct for residual trajectory errors. This was followed by TCM-10 on 19 July 1999, TCM-11 on 2 August 1999, and TCM-12 on 11 August 1999. TCM-12 was the final adjustment before the 17 August 1999 flyby of Earth. The vessel was now precisely on course to pass Earth at an altitude of 1,166 kilometers (725 miles) over the eastern South Pacific, above such landmarks as Pitcairn Island and Easter Island. Nine of the Orbiter’s 12 scientific instruments were set to take observations of the Earth-Moon system, including studies of Earth’s magnetic environment and imagery of the Moon.57 The Huygens Probe’s six instruments remained dormant during the flyby. 7.3.2

Concerns about plutonium contamination surface again

Before the Cassini-Huygens launch, anti-nuclear activists expressed strong concern that an accidental impact of the spacecraft with Earth during its gravity-assist might break open the RTGs and spread plutonium through Earth’s biosphere. Numerous precautions had been taken to prevent a release of plutonium, many of which were discussed in Chapter 6. An additional precaution taken to greatly lower any chance of an accidental impact during the gravity assist had to do with the trajectory. The spacecraft was initially biased (aimed) so that it would pass far from Earth, thereby making a collision “virtually impossible.”58 As the spacecraft approached Earth, the mission team carefully reduced the bias using a series of TCMs, until the trajectory reached the proper flyby altitude for the gravity assist. 7.3.3

Orbiter calibration operations and scientific measurements at Earth

While the principal purpose of the flybys in the inner solar system was to provide sufficient velocity boost for the spacecraft to reach Saturn, these encounters also furnished opportunities to perform several useful activities: • • •

Calibrate Orbiter instruments Validate their capabilities Conduct those scientific observations best carried out during a flyby.

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7.3.3.1

Calibration activities

The Orbiter’s 11 meter (36 foot) magnetometer boom was extended in order to use details of Earth’s well-studied magnetic field to precisely calibrate the MAG. A day before closest approach to Earth, the spacecraft rotated so that its Imaging Science Subsystem (ISS), Ultraviolet Imaging Spectrograph (UVIS), and Visual and Infrared Mapping Spectrometer (VIMS) optical remote sensing instruments could observe the Moon and calibrate against another well-known target. For example, the ISS used its cameras to generate video clips and composite images of a face of the Moon nearly identical to that seen from Earth. Calibrating against such a target would help later on in the mission in interpreting unfamiliar targets and objects.59

7.3.3.2

Science observations and instrument performance evaluations

While numerous spacecraft orbit our planet, flybys are rare. The Cassini-Huygens flyby provided an opportunity that orbiting satellites did not have: it enabled a rapid transit of Earth’s entire magnetosphere in just a few hours, allowing a snapshot of sorts to be taken of it. And the flyby also allowed a comparison, using the same set of instruments, of familiar terrestrial phenomenology and, later on, Saturn-system processes. The Radio and Plasma Wave Science instrument (RPWS) operated in Earth’s vicinity from 15 August through 14 September 1999. It returned a useful plasma density profile from its transit of the magnetosphere and was able to determine the propagation direction and the polarization of various plasma waves. The instrument demonstrated good performance and its ability to analyze wave phenomena across a large range of frequencies, extending from 1 cycle per second (1 Hz) to 16 million cycles per second (16 MHz).60 The Magnetospheric Imaging Instrument (MIMI), designed to study energetic neutral and charged particles, provided tests of various models of energetic particle distribution within our planet’s magnetosphere. Two complete substorm cycles occurred during the Earth flyby, magnetospheric disturbances lasting several hours which involve interaction with the interplanetary field and energy transfer from the solar wind. Detailed analyses of particle behaviors were able to be made of these phenomena.61 The Cassini Plasma Spectrometer (CAPS) used one of its subsystems, its electron spectrometer (ELS), for almost 10 hours of solar wind observations as the spacecraft was approaching Earth, followed by nearly 9 hours of magnetosphere measurements. During the Earth flyby, ELS analyzed electrons in the bow shock and various other parts of the terrestrial magnetosphere.62 The RADAR instrument tests verified that it was capable of obtaining interesting and useful data for both solid and liquid planetary surface features, boding well for the work it was to do at Saturn, particularly in studying the satellite Titan’s land and lake characteristics.63 Earth’s magnetotail is the long, trailing limb of the magnetosphere on the side of our planet facing away from the Sun. The previous spacecraft that penetrated this region made their observations at a single location in the tail. But Cassini-Huygens was able, for the

7.4

Huygens Probe activities during the cruise

195

first time, to make nearly continuous measurements at a range of downtail distances, and this provided a better understanding of the charged particle flows there. In the period between 2 and 8 hours after closest approach, the space vessel traveled from 20 Earth radii (20 RE) downtail to 70 RE, using CAPS to map various electron and ion flows. Several enhanced particle fluxes were observed in a boundary layer. Flows of returning electrons were also observed that traveled down field lines, passed through the magnetopause (the boundary between Earth’s field and the interplanetary solar wind), reversed their direction, and came back.64 7.3.4

The gravity assist

Engineers at ESA’s control center in Darmstadt in Germany and at NASA’s JPL monitored the space vessel as it passed 1,170 kilometers (728 miles) above the eastern South Pacific region on 18 August 1999, possibly visible from Pitcairn and Easter Islands as well as from other small islands in the area. Earth’s gravitational force gave the spacecraft a 5.5 kilometer per second (12,000 mile per hour) boost in speed, helping to propel it toward a rendezvous with Jupiter and eventually Saturn, nearly 1 billion miles distant.65 Jean-Pierre Lebreton, ESA project scientist for the Huygens Probe, commented that during the flyby, the spacecraft “worked perfectly and we’re very happy.”66 7.3.5

TCM-13: Preparing for Jupiter

Shortly after the Earth gravity assist, the mission team began preparing the craft’s trajectory for Jupiter. On 31 August 1999, the spacecraft executed TCM-13 with a burn of 72 seconds that achieved a speed increment of 6.7 meters per second (15 miles per hour).67

7.4

HUYGENS PROBE ACTIVITIES DURING THE CRUISE

During the cruise phase, mission staff activated the Huygens Probe for its scheduled biannual health exams, called Probe checkouts. These 3 to 4 hour assessments were designed to simulate the Probe’s Titan descent scenario and were useful for carrying out necessary instrument maintenance and payload sensor calibrations. Additionally, the checkouts provided opportunities to perform end-to-end testing of the Huygens telecommunication system receiving elements. This testing was aided by the Earth-based Deep Space Network (DSN) antennas, which mimicked radio transmissions that would be carried out after arrival at Saturn. Use of the DSN antennas was not part of the normal Probe checkout operation, but was something mission staff had decided to add as a last minute thing. Such a test had been proposed previously, but had been denied for political reasons.68 These examinations of the telecommunication system proved to be tremendously important procedures to have added, as the Huygens Probe receiver anomaly, an oversight that nearly ended its mission, was discovered as a result of an end-to-end test performed in February 2000.69 This is discussed in detail in the next chapter.

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7.4.1

Through the asteroid belt

After Cassini-Huygens left Earth on its journey outward through the solar system, it first passed by the orbit of Mars and then transited the asteroid belt. This is a region of space between Mars and Jupiter that contains over 670,000 asteroids greater than 1 kilometer (0.6 miles) in size.70 On 23 January 2000 on its way through the belt, the craft captured an image of asteroid 2685 Masursky. Although this picture was taken at a distance of 1.6 million kilometers (960,000 miles, approximately four times the distance from Earth to the Moon), the resolution was high enough to determine that 2685 Masursky, named for deceased planetary geologist Harold Masursky, was 15 to 20 kilometers (9 to 12 miles) across.71 New data was also collected on Masursky’s reflectance and rotation rate. Asteroids are classified according to their surface brightness and color, and these properties relate to their composition. S-types are predominantly silicaceous, or in other words they have large amounts of silicon. They are the second most common type, comprising approximately 17% of all known asteroids. Until Cassini-Huygens analyzed 2685 Masursky, space scientists believed that it was an S-type asteroid, for its orbit placed it in the Eunomia family, a large grouping of S-types. But Cassini-Huygens’ observations cast some doubt on this classification. The data permitted a preliminary calculation of 2685 Masursky’s reflectance, which seemed wrong for an S-type asteroid. But later groundbased spectroscopy supported its classification as of this type.72 At the beginning of February 2000, while still in the asteroid belt, the Cassini-Huygens spacecraft began to use its high-gain antenna (HGA) for communication purposes. During the inner solar system portion of the cruise, the HGA had been oriented toward the Sun in order to shade the spacecraft and keep its temperatures within required limits. During this period the craft’s small low-gain antennas had been used for communication. The vessel was now far enough away from the Sun that shading it was no longer necessary. The increased performance supplied by the 4 meter diameter dish allowed the high data rates that would be required for future mission activities.73 Our solar system’s asteroid belt used to be portrayed in science fiction novels and movies as a dangerous region full of large pieces of rock “jostling for an opportunity to smash a hapless spacecraft.”74 In actuality the asteroid density is far lower. Space scientists no longer consider crossing the belt to be particularly hazardous. Cassini-Huygens was, in fact, the seventh spacecraft to pass through the asteroid belt, all of which did so without harm.75 Nevertheless, Cassini Program Manager Bob Mitchell was relieved when, in April 2000, the craft completed its transit of the belt intact and entered the outer solar system.76 # It was during the transit of the asteroid belt that the serious Huygens Probe receiver anomaly was discovered. This problem arose not because of any interaction with the asteroid belt, but due to oversights and miscommunications on Earth years earlier. A detailed explanation of the issue is given in Chapter 8 7.4.2

Preparing for the Jupiter flyby

On 14 June 2000, as the spacecraft sped toward Jupiter, the mission team uplinked the instructions for TCM-14. This course correction was carried out by a short main engine

7.5 The Jupiter flyby

197

burn of 6 seconds, giving the craft a speed boost of 0.58 meters per second, a small increment meant to optimize the upcoming Jupiter gravity assist at the end of December 2000. This assist would send the vessel on a new trajectory providing an excellent opportunity to investigate Saturn’s satellite Phoebe, three and a half years later.77 The TCM was designed to achieve a 2,000 kilometer (1,200 mile) flyby of Phoebe approximately three weeks prior to Saturn Orbit Insertion in 2004. It would be a unique opportunity to study this interesting moon at close range, because once Cassini-Huygens entered orbit around Saturn it would never again venture as far from the planet as Phoebe’s orbit. If TCM-14 had not been executed, the original trajectory would have taken the spacecraft no closer than 56,000 kilometers from Phoebe.78 Two further TCMs (15 and 16) that had been scheduled prior to the Jupiter flyby, proved unnecessary and were canceled.79 No further TCMs were required until well after the spacecraft had passed Jupiter.

7.5

THE JUPITER FLYBY: PARTNERING WITH GALILEO

On 30 December 2000, on its way to Saturn, the Cassini-Huygens spacecraft flew by Jupiter and received its fourth and final planetary gravity assist, boosting its speed by 2 kilometers per second (4,500 miles per hour). The encounter also gave Cassini-Huygens an opportunity to work with the Galileo Orbiter, which had been at Jupiter since 1995, and NASA took advantage of this rare chance to have two vessels make simultaneous, close range observations of a planet other than Earth.80 7.5.1

Imaging science

For six months, Cassini-Huygens’ instruments, more advanced than those of the older Galileo spacecraft, observed Jupiter’s atmosphere, magnetosphere, satellites, and rings. The Imaging Science Subsystem (ISS) took about 26,000 images of the Jupiter system and obtained valuable data on the planet’s atmospheric motions and meteorology. Data collection for the flyby started on 1 October 2000 and continued to 22 March 2001. The point of closest approach was on 30 December at a range of 136 Jovian radii (9.7 million kilometers or 6.0 million miles). Unsurprisingly, this encounter was dubbed the Jupiter Millennium Flyby. The long six month period of observations and ISS’s wide spectral range from the ultraviolet into the near-infrared enabled the defining of multi-level clouds, aerosols, and hazes, as well as monitoring evolving cloud structures and winds that generated clues to the planet’s atmospheric dynamics and chemistry.81 The spacecraft’s relatively gradual approach to and departure from Jupiter permitted its optical instruments to make timelapse videos of planetary atmospheric phenomena and activity on its satellites. ISS captured several time-lapse image sequences of Io, Europa, and Ganymede being eclipsed by Jupiter, with atmospheric emissions detected from both Io and Europa. Of the many sets of data returned, particularly interesting were image sequences of two giant plumes on the volcanically active moon Io. These sequences were of Pele, a 390 kilometer (240 mile) tall equatorial plume discovered by Voyager, and a similarly sized plume over the moon’s north pole that had never been seen before.82 Galileo spacecraft data supported these observations, which confirmed the existence of a class of sizable although short-lived

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plumes at high latitudes on Io that occurred in conjunction with “voluminous outpourings of lava,”83 perhaps indicating the existence of tidal heating in Io’s deep mantle. 7.5.2

Jovian storms

One mystery of Jupiter’s weather system is why its storms last so long. Storms on Earth might last a week before they break up and get replaced by other storms. But around Jupiter’s midsection, “long-lived storms and globe-circling belts of clouds are familiar features.”84 The Great Red Spot, for instance, is a storm that has been observed for over 300 years. And Cassini-Huygens confirmed that even closer to the planet’s poles, where the banded cloud patterns of equatorial latitudes give way to a chaotic-appearing, mottled patterns from numerous interacting vortices, the storms also last longer than on our planet. According to Caltech planetary scientist Andy Ingersoll, a member of the Cassini imaging team, “There are thousands of storms there the size of the biggest storms on Earth. Until now, we didn’t know the lifetime of those storms.” A movie clip made from CassiniHuygens images taken over an interval of 70 days showed the storms in this chaotic upperlatitude region usually persisting for the entire period, and typically moving together within each latitude band. Ingersoll addressed the mystery of long-lived Jovian storms in an interesting way, suggesting that it actually might be Earth that is unusual: “Perhaps we should turn the question around and ask why the storms on Earth are so short lived,” he said. “We have the most unpredictable weather in the solar system, and we don’t know why.”85 Analyses of data from joint Cassini-Huygens and Galileo observations suggested a relationship between Jovian storms of different sizes and atmospheric flows deep in the atmosphere. Large storms on Jupiter appear to “gain energy from swallowing smaller storms.”86 And the smaller storms, in turn, pull their energy from the lower depths of the Jovian atmosphere, below the cloud surface. 7.5.3

Radio emissions and aurorae

In the 1950s, Jovian radio emissions provided the first evidence that the planet had a strong magnetic field.87 Jupiter also proved to have a large magnetosphere and polar aurorae, similar in many respects to Earth’s aurorae.88 Planetary scientists believed Jupiter’s radio emissions were generated along high-latitude magnetic field lines by the same electrons producing the aurorae. Jovian aurorae in the extreme-ultraviolet range as well as radio emissions in the hectometric frequency range (roughly 300 kilohertz to 3 megahertz) have been observed to vary considerably, but the causal mechanism for this was not well understood. The unprecedented opportunity of using two spacecraft – Cassini-Huygens and Galileo – to simultaneously observe Jupiter’s surroundings shed light on this issue. One spacecraft was able to characterize the solar wind at the same time as the other sensed the response of the planet’s magnetosphere. The spacecraft traded off these roles as their positions changed. Inbound toward Jupiter, Cassini-Huygens was in a position to monitor the solar wind while Galileo analyzed the magnetosphere from within. On Cassini-Huygens’ outbound leg, it observed Jupiter’s nightside aurora and fluctuations in its magnetosphere’s

7.5 The Jupiter flyby

199

shape and width, while Galileo monitored the solar wind. During this whole process, the Hubble Space Telescope observed Jupiter’s aurorae from its vantage point in Earth orbit. A key goal of this teaming approach was to distinguish between solar wind influence on auroral intensity and morphology and changes due to internally driven processes such as rotation of the magnetosphere. The multi-spacecraft study caught the Jovian magnetosphere being compressed by an interplanetary shock wave propagating outward from the Sun and carried by the solar wind.89 And measurements of hectometric radio and extreme-ultraviolet auroral emissions from Jupiter suggested that both types of emission were being triggered by the shock wave, which initiated a major reconfiguration of the Jovian magnetosphere, generating strong forces that resulted in electron acceleration along auroral magnetic field lines. This acceleration produced changes in electromagnetic fields leading to radio emissions. And the electrons racing along the magnetic field lines, according to the model, struck the Jovian atmosphere and produced aurorae. Similar processes are known to occur at Earth during geomagnetic storms.90 Further evidence for the connection between solar wind shock waves and Jovian auroral displays was provided by Cassini-Huygens as it approached Jupiter. During the two months preceding its arrival, the spacecraft noted three interplanetary shock waves traveling in the solar wind. Such shocks are not uncommon, because pressure variations in the solar wind tend to steepen into shock waves as they propagate away from the Sun. What was significant, however, was that all three of the shock waves propagating in the direction of Jupiter were followed by “distinctive brightenings of Jupiter’s aurorae … and associated radio emissions.”91 7.5.4

Jovian ring observations

Cassini-Huygens took images of Jupiter’s tenuous ring system to help establish the sources and sinks of its material, in addition to its interactions with magnetospheric plasma flows. During the spacecraft’s half-year encounter with Jupiter, the Imaging Science Subsystem (ISS) took almost 1,200 images of the ring system. This study focused on the main ring, because the halo and gossamer rings were too faint to be observed. It identified possible 1,000 kilometer-scale clumps within the main ring and imposed an upper limit on the ring’s thickness of 80 kilometers. Results from the study suggest that particles from either one or both of the small inner satellites Metis and Adrastea are, in part, the sources of Jupiter’s main ring.92 7.5.5

Satellite analyses

The mission team used the Visual and Infrared Mapping Spectrometer (VIMS) to obtain improved surface composition data for Jupiter’s four largest satellites: Io, Europa, Ganymede, and Callisto. These were discovered by the astronomer Galileo Galilei in 1610 and are collectively known as the Galilean satellites. VIMS, which had nearly double the spectral resolution of the Galileo spacecraft’s Near Infrared Mapping Spectrometer (NIMS), confirmed the presence of carbon dioxide on the surfaces of Callisto and Ganymede, as well as cyanide on Callisto.

200 7.5.6

The interplanetary journey Io’s emissions

The Ultraviolet Imaging Spectrograph (UVIS) picked up oxygen emissions related to Io’s sulfur dioxide atmosphere and Europa’s oxygen atmosphere. In addition, the ISS instrument observed spectral signatures of Io’s atmospheric gases during four of the moon’s eclipses. The fact that spectral emissions from various Io atmospheric constituents, including potassium, sodium, oxygen, and sulfur compounds, were at different altitudes was evidence of stratification in the atmosphere. 7.5.7

Himalia

The spacecraft sighted on the small Jovian moon Himalia for 6 hours at a range of 4.4 million kilometers (2.7 million miles), detecting the possible presence of ice.93 Space scientists believe that irregularly shaped Himalia was not created in the swirl of gas that formed Jupiter, but was most likely an asteroid captured into orbit by the planet.94 7.5.8

A wind of neutral particles

The Magnetospheric Imaging Instrument (MIMI) saw a fast, hot “wind” of neutral particles moving at a speed of over 100 kilometers per second (60 miles per second) and extending more than 0.5 astronomical units95 (over 75 million kilometers, or 46 million miles) from Jupiter. The data indicated that this wind might have originated in gases emitted by the volcanically active moon Io, then evolved through various electromagnetic interactions, escaped Jupiter’s magnetosphere, and finally populated a region more distant from the planet, forming a sort of nebula extending outwards over hundreds of Jovian radii.96 7.5.9

Synchrotron radiation

During the first three months of 2001, the RADAR instrument observed Jupiter’s synchrotron emission. This is electromagnetic radiation from high-energy charged particles moving close to the speed of light in curving paths as they travel along the lines of magnetic or electric fields. The observations were made jointly in space by Cassini-Huygens and on Earth by the antennas of the Deep Space Network (DSN) and Very Large Array (VLA) telescope. Jupiter’s synchrotron emission originates with electrons trapped within its inner radiation belts. Monitoring this can provide significant insights into Jupiter’s magnetosphere. Cassini-Huygens could observe the synchrotron radiation at a frequency of 13.8 gigahertz (GHz), which is not possible using Earth-based telescopes.97 7.5.10

Gravity assist summary

To get some sense of the magnitude of help that Venus, Earth, and Jupiter gravity assists gave the spacecraft, for an unassisted vessel to reach Saturn it would have needed to attain a velocity of 10.3 kilometers per second (6.4 miles per second) in the act of escaping from Earth’s gravity. But Cassini-Huygens only had to attain a speed of 4.1 kilometers per second (2.5 miles per second).98 The combined gravity assists given by Venus (twice), Earth, and Jupiter provided the spacecraft with the energy equivalent of 75 tons of rocket propellant.99,100

7.6

Last leg of the cruise 201

The entire gravity-assisted journey to Saturn, a complex arced trajectory which included four planetary flybys, was an amazing feat of engineering comparable to the “extremely long windup of a celestial baseball pitcher before throwing a curve ball 2 billion miles.”101

7.6

LAST LEG OF THE CRUISE

Although the closest point of the Jupiter flyby occurred on 30 December 2000, the scientific investigations of the planetary system by Cassini-Huygens continued for months afterward. The Jupiter phase of the interplanetary cruise officially ended on 29 April 2001. This was followed by the Quiet Cruise, a 14 month period during which routine maintenance, engineering, and navigation functions were performed. Some science activities also took place during this time, including a December 2001 search for gravitational waves – a phenomenon hypothesized to propagate gravity at the speed of light – and a solar conjunction experiment in June 2002.102 7.6.1

Gravity waves

In the theory of general relativity, one of the most supported theories in physics, propagating gravitational waves can change the distance between test masses. The December 2001 gravity wave experiment used Earth and Cassini-Huygens as test masses. Because of instrument upgrades both on the spacecraft and at Earth, this joint NASA-Italian Space Agency (ASI) effort was the most sensitive gravitational tracking experiment of its type to date. Unfortunately, an instrument failure impacted the results. The Ka-band translator in the Radio Science Subsystem malfunctioned during the experiment owing to a problem with an electronic component. This was a disappointment, but the system functioned well for two of the three searches and useful data were obtained.103 The results suggested approaches that could improve the sensitivity of future searches by an order of magnitude. Multi-spacecraft arrays might further improve the sensitivity of such observations.104 Although gravity waves have not yet been detected, astronomical observations of binary pulsars support their existence.105 7.6.2

Solar conjunctions

The spacecraft’s solar conjunctions occurred when it was on the opposite side of the Sun from Earth. The June 2002 conjunction was used to measure how radio signals that passed close to the Sun on their way from the spacecraft to Earth were affected by the Sun’s mass. According to general relativity, the waves should be deflected by the curvature of spacetime produced by the solar mass, and therefore delayed. Their frequency should be altered as well. Measurement of this Doppler shift agreed with the predictions, supporting the principles of general relativity.106 In July 2002, two years ahead of Saturn Orbit Insertion (SOI), the spacecraft began its year-and-a-half Space Science period, which included two more gravity wave experiments and another solar conjunction experiment. In January 2004, six months before SOI, the Approach Science period was initiated, involving Saturn science observations,

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preparations for the Phoebe flyby, and preparations for the Saturn tour. The spacecraft’s reaction wheels were turned on at the beginning of the Approach Science period in order to furnish a more stable viewing platform than the thrusters had been providing. The imaging system began taking long-range pictures and atmospheric videos of the planet. Four months from SOI, Saturn filled one-third to one-half the field of view of various instruments. As the spacecraft drew closer, VIMS gathered data about temperature differences on Saturn. Meanwhile the fields, particles, and waves instruments collected solar wind data and recorded planetary emissions.107 7.6.3

The Phoebe flyby

The small Saturnian satellite named Phoebe is only 220 kilometers (140 miles) wide. It was discovered in 1898 by the U.S. astronomer William Henry Pickering.108 On 27 May 2004, two weeks before the Phoebe flyby, mission staff performed TCM-20 to fine tune the approach. On 11 June the vessel swung by the satellite at an altitude of 2,000 kilometers (1,200 miles).109 The spacecraft’s imaging system revealed “a scarred moon pounded by massive impacts that tossed building-sized rocks out on to its surface.”110 Its surface was rife with large and small craters (Figure 7.3). One well-defined crater exhibited at least two layers of alternating icy and dark material. The observations strongly suggested that Phoebe contained icerich material, overlain with a thin layer of darker material, perhaps 300 to 500 meters (1,000 to 1,600 feet) thick. Imaging team member Alfred McEwen of the University of Arizona commented that “Phoebe is a world of dramatic landforms, with craters everywhere, landslides, and linear structures such as grooves, ridges, and chains of pits.”111 It is markedly different from rocky asteroids observed by spacecraft at comparable resolution, such as Ida, Mathilde, and Eros. Those did not have the bright speckling associated with Phoebe’s small craters. Torrence Johnson, an imaging team member from JPL who served as the Galileo mission’s project scientist, thought that Phoebe might be “an ancient remnant of the bodies that formed over four billion years ago in the outer reaches of the solar system.”112 It could be a wanderer that was captured by giant Saturn while still a young object. This logic is supported by the fact that the plane of its orbit is closer to that in which the planet orbits the Sun than to the equatorial plane of the planetary system, and it travels around the planet in the opposite sense to the other moons.113 Once the data from Phoebe had been analyzed in the year following the flyby, scientists concluded that it was an object related to Pluto and other members of the Kuiper Belt – the region of the solar system beyond the planets, extending from the orbit of Neptune, which is 30 astronomical units (AU) from the Sun, to as far as 55 AU. Cassini-Huygens interdisciplinary scientist Jonathan Lunine made the point that “Phoebe is quite different from Saturn’s other icy satellites, not just in its orbit but in the relative proportions of rock and ice. It resembles Pluto in this regard much more than it does the other Saturnian satellites.”114 Phoebe’s estimated density is consistent with that of other Kuiper Belt objects and suggests a similar composition of ice and rock to that of Pluto as well as of Neptune’s moon Triton, which also travels around its primary in the “wrong” direction.115

7.7

Arrival at Saturn

203

Figure 7.3 Phoebe. Notice how beat up it is, pockmarked by large and small craters.

7.7

ARRIVAL AT SATURN

As a dramatic finale to its seven year journey through interplanetary space, CassiniHuygens “soared up through Saturn’s rings … settled into orbit [around Saturn] … and … swooped back down through the rings.”116 Within hours of its capture by the planet’s gravitational field, the spacecraft started relaying images of the Saturn system back to Earth that were so spectacular they shocked even the scientists who knew what to expect. Carolyn Porco, imaging team leader, pointed out “the beauty and clarity of these images” and the “structures, literally, that we’ve never imaged before.”117 These pictures not only

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transmitted a wealth of science data about the planetary system, they also communicated a sense of the sheer splendor of Saturn and its rings (Figure 7.4). The aesthetic and emotional effect these haunting images had on viewers around the world helped convey in a

Figure 7.4 Images taken by the spacecraft when it arrived at Saturn. (a) A half-lit Saturn. (b) Saturn’s rings draped by the shadow of the planet. (c) Rings viewed from underneath, after spacecraft plunged through the ring plane. The image shows details of the Encke gap in Saturn’s A ring. The wavy inner edge of the gap was sculpted by the tiny moon Pan that orbits in the middle of the gap (Many more details about the rings will be discussed later in the book).

7.7

Arrival at Saturn

205

Figure 7.4 (continued)

visceral manner the import of sending tiny ships billions of miles away from Earth to examine strange places we do not fully understand. During Saturn Orbit Insertion (SOI) on 1 July 2004, the spacecraft made a close approach to the planet, skimming by at an altitude of only 0.3 Saturn radii, or 18,000 kilometers (11,000 miles). The SOI maneuver had to be performed correctly, for it was “the one activity that [was] absolutely mission critical and essential for the success of the entire program.”118 In order for both the Cassini Orbiter and Huygens Probe to fulfill their respective missions, SOI needed to be carried out precisely as planned, and the flight operations team had just one chance to achieve it. To enter into orbit around the planet, the main engine had to be fired for 95 minutes to slow the vehicle by 630 meters per second (1,400 miles per hour). This burn successfully put the spacecraft on a path that would circumnavigate Saturn every 148 days.119 Among the surprising observations taken shortly after SOI was the detection of an atmosphere-like environment around Saturn’s main rings, consisting of O+ ions as well as ions of molecular oxygen – two atoms of oxygen bonded together, like in Earth’s atmosphere.120 The ring atmosphere differed drastically from that of Saturn, which consists of 91% hydrogen.121 Observing at different frequencies with various instruments, the spacecraft’s first (and indeed subsequent) examinations of the ring system sought to answer

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questions about its composition as well as its history. Were the rings formed by the pieces from various asteroid collisions with Saturn’s moons? Or did they originate from comets drawn in close to Saturn and then fragmented by the planet’s strong gravity? Also, what were the roles of various moons in defining the characteristics and boundaries of the different rings? SOI marked the start of the spacecraft’s Prime Mission, a 4 year tour in which it would swoop closely by many of the icy satellites of Saturn, revealing them to have surprisingly diverse geological processes and appearances. Some moons were pock-marked, others pristine, one was spongy, and another two-faced. One was actively spewing out water jets. Another had probably been captured from the far regions of the solar system.122 On each of its orbits, the vehicle returned to Titan for a gravity-assisted trajectory change. During these flybys it made in-depth observations of the large moon, including extensive radar mapping of its surface which revealed lakes, shorelines, streambeds, and possible cryovolcanism. During these years the craft also studied Saturn’s dramatic meteorology, including its persistent storms, and observed features of the ring system and its strange interactions with various satellites. The planet’s magnetosphere was mapped and its interfaces with the solar wind analyzed. Slowly over the course of the mission, the Orbiter’s observations uncovered many of the processes that formed the diverse features of the Saturn system.

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31. Ken Stowers, “Trajectory Correction Maneuver 5,” final report, D-48431, 5 Feb. 1999, JPL Cassini-Huygens CASTL archive; NASA-JPL, “Trajectory Adjustment Prepares Cassini for Second Venus Flyby,” JPL Media Relations Office, news releases—1998, 3 December 1998; Bob Mitchell email to author, 17 Jan. 2012. 32. NASA-JPL, “Cassini Executes Instrument Checkout,” JPL Media Relations Office, news releases—1999, 15 Jan. 1999. 33. ESA, “The Flybys Around Venus and the Earth Provided a Calibration Opportunity for the Instruments Aboard Huygens and Cassini,” http://sci.esa.int/science-e/www/object/index. cfm?fobjectid=18630, ESA Science & Technology Web site, 4 May 2000. 34. Michael Meltzer, Mission to Jupiter: A History of the Galileo Project (Washington D.C.: NASA SP-2007-4231, 2007), p. 246. 35. NASA-JPL, “Cassini Executes Instrument Checkout.” 36. A. Mark memo to E. Maize, “AACS TCM 6 Final Report,” IOM SCO-99-010, 11 March 1999, JPL Cassini-Huygens archive, WH cabinet 13. 37. Mark Guman and Fernando Peralta memo to Jerry Jones, “Cassini Navigation Reconstruction of TCM-7,” IOM 312.A/013-99, 20 Sept. 1999, JPL Cassini-Huygens archive, WH cabinet 13. 38. Troy D. Goodson, Donald L. Gray, Yungsun Hahn, and Fernando Peralta, “Cassini Maneuver Experience: Finishing Inner Cruise,” available at http://hdl.handle.net/2014/13680 or www. ltas-vis.ulg.ac.be/cmsms/uploads/File/CassiniManeuverExperience_FinishingInnerCruise. pdf, Spaceflight Mechanics Meeting, Clearwater, Florida, 23 Jan. 2000, JPL Beacon eSpace archive, collection JPL TRS 1992+, file 00-0046.pdf. 39. Troy Goodson, “TCM-8 Strategy Meeting: Venus-2 Minus 30 Days Orbit Determination,” in “TCM8 Strategy Meeting Package,” 26 May 1999, JPL Cassini-Huygens archive, WH cabinet 13; Guman and Peralta. 40. JPL, “Significant Event Report for Week Ending 6/30/1999,” http://saturn.jpl.nasa.gov/news/ significantevents/sigevent19990630/, JPL Media Relations Office, 30 June 1999, accessed 12 March 2009; Goodson et al., “Cassini Maneuver Experience: Finishing Inner Cruise.” 41. Science@NASA, “Venus Lends a Helping Hand,” http://science.nasa.gov/newhome/headlines/ast24jun99_1.htm, 25 June 1999, accessed 11 March 2009. 42. John Aiello, “Mission Planning Summary Package for Venus 2-Earth Subphase,” JPL CAS/ ULO-311-99-477 (23 Dec. 1998); Bob Mitchell review of manuscript, Feb. 2011. 43. M.E. Burton, B. Buratti, D.L. Matson, and J.P. Lebreton, “The Cassini/Huygens Venus and Earth Flybys: An Overview of Operations and Results,” J. of Geophys. Research 106(A12) (1 Dec. 2001):30099-30107. 44. Martha Heil and Gary Galluzo, “Cassini Scientists See No Sign of Lightning on Venus,” http:// saturn.jpl.nasa.gov/news/newsreleases/newsrelease20010119/, JPL Media Relations Office, 19 Jan. 2001, access 12 March 2009. 45. Heil and Galluzo, “Cassini Scientists See No Sign of Lightning on Venus.” 46. D.A. Gurnett et al., “Non-Detection at Venus of High Frequency Radio Signals Characteristic of Terrestrial Lightning,” Nature 409 (18 Jan. 2001):313–315. 47. Michael Meltzer, Mission to Jupiter: A History of the Galileo Project (Washington D.C.: NASA SP-2007-4231, 2007), pp. 154–155. 48. Burton et al., “The Cassini/Huygens Venus and Earth Flybys.” 49. S.M. Krimigis et al., “Preliminary Results from MIMI Observations During Cassini’s Venus-2 Flyby on June 24, 1999,” Bull. Am. Astron. Soc. 31(4) (1999). 50. Burton et al., “The Cassini/Huygens Venus and Earth Flybys.” 51. Kevin H. Baines et al., “Detection of Sub-Micron Radiation from the Surface of Venus by Cassini/VIMS,” Icarus 148 (2000):307–311; D.A. Allen and J.W. Crawford, “Cloud Structure on the Dark Side of Venus,” Nature 307 (1984), as reported in Baines et al.

References 209 52. Baines et al., “Detection of Sub-Micron Radiation.” 53. A.I.F. Stewart et al., “Venus’s Airglow as Observed by the Cassini Ultraviolet Imaging Spectrometer,” Bulletin of the American Astronomical Society 32 (Oct. 2000):1120. 54. Burton et al., “The Cassini/Huygens Venus and Earth Flybys.” 55. ESA, “The Flybys Around Venus and the Earth Provided a Calibration Opportunity for the Instruments Aboard Huygens and Cassini,” http://sci.esa.int/science-e/www/object/index. cfm?fobjectid=18630, ESA Science & Technology Web site, 4 May 2000. 56. Burton et al., “The Cassini/Huygens Venus and Earth Flybys.” 57. A. Mark memo to E. Maize, “AACS TCM 9 Final Report – Rev C,” JPL IOM SCO-99-051, 5 Aug. 1999, JPL Cassini archives, WH cabinet 13; E.H. Maize memo to J.M. Millard/A. Avila, “SCO PTT TCM-10 Final Report,” JPL IOM SCO-99-050, 30 July 1999, JPL Cassini archives, WH cabinet 13; E.H. Maize memo to J.M. Millard/A. Avila, “SCO PTT TCM-11 Final Report,” JPL IOM SCO-99-053, 12 Aug. 1999, JPL Cassini archives, WH cabinet 13; E.H. Maize memo to J.M. Millard/A. Avila, “SCO PTT TCM-12 Final Report,” JPL IOM SCO-99-060, 13 Sept. 1999, JPL Cassini archives, WH cabinet 13; JPL “On Target for Earth Flyby,” http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease19990811/, 11 Aug. 1999, accessed 13 Mar. 2009. 58. Burton et al., “The Cassini/Huygens Venus and Earth Flybys.” 59. JPL, “Cassini’s Moon Shots,” Universe 29(17) (3 Sep. 1999), NASA NHRC 17902 Cassini (since launch). 60. W.S. Kurth et al., “An Overview of Observations by the Cassini Radio and Plasma Wave Investigation at Earth,” J. of Geophys. Res. 106(A12) (1 Dec. 2001):30239–30252. 61. A. Lagg et al., “Energetic Particle Measurements During the Earth Swing-By of the Cassini Spacecraft in August 1999,” J. of Geophys. Res. 106(A12) (1 Dec. 2001):30209–30222; Southwest Research Institute, “Substorm,” http://pluto.space.swri.edu/image/glossary/substorm.html, IMAGE Web site, accessed 19 Mar. 2009. 62. A.M. Rymer et al., “Cassini Plasma Spectrometer Electron Spectrometer Measurements During the Earth Swing-By on August 18, 1999,” J. of Geophys. Res. 106(A12) (1 Dec. 2001):30177–30198. 63. R.D. Lorenz et al., “Cassini Radio Detection and Ranging (RADAR): Earth and Venus Observations,” J. of Geophys. Research 106(A12) (1 Dec. 2001):30271–30279. 64. G.A. Abel et al., “Cassini Plasma Spectrometer Observations of Bidirectional Lobe Electrons During the Earth Flyby, August 18, 1999,” J. of Geophys. Research 106(A12) (1 Dec. 2001):30199–30208; David P. Stern and Mauricio Peredo, “The Magnetopause,” http://wwwistp.gsfc.nasa.gov/Education/wmpause.html, no. 19 on the Exploration of the Earth’s Magnetosphere Web site, NASA-GSFC, last updated 5 June 2008, accessed 20 Mar. 1999. 65. NASA, “Cassini Gets a Boost from Earth Flyby,” NASA Headquarters Bulletin (23 Aug. 1999):3. 66. ESA, “Cassini-Huygens Swings BY Earth and Accelerates Toward Saturn,” press release no. 34–99, 18 Aug. 1999, NASA NHRC 17902 Cassini (since launch). 67. NASA-JPL, “Significant Event Report for Week Ending 9/3/1999,” http://saturn.jpl.nasa.gov/ news/significantevents/sigevent19990903/, accessed 3 April 2009. 68. Bob Mitchell review of manuscript, Feb. 2011. 69. K.C. Clausen et al., “The Huygens Probe System Design,” Space Science Reviews 104 (2002):157–158. 70. R. Tagle et al., “On the Nature of S-type Asteroids and the Terrestrial Impactor Population,” 38th Lunar and Planetary Science Conference, League City, Texas, LPI contribution no. 1338 (Mar. 2007):2216.

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71. Lori Stiles, “Cassini Cameras Photograph Asteroid 2685 Masursky,” http://uanews.org/ node/2841, UA News, Univ. of Arizona, 11 Feb. 2000, accessed 24 Mar. 2009. 72. Planetary Society, “Space Topics: Asteroids and Comets,” http://www.planetary.org/explore/ topics/asteroids_and_comets/facts.html, accessed 24 Mar. 2009. 73. NASA-JPL, “Significant Event Report for Week Ending 2/4/2000,” http://saturn.jpl.nasa.gov/ news/significantevents/sigevent20000204/, accessed 3 April 2009. 74. Keith Cowing, “Cassini Completes Trip Through Asteroid Belt. Jupiter is Next, Then Saturn and Titan,” SpaceRef.com, 16 Apr. 2000, NASA NHRC 17902 Cassini (since launch). 75. Robert T. (Bob) Mitchell, “The Cassini/Huygens Mission to Saturn and Titan,” http://trs-new. jpl.nasa.gov/dspace/bitstream/2014/16028/1/00-1883.pdf, International Astronomical Union Congress, IAF-00-Q.2.02 (2000). 76. Gretchen O’Brien and Tom Wiseman, “Cassini on to Saturn After Crossing Asteroid Belt,” Space News (8 May 2000), NASA NHRC 17902 Cassini (since launch); NASA-JPL, “Significant Event Report for Week Ending 4/21/2000,” http://saturn.jpl.nasa.gov/news/significantevents/sigevent20000421/, accessed 3 April 2009. 77. NASA-JPL, “Significant Event Report for Week Ending 6/16/2000,” http://saturn.jpl.nasa.gov/ news/significantevents/sigevent20000616/, accessed 3 April 2009. 78. Robert T. (Bob) Mitchell, “The Cassini/Huygens Mission to Saturn and Titan,” http://trs-new. jpl.nasa.gov/dspace/bitstream/2014/16028/1/00-1883.pdf, International Astronomical Union Congress, IAF-00-Q.2.02 (2000). 79. Todd Barber and Richard Cowley, “Initial Cassini Propulsion System In-Flight Characterization,” AIAA-2002-4152, 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Indianapolis, Indiana, 7–10 July 2002. 80. JPL, Cassini Mission Plan, Revision N (PD 699-100), JPL D-5564, 2002, as reported in NASAJPL,“MissionInformation,” http://starbrite.jpl.nasa.gov/pds/viewMissionProfile.jsp?MISSION_ NAME=CASSINI-HUYGENS, accessed 4 Apr. 2009; NASA-JPL, “Jupiter Millennium Flyby,” http://www.jpl.nasa.gov/jupiterflyby/, accessed 22 Mar. 2009. 81. Larry Esposito, “Cassini Imaging at Jupiter,” Science 299 (7 March 2003):1529–1530; Carolyn C. Porco et al., “Cassini Imaging of Jupiter’s Atmosphere, Satellites, and Rings,” Science 299 (7 March 2003): 1541 – 1547. 82. NASA-JPL, “Galileo and Cassini Image: Two Giant Plumes on Io,” http://www.nasaimages. org/luna/servlet/detail/NVA2~14~14~26099~124675:Galileo-and-Cassini-Image – Two-Gian, NASA Images Web site, accessed 23 Mar. 2009. 83. Porco et al., “Cassini Imaging.” 84. Guy Webster and Maria Martinez, “Seventy-Day Jupiter Movie Pulls Patterns Out of Chaos,” JPL Media Relations Office, 16 July 2001. 85. Webster and Martinez, “Seventy-Day Jupiter Movie.” 86. ScienceDaily, “Cassini Visualizes The Invisible, Tracks Giant Storms On Jupiter,” http://www. sciencedaily.com/releases/2001/01/010101103713.htm, (Jan. 1, 2001), accessed 27 Mar. 2009. 87. B.F. Burke and K.L. Franklin, “Observations of a Variable Radio Source Associated with the Planet Jupiter,” J. Geophys. Res. 60 (1955):213–217; F.D. Drake and S. Hvatum, “Non-Thermal Microwave Radiation from Jupiter,” Astron. J. 64 (1959):329–330, as reported in D. A. Gurnett et al., “Control of Jupiter’s Radio Emission and Aurorae by the Solar Wind,” Nature 415 (28 February 2002):985–987. 88. Aurorae are the atmospheric bands of light generated by charged solar particles following a planet’s magnetic field lines. 89. Candice J. Hansen et al., “The Cassini-Huygens Flyby of Jupiter,” Icarus 172 (2004):1–8. 90. D. A. Gurnett et al., “Control of Jupiter’s Radio Emission and Aurorae by the Solar Wind,” Nature 415 (28 February 2002):985–987.

References 211 91. Thomas W. Hill, “Magnetic Moments at Jupiter,” Nature 415 (28 Feb. 2002):965–966. 92. H. Throop et al., “Cassini Imaging Observations of Jupiter’s Rings,” American Astronomical Society, DPS meeting #35, #11.07 (2003); Carolyn C. Porco et al., “Cassini Imaging of Jupiter’s Atmosphere, Satellites, and Rings,” Science 299 (7 March 2003):1541–1547; H. B. Throop, C. C. Porco, “Cassini Observations of Jupiter’s Rings,” http://aas.org/archives/BAAS/ v33n3/dps2001/552.htm , DPS 2001 meeting, Session 32 – Rings II (Nov. 2001). 93. Candice J. Hansen et al., “The Cassini-Huygens Flyby of Jupiter,” Icarus 172 (2004):1–8. 94. NASA/JPL/University of Arizona, “Himalia, a Small Moon of Jupiter,” http://saturn.jpl.nasa. gov/photos/imagedetails/index.cfm?imageId=656, 23 January 2001. 95. An astronomical unit, or AU, represents a length equal to the distance from the Sun to Earth, about 150 million kilometers or 93 million miles. 96. Stamatios M. Krimigis et al., “A Nebula of Gases from Io Surrounding Jupiter,” Nature 415 (28 February 2002):994–996. 97. NASA-JPL, “Significant Event Report for Week Ending 1/5/2001,” http://saturn.jpl.nasa.gov/ news/significantevents/sigevent20010105/, accessed 4 April 2009. 98. Robert T. Mitchell email to author, 8 Dec. 2010. 99. This number serves as a means of communicating how much energy gravity assists can give to a spacecraft. It must be noted, however, that if gravity assists had not been available and more fuel had been needed for the voyage, a bigger launch vehicle would likely have been employed so that the spacecraft could have gone direct from Earth to Saturn without all the loops around the Sun on the way, and this more direct route would not have required 75 tons of propellant. Robert T. (Bob) Mitchell email to author, 15 Nov. 2011. 100. Associated Press, “Cassini-Navigation,” 14 Aug. 1999, in a package beginning with Matthew Fordahl, “Plutonium Powered Spacecraft to Zoom By Earth on Way to Saturn,” NASA NHRC 17902 Cassini (since launch). 101. Warren E. Leary, “En Route to Saturn, A Boost from Earth: Cassini Craft’s Flyby is Exactly on Course,” NY Times (17 Aug. 1999):D1 102. JPL, Cassini Mission Plan, Revision N (PD 699-100), JPL D-5564, 2002, as reported in NASA-JPL, “Mission Information,” http://starbrite.jpl.nasa.gov/pds/viewMissionProfile. jsp?MISSION_NAME=CASSINI-HUYGENS, accessed 4 Apr. 2009. 103. Bob Mitchell interview with author, JPL, 26 Oct. 2010. 104. S.F. Abbate et al., “The Cassini Gravitational Wave Experiment,” Astronomical Telescopes & Instrumentation (22 Aug. 2002), JPL Beacon eSpace archive, collection JPL TRS 1992+, file 02-1711; J.W. Armstrong, “Low-Frequency Gravitational Wave Searches Using Spacecraft Doppler Tracking,” Living Reviews in Relativity 9 (2006):1. 105. Dave Kornreich, “Have Gravitational Waves Been Proven to Exist?” http://curious.astro.cornell.edu/question.php?number=83, Ask an Astronomer Web site, Astronomy Department, Cornell University, last updated 2 May 2002, accessed 4 Apr. 2009. 106. B. Bertotti1, L. Iess, and P. Tortora, “A Test of General Relativity Using Radio Links with the Cassini Spacecraft,” Nature 425 (25 September 2003):374–376; L. Iess et al., “The Cassini Solar Conjunction Experiment: a New Test of General Relativity,” IEEE Aerospace Conference Proceedings 1 (8–15 March 2003). 107. JPL, Cassini Mission Plan, Revision N (PD 699-100), JPL D-5564, 2002, as reported in NASA-JPL, “Mission Information,” http://starbrite.jpl.nasa.gov/pds/viewMissionProfile. jsp?MISSION_NAME=CASSINI-HUYGENS, accessed 4 Apr. 2009 108. BBC News, “Cassini Pass Reveals Moon Secrets,” http://news.bbc.co.uk/1/hi/sci/tech/3798485. stm, 14 June 2004. 109. Troy Goodson et al., “Cassini-Huygens Maneuver Experience: Cruise and Arrival at Saturn,” AAS/ AIAA Astrodynamics Specialist Conference, Lake Tahoe, CA, Aug. 2005, Paper AAS 05-286.

212 110. 111. 112. 113. 114. 115. 116. 117. 118. 119.

120. 121. 122.

The interplanetary journey BBC News, “Cassini Pass Reveals Moon Secrets.” ScienceDaily “Phoebe’s Surface Reveals Clues To Its Origin,” (15 June 2004). ScienceDaily “Phoebe’s Surface Reveals Clues To Its Origin,” (15 June 2004). David Michael Harland, Cassini at Saturn: Huygens Results (Springer Praxis, Praxis; 1 edition (2007):33. ScienceDaily, “Scientists Discover Pluto Kin Is A Member Of Saturn Family,” http://www. sciencedaily.com/releases/2005/05/050507095634.htm (7 May 2005). Paolo Ulivi with David Michael Harland, Robotic Exploration of the Solar System: Part 1 – The Golden Age, 1957–1982 (Springer Praxis, Praxis; 1 edition (2007):423. Guy Gugliotta, “Scientists Marvel Over Planet Rings,” Washington Post (2 July 2004):A3, NASA NHRC 18426 Cassini/Huygens Saturn Arrival (June 30, 2004). Leonard David, “Cassini Delivers ‘Shocking’ Images of Saturn’s Rings,” SpaceNews (5 July 2001):7, NASA NHRC 18426 Cassini/Huygens Saturn Arrival (June 30, 2004). Mona M. Witkowski et al., “Managing Risk to Ensure a Successful Cassini/Huygens Saturn Orbit Insertion (SOI),” SpaceOps 2004 Conference, Montreal, Canada (17–21 May 2004). JPL, Cassini Mission Plan, Revision N (PD 699-100), JPL D-5564, 2002, as reported in NASA-JPL, “Mission Information,” http://starbrite.jpl.nasa.gov/pds/viewMissionProfile. jsp?MISSION_NAME=CASSINI-HUYGENS, accessed 4 Apr. 2009. R.E. Johnson et al., “Production, Ionization and Redistribution of O2 in Saturn’s Ring Atmosphere,” Icarus 180 (2006):393–402. Bjorn Carey, “Cassini Data Suggests Atmosphere for Saturn’s Rings” Space News (29 Aug. 2005), NASA NHRC 18337 Cassini 2002-. NASA-JPL, “Saturn’s Mingling Moons May Share A Dark Past,” http://www.sciencedaily. com/releases/2008/02/080219122014.htm, ScienceDaily (21 Feb. 2008), accessed 26 May 2009.

8 How a few people can make a big difference: The Doppler shift problem that nearly ended the Huygens mission “When the Cassini-Huygens mission blasted off from Cape Canaveral in October 1997, no one suspected that a critical design flaw was lurking deep within the telemetry system onboard Cassini that was dedicated to harvesting Huygens’s broadcast.” – James Oberg, “Titan Calling,” IEEE Spectrum (Oct. 2004)

A critical problem in the radio link between Huygens Probe and Cassini Orbiter was discovered during the cruise to Saturn. The issue was related to the complexities of an international mission with many players and with engineering data that could not be freely shared among all parties. The problem threatened to block vital Huygens Probe scientific findings from ever reaching Earth. For the Cassini-Huygens mission to succeed, communication within and between NASA, ESA, and ASI needed to be reliable and efficient. In almost all instances, communication was excellent, but in one notable exception, a series of incidents came disturbingly close to cancelling out the benefits of the Huygens Probe exploration of Titan.

8.1

THE PROBE-ORBITER TRANSMITTER LINK

A serious flaw with the Probe-Orbiter transmitter-receiver link that had potentially catastrophic consequences for the mission’s science return came to light a full three years after the spacecraft launched, while cruising through the asteroid belt toward Jupiter. This anomaly was totally missed by the exhaustive prelaunch checks and tests. ESA contractor Alenia Spazio designed and built the radio link between the Probe and the Orbiter. It was composed of two basic parts: the Probe Transmitter Terminal and the Orbiter Receiving Terminal.1 In February 2000, the mission team conducted an in-flight check of the Probe to characterize its support equipment performance in realistic flight © Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_8

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214 How a few people can make a big difference… conditions. Mission engineers transmitted a test signal from NASA’s Goldstone Deep Space Network (DSN) ground station in California to the Cassini Orbiter’s receiver. This test revealed that the instrument might be unable to receive and decode data transmitted by the Huygens Probe as it descended by parachute through the atmosphere of Titan. This would be an enormous setback for the mission. Although the problem had eluded the mission team while Cassini-Huygens was on the ground, the continuing battery of tests and simulations as well as the intelligence of mission personnel did eventually catch it. Fortunately, the vessel was still 4 years out from Saturn and there was time to work the issue. Claudio Sollazzo, the ground operations manager for Huygens at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany, confessed that he’d “had a nagging worry about the lack of a full-up communications systems test”2 and the problems that might have been missed as a result. During the spacecraft’s long and fairly uneventful cruise, Sollazzo fought for and eventually received permission to conduct an in-depth analysis of the Probe-to-Orbiter communications capability. In January 1998 he selected one of his senior engineers – Boris Smeds, a 26 year ESA veteran from Sweden – to carry out the investigation. He designed a signal to be beamed from Earth that would mimic a radio transmission from the Huygens Probe during its descent to Titan. Smeds was a good choice for this task, for he specialized in communications link issues between ESA’s global ground antenna network and its eleven active scientific spacecraft. During the cruise, it was not feasible to directly test the radio link using the Probe and Orbiter. They had been tightly mated together before launch and communicated not by radio but through a cable. Even if the Probe could be made to transmit to the Orbiter, successfully sending a radio signal a few centimeters would hardly simulate the conditions during the Titan descent, when the distance between the two vessels would be rapidly changing.3 After the spacecraft reached Saturn, it was to go into orbit around the planet. The mission plan was for the Orbiter then to release the Probe on a trajectory that would intercept Titan. The Orbiter would maneuver for a low-altitude, high-velocity flyby of the moon. According to the plan, the Probe would reach Titan in advance of the Orbiter, enter the atmosphere, and slowly descend by parachute. As the Orbiter flew overhead at a relative speed of 21,000 kilometers per hour (almost 6 kilometers per second, or about 13,000 miles per hour) there would be a Doppler shift increase of about 38,000 cycles per second (38 kilohertz) in the frequency of the radio waves transmitted by the Probe, as observed by the Orbiter’s receiver. The basics of a Doppler frequency shift can be understood by considering a speeding ambulance with its siren blasting. When the siren on a moving ambulance emits a sound wave, an observer in front of the object experiences an increase in the wave’s frequency (and a higher pitch) as its peaks and troughs arrive more quickly, or in other words, at a higher frequency, due to the object’s motion. Conversely, an observer behind the moving object notices a decrease in the wave’s frequency as the object speeds away, causing its peaks and troughs to arrive less often. The faster the object is moving, the greater the frequency shift. This mechanism is what produces the familiar rise and fall in pitch of an ambulance’s siren as the vehicle speeds by. It is also what produces a frequency shift in a radio signal – an electromagnetic wave – passing between two space vessels in relative motion. And if one of the vessels is equipped to receive only a narrow band of radio

8.1

The Probe-Orbiter transmitter link

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frequencies, then a Doppler shift might alter the frequency of the transmitted signal so much that it simply cannot be picked up by the receiver on the other vessel. At Titan, the Probe was going to communicate with the Orbiter by transmitting a carrier signal with the data modulated onto it. But at the time that Sollazzo brought Smeds into the communications link testing effort, the plan to test the Orbiter’s receiver had been only to transmit the carrier wave (without data, or “telemetry” modulated onto it) to the Orbiter from Earth. If the Orbiter receiver could read the carrier wave, the system would pass the test. Smeds, however, insisted that a much closer simulation of the expected mission conditions at Titan was necessary. His proposal was to send a signal that had telemetry modulated onto the carrier wave. At first, Smeds met with opposition from many project personnel who thought that such a detailed analysis was unnecessary, and the scheme was rejected. The project budget was always a factor, and decisions had to continually be made that attempted to balance economy with the perceived importance of the procedure. But fortunately Smeds continued to push for his detailed investigation. In the end, with the support of Sollazzo and Huygens Project Scientist Jean-Pierre Lebreton, mission management accepted Smeds’ plan, not because they thought it necessary, but largely because it would be easy and relatively cheap to perform.4 Smeds used ESOC’s engineering model of Huygens, “an exact duplicate of the lander down to the last bolt and transistor,”5 to create a representative stream of telemetry which he then modulated onto a special test signal he developed on his computer. He also simulated an expected Titan atmospheric condition with his test signal. As the Huygens Probe descended, it was expected to get buffeted around by Titan’s atmosphere, changing the orientation of the Probe transmitter. The receiver on the Orbiter would experience this as variations in the signal amplitude. Smeds’ way of simulating this was for his test signal pattern to be broadcast from Earth to the Orbiter in a sequence of varying power levels that replicated the Probe and its transmitter being tossed around in Titan’s atmosphere. As he was developing the test signal, Smeds had the insight to carefully adjust its frequency so that when it arrived at the Cassini Orbiter it would match the Doppler-shifted signal expected from the Probe. Smeds would then verify the signal received by the Orbiter by matching it against the original telemetry that he used to create the test signal. Using an antenna in the Goldstone DSN array, Smeds sent the broadcast to the Cassini Orbiter’s receiver. The Cassini Orbiter then sent its received signal back to Earth. If the system was working, the data signal received on Earth should reproduce the original signal. It did not. When the DSN sent the test signal, the spacecraft was 430 million kilometers (270 million miles) from Earth, and it took 48 minutes for the signal to complete the round trip back to Goldstone. When NASA relayed the signal to ESOC for analysis, it became apparent that something was amiss. Although the data in the returned signal “was a mess,”6 the situation was puzzling because occasionally there were bursts of well-reproduced data. Smeds had expected that when he increased the power level in his transmitted signal, more data would get to the spacecraft and what was returned back to Earth would be of higher quality. Initially, this was the case. But then when he increased the power level even more, the data again came back corrupted.

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During his hour-long drive to his motel from Goldstone, Smeds kept thinking about the returned data, sometimes scrambled and sometimes good. He wondered if the problem had nothing to do with varying power levels of the signal, but only with its Doppler frequency shift. So when he returned to Goldstone, he told the signal processing center there to send the test signal with a zero simulated Doppler shift. This was done. After the Orbiter sent back the signal and it was analyzed, Smeds clearly saw an improvement in the returned data. He did further tests, confirming that the presence of a Doppler shift in the signal from the Probe would dramatically degrade the data quality received by the Orbiter. Further analysis of the Cassini-Huygens craft performed by Alenia Spazio, the Romebased company that supplied the radio link apparatus for the Probe and the Orbiter – as well as for ground tests conducted at ESOC – confirmed that a Doppler frequency shift, such as would occur during the mission, would indeed cause the Probe’s data signal to fall outside the bandwidth of the Orbiter receiver’s narrow-band detector. This anomaly in the communication subsystem threatened to prevent reception of most of the data transmitted by the Probe during its descent through the atmosphere of Titan. If this happened, it would negate one of the major elements of the mission.7

8.2

THE STRUCTURE OF PROBE TRANSMISSIONS

As mentioned above, transmissions by the Probe involved carrier waves on which sets of telemetry were modulated and broadcast at a rate of 8,192 bits per second. For the Orbiter to receive and read this telemetry required immaculate timing. The Orbiter receiver needed to break the incoming signal into 8,192 parts every second and its decoder needed to read the binary code embedded in them. The receiver’s decoder could accommodate small frequency shifts in the incoming data rate, but with a Doppler shift such as would be produced by the relative speed between the Orbiter and the Probe, the decoder would generate a stream of binary junk. And to make matters worse, the Orbiter’s Alenia-built receiver contained internal firmware – coded instructions stored permanently in read-only memory – that implemented the receiver’s timing sequence. They were unalterable once the spacecraft was off the ground, and so could not be changed to accommodate large Doppler frequency shifts in transmissions from the Probe. It was therefore vital to devise a scheme to enable the data transmitted by the Probe to be correctly received by the apparatus aboard the Orbiter.8

8.3

ENQUIRIES INTO THE DOPPLER PROBLEM

An investigation team established in the spring of 2000 from ESA, NASA/JPL, and industry representatives sought to definitively assess the problem’s potential impacts on the Huygens mission. This team conducted ground tests using reference models of the Probe, particularly, the “engineering” and “spare” models, and confirmed the communication subsystem’s design flaws and the adverse impact which they would have on data recovery during the descent through Titan’s atmosphere. In response to the crisis, the Director General of ESA convened an independent Huygens Probe Communications Link Enquiry Board. This was unable to find any direct reference at any level of the project requirements or design specifications relating to

8.3

Enquiries into the Doppler problem

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Doppler frequency shifts on the data sent in the radio signal transmitted from the Huygens Probe to the Cassini Orbiter. This was a serious error of omission and it was perpetuated throughout the life of the project before launch. Not a single recorded question had been raised on the subject in any ESA, NASA, or independent review.9 8.3.1

Issues of trust, economics, and geography

The communication system’s Doppler problem provided a good illustration of how mission-damaging predicaments can arise when two vital parts of a spacecraft are constructed on different continents by different groups of engineers, and when there are issues of trust between those groups. The builders of the spacecraft felt confident in their work: both the Cassini Orbiter and the Huygens Probe had been extensively tested prior to launch, both independently and together. But a proposal to conduct a detailed test of the radio link from the Probe to the Orbiter, in which every system would be subjected to a simulation of the exact signals and conditions during flight, had been rejected because it would have required disassembling and reassembling the communication system components. According to Cassini-Huygens Program Manager Bob Mitchell, “budget was a key part” of this decision, for the reassembled spacecraft would then have had to undergo exhaustive and expensive recertification. There were many checks performed during spacecraft construction, but none were the ones needed to catch this particular anomaly. As John Credland, head of Space Science Projects at ESA explained, “We had three safety nets set up to catch things like Cassini-Huygens’ communications problem, and it now appears that we fell through all three.”10 The Doppler problem, and the near-disaster it caused in terms of vital data loss, illustrated the requirement for an end-to-end assessment of spacecraft performance during the ground testing phase by simulating post-launch conditions as closely as possible. The Doppler problem did not surface during any of the ESA, NASA, or independent reviews carried out during the project’s life cycle. Two independent reviews were carried out in which the reviewers had face-to-face interviews with engineers across the project, and these procedures were quite detailed. But still the effect of Doppler shift was missed. The Doppler problem also pointed to the need for an organizational structure in which critical information can be more freely shared and more easily accessed. It was unfortunate that vital data on the design of the receiver was not made readily available to NASAJPL reviewers, because this restriction probably played a part in missing the Doppler anomaly. ESA and its contractor Alenia had an agreement that certain design information could only be viewed on an Alenia site, not sent to JPL. Bob Mitchell explained that Alenia considered JPL a competitor and thus treated its radio design as proprietary. Richard Horttor, a NASA interplanetary probe project manager and a chief of telecommunications engineering at JPL, had further insight into this issue. While NASA-JPL probably could have insisted on seeing the design, it would first have had to sign nondisclosure agreements. JPL was very busy with mission planning and did not consider the whole effort or expense of accessing and analyzing the design documents at Alenia worthwhile. JPL assumed, erroneously, that Alenia would have built the capability to compensate for a changing data rate into its radio equipment.11

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8.4

SAVING THE MISSION

“In any complex space mission problems may arise. The measure of an organization is the manner in which it recovers.” – John Credland, Head, ESA Space Science Projects Department12 Uncovering the organizational reasons why the Doppler anomaly was never noticed until 3 years after launch was useful for preventing such issues in future flights, but it didn’t solve the present problem. Mission staff could not alter the firmware in the receiver onboard the Orbiter, so this apparatus could not be reprogrammed to “hear” the Dopplershifted signal from the Probe. What could be done to capture the vital data from Titan? By the time the mission team fully understood the communication link problem, the team’s navigators had already developed a complex, multi-year flight plan that would involve 44 flybys of Titan, 8 close passes of smaller moons, and between 50 and 100 distant inspections of the smaller moons. Its production had been laborious and timeconsuming, and reconstructing it would seriously increase mission costs. The mission team needed to find a way to reduce the Doppler shift in the Probe’s signal sufficiently to keep it within the recognition range of the Orbiter’s receiver, and achieve this in a way that caused the minimum of revision to the existing flight plan. After six months of analysis by a joint ESA-NASA Huygens Recovery Task Force, senior management from both agencies and members of the Cassini-Huygens scientific and engineering community developed a way to proceed. The spacecraft would, as planned, go through its Saturn Orbit Insertion (SOI) maneuver on 1 July 2004. But the revised scheme called for a new 32 day orbit to be inserted early in the Prime Mission tour in order to create a modified trajectory for the Orbiter while the Probe performed its mission at Titan. Vital to this strategy was that the additional orbit could be added to the beginning of the tour without modifying the subsequent sequence of Titan flybys.13 Thus, the original flight plan, labored over and refined for so long, could remain intact and the science that the Orbiter tour would generate could be preserved. On the modified tour, Cassini-Huygens would fly by Titan on 26 October 2004, followed by another close encounter on 13 December. Then on 25 December, the Orbiter would release the Probe toward Titan using its spring-loaded spin-eject device, and the Probe would enter the moon’s atmosphere 22 days later. The key was that the Orbiter would be given a new trajectory that minimized the relative velocity between the Probe and the Orbiter, and thus also minimized the Doppler shift, since that was dependent on the relative velocity.14 Using the original trajectory, the Orbiter would have plunged toward Titan for a close flyby of 1,200 kilometers (750 miles). That path would have produced a relative velocity peaking at about 6 kilometers per second (13,000 miles per hour) and a Doppler shift that the Orbiter could not handle. The new trajectory employed a slower Orbiter pass over Titan that approached no closer than 60,000 kilometers (37,000 miles) from the surface. On this new trajectory, the relative velocity never rose much above 4 kilometers per second, and the smaller Doppler shift would allow the Orbiter to read the data from the Probe.15

References 219 The price paid for a 60,000 kilometer flyby of Titan instead of one at an altitude of 1,200 kilometers was that Titan’s gravitational field, which falls off rapidly with distance from the moon, would not bend the Orbiter’s path nearly so much as in a close flyby. An extra flyby was added to the mission so that the trajectory could be adjusted to return the vessel to Titan, which was essential in order to receive further gravity assists.16 The new scheme also called for several procedures to maximize the efficiency of the communication system on the Probe. These included pre-heating the electronics to improve tuning of its transmitted signal, continuously monitoring and controlling the receiver on the Orbiter in order to maximize its performance, and improving the Probe’s onboard software. The new trajectory had other advantages besides reducing the Doppler shift to a level that the Orbiter’s receiver could handle. It gave earlier observations of Titan’s upper atmosphere, improving our knowledge of its structure and composition well before releasing the Probe. In particular, these early observations put better bounds on the atmosphere’s argon concentration and indicated that methane was not present in sufficient amounts to adversely affect the Probe’s entry. And they provided data on the winds that the Probe was likely to encounter. The new plan also permitted the Orbiter to smoothly pursue the 4 year trajectory that had been planned for it after the Probe had achieved its mission at Titan. Thus, disruptions to the carefully designed Orbiter tour of Saturn and its moons were kept to a minimum.17

REFERENCES 1. G. Boscagli and M.C. Comparini, “Deep Space Receiver for Cassini-Huygens Radio Link,” European Microwave Conference, Helsinki, Finland, vol. 1 of proceedings (October 1992): 119–124. 2. James Oberg, “Titan Calling,” IEEE Spectrum Magazine, October 2004. 3. D.C.R. Link (chair), Huygens Communication Link Enquiry Board Report, NASA, 20 December 2000. 4. Oberg, “Titan Calling.” 5. Oberg, “Titan Calling.” 6. Oberg, “Titan Calling.” 7. D.C.R. Link (chair), Huygens Communication Link Enquiry Board Report, NASA, 20 December 2000. 8. Oberg, “Titan Calling.” 9. Link, Huygens Communication Link Enquiry Board Report. 10. Both quotes on this page are from James Oberg, “Titan Calling.” 11. Link, Huygens Communication Link Enquiry Board Report; James Oberg, “How Huygens Avoided Disaster,” http://www.thespacereview.com/article/306/1, Space Review (17 Jan. 2005). 12. “ESA and NASA Agree on a New Mission Scenario for Cassini-Huygens,” press release (2 Jan. 2001). 13. Nathan J. Strange et al., “Cassini Tour Redesign for the Huygens Mission,” AIAA Astrodynamics Specialist Conference, Monterey, CA, 5 Aug. 2002, JPL Beacon eSpace archive, collection JPL TRS 1992+, file 02–1385.pdf.

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14. Jean-Pierre Lebreton et al., “An Overview of the Descent and Landing of the Huygens Probe on Titan,” Nature 438 (8 December 2005):758–764. 15. Strange. 16. Strange. 17. Dolores Beasley, Guy Webster, and Franco Bonacina, “European Space Agency and NASA Set New Cassini-Huygens Plan,” NASA News release 01–132, 29 June 2001.

9 The Titan Huygens Probe mission “Think of Titan as the Peter Pan of the solar system … it’s the … world that never grows up. It’s kind of stuck in time. It’s got all the essential elements for life, the big four, H, C, O, N, but it’s too cold.” – Toby Owens, Cassini investigator studying the origin and composition of planetary atmospheres

Targets of NASA and ESA missions are typically identified years before a project ever kicks off. Cassini-Huygens was no exception to this. As Trina Ray, co-chair of the Titan Orbiter Science Team explained, “Each generation teaches the next one what are the questions.”1 And each generation teaches the next how to design its spacecraft and what to aim for. Voyager and Galileo taught Cassini-Huygens what instruments to take, what experiments it needed to perform, what design problems it had to avoid, and what celestial targets it had to explore. Titan was high on the list of targets. In the early 1980s, NASA convened the Solar System Exploration Committee (SSEC) to recommend the most important types of future space expeditions. SSEC also focused on lowering mission expense. Its “marching orders were to try to figure out a cheaper way to go exploring the solar system.”2 A subcommittee on the outer solar system quickly identified Titan as a critically important object to investigate. There were many fascinating and compelling unanswered questions about the moon that the Voyagers had only begun to research when they flew past in 1980 and 1981. But numerous engineering challenges had to be surmounted to successfully explore Titan. One of them was to design a probe that could make in situ observations of the atmosphere and surface of the satellite. # After the Huygens Probe, attached to the Cassini Orbiter, launched from Cape Canaveral Air Force Station, it had to achieve several milestones before it could touch down on Titan: • • •

Cruise from Earth to the Saturn system, while attached to the Orbiter Separate from the Orbiter Coast to Titan (since it didn’t have engines of its own)

© Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_9

221

222 The Titan Huygens Probe mission • • •

Enter Titan’s atmosphere and rapidly decelerate Obtain and report observations of the atmosphere as it descended on its parachute Survey what it could from its landing spot on the surface (should it manage to survive its touchdown).

In addition to the Huygens Probe’s actions, the Cassini Orbiter performed several procedures during the mission that were critical for the Probe’s success: • • •

Adjust its trajectory and orientation so that as the Probe separated, it would be pushed onto the proper path to intercept Titan Initiate the separation sequence at exactly the right time Alter its trajectory to ensure that it would not follow the Probe into Titan’s atmosphere and instead would be positioned to receive the data transmitted by the Probe during the operational phase of its mission (there would be no contact during the Probe’s three week cruise).

These and other Probe and Orbiter actions are discussed below.

9.1

9.1.1

FROM EARTH TO TITAN: THE CRUISE, SEPARATION, AND COAST PHASES Cruise phase

Once launched, the Cassini-Huygens spacecraft, which included both Orbiter and Probe, began its cruise phase toward the Saturn system. This journey, including its gravity assists and Orbiter observations of various planets, was described in detail earlier in the book. Although the Probe remained dormant throughout most of the interplanetary cruise, mission staff did periodically revive it to monitor the health of its subsystems and scientific instruments.3 Once the spacecraft was in orbit around Saturn, the time finally came for the Probe to conduct its own mission at Titan. 9.1.2

Separation phase

Approximately 50 days after the Saturn Orbit Insertion (SOI) maneuver on 1 July 2004, the Orbiter conducted a maneuver using its propulsion system that altered the vessel’s orbit around Saturn so that it would pass relatively close to Titan (the Probe was still attached to the Orbiter at this time). In early December 2004, as the planned separation date neared, NASA and ESA management and engineering teams met to verify that the Probe, Orbiter, and their support systems on Earth were healthy and ready. Simultaneous with this meeting, mission staff conducted risk review analyses as additional decision exercises in the process to determine whether separation should proceed. Both of these events produced positive recommendations to go ahead with the separation. A final NASA-ESA decision exercise then polled personnel as to space vehicle health and ground support system readiness. Finding no reason not to proceed, the mission team configured the Probe and Orbiter for separation.4 However, before the Probe could be released, the mission team needed to again modify the spacecraft’s trajectory. Since SOI, the craft had swung past Titan on two orbits. The second flyby, on 13 December 2004, had put it on a trajectory which, if left uncorrected,

9.1

From Earth to Titan 223

would lead to a subsequent flyby at a distance of 4,600 kilometers (2,900 miles) from the moon. But the Probe needed to intercept Titan, not fly by it! And once the Probe was released, its trajectory could no longer be changed because it had no propulsion system. So the Orbiter had to release the Probe in precisely the right direction and at just the right speed to reach Titan, and in the right orientation for atmospheric entry. The first step was the targeting maneuver on 17 December that placed the entire Cassini-Huygens spacecraft on an impact trajectory with the moon. The trajectory was fine-tuned on 22 December.5 These maneuvers weren’t quite enough to prepare Huygens for atmospheric entry, however. The spacecraft now needed to rotate to the appropriate orientation so that the spin-eject device could push the Probe onto exactly the desired trajectory. This orientation was such that the Orbiter’s fixed-in-place high-gain antenna would not point toward Earth during the separation process, preventing any data transmission to Earth until after the Probe was free and the Orbiter rotated again, reorienting its antenna. Prior to separation, the mission team on Earth controlled the Probe functions by sending commands to the Orbiter, which passed them on, along with power, to the Probe through an umbilical cable. But once separation occurred, the Probe would be on its own. It would have to rely on its own power source, a set of lithium batteries, and would communicate with the Orbiter via radio. This communication would be one-way to the Orbiter. No commands could be sent to the Probe. And there would be no transmission from the Probe during its 3 week cruise. The first that would be heard from the Probe would be when it deployed its parachute in the atmosphere of Titan.6 On 25 December 2004, the spacecraft performed the separation sequence. This involved firing pyro bolts – fasteners incorporating small explosive charges. When these charges blew, explosions severed the bolts that held the Probe to the Orbiter. Three push-off springs, held compressed for the entire journey thus far, shoved the Probe away. It rolled along three spiral tracks that gave it a spin of 7.5 revolutions per minute, which helped stabilize it. The Probe coasted away with a relative speed of 0.35 meters per second (0.8 miles per hour, or about 1 foot per second). As part of this separation process, the umbilical cable was severed, leaving the Probe truly isolated from its mother ship. All actions between the time the separation command was delivered and the time the two vehicles were no longer in physical contact took place in an interval of just 0.15 seconds.7 9.1.3

Coast phase

The Probe now entered a 21 day coast phase for a distance of 4 million kilometers (2.5 million miles). NASA and ESA deemed that the coast phase would end when the Probe reached an altitude of 1,270 kilometers (789 miles) above Titan’s surface. This was not actually the upper limit of Titan’s atmosphere, it was just the location, negotiated between NASA and ESA, for which the probe’s trajectory was targeted. At that point, responsibility for its trajectory would pass from NASA to ESA.8 After separation and during the Probe’s entire coast phase, in order to conserve the power in its batteries, the craft remained in a low-power sleep mode and did not communicate with the Orbiter. The only system operating during this period was a timer counting down to the moment at which the craft had to wake up, immediately before it entered Titan’s atmosphere. With almost all its systems sleeping, the Probe would have run a risk of getting too cold for its crucial components to survive, but this danger was averted by the thermal energy from its plutonium-fuel radioisotope heater units.9

224 The Titan Huygens Probe mission 9.2

ORBITER ACTIVITIES DURING THE PROBE’S COAST PHASE

On 28 December 2004, while the Probe coasted toward Titan, the Orbiter used its main engines to alter its trajectory. Such an action was mandatory because before separation, to put the Probe on an interception course with Titan, the Orbiter had to put itself on the same collision trajectory. Now, in order not to crash on Titan, the Orbiter needed to alter its path.10 In addition to missing Titan, the new trajectory had to satisfy two additional objectives. First, it had to enable the Orbiter, with its big antenna and powerful transmitter, to serve as a radio relay link for the Probe’s data because the Probe did not have the capability to send its data directly to Earth. This relaying function was not done in real time, but rather the data that the Probe was transmitting were recorded on board the Orbiter in solidstate memory. After the full data had been received, the Orbiter would turn to point its high-gain antenna at Earth to download the data.11 Secondly, the Orbiter’s new path had to be compatible with starting the tour of the Saturn system. To achieve these objectives, the Orbiter performed two procedures. A deflection maneuver ensured that its closest approach to Titan would occur 2 hours after the Probe reached its atmospheric entry point. A cleanup procedure then corrected any execution errors in the previous maneuver.12

9.3

9.3.1

THE PROBE’S ENCOUNTER WITH TITAN: ENTRY, DESCENT, AND SURFACE OPERATIONS Atmospheric entry phase

On 14 January 2005, the Probe initiated the final and most important phase of its journey when it penetrated the upper layers of Titan’s atmosphere at a speed of 6 kilometers per second (13,000 miles per hour). This entry phase of the Huygens mission involved a period of radical deceleration in which the heat shield airfoil was intensely heated. The peak heating occurred at an altitude of about 400 kilometers (250 miles), when the temperature of the shock wave in front of the Probe exceeded 1,000°C. The front heat shield (discussed in Chapter 4) was critical in protecting the instruments from the high temperatures. The aerodynamic drag slowed the Probe by some 90% over a 3 minute period, from 18,000 to 1,400 kilometers per hour, or from approximately 4 miles per second to less than 0.3 miles per second.13 9.3.2

Descent phase through the atmosphere

At an altitude of about 160 kilometers (100 miles), after the Probe had been slowed sufficiently, it activated its parachute system and began the descent phase.14 The appropriate instant for parachute deployment was based on measurements from the accelerometers that monitored the vessel’s deceleration. Parachute deployment was initiated by pyrotechnic devices. A mortar was fired that pulled out the pilot chute, which in turn pulled loose the back cover (necessary for data to be transmitted) and the main parachute.15 The main chute’s large diameter of 8.3 meters (27.2 feet) was needed in order to provide sufficient drag to pull the descent module away from the front heat shield, so the instruments in the descent

9.3 The Probe’s encounter with Titan 225 module could operate correctly. After a 30 second delay built into the Probe’s command sequence to ensure that the front heat shield had fallen far enough to not contaminate the instruments, the Gas Chromatograph Mass Spectrometer (GCMS) and Aerosol Collector and Pyrolyzer (ACP) inlet ports were opened and the Huygens Atmospheric Structure Instrument (HASI) boom was deployed. Two minutes after that, the Descent Imager/ Spectral Radiometer (DISR) cover was ejected. At this point, the two redundant channels in the Probe’s transmitter commenced sending data to the Orbiter, flying far above. Design and development efforts for the Huygens mission were centered on the performance of the Probe during its descent through Titan’s atmosphere, which all calculations predicted would last 2.5 hours. A hoped-for bonus was that the Probe would survive for several more minutes on the surface, with its instruments intact and its transmitter continuing to send data to the Orbiter.16 During the descent, the main chute could only stay attached for a short period. If it had remained attached until touchdown, the Probe would have descended far too slowly, taking 5 to 8 hours to reach the surface. Long before that, the Orbiter would have disappeared over the horizon, missing any data sent by the Probe regarding the lowest part of the atmosphere and the surface encounter. To prevent such a situation, the mission team detached the main chute after only about 15 minutes and a smaller 3 meter (10 foot) stabilizing chute took over.17 While the Probe’s camera was imaging Titan, the five other instruments in the payload sampled and analyzed its atmosphere (the instruments were discussed in Chapter 3). Mission engineers and scientists designed the suite of instruments to perform a range of measurements during their trip through Titan’s atmosphere in order to determine: • • • • • • • •

atmospheric composition energy sources for atmospheric chemistry aerosol properties and aspects of cloud physics wind characteristics temperature profiles properties of the surface internal structure of the satellite upper atmosphere and ionosphere characteristics.

While the Probe descended through the hazy atmosphere, two redundant radars provided altitude measurements, against which properties of the atmosphere could be plotted. Aerosol samples were collected between 125 and 20 kilometer altitudes and analyzed on board. Titan’s surface became increasingly visible, although not as quickly as expected. Clear images began to be obtained as the Probe passed below an altitude of 40 kilometers (25 miles) and they revealed “an extraordinary world, resembling Earth in many respects, especially in meteorology, geomorphology and fluvial activity.”18 The images showed what looked like erosion patterns caused by liquid flows. The Probe detected methane clouds at about 20 kilometers altitude and methane or ethane fog near the surface. With about 2 minutes remaining to touchdown, the Surface Science Package (SSP) turned on its acoustic sounder, a small sonar device that was meant to send out sound pulses, or pings, and measure distances by using the times for the echoes of the pings to return. The SSP began sending a rapid series of pings toward Titan’s surface. A powerful lamp illuminated the surface during the final part of the descent and DISR made spectral measurements of the material on which the Probe was about to land.

226

The Titan Huygens Probe mission

Mission engineers had designed the Probe to withstand either a splashdown into liquid or a landing on a solid surface. ESA and NASA hoped it would continue to transmit no matter what it touched down upon. If some surface material vaporized upon contact with the Probe’s GCMS, the instrument might then be able to analyze it. If the Probe landed in liquid, the acoustic sounder would attempt to detect an echo from the bottom to determine its depth while other sensors measured the properties of the liquid and sought evidence of wave action. 9.3.3

Touchdown on Titan’s surface

“Titan was always the target in the Saturn system where the need for ‘ground truth’ from a probe was critical.” – David Southwood, ESA Science Director19 The Huygens mission was going to end with loss of signal from the Probe at some point after it reached the surface of Titan.20 An in situ investigation of this moon was “at the very heart of the Cassini/Huygens mission”21 and generated a detailed, extremely useful data set that shed light on the thick, hazy, and chemically active atmosphere and the mysterious surface beneath. On 14 January 2005, after a descent of 2 hours 28 minutes, the Probe touched down on a solid surface at a speed of 4.6 meters per second (10 miles per hour) to become “the most distant manmade object to land on another world.”22 The Probe continued transmitting data to the Orbiter, which had pointed its high-gain antenna toward the predicted impact site. The transmission was received for a further 1 hour 12 minutes, ending when the Orbiter passed below the horizon. A network of radio telescopes on Earth was able to detect its 2 gigahertz (GHz) carrier signal (although they could not read the data) and they found that the Probe continued to transmit for at least another 2 hours. Post-flight analysis indicated the Probe probably continued transmitting its carrier signal until its batteries fully discharged, sometime after its last radio signals were received on Earth.23 After it passed out of view of the Probe, the Orbiter maneuvered to aim its high-gain antenna to Earth and transmitted the Probe data it had just stored. At that time, the Orbiter was 1,200 million kilometers (750 million miles) from Earth, so it took 67 minutes for its transmission, traveling at the speed of light, to span this distance. The Probe data was analyzed by ESA’s European Space Operation Centre (ESOC) in Darmstadt, Germany. ESA Science Director David Southwood said of the Probe’s science instrument performance, “The torch has now been passed from the engineers who delivered the Probe and got the data sent to Cassini to the scientists who will evaluate the data.”24 The data collected over a period of several hours would require years of analysis to understand in depth.

9.4

WHAT DID THE HUYGENS PROBE TELL US ABOUT TITAN?

“Even though we have only four hours of data, it is so rich that … we have yet to retrieve all the information it contains.” – François Raulin, Huygens Interdisciplinary Scientist25

9.4 What did the Huygens Probe tell us about Titan? 227 The Probe sent back an exciting data set that revealed eagerly awaited insights into the atmospheric and surface characteristics of Titan. More than 474 megabits of data were received, including about 350 pictures collected during the descent and on the ground which showed a landscape amazingly like that of Earth.26 This data set also helped unlock the secrets of Titan’s atmosphere, rich in nitrogen and methane, and its haze-shrouded surface, both of which contain many of the chemicals which were present on our own planet when it was young, some 4.5 billion years ago.27,28 9.4.1

Titan’s atmospheric winds

During Cassini-Huygens’ development, the planners realized that the Probe’s motion during its descent could be employed to study Titan wind characteristics. This led to inclusion of the Doppler Wind Experiment (DWE) on the craft, which was supposed to provide a high resolution vertical profile of the satellite’s winds with an estimated accuracy better than 1 meter per second.29 Two key assumptions about the wind-driven horizontal motion of the Probe underlay DWE’s design. The first was that the horizontal drift of the Probe followed the horizontal wind after a negligible response time. The second assumption, based on theoretical modeling that implied dominance of east–west atmospheric circulation, was that any north–south drift of the Probe was negligible. 9.4.2

The DWE problem

As the Orbiter started relaying Probe data to the main control room of ESA’s ESOC facility in Darmstadt, Germany, cheers rang out and “some two hundred journalists, reporters, and camera operators”30 besieged the excited scientists. But as Huygens data spilled in, it soon became apparent that one of the data channels had a serious problem. In keeping with redundancy of design, the Huygens Probe was equipped with two separate radio transmitters. Similarly, the Orbiter carried two individual radio receivers. Hence, there were two separate conduits of data flowing from the Probe to the Orbiter. The mission team referred to them as channels A and B. This redundancy was to prevent the loss of all mission data from a single failure aboard either vehicle. Mission control room staff determined that channel A had a severe malfunction, because data from the Probe were not reaching that channel. The problem turned out not to be due to a mechanical or electrical failure of some component, but rather to an error of omission, and this error impacted operation of the receiver on the Cassini Orbiter, not the transmitter on the Huygens Probe. That detail turned out to be very important. For the Orbiter to measure tiny Doppler shifts in the Probe’s signal to enable the wind speeds to be calculated, the frequencies of signals transmitted from the Probe to the Orbiter on at least one of the channels had to be extremely precise and stable, and channel A had been selected for this purpose. Radio transmitters and receivers employ a device called an oscillator to generate and measure a signal’s frequency. Typical quartz oscillators would not have provided the stability required for DWE. Instead the team designed and built an ultra-stable oscillator (USO) that contained a special rubidium element. This was the first time that this particular USO was used on a deep space mission.31 Two were needed for the radio link on channel A, one on the Probe’s transmitter and the other on the Orbiter’s

228

The Titan Huygens Probe mission

receiver. But channel A had a second oscillator as well in its receiver. There had been a concern during the design process that the USO for receiving telemetry data for DWE might not work, and so a simpler unit was installed and available if needed. In the end, mission staff decided to use the USO. But two commands were required for this: one to select the USO as the oscillator to be used, and another to power it on. The critical software contained the USO selection command, but not the command to turn the device on.32 The plan was for DWE to create a profile of Titan wind velocity versus altitude, but since no command had been given to the Orbiter to turn on its USO the receiver on the Orbiter could not detect the channel A signal, and all the data it carried sped right by the Orbiter. ESA took responsibility for this error. The missing command should have been part of a software sequence developed by ESA and delivered to the Orbiter. DWE personnel were understandably crushed when they learned that the Orbiter had not received the data that they had been looking forward to for years. Michael Bird, the principal investigator, said, “I’ve never felt such exhilarating highs and dispiriting lows than those experienced when we first detected the signal … indicating ‘all’s well,’ and then discovering that we had no signal … indicating ‘all’s lost.’.”33 Other teams also wondered what impact the loss of channel A would have on their data. While some teams had duplicated their data by sending it all over both channels, others had taken the more risky approach of sending different information on channels A and B. If both channels worked, they would receive twice the data, but they also risked losing half of their data if one channel ran into problems. This was the case with the Descent Imager/Spectral Radiometer (DISR) experiment.34 This story sounds grim and hopeless, but it had a mostly happy ending because the DWE team had arranged for powerful radio telescopes on Earth to also attempt to receive the Probe’s signal directly, even though over such a distance it would be very weak. The Probe sent out a constant carrier signal, an electromagnetic wave of steady base frequency on which data could be imposed by modulating the signal strength, or the base frequency itself, or the wave phase. The constant carrier wave helped the telescopes to find and lock onto the signal the Probe was sending to the Orbiter.35 The idea was that two independent measurements of the Probe’s Doppler-shifted signal received from slightly different directions would tell something about small north–south components of Titan’s winds. With the loss of channel A, that couldn’t be done, but radio telescopes on Earth were able to detect sufficient carrier signal directly from the Probe to reconstruct its Doppler shifts and build up a pretty good profile of Titan’s winds as a function of altitude.36 The project had sent DWE co-investigator Sami Asmar to the Green Bank Telescope with a special receiver for this task.37 While redundancy of instrumentation was key in recovering the Titan wind data, plain luck also played a part. The Green Bank Telescope in West Virginia collected critical data directly from the Probe, but poor local weather had almost forced this telescope off-line before the data came in. The data from the Probe indicated that Titan’s winds blew in the direction of the moon’s rotation (prograde) during most of the descent phase, and provided in situ confirmation of super-rotation, where a planet’s atmosphere rotates independently of, and much more rapidly than, its surface. The Probe also encountered a layer of surprisingly slow wind at altitudes between about 60 and 100 kilometers (40 to 60 miles) where the speed fell almost to zero. In the lowest layer of the atmosphere, within 5 kilometers (3 miles) of the surface,

9.4 What did the Huygens Probe tell us about Titan? 229 the Probe observed weak winds that were typically 1 meter per second.38 Further analysis revealed that the atmosphere was an enormous conveyor belt which, at that time, was circulating its gas from the south pole to the north and back again.39 9.4.3

Titan’s weather: Haze, aerosols, clouds, and rain

The Probe determined that owing to the presence of dust particles, termed aerosols, Titan’s atmosphere was rather hazier than predicted. These aerosols were not like those found on Earth, but contained solid cores of materials that included ammonia and cyanide.40 It was not until the Probe dropped below 40 kilometers in altitude that the haze cleared sufficiently for the cameras to take their first distinct images of the surface. They revealed a landscape showing strong evidence that a liquid had flowed on the surface, causing erosion. These images provided the ground truth against which data from Orbiter observations on later flybys were compared. Together, the data helped determine how the environment on Titan might have carved out such a landscape.41 The Probe used its Gas Chromatograph and Mass Spectrometer (GCMS) coupled to its Aerosol Collector and Pyrolyzer (ACP) to analyze the aerosol particles. As the Probe descended, the ACP instrument collected atmospheric samples over separate altitude ranges: 130 down to 35 kilometers (81 to 22 miles) and 25 to 20 kilometers (16 to 12 miles). The GCMS then conducted a three-stage analysis of the samples’ gaseous products. First, it examined the most volatile portion of the aerosols at the ambient collection temperature. Then the ACP heated the particles to 250°C (480°F) to volatize more of the aerosols. Finally, what remained of the sample was heated to 600°C (1,100°F) to vaporize all of the other volatile components. As predicted by theoretical models, the ACP results were the first hard evidence of complex organic material in Titan’s atmosphere. The data also clearly indicated that nitrogen, the predominant atmospheric gas, was incorporated into the chemical structure of the aerosols, which suggested they functioned as sinks for atmospheric nitrogen. The presence of both ammonia (NH3) and hydrogen cyanide (HCN) was evidence that nitrogen was involved in the aerosols in different ways. Probe results also indicated that the aerosols carried the complex organic matter irreversibly to the surface. Hence organics should contribute to the spectral signature of the surface.42 On Earth, nitrogen compounds play important roles in atmospheric processes, such as smog formation, which they also seem to do on Titan. But on Earth, the presence of nitrogen compounds in living matter is also very important. The extent to which nitrogen participates in processes on Titan has yet to be determined. In situ methane concentration and temperature data in Titan’s troposphere – the lower part of its atmosphere, where most weather occurs – indicated the persistent presence of thin, layered clouds, in particular an upper cloud of methane ice and a lower, barely visible cloud of liquid methane-nitrogen. The data suggested that the liquid cloud could produce a persistent methane drizzle wherever there was a slow upward motion, and that this drizzle reached the surface and affected its geological features.43 A question arose regarding what happened to the methane after it rained onto Titan’s surface. Did it add to large reservoirs on the moon? Or did it return to the atmosphere? An important clue to the nature of Titan’s hydrologic cycle was that the relative humidity of methane just above the surface was substantial at about 50%, suggesting that at least a fraction of the precipitation was evaporating back into the atmosphere.44

230 The Titan Huygens Probe mission 9.4.4

Chemical characteristics of the atmosphere

The Probe’s descent through Titan’s atmosphere revealed surprising similarities, as well as dramatic differences, with that of Earth. Both atmospheres are dominated by nitrogen. But Titan, ten times farther from the Sun than Earth, is considerably colder, resulting in methane (CH4) becoming the atmosphere’s dominant carbon-bearing gas rather than carbon dioxide (CO2), as on Earth. The isotope 40Ar, which constitutes 1% of Earth’s atmosphere, made up only 43 parts per million (0.0043%) on Titan. At middle altitudes, a process similar to photochemical smog formation on Earth has created the thick layers of organic haze which mask the surface at visible wavelengths.45 The primordial formative processes that gave rise to the present atmosphere on Titan depended to a great extent on the interactions of methane, molecular nitrogen (N2, a very stable, diatomic, or 2-atom, form of the gas), and their byproducts. Key to understanding the evolution of Titan’s atmosphere was a profile of methane and nitrogen concentrations versus altitude, as well as the isotopic ratios of 12C/13C and 14N/15N, amongst others. The 40 Ar measurement was important as well, because it provided information on the interaction between the atmosphere and interior processes. ESA and NASA engineers designed the Probe’s GCMS to shed light on the abovementioned areas of inquiry. Space scientists have identified two principal methods in which atmospheres of solar system bodies form. One is by accretion of a part of the original solar nebula, or by a subnebula in the neighborhood of a gas giant planet such as Saturn. A second way is by impacts of gas-rich planetesimals. A rarity of noble gases in an atmosphere suggests, according to many scientists, the planetesimal influx mechanism of atmosphere formation. The dearth of noble gases in Earth’s atmosphere supports the planetesimal influx hypothesis, as does the near absence of noble gases from Titan.46 9.4.5

Temperature, pressure, and density

The Huygens Atmospheric Structural Instrument (HASI) determined temperature and density profiles from an altitude of 1,400 kilometers (870 miles) down to the surface. Previous to this, data obtained by the Voyager 1 Ultraviolet Spectrometer during the November 1980 flyby of Titan was the primary source of information on the moon’s upper atmosphere. Titan’s upper atmosphere was defined as the region above 160 kilometers (100 miles). HASI measured upper atmosphere density indirectly by calculating it from the deceleration of the Probe due to aerodynamic drag forces. HASI then inferred temperature from this. Such an indirect approach needed to be used in this region because the Probe’s instruments had to remain enclosed in the aeroshell during the deceleration phase of atmospheric entry; they would operate in the descent phase. In the upper atmosphere, density and temperature turned out to be higher than model profiles, with wave-like swings of 10-20°C in the temperature profile as a function of altitude. Upper atmospheric temperature can be affected by particles in the solar wind, in Saturn’s magnetosheath, or in its magnetosphere. For example, theoretical calculations suggested that the heating of Titan’s upper atmosphere by magnetospheric ions could produce a temperature effect up to 30°C. An additional heat source in Titan’s atmosphere is high

9.4 What did the Huygens Probe tell us about Titan? 231 altitude haze. Studies have indicated that monomer haze particles absorb sunlight and then emit thermal (infrared) radiation that can impact temperatures of the surrounding gas.47 It was important to determine Titan’s upper atmosphere conditions because they influenced both the entry trajectory of the Probe and the Orbiter’s subsequent low flybys at an altitude of around 1,000 kilometers. By the time the Probe had descended to about 160 kilometers, it was traveling slowly enough to safely deploy its large parachute. The resulting atmospheric drag helped separate the descent module from the front heat shield, thereby exposing the temperature and pressure sensors and enabling them to take direct measurements of the ambient environment. The sensors remained exposed during the remainder of the descent.48 The temperature measurements within the lower stratosphere were in very good agreement with Voyager 1’s remote sensing observations. Considering the very long radiative time constants associated with much of the lower atmosphere, the lack of change over this interval was not surprising. Radiative time constants are indicators of the time that it takes for part of an atmosphere to react to changes in, for instance, insolation levels. The radiative time constants in Titan’s lower atmosphere are of the order of 60 years at an altitude of 50 kilometers and 300 years at altitudes of 10 to 20 kilometers. Since these are considerably longer than a Saturnian year (29.5 Earth years), Titan’s lower atmosphere would not be particularly sensitive to the annual variation in insolation.49 There are regions of the atmosphere where time constants may be considerably shorter than those mentioned above. Titan surface temperatures appear to respond to the annual cycle of temperature changes, particularly in the polar regions. Scientists believe there could be a thin atmospheric boundary layer in contact with the surface that also has a shorter time constant than that at altitudes in the 10 to 20 kilometer altitude range.50 9.4.6

Atmospheric electricity

Once the Probe descended below 140 kilometers (87 miles), the Permittivity, Wave, and Altimetry (PWA) package on the Huygens Atmospheric Structure Instrument (HASI) began measuring electrical properties of the atmosphere. PWA utilized two different types of instruments to determine the atmosphere’s electrical conductivity profile, due both to electrons and ions. Its instruments found a peak in conductivity at 63 kilometers (39 miles) above the surface, typified by a sharp rise in electron density. To some extent, this conducting layer splits the atmosphere of Titan into two separate resonating regions, one from the ionosphere (at an altitude of about 1,200 kilometers) down to 63 kilometers, and the other from there down to the surface (Figure 9.1). Indications of possible lightning discharges were also observed.51

9.4.6.1

A slowly oscillating tool for seeing below Titan’s surface

The PWA also detected an extremely low frequency (ELF) radio wave during the descent that oscillated at only 36 cycles per second (36 Hz). If such occurrences are natural phenomena and not artifacts of the instrument’s operation,52 then they might constitute a powerful new way to examine not just Titan’s atmosphere, but also its subsurface.

232 The Titan Huygens Probe mission

Figure 9.1 Electron density in Titan’s atmosphere plotted versus altitude, as obtained from the HASI instrument onboard Huygens. The density of electrons is related to the electrical conductivity in Titan’s atmosphere.

Earth is the only other world on which ELF waves have been detected. On Earth, they are reflected by both its surface, which has some electrical conductivity, and its ionosphere, where most particles are electrically charged. Our atmosphere serves as a giant enclosure in which certain ELF waves resonate and are reinforced. Owing to its low electrical conductivity, the surface of Titan is a poor reflector of ELF waves and instead of being reflected they are able to penetrate the interior. According to Fernando Simões of the PWA team, theoretical models of Titan predict the existence of a subsurface ocean of water and ammonia that would be electrically conductive to some extent. An ELF wave would get reflected by the liquid-ice boundary of such an ocean, and could be used to map the boundaries of this body of liquid (Figure 9.2).53 9.4.7

Titan’s surface

“It was eerie. We saw bright hills above a dark plain, a weird combination of light and dark. It was like seeing a landscape out of Dante.” – Jonathan Lunine, Cassini-Huygens Interdisciplinary Scientist54 The Probe’s imaging system revealed Titan to have surprisingly Earth-like features. Many familiar terrestrial geophysical processes appear to be operating on Titan, but with different substances. Instead of liquid water, Titan’s much colder liquid hydrocarbons, such as methane, helped carve its topography. Instead of Earth’s silicate rocks, Titan’s stones are made of frozen water ice. And instead of Earth-type dirt, there are hydrocarbon particles that settled out of Titan’s atmosphere.

9.4 What did the Huygens Probe tell us about Titan? 233

Figure 9.2 How an ELF wave might reflect off of, and thus define, the boundary of a subsurface Titan ocean.

Before the Huygens Probe mission, space scientists thought that Titan might be a place of astrobiological interest. The mission confirmed this, for the data suggested Titan was “a planetary-scale laboratory for studying pre-biotic chemistry.”55

9.4.7.1

Riverbeds?

As the Probe approached Titan’s surface, the Descent Imager/Spectral Radiometer (DISR) revealed channel networks reminiscent of Earth. Two distinct patterns of channel networks were seen: those with multi-branched dendritic features, such as in Figure 9.3, and those with short, stubby features. Regions of stubby networks are significantly brighter than dendritic regions. The stubby channels are shorter, wider, relatively straight, and often begin or end in dark circular areas suggestive of ponds or pits. Their morphologies are consistent with spring-fed channels or arroyos.56 These channel systems are discussed further in Chapter 14.

9.4.7.2

Methane sources

One of the major questions about Titan had to do with the sources of methane that countered its ongoing loss at high altitudes as a result of solar-driven photochemical processes. Possible sources of methane include open pools of liquid hydrocarbons on the surface (although DISR did not observe such liquid bodies) and subsurface reservoirs combined with cryovolcanism, the eruption of liquid or vapor-phase volatiles from the subsurface.57

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The Titan Huygens Probe mission

Figure 9.3 Fluvial channels? This mosaic of three frames from the Huygens Descent Imager/ Spectral Radiometer (DISR) shows a high ridge area and what appears to be the flow down into a major dendritic fluvial channel network.

9.4.7.3

Surface Science Package observations

In the final part of the Probe’s descent and after landing, the nine instruments of the Surface Science Package (SSP) played a major role in characterizing the surface of Titan. Acoustic sounding during the last 90 meters revealed a fairly smooth but not completely flat surface surrounding the landing site. As the Probe touched down, the nature of the ground was determined by the SSP accelerometer measuring the rate of slowing and the penetrometer, configured to be the first part of the vehicle to make contact, determining the ease of penetrating the surface material. The mission team selected an accelerometer because it would be able to estimate surface hardness for all touchdowns in which the Probe survived. However, for a hard surface it was likely that the structure of the descent module would buckle and collapse, damping the measured deceleration and yielding an erroneous estimate of surface hardness. In this situation, a penetrometer designed to provide data on the strength, texture, and penetrative resistance of the material would be more useful. The two instruments together covered the range of surface types likely to be encountered, from liquids and very soft solids to hard ice. Adding to impact data from the SSP accelerometer and penetrometer were those from an SSP tilt sensor and the three accelerometers of the Huygens Atmospheric Structure Instrument (HASI).58 Aspects of the surface could be surmised from some of the Probe’s data. The SSP accelerometer, for instance, gave data on change in velocity versus time after impact. Many years before the mission, in preparation for the Huygens Announcement of Opportunity in 1989, engineers had performed a series of laboratory simulations of the touchdown, using 1/3rd and 1/9th scale models of the Probe. These experiments included dropping the model

9.4 What did the Huygens Probe tell us about Titan? 235 into water and sand, with an accelerometer generating data, one video camera recording altitude versus time, and another monitoring the penetration. Deceleration characteristics were observed for a variety of substances. As an example, the Probe’s velocity decreased much more rapidly when it impacted sand than water. It reached a maximum deceleration of roughly 350 meters/second2 in dry sand, but only about 35 meters/second2 in water. This difference in observed deceleration could be employed as a means of discriminating the actual substance at the touchdown site on Titan. The experimental data also showed that after an impact with water, there was a characteristic up and down bobbing that was not present in sand. Planetary scientist and Titan specialist Ralph Lorenz recognized that the period of this bobbing action could be used to calculate the density of an unknown liquid, for instance if the Probe splashed into some sort of hydrocarbon lake or stream.59 The impact data returned from actual touchdown indicated that the surface at the landing site was not hard like ice, nor particularly compressible, such as it might be if composed of blankets of fluffy aerosol deposits. Instead, the surface resembled a relatively soft material such as wet clay, lightly packed snow, or wet or dry sand. Less elegantly, ESA described the Probe as landing with a “splat” in “Titan mud”60 whose composition proved to be a mixture of dirty water ice and hydrocarbon ice. The ground-level temperature at this site was a chilly −180°C, or −290°F, far colder than even the polar regions of Earth. Images taken by the Probe depicted the area around the landing site as a smooth plain with rounded rocks on it (Figure 9.4). Rounded rocks suggested a river which rolled chunks of stone about, breaking off their corners and making them into more curved shapes. And the landscape was reminiscent of a terrestrial stream bed, lake bed, or perhaps a beach with the tide out.61

Figure 9.4 The view from the Probe after landing: a flat plain with rounded stones. This was the first color view of Titan’s surface. The two rock-like objects just below the middle of the image are, from left to right, 15 centimeters (6 inches) and 4 centimeters (1.5 inches) across, and lie at a distance of 85 centimeters (33 inches) from the Huygens Probe. Titan’s surface is darker than was originally expected, consisting of a mixture of water and hydrocarbon ice. There is also evidence of erosion at the base of these objects, indicating possible fluvial activity.

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9.5

SIGNIFICANCE OF HUYGENS PROBE DATA

The Huygens Probe data set was by no means all that the mission collected about Titan. During later flybys the Orbiter would use its impressive suite of instruments to carry out extensive observations and measurements of the moon. But Probe data constituted the in situ information. Taken together, “the six scientific instruments carried by Huygens provided the ‘ground truth’ which now forms the bedrock of all subsequent investigations into Saturn’s largest moon.”62 Remote sensing data obtained by the Orbiter since the Probe conducted its mission continue to be checked against these “on-the-scene” records. The Huygens mission resulted in many honors for Europe and its space agency, whose planetary exploration efforts were rapidly expanding. Indications of the high regard that various heads of government held for the mission included a visit by the U.K. Prime Minister Tony Blair to John Zarnecki, the principal investigator of the SSP experiment, and an invitation by French President Jacques Chirac to several French Huygens scientists to meet him at the Élysée Palace.

REFERENCES 1. Trina Ray interview by author, JPL, 22 October 2008. 2. Toby Owen telephone interview by author, 31 July 08. 3. Jean-Pierre Lebreton, Olivier Witasse, Claudio Sollazzo, Thierry Blancquaert, Patrice Couzin, Anne-Marie Schipper, Jeremy B. Jones, Dennis L. Matson, Leonid I. Gurvits, David H. Atkinson, Bobby Kazeminejad and Miguel Pérez-Ayúcar, “An Overview of the Descent and Landing of the Huygens Probe on Titan,” Nature 438 (8 Dec. 2005):758–764. 4. Jean-Pierre Lebreton et al., “An Overview of the Descent and Landing of the Huygens Probe on Titan,” Nature 438 (8 December 2005):758–764. 5. ESA, “Huygens Begins its Final Journey into the Unknown,” ESA press release 67–2004 (25 Dec. 2004). 6. Bob Mitchell review of manuscript, Feb. 2011. 7. ESA, “Huygens Probe Separation and Coast Phase,”http://sci.esa.int/science-e/www/object/ index.cfm?fobjectid=34956, ESA Science & Technology Web site (19 Jan 2005), accessed 16 Apr. 2009; Ralph Lorenz and Jacqueline Mitton, Titan Unveiled (Princeton and Oxford: Princeton University Press, 2008), p. 121; Bob Mitchell review of manuscript, Feb. 2011. 8. Bob Mitchell review of manuscript, Feb. 2011. 9. ESA, “Second Space Christmas for ESA: Huygens to Begin Its Final Journey to Titan,” ESA press release no. 63–2004, 7 Dec. 2004, NASA NHRC 18337 Cassini 2002-; Aviation Week, “Europe’s Huygens Probe Nears End of its Journey to Titan,” Aviation Week’s Aerospace Daily & Defense Report 213(8) (13 Jan. 2005), NASA NHRC 18337 Cassini 2002-; AstronomyOnline. org, “Huygens Mission to Titan,” http://astronomyonline.org/Astrobiology/HuygensProbe.asp, accessed 27 Apr. 2009; Bob Mitchell review of manuscript, Feb. 2011. 10. ESA, “Huygens Begins its Final Journey.” 11. Bob Mitchell review of manuscript, Feb. 2011. 12. C. Collet, Huygens User Manual Description, Aerospatiale doc. no. HUY.AS/c.100.OP.0201, issue 4, rev. B, 15 Sep. 1997, JPL Cassini Archive, wood cabinet; ESA, “Huygens Probe Separation.”

References 237 13. Kai Clausen (Project Manager), Huygens System Requirements Document, ESA reference no. HUY-SRD-001, issue 2, Dec. 1990; ESA, “Huygens Begins its Final Journey;” AstronomyOnline. org, “Huygens Mission to Titan,” http://astronomyonline.org/Astrobiology/HuygensProbe.asp, accessed 27 Apr. 2009; Lebreton et al., “An Overview of the Descent and Landing”; Lorenz and Mitton, Titan Unveiled. 14. Clausen, Huygens System Requirements Document, pp. 14–15. 15. Jean-Pierre Lebreton et al., “An Overview of the Descent and Landing of the Huygens Probe on Titan,” Nature 438 (8 Dec. 2005):758–764. 16. Bob Mitchell review of manuscript, Feb. 2011. 17. AstronomyOnline.org, “Huygens Mission to Titan,” http://astronomyonline.org/Astrobiology/ HuygensProbe.asp, accessed 27 Apr. 2009; Lebreton et al., “An Overview of the Descent and Landing”; Lorenz and Mitton, Titan Unveiled. 18. Lebreton et al., “An Overview of the Descent and Landing.” 19. ESA, “Europe Reaches New Frontier—Huygens Lands on Titan,” press release no. 03–2005 (14 Jan. 2005), NASA NHRC 18337 – Cassini 2002-. 20. J.-P. Lebreton and D.L. Matson, “The Huygens Probe: Science, Payload and Mission Overview,” http://www.esa.int/esapub/bulletin/bullet92/b92lebre.htm, ESA Bulletin No. 92, Nov. 1997; ESA, “Huygens,” http://huygens.esa.int/science-e/www/area/index.cfm?fareaid=12, accessed 4 Nov. 2008. 21. Lebreton and Matson. 22. ESA, “Celebrating the Fifth Anniversary of Huygens’ Titan Touchdown,” http://www.esa.int/ SPECIALS/Cassini-Huygens/SEM5KSLJ74G_0.html (14 Jan. 2010). 23. Lebreton et al., “An Overview of the Descent and Landing.” 24. Peter de Selding, “Huygens Probe Returns First Images of Titan’s Surface,” http://www.space. com/missionlaunches/huygens_images_050114.html, Space News (14 Jan. 2005). 25. ESA, “Building Our New View of Titan,” http://www.esa.int/SPECIALS/Cassini-Huygens/ SEMHME9RR1F_0.html, ESA News, 1 June 2007, accessed 5 May 2009. 26. ESA, “More of Titan’s Secrets to be Unveiled on 21 January,” http://www.esa.int/SPECIALS/ Cassini-Huygens/SEM15Y71Y3E_0.html, PR 04–2005 (18 January 2005). 27. ESA, “Europe Reaches New Frontier—Huygens Lands on Titan,” press release no. 03–2005 (14 Jan. 2005), NASA NHRC 18337 – Cassini 2002-. 28. Lebreton et al., “An Overview of the Descent and Landing.” 29. M.K. Bird et al., “The Huygens Doppler Wind Experiment: Titan Winds Derived from Probe Radio Frequency Measurements,” Space Science Reviews 104 (July 2002): 613–640. 30. Lorenz and Mitton, Titan Unveiled, p. 138. 31. Emily Lakdawalla, “Radio Astronomers Rescue Science Results for Huygens’ Doppler Wind Experiment,” http://www.planetary.org/news/2005/0209_Radio_Astronomers_Rescue_ Science.html, Planetary News, The Planetary Society (9 Feb. 2005). 32. Bob Mitchell review of manuscript, Feb. 2011. 33. Michael Bird comment to author, relayed by Bob Mitchell email, 19 Jan, 2011; “Wind on Titan: First Measurements from the Huygens Descent,” http://www.jive.nl/news/huygens/pr_feb09_ en.html, Joint Institute for VLBI in Europe, 9 Feb. 2005, accessed 2 May 2009. 34. M. G. Tomasko et al., “Rain, Winds and Haze During the Huygens Probe’s Descent to Titan’s Surface,” Nature 438 (8 December 2005):765–778; Lorenz and Mitton, Titan Unveiled, pp. 140–141. 35. William Folkner, “The Huygens Doppler Wind Experiment: Results from Titan,” http://www. mrc.uidaho.edu/~atkinson/Huygens/DWE_Presentations/AGU_2005/DWE.AGU.23may05. pdf, American Geophysical Union (AGU) conference, New Orleans, 23 May 2005; Lorenz and Mitton, Titan Unveiled, pp. 154–156.

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36. Lorenz and Mitton, Titan Unveiled, pp. 154–156. 37. Sami Asmar comments to author, by way of Bob Mitchell email, 19 Jan. 2012. 38. M. K. Bird et al., “The Vertical Profile of Winds on Titan,” Nature 438 (8 December 2005):800–802. 39. ESA, “Building Our New View of Titan,” http://www.esa.int/SPECIALS/Cassini-Huygens/ SEMHME9RR1F_0.html, 1 June 2007, accessed 26 Apr. 2009. 40. G. Israël et al., “Complex Organic Matter in Titan’s Atmospheric Aerosols from In Situ Pyrolysis and Analysis,” Nature 438 (8 December 2005):796–799. 41. ESA, “Building Our New View of Titan,” http://www.esa.int/SPECIALS/Cassini-Huygens/ SEMHME9RR1F_0.html, ESA News, 1 June 2007, accessed 5 May 2009. 42. Sébastien Lebonnois et al., “Transition from Gaseous Compounds to Aerosols in Titan’s Atmosphere,” Icarus 159(2) (October 2002):505–517; Christopher P. McKay, “Elemental Composition, Solubility, and Optical Properties of Titan’s Organic Haze,” Planetary and Space Science 44(8) (August 1996):741–747; Israël et al., “Complex Organic Matter in Titan’s Atmospheric Aerosols.” 43. Tetsuya Tokano et al., “Methane Drizzle on Titan,” Nature 442 (27 July 2006):432–435. 44. M. G. Tomasko et al., “Rain, Winds and Haze During the Huygens Probe’s Descent to Titan’s Surface,” Nature 438 (8 December 2005):765–778. 45. Tobias Owen, “Planetary Science: Huygens Rediscovers Titan,” Nature 438 (8 December 2005):756–757. 46. H.B. Niemann et al., “The Abundances of Constituents of Titan’s Atmosphere from the GCMS Instrument on the Huygens Probe,” Nature 438 (8 Dec. 2005):779–784. 47. B. Kazeminejad et al., “Temperature Variations in Titan’s Upper Atmosphere: Impact on Cassini/Huygens,” Annales Geophysicae 23 (2005): 1183–1189; M. Fulchignoni, “Results on Titan’s Atmosphere Structure by the Huygens Atmospheric Structure Instrument (HASI),” presentation at Space Week, Moscow, Russia, Space Research Institute (IKI), Russian Academy of Sciences (2 Oct. 2007). 48. M. Fulchignoni et al., “In Situ Measurements of the Physical Characteristics of Titan’s Environment,” Nature 438 (8 Dec. 2005):785–791. 49. Darrell F. Strobel et al., “Atmospheric Structure and Composition,” Chapter 10 in Robert H. Brown et al. (eds.), Titan from Cassini-Huygens (Springer, 2009):237. 50. Darrell F. Strobel email to author, 24 Oct. 2010. 51. M. Hamelin et al., “Electron Conductivity and Density Profiles Derived from the Mutual Impedance Probe Measurements Performed During the Descent of Huygens Through the Atmosphere of Titan,” Planetary and Space Science 55 (2007):1964–1977; Fernando Simões, Michel Hamelin, and Jean-Pierre Lebreton, “Titan’s Mysterious Radio Wave,” http://www.esa. int/SPECIALS/Cassini-Huygens/SEM17F9RR1F_0.html, ESA news release (1 June 2007); Fulchignoni, “In Situ Measurements.” 52. F. Simoes et al., “A New Numerical Model for the Simulation of ELF Wave Propagation and the Computation of Eigenmodes in the Atmosphere of Titan: Did Huygens Observe any Schumann Resonance?” Planetary and Space Science 55 (2007):1978–1989. 53. ESA, “Titan’s Mysterious Radio Wave,” http://www.esa.int/SPECIALS/Cassini-Huygens/ SEM17F9RR1F_0.html, 1 June 2007, accessed 26 Apr. 2009. 54. Jonathan Lunine in Jia-Rui C. Cook, “Land Ho! Huygens Plunged to Titan Surface 5 Years Ago,” http://www.jpl.nasa.gov/news/features.cfm?feature=2448&cid=feature_2448, JPL news release, 14 Jan. 2010. 55. Lebreton et al., “An Overview of the Descent and Landing.” 56. M. G. Tomasko et al., “Rain, Winds and Haze During the Huygens Probe’s Descent to Titan’s Surface,” Nature 438 (8 December 2005):765–778.

References 239 57. Fernanda Scuderi, “Cryovolcanism: Ice as Lava,” American Geophysical Union paper number 2002GL012345 (2002); Tomasko et al., “Rain, Winds and Haze.” 58. John C. Zarnecki et al., “A Soft Solid Surface on Titan as Revealed by the Huygens Surface Science Package,” Nature 438 (8 December 2005):792–795; ESA, “More of Titan’s Secrets to be Unveiled on 21 January,” http://www.esa.int/SPECIALS/Cassini-Huygens/ SEM15Y71Y3E_0.html, PR 04–2005 (18 January 2005). 59. A. Seiff et al., “Determination of Physical Properties of a Planetary Surface by Measuring the Deceleration of a Probe Upon Impact: Application to Titan,” Planetary and Space Science 53 (2005):594–600; R.D. Lorenz, “Huygens Probe—the Surface Mission,” Proceedings of the Symposium on Titan, Toulouse (Noordwijk, The Netherlands: European Space Agency SP-338, Sep. 2001). 60. ESA, “Huygens Lands in Titanian Mud,” http://www.esa.int/esaCP/SEM5YW71Y3E_index_0. html, (18 January 2005). 61. Lorenz and Mitton, “Titan Unveiled,” p. 144–146, 172; Caitlin A. Griffith et al., “Titan’s Tropical Storms in an Evolving Atmosphere,” Astrophysical Journal Letters 687 (2008). 62. ESA, “Celebrating the Fifth Anniversary of Huygens’ Titan Touchdown,” http://www.esa.int/ SPECIALS/Cassini-Huygens/SEM5KSLJ74G_0.html (14 Jan. 2010).

10 The Saturn tour: Decision-making processes, trajectory design, and changes of management “On Cassini, you generally have one team that is happy and eleven that are not.” – Bob Mitchell, Cassini-Huygens Program Manager1

This chapter gives details of the Orbiter’s Prime Mission tour, the Cassini Equinox Mission, and the Solstice Mission, including descriptions of Saturn orbits, Titan flybys, close and distant flybys of the other icy satellites, and studies of the ring system. It discusses the process of choosing spacecraft trajectories and assigning research opportunities. Planning and management issues on the Galileo mission to Jupiter are contrasted with those on Cassini-Huygens. The spacecraft’s orbital dynamics are examined, along with how it employed Titan gravity assists for fuel conservation and guidance, and the planetary protection issues pertaining to each body that the Orbiter flew by, especially those that could potentially harbor life. The impact of mission management changes is examined as the operational focus of the project shifted to overseeing the Saturn tour.

10.1

CHOOSING TRAJECTORIES AND ASSIGNING TIME ON THE SPACECRAFT

“When you have a multidisciplinary spacecraft, fully functional in such a target rich environment, it is a whole different world.” – Trina Ray, Science Lead for the Titan Orbiter Science Team2 While the Huygens Probe’s descent to Titan provided invaluable in situ data about that moon, the Cassini Orbiter’s subsequent sojourn through the Saturnian system generated years of amazing information about the mother planet itself, many of its satellites, and its rings, particles, and magnetic field. The Saturn tour, focus of the Orbiter’s mission, included many smaller tours, or spacecraft trajectories, that were needed to collect “radically different observations of Saturn and its system from a wide variety of perspectives”3 in order to address the mission’s scientific goals. © Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_10

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242

The Saturn tour

Cassini’s tour designers – talented engineers who formulated Orbiter trajectories – had a hard, highly constrained task before them. They needed to find feasible ways of sending the spacecraft to the many targets that were of interest to over 200 scientists, while using as little time, fuel, and other resources as possible. Neither the orbital dynamics nor the internal politics of developing a planetary system tour was straightforward, but in the case of Cassini-Huygens the effort was monumental. The tour designers did their utmost to fulfill the hopes and dreams of scientists who had been preparing their experiments for years or even decades, but not everyone could get what they wanted. John Smith explained his job like this: “A tour designer is … like someone who takes a group of really enthusiastic scientists to Disneyland. Just like anyone else, the scientists want to go on every ride … but the constraint is that we all have to move together as a group, because on the spacecraft, all the instruments are all attached to the spacecraft. The tour designer is … trying to make everyone pretty happy, but we pretty much only have one day at Disneyland.”4 The tour designer’s task was made significantly more complex by the elimination of a scan platform for remote sensing instruments, turntable for fields and particles instruments, and articulating radar antenna (the impacts of doing away with these devices are discussed in some detail in Chapter 1). NASA canceled the development of these pieces of equipment in the 1990s to cut down on capital costs. As a result, instead of instruments being able to turn and point independently of each other, the whole spacecraft usually had to be rotated in order to orient an experiment correctly. And when one instrument got to point toward its target of choice, other instruments were unable to do so. To operate within this highly constrained situation, a complex decision-making structure arose, complete with multi-tiered scientific groups, to assign pointing time in a manner that was fair and respectful of the different scientific goals of various research teams. In order to make their observations and obtain their samples, Cassini’s various scientific instruments required specific spacecraft trajectories and orientations. And to run its experiment for the maximum amount of time, each scientific group needed to make a case for why its use of the spacecraft was vital to mission objectives and scientific goals. The arguments sometimes changed as the mission progressed. New discoveries could raise the value of studying a particular target. For example, after the spacecraft found evidence of liquid water within the moon Enceladus, visiting it took on increased significance and mission staff scheduled additional flybys. 10.1.1

The art of tour design

Cassini tour designers compared the process of mapping out a path through the Saturn system to a would-be vacationer visiting AAA and planning out a route. The vacationers (or in NASA’s case, the mission’s scientific groups) stand on one side of the counter and ask AAA staff (the tour designers) to make up a TripTik® that will take the vacationers to some particular places they want to see. The scientists specify what they are after in terms of scientific objectives and the geometry requirements to meet those objectives. The tour designers then “take that off for a couple months, apply astrodynamics,”5 and find a route that conserves fuel and still gets to as many locations as possible.

10.1

Choosing trajectories and assigning time on the spacecraft 243

Sometimes a request simply cannot be fulfilled. This was the case for an additional flyby of Iapetus sought by scientists who discovered interesting new characteristics of the satellite. Perturbations possibly caused by Iapetus in the surrounding magnetic field lines implied to some scientists that an internal, electrically conducting ocean might exist, while others in the Cassini community believed this to be unlikely.6 A close flyby could have explored this question further. But the request was raised late in the planning process. Reaching faraway Iapetus with the spacecraft would have been very difficult and would have made it quite challenging to achieve other important objectives, one of which was to study Saturn’s F ring. There simply was not time in the mission to do everything, and the request for another Iapetus flyby was denied.7 Different scientific disciplines on the mission wanted different types of tours. Ring scientists, for instance, desired that the Orbiter fly high above the ring plane, giving them a commanding view of ring processes. Scientists studying icy moons wanted quite a different trajectory. It was best for their observations if the vehicle flew in the plane of the ring system, which is also the equatorial plane of the planet in which most of the satellites orbit. Such a path would allow the spacecraft to fly close to many of their targets of interest. These different needs set up a tug-of-war between the two groups. As Jonathan Lunine, one of the mission’s interdisciplinary scientists, explained, “What the tour designers have to do is take all of these different inputs from hundreds of scientists, who say, we have to observe this, we want to look at that, and put that together”8 into a tour. How did the tour designers choose the best way of putting everything together? For one thing, they tried to provide value to every group needing time on the Orbiter. According to John Smith, “We show a variety of different tours that are kind of in different flavors. One makes the rings people a little more happier than the moons people,”9 and vice versa. At Project Science Group (PSG) meetings, which are held three times every year, representatives of the different science groups would rate the options as green, yellow, or red – preferred, acceptable, or unacceptable. Sometimes shades of these colors were used as well. The rating exercise was typically followed by an executive session in which leaders of the science groups selected the final tour. The tour designers worked well with the scientists and could generally identify a tour that all of the science groups were able to live with. But if a decision was not able to be reached, the mission’s project scientist made the final decision. Also critical to designing a tour were the fuel requirements. If the Orbiter could have carried an infinite amount of fuel, nearly any target in the Saturn system would have been able to be visited. But the fuel capacity was limited and the scientists and engineers thus needed to carefully weigh the value of the science conducted during the tour to the cost in terms of fuel. During the 4 year Prime Mission, the intent was to achieve Prime Mission objectives and fuel was spent to do so. One example was overcoming the Doppler shift problem by redesigning the initial orbits to radically increase the Orbiter’s distance from Titan during the Huygens Probe descent. This redesign was expensive in terms of fuel, but it was a necessary action to achieve the objectives of the Prime Mission. But during the extended missions the objective was to conduct long term observations of the Saturn system through another season, and hence “fuel economy was the name of the game.”10 Being too spendthrift with the available fuel early on might have prevented vital targets from being visited later.

244 10.1.2

The Saturn tour A valuable commodity: Pointing time on the Orbiter

After the trajectory had been decided, decisions had to be made concerning which instruments would get to point at their targets during which periods. For instance, at a particular time during a flyby of Titan, would the cameras get to point at the moon and take pictures? Or would RADAR get the pointing opportunity? Or perhaps the Ion and Neutral Mass Spectrometer? Or would the Radio Science Team get to point the high-gain antenna at Earth to collect gravity data? Pointing time was the most valuable shared resource aboard the Orbiter, and it was apportioned by project-formed discipline teams, with each team representing one of the mission’s five foci of scientific research: • • • • •

Saturn Rings Magnetosphere Icy satellites Titan.

Various of the Orbiter’s instruments were used to study each of these foci. A discipline team included personnel from each instrument team which operated an experiment pertinent to its focus. The Titan Orbiter Science Team (TOST) – the discipline team studying Saturn’s largest moon – had a wide range of interests and included representatives from all instrument teams.11 It was important that each discipline team made its own spacecraft pointing and data collection decisions concerning its focus of research. On TOST, for instance, personnel studying Titan were the ones who chose what data would be taken on a given Titan flyby. Of course, the discipline teams did not deliberate in a vacuum. They received important input from other personnel that helped guide their actions. As an example, the Cassini mission (as well as various other NASA space projects) included personnel with extensive knowledge of a variety of fields who served as interdisciplinary scientists (IDS). They were to ensure that the mission met its scientific objectives. In order to accomplish this, IDSs observed and participated in each discipline team’s pointing decision processes to make sure that each scientific instrument was given appropriate observing time to meet its particular objectives.12 Occasionally conflicts arose that a discipline team could not resolve. In those rare cases, the responsibility for making a decision usually went to the project scientist. In planning the 20th flyby of Titan (T20), for instance, both the RADAR team and the Visual and Infrared Mapping Spectrometer (VIMS) team felt this chance to take data relevant to its own instrument was so important that it could not be passed up. When TOST was unable to decide which team should be awarded pointing time, presentations were given to Project Scientist Dennis Matson. Each side also wrote a white paper. Matson pondered the issues, asked questions, and eventually decided that the VIMS team was the best one to receive the T20 opportunity. This ended the matter. Discipline team decision-making processes have evolved over the years as their objectives were refocused. TOST was set up during the interplanetary cruise phase of the mission. By the time the spacecraft reached Saturn, all 44 satellite flybys of the Prime Mission had been planned out. But TOST realized that it required further data on how the science

10.1

Choosing trajectories and assigning time on the spacecraft 245

instruments would actually operate during those flybys. For instance, would the Imaging Science Subsystem (ISS) even be able to see Titan’s surface through the atmospheric haze? It was decided that for the first twelve flybys of Titan, each instrument’s observations would be carefully analyzed. TOST would use this data to determine which instruments should optimally be allocated time on a given flyby and, if necessary, revise some of the flyby plans that had been made in order to achieve the best possible science returns. It turned out, however, that very little needed to be changed, because all of the instruments returned quite high value data.13 10.1.3

Planning activities on Galileo versus Cassini-Huygens

At first glance, the Galileo mission to Jupiter appeared to be a sort of a sister mission to Cassini-Huygens. Each robot expedition visited a gas giant planet and launched a probe from an orbiter that then toured the planet’s satellites for years. But Galileo’s development had been fraught with adversity, and this impacted on the rest of the mission. Although the Jovian satellite tour concept was conceived in 1973 and early planning envisioned a Jupiter orbiter/probe launch in 1980, serious technical issues, such as that the Space Shuttle and its Inertial Upper Stage (IUS) were not developed on time, led to launch delays. Repeated flip-flops in upper stage design between the hydrogen-powered Centaur and a solid-fueled IUS necessitated significant rework and further delays. The Galileo mission also ran into political opposition from the Reagan White House and Congress. Then the Challenger disaster required a major redesign of the Galileo spacecraft and its trajectory, and pushed back the launch by another three and a half years. By the time Galileo finally left Earth in 1989, many of its scientists had been developing their experiments for over a decade and some had been involved in mission planning for even longer. They had all given major chunks of their careers to the project, and wanted ample scientific return for their efforts.14 Galileo was not initially a “resource-rich” mission, even though it carried a scan platform that allowed more than one instrument to aim at its target at the same time. Only eleven orbits of Jupiter were included in the Prime Mission, because mission engineers feared that the intense radiation environment might degrade the onboard equipment so severely that more orbits would not add value. It was actually a surprise that the spacecraft operated well beyond the period of its Prime Mission. But even though it withstood intense radiation longer than envisioned, the craft experienced a series of technical problems during its cruise through interplanetary space and afterwards during the Jupiter tour. When the high-gain antenna failed to open and the spacecraft had to rely on the low-gain antenna with its tiny data rate, mission scientists were intensely disappointed and frustrated. The technical issues called into question whether the spacecraft would be able to complete its mission objectives. Project scientists were thus justifiably concerned that, with the relatively small number of orbits initially available and the continuing problems, they might not get the Jovian-system data for which they had been waiting so long. What arose on Galileo was a very different vision of the mission’s likely future than experienced by Cassini-Huygens scientists. Galileo seemed to be a mission of limited resources with an uncertain future. Scientists fought ferociously to ensure that their experiments got to run and collect data. Some of the scientists remember tour design meetings

246 The Saturn tour as being cantankerous, combative affairs, with personnel vying strenuously to win operating time on the spacecraft for their instruments.15 On the face of it, designing the Cassini Orbiter tour should have been even more difficult and contentious. For Galileo, there were three different scientific working groups: fields and particles, satellites, and the Jovian atmosphere. Cassini had those three teams plus teams for Titan and the ring system. And Cassini was more of an international effort with many additional scientists participating, all of whom had their own agendas, and who had to wait from the mission’s official kickoff in 1990 to its arrival at Saturn in 2004 to begin their observations of the planetary system. Kevin Baines, a scientist on both missions, noted that planning efforts on CassiniHuygens were actually quite smooth and harmonious, more so than on Galileo, and suggested some reasons for this. For one, Cassini-Huygens did not experience the disappointment of a broken high-gain antenna. The spacecraft operated pretty much as hoped and there was an expectation that it would continue to do so. Second, more advanced communications technology played a part. Ubiquitous laptop computers and presentation software such as PowerPoint enabled scientists to access virtually any mission information they wanted, whether they were on the road or in their labs. And a minutes-old presentation of brand-new information could be instantly sent to anybody around the world. What a difference from life on the Galileo project! If scientists on that mission were not at JPL and not physically attending a vital meeting, they were out of the loop and could lose an opportunity to operate their instruments aboard the vehicle and collect vital data. That put pressure on personnel to drop everything and fly out to JPL when a critical session was called. This was very stressful and quite different from the Cassini world, in which scientists could simply join a telephone conference call or send and receive emails and remain virtually present. Another thing that made Cassini different from Galileo was that it truly was a resourcerich mission. It did not have just eleven orbits planned during its Prime Mission; it had over 75. And as mentioned above, the Cassini Orbiter continued to operate with relatively few technical problems. There wasn’t as much concern that the mission would end before its objectives had been met. With many more orbits and a functioning high-gain antenna, the competition for time on the spacecraft was significantly reduced. If two scientific teams both wanted pointing time, they were generally willing to negotiate for, and trade, their observation opportunities. There were ample opportunities on the mission for teams to usually get what they needed.

10.2

USING TITAN AS THE “TOUR ENGINE” FOR CHANGING SPACECRAFT ORBITS

“We use Titan as our tour engine, to change our orbit and explore Saturn.” – Linda Spilker, Cassini-Huygens Deputy Project Scientist16 The Cassini Orbiter used gravity assists to get around the Saturn system in the same way that the spacecraft employed Venus, Earth, and Jupiter to get to Saturn. Huge amounts of propellant would have been needed if these gravity assists had not been available.

10.2

Using Titan as the “tour engine” for changing spacecraft orbits

247

On the way to Saturn, the craft was able to alter its velocity and direction in a major way without the need for additional propellant, simply by flying by Venus, Earth, and Jupiter and receiving a miniscule amount of each planet’s enormous momentum. But once the vehicle reached Saturn and began orbiting it, there was only one other body in the planetary system with sufficient mass, and thus a strong enough gravitational field, to provide hefty gravity assists. That body was Titan. A single Titan flyby, in fact, could alter the velocity of the spacecraft by more than 800 meters per second (1,800 miles per hour). A serious constraint was that each Titan flyby had to aim the spacecraft so that after at most several more orbits around Saturn, it would return to Titan for another flyby. By doing this, the craft’s trajectory could be appropriately manipulated for its next orbit of Saturn.17 Tour designers planned each Titan encounter to ensure that it created what was referred to as a free return to Titan, one that did not require large propulsive maneuvers, because propellant was a very limited resource. From a scientific point of view, the frequent returns to Titan were by no means a waste of time. Just the opposite, because this satellite is one of the most fascinating objects in the Saturn system. The repeated flybys provided a golden opportunity to study its thick atmosphere, map its surface, and investigate its geological processes. These include many that are Earth-like, as well as the phenomenon of cryovolcanism (described in Chapter 9) which does not occur on Earth. One of the chief features of the Saturn tour was its sequence of high inclination orbits. To appreciate what this term means, picture Saturn’s main rings lying in the equatorial plane of the planet (Figure 10.1). If the spacecraft orbited Saturn within its equatorial plane, then the orbit’s inclination would be zero. But if the spacecraft flew a path that took it sharply above and below the equatorial plane, its orbit would be highly inclined. The angle between the spacecraft’s orbit and the equatorial plane is the inclination of the vessel’s orbit.

Figure 10.1 Saturn’s main rings, lying in the planet’s equatorial plane.

248 The Saturn tour Most of Saturn’s moons orbit in or close to the equatorial plane, but some follow highly inclined paths. To perform a close flyby of such a moon, the Orbiter would typically have to leave the equatorial plane as well – unless the flyby was timed to occur when the moon passed through the equatorial plane. Also, to make thorough observations of the planet’s atmosphere and magnetic field near its north and south poles, the spacecraft needed to sharply incline its orbit. Such high inclinations were achieved using Titan gravity assists and a relatively small amount of propellant, but this needed to be done over a considerable amount of time and in small steps, since there are limits on the amount of inclination change that can be accomplished by a single Titan encounter.18 In its multi-year soirée through the Saturn system, the Cassini spacecraft revised the size, period, and inclination of its trajectory multiple times in order to view the planet and its rings, satellites, and magnetosphere from a range of different angles and distances. To set up a satellite flyby, the spacecraft needed to fine-tune its path by firing its rockets. Any moon encounter set up by such a rocket firing was termed a targeted flyby. These usually had close approach distances within 2,000 kilometers (1,200 miles), and sometimes as low as 25 kilometers (15 miles). The targeted flybys of Titan had to approach no closer than roughly 900 kilometers (600 miles) however, owing to its extended atmosphere. In addition to these targeted flybys, the Orbiter took advantage of fortuitous alignments when its trajectory happened to carry it relatively close to a moon. These were termed nontargeted flybys and were typically far more distant than the targeted variety. One example was the encounter with Mimas that occurred just days before the SOI maneuver, when the vehicle passed 76,000 kilometers (48,000 miles) above the satellite’s heavily cratered surface.19 During the Prime Mission, which began when the spacecraft arrived at Saturn, it made a 4 year tour of the system. While this tour has been given various names, in this book it is called the Prime Mission tour. It was followed in turn by a 27 month extended mission called the Cassini Equinox Mission and then by a 7 year Extended-Extended Mission or “XXM,” also known as the Solstice Mission.

10.3

THE CASSINI-HUYGENS PRIME MISSION TOUR

After the spacecraft reached Saturn on 1 July 2004, it began its Prime Mission tour with an elongated, elliptical journey around Saturn, depicted in Figure 10.2. During this circuit, the vessel swung rather close to Saturn, then flew quite far away, well beyond the orbit of Iapetus, and finally returned on 26 October 2004 to fly by Titan. Part of the motivation behind the large orbit was to use as little propellant as possible during the rocket firing that accomplished Saturn Orbit Insertion (SOI). The closer the orbit’s periapse was to Saturn, the less propellant was needed. There was a tradeoff to this strategy, however. In order to be captured by Saturn, the craft flew near the planet and passed through its ring plane. For safety, mission planners aimed it to cross the ring plane inside the G ring but outside the F ring, in what they believed to be a region relatively clear of particles, and this turned out to be the case. But the planners, concerned about particle dangers to the spacecraft, did not want it to try another ring plane crossing in that area, especially so early in the mission. Hence they

10.3

The Cassini-Huygens Prime Mission tour 249

Figure 10.2 SOI and initial orbits of the spacecraft around Saturn.

developed a periapse raise maneuver – a procedure to move the periapse (the closest point to Saturn) in the spacecraft’s following orbits farther out from the planet and its region of dense rings. The most efficient way to carry out this maneuver was to perform a trajectory correction at the elongated orbit’s apoapse, its farthest point from Saturn, where the spacecraft’s speed was significantly slower. At the periapse during SOI the spacecraft was zipping along at over 30 kilometers per second, but out near the distant apoapse at the start of the periapse raise maneuver, it was toddling along at only 0.4 kilometers per second. The slower that the spacecraft was travelling, the less propellant was required to change its trajectory to adjust the periapse.20 Subsequent orbits were far less elongated, did not pass so close to the planet’s dense rings, and had typically less than half the period of that initial circuit. Some orbits had periods as short as 7 days (which was half the period of Titan’s orbit). These shorter, faster orbits allowed more flybys of Titan, Saturn and its rings, and other satellites. 10.3.1

The unusual numbering system of Prime Mission orbits

Cassini-Huygens had 75-1/2 orbits in its Prime Mission, with the half orbit arising from the convention that orbit numbers start at the first apoapse. The 180° arc from SOI to the first apoapse was labeled orbit zero. Originally the mission had 74-1/2 planned orbits. Then (as described in Chapter 8) another orbit was added in order to solve the problem with the Doppler-shifted signal sent by the Probe to the Orbiter. But rather than renumbering all of the planned orbits, mission engineers replaced the Orbiter’s first two circuits of Saturn with orbits a, b, and c, and then, after Huygens had completed its mission, the Orbiter picked up the tour at orbit 3. So what was called orbit 3 was actually the 4th circuit

250

The Saturn tour

of the planet, and so on. The final Prime Mission orbit was numbered 74, but was actually the 75th time around. This unusual way of numbering was used because there were many documents and computer files based on the original numbering scheme and it would have been more difficult to deal with the confusion resulting from changing all of that work to reflect new orbit numbers.21 10.3.2

Occultations

During the Prime Mission tour’s 75-1/2 circuits of Saturn, the Orbiter made many flybys of icy moons. The tour designers also built a hiatus from satellite encounters into their plans, when the Orbiter could undertake different types of investigations. During its first year at Saturn, from mid-April to mid-July 2005, the Orbiter passed six times behind Saturn and its rings as seen from Earth, and the manner in which the signal from the Orbiter was altered during these occultations enabled the Radio Science Team to determine characteristics of the atmosphere of the planet and its rings.22 10.3.3

The magnetometer issue

The Cassini Orbiter functioned beautifully during most of its Prime Mission tour, although a serious issue did arise with the Magnetometer (MAG) that measured the strength and direction of magnetic fields. The magnetometer contained two different types of sensor to enhance the measurements it could obtain. The different sensors did different things, but were quite complementary.23 Cassini’s design constituted a departure from Galileo’s approach, which used a pair of flux gate magnetometers. Sticking out from the edge of the Cassini Orbiter was an 11 meter boom. Half way along the boom was the flux gate magnetometer (FGM) and at its far end was the vector/scalar helium magnetometer (VHM/ SHM). The VHM/SHM was a more stable instrument and could help calibrate readings from the FGM instrument. The VHM/SHM was particularly useful for measuring Saturn’s internal magnetic field due to its accuracy in determining absolute field magnitudes. The FGM was installed a different distance away from the spacecraft than the VHM/SHM for an important reason. This allowed interference from the vehicle’s other electronic components to more easily be calculated and subtracted out of the final reading, ensuring that it was the true magnetic field that was being measured. This was a vital capability, because the magnetometers were extremely sensitive and even though on a long boom, they could still sense the fields produced by other instruments.24 As a cost reduction measure, the flight spare helium magnetometer from the ESA-NASA Ulysses mission was used as the core of the Cassini-Huygens mission’s flight model helium magnetometer. Only slight modifications were needed to enable it to operate on Cassini.25 Both magnetometers operated well in pre-launch testing and during the early mission, including on the Earth flyby and the cruise phase toward Saturn. The system generated good data, but the MAG team eventually noticed disturbing behavior of the helium magnetometer. A year and a half into the mission, the team observed that the instrument’s temperature and power draw was slowly changing. The instrument’s sensitivity began to get worse and worse, and by November 2005 the helium magnetometer stopped taking useful data.26

10.4

The Cassini Equinox Mission 251

Fortunately the flux gate magnetometer continued to function at full capacity. But it no longer had the helium magnetometer to work with, and so a new method of calibration needed to be developed. The spacecraft was periodically placed into rolls about different axes. As the orientation of the axis changed, the measurements made by its magnetometer were used to calibrate the instrument. Because mission scientists knew what the field was supposed to be doing, comparing it to the actual readings told them how to calibrate the system and determine the true zero level of the instrument. Consequently, even without the helium magnetometer, the flux gate magnetometer could continue taking the data vital to various phases of the mission. The down side, of course, was the effort required to keep the instrument calibrated. A team of three personnel at Imperial College London, who were responsible for the operation and safety of the instrument, spent roughly 50% of their time making sure the flux gate magnetometer’s calibration was correct.27 What caused the failure of the helium magnetometer? It is difficult to determine for sure, with the vehicle nearly a billion miles from Earth. Helium magnetometer measurements utilized the precession frequency of helium atoms about a magnetic field, or in other words, the rate at which the atoms wobbled around the field lines, rather like the wobble that occurs with a spinning top. This rate of wobble, called the Larmor frequency, is proportional to the magnetic field strength. Thus if the Larmor frequency can be determined, “it becomes a very accurate measure of the [magnetic] field magnitude.”28 The MAG team believed that a slow leak in the magnetometer’s helium vessel was the most likely explanation for the degradation of performance, because as the quantity of helium decreased, so would the number of precessing atoms capable of measuring the field through their Larmor frequencies. The leak could have resulted from a small puncture or crack in the glass vessel, possibly from a micrometeoroid impact that went undetected.29

10.4

THE CASSINI EQUINOX MISSION

The Orbiter performed admirably during the Prime Mission, completing it in good shape and with ample propellant remaining. So in April 2008, NASA announced a 27 month extension through September 2010. NASA called it the Cassini Equinox Mission, after the impending Saturnian equinox in August 2009. During this event the Sun, which had been shining on Saturn’s southern hemisphere and the southern face of the rings, shone directly on the planet’s equator and on the edges of its rings. Immediately afterward, it began to illuminate the northern hemisphere and the rings’ northern face. One of the Orbiter’s tasks was to observe seasonal changes brought about by the changing Sun angle on the planet and its rings and moons.30 The Cassini Equinox Mission kicked off in July 2008. It built upon the successes of the Prime Mission and focused on scientific inquiries that included: • • • • •

Continued study of Titan Further study of the icy moons, especially Enceladus Monitoring seasonal phenomena on Titan and Saturn Analyzing new magnetosphere regions Observing the rings during the Saturn equinox in August 2009, when they were illuminated edge on.31

252

The Saturn tour

10.4.1

A major management change

It has been standard practice on many of NASA’s long missions for a changing of the guard when a new phase is entered. After his very successful tenure as project scientist, Dennis Matson left the Cassini team at the end of the Prime Mission. The team kicked off its Equinox Mission with the addition of Bob Pappalardo, whose research background was ideal for many of the challenges ahead. As a planetary geologist and senior research scientist in JPL’s Planetary Ices Group, his research focused on the processes that shaped the icy moons of the outer solar system, in particular the mechanisms that powered the water jets spewing from Enceladus.32 In his own words, Pappalardo was assigned the position partly for interpersonal reasons. Demands of a mission as intense and long-lasting as Cassini-Huygens can put considerable stress on its personnel. Sometimes this manifested in the form of strained relationships and animosities, although on the whole, Cassini’s personnel operated together quite beautifully despite it being a high pressure, high-tech work environment that demanded long hours and a large degree of commitment. Dennis Matson’s role in creating and maintaining the mission’s collegial environment was acclaimed by some personnel associated with the mission, although others saw him as simply staying out of the way rather than proactively developing good working relations. Pappalardo was a fairly new face on the mission who was not even hired by JPL until 2006. When he was made project scientist, he did not have a history with most of the managers on the project. While this made his new position quite challenging, Pappalardo saw it as a distinct advantage. His objective was to “gently push here and pull there and see if we can do the best to make up the most cooperative project that it can be.”33 He strove to achieve the ideal of unselfish cooperation in science, and would approach personnel in conflict with the simple statement, “Here is the right thing to do. Here is the way we should go forward.”34 Program Manager Bob Mitchell had a different memory of Pappalardo’s tenure. Friction between Pappalardo and other scientists on the mission arose because of the stark differences in the management styles applied by Matson and Pappalardo. Dennis Matson’s style was “very much hands off and behind-the-scenes.”35 In The Titans of Saturn, Groen and Hampden-Turner referred to him as “Soft-spoken, and always calm and confident, his adage was, ‘I try to avoid deciding.’”36 Matson strongly believed that in the meetings he chaired, no scientist’s concerns should be omitted from consideration. He also held that conflicts were ideally resolved at the interfaces where the different scientific disciplines intermingled, rather than with a decision originating from upper management. Matson’s approach was very much in keeping with the principle of subsidiarity, described earlier in the book, that Jean-Pierre Lebreton borrowed from the European Union and applied to congresses of Huygens scientists. This required every decision to be made at the lowest level for which full information was available, and made on the basis of good science rather than politics or seniority. Matson had developed the right approach for managing “hundreds of talented, independent, specialized experts, with an unrivaled knowledge of their own disciplines.”37 Under his leadership, the mission thrived. Personnel recognized him as a senior scientist who had worked in the planetary science field a very long time and had been with the Cassini-Huygens mission since 1989.

10.4

The Cassini Equinox Mission 253

Bob Pappalardo, on the other hand, was “this new guy who was relatively young, quite inexperienced, at least in this kind of an arena, and [with] a very different style and personality. Very much hands on and controlling.”38 For example, when media relations personnel would write a news release on exciting work that a scientist had done, Matson usually did not feel the need to get involved in the editing process. He would leave that to the scientists and reporters. Pappalardo, on the other hand, would inspect the drafts quite carefully and go through numerous iterations with personnel involved. Pappalardo would edit, shape, and modify the draft to the point of causing some scientists to get rather irritated with him. These scientists felt censored in what they were permitted to say. Pappalardo was perceived as constraining the breadth of scientific opinions that could be released to the public, which flew in the face of the freedom of expression that was part of mission culture under Matson. Pappalardo brought both considerable talent and controversy to his role as project scientist, but he did not stay long. After only 20 months in post, he left, as planned, for another very influential and important position – project scientist for the Europa Jupiter System Mission under development at JPL.39 He was succeeded by his deputy, Linda Spilker, who had been deeply involved in Cassini science activities since 1988, even before it was selected as a flight mission. Before that she had worked on Voyager, another endeavor that investigated the Saturn system.40 Trina Ray, the science lead for TOST, expressed her excitement at having Linda be chosen the project scientist, noting that she had been on the mission forever and understood all its subtleties.41 Bob Mitchell saw Linda as somewhere in between the two previous project scientists in management style – more hands-on than Matson and less so than Pappalardo, and Mitchell thought that this would be a good thing.42 10.4.2

An aging spacecraft: Trouble with the Orbiter’s thrusters

During the Cassini Equinox Mission, a problem developed with the spacecraft’s propulsion system that indicated it was feeling some of the effects of middle age. The first clue that something was amiss came from the Orbiter’s small, 1 newton43 thrusters. These made small corrections in the trajectory, controlled the attitude of the vehicle, and adjusted the angular momentum in the reaction wheels (Chapter 3 discusses thrusters and reaction wheels). The Orbiter had a total of 16 thrusters, but only one set of 8 was needed at any given time. In keeping with NASA’s policy of redundancy, the other 8 constituted an identical set of backup thrusters. They were split across two branches, with 8 on the primary A-branch and 8 on the backup B-branch. The thrusters were being used every few weeks for maneuvers. When mission personnel used the A-branch thrusters in fall 2008 for a maneuver, they found that one performed at a significantly impaired level. The thrusters on the A-branch had operated satisfactorily for a long time; in fact, all the time since launch in October 1997. After the issue surfaced, mission engineers initiated an extensive analysis of the system in coordination with JPL’s propulsion experts, Lockheed Martin Space Systems of Denver, Colorado (the propulsion system contractor), and Aerojet of Sacramento, California (the thruster manufacturer). Impairment of thruster performance could lead to serious problems, as will be explained below. When the thrusters were used for attitude changes or for reaction wheel momentum

254 The Saturn tour management, the spacecraft itself was able to directly measure the modification. But when altering the vehicle’s velocity for course corrections, no such measurement of the achieved change was available on board.44 Instead, the Orbiter’s thrusters were commanded by mission control on Earth to operate for a preset amount of time, based on the amount of force they were expected to exert. If that force decreased and the engineers were not aware of it, the thruster would not change the spacecraft’s velocity as predicted and significant amounts of fuel would require to be expended later to rectify the problem. On one maneuver, the thruster issue caused the desired velocity change to be reduced by 15 millimeters (roughly half an inch) per second. This might sound like a small amount, but by the time of the first opportunity to correct the error, fixing it required a correction of 5 meters (16 feet) per second and consumed propellant which the mission could ill afford to waste. If such anomalies became common, the mission would have to be terminated prematurely.45 Then a second thruster exhibited a similar problem. The vehicle was not getting the thrust expected, and mission engineers were having to guess at the actual thrust that would be delivered. This quantity did not remain constant, because the amount of thrust available was continually diminishing as the thrusters deteriorated. If the engineers guessed wrong as to how much thrust was available, errors such as the one described above would keep occurring. Understandably, therefore, the industry-JPL team recommended replacing the A-branch as soon as possible. The mission’s engineering team prepared and tested a sequence of commands, then sent them to the spacecraft. Over a 7 day window in March 2009, the Orbiter switched to its B-branch thrusters. The engineers had chosen this time for the task because there were no high priority science observations scheduled, nor navigation maneuvers to maintain the vehicle’s trajectory. The swap involved commanding a latch valve to open to allow hydrazine to flow to the B-branch, then powering on thruster control electronics. No pyrotechnic devices were involved, and the action was fully reversible if necessary. This was only the second time in the spacecraft’s 11 years of flight that it switched to a backup system. The other event had occurred when the backup reaction wheel was activated several years earlier.46 The reaction wheel exchange was particularly significant because there was only one spare wheel aboard the spacecraft. The industry-JPL team also conducted an analysis of what went wrong with the A-branch thrusters, hoping to prevent such an occurrence on the B-branch. For if they developed the same problems as the A-branch thrusters, that could impair the vehicle’s ability to finish its second project extension, the Solstice Mission.

10.5

THE SOLSTICE MISSION

In February 2009, the Cassini team recommended to NASA yet another mission extension. This was called the Solstice Mission as well as the Extended-Extended Mission, or “XXM.” If the Orbiter remains healthy, the Solstice Mission could last until 2017. This will be an auspicious time, because during May 2017, the Saturn system will be enjoying its northern midsummer, or in other words, its summer solstice. Major differences exist between it and the terrestrial summer solstice. A Saturnian year spans 29.5 terrestrial

10.5

The Solstice Mission

255

years, and the tilt of Saturn’s axis is a rather significant 27°, as against 23.5° for Earth. Axial tilt is principally responsible for solar illumination (insolation) variations that cause seasonal changes in weather and temperature. The slow progression of seasons has major impacts on both Saturn and Titan phenomena, such as giving rise to the ghostly spokes of the ring system and powering methane storms at Titan’s poles.47 The planning for the Solstice Mission required a complex application of orbital mechanics. Cassini’s tour designers (whose work was discussed earlier) needed to devise trajectories that would maneuver the Orbiter to observe a myriad of targets, including moons, rings, and Saturn itself, in a manner that would fulfill as many of the project’s science research goals as possible. And this had to be achieved using the minimum of propellant. The planners were to keep the Orbiter working for an additional 7 years, but by the time this phase of the project began, the vehicle had already used up about 80% of the propellant originally available for maneuvering during tours. The task of the tour designers was thus to work out how to more than double the duration of the Orbiter’s exploration of the Saturn system, using only a fifth of its original propellant volume. Brent Buffington, one of the tour designers, compared this task to “plotting a seven year road trip around the United States for more than 200 scientists, all with different interests and all wanting to see different things. … Oh, and the targets they want to see are moving.”48 10.5.1

The final orbits of the spacecraft

If all goes as planned, the Cassini Orbiter will enter the final segment of the Solstice Mission, and of its life, in November 2016. The final activities of the Orbiter can be divided into three phases: • • •

F ring phase Proximal orbit phase Planetary protection phase.

10.5.1.1

F ring phase

Starting in November 2016, the spacecraft will fly around Saturn 20 times with its periapses (the closest points in its orbits to the planet) just outside the F ring. This will provide valuable opportunities to make high resolution measurements of the F and A rings, observe solar and Earth occultations by the rings, and study auroral phenomena. This phase will also position the spacecraft for its April 2017 jump to proximal (close-to-Saturn) orbits with periapses just inside the D ring.49

10.5.1.2

Proximal orbit phase

The Orbiter will dive into the 3,000 kilometer clear region between the inner edge of the D ring and Saturn’s upper atmosphere, then carry out at least 22 circuits of the planet. By flying between the planet and its ring system, the spacecraft will seek to distinguish the

256

The Saturn tour

ring’s gravitational effects from those of Saturn itself, providing a reliable estimate of the total mass of Saturn’s ring system. Such a measurement is crucial for understanding the rings’ ages and evolutionary details: the more massive the rings are, the more likely it is they have survived from primordial times – when Saturn first formed. But scientists currently don’t know the mass of the ring system to better than a factor of ten. During this phase, the Orbiter also may be able to collect important data on the planet’s magnetic fields, internal rotation rate, and internal structure – especially the nature and depth of its metallic core. In addition, the proximal orbit phase will offer a chance to make in situ measurements of Saturn’s ionosphere, innermost radiation belts, D ring, and auroral region.50

10.5.1.3

Planetary protection phase

If the spacecraft were to run out of propellant, mission control would no longer be able to define its trajectory and it could eventually strike one of Saturn’s satellites. The vehicle was not sterilized before it left Earth, so could conceivably contaminate one of the satellites with terrestrial microorganisms. It is particularly important to avoid compromising Enceladus, because its interior might include an ocean that is capable of harboring life. If the spacecraft crashed on Enceladus, the heat from the plutonium-powered RTGs might conceivably melt through the icy shell (although it would have to be thin ice for this to happen) and come into contact with the water underneath. It is difficult to assess the risk of this scenario because, if liquid water does exist within Enceladus, we do not yet know at what depth it is located. We do know this would compromise future searches for extraterrestrial life. If liquid water exists inside Titan, on the other hand, scientists currently believe the crust is too thick for Cassini’s RTGs to melt through it.51 To guard against potential contamination by Earth’s microorganisms, a planetary protection phase of the mission has been proposed. The proximal orbits will be carried out in such a manner that the ship receives regular gravitational bumps from Titan. These will eventually cause the ship to plunge into Saturn’s atmosphere and vaporize, preventing accidental biological contamination of any of the satellites. The currently planned Saturn impact date is 15 September 2017, but if sufficient propellant is available it would be possible, if NASA wished to further support the mission, to execute a maneuver that would delay atmospheric entry for several more orbits.52 Figure 10.3 provides a summary of the targets and orbits of the Prime, Equinox, and Solstice Missions, with information on the number of encounters with various satellites (and the G ring arc) in each year of the mission, as well as the orientation of Saturn in that year. Note that during equinox, Saturn is oriented so that its rings are edge on to the Sun. The figure gives not only past encounters with satellites, but also those projected to occur through the end of the mission. In addition, it gives the proximal orbits of Saturn that are planned for the final stage of the mission.

References 257

Figure 10.3 The targets and orbits of the Prime, Equinox, and Solstice Missions.

REFERENCES 1. Bob Mitchell interview by author, JPL, 5 February 2008. 2. Trina Ray interview by author, 22 October 2008, JPL. 3. Manuel Grande, “The Saturn Tour,” http://www.sstd.rl.ac.uk/news/cassini/mission/tour.html, U.K. Cassini Home Page, Space Science and Technology Dept. at Rutherford Appleton Laboratory, last updated 1 July 2004, accessed 20 May 2009. 4. Amy Roth (from Skepchick.org) interview with John Smith, Cassini Mission Planner, “Keep Your Day Job: Cassini Tour Designer,” found in the NHRC folder, U:\PA\Codeiq\contract historians electronic files. 5. Brent Buffington, speaking in the NASA-JPL Web video, “The Tour Designers: Charting Cassini’s Next Moves,” http://www.jpl.nasa.gov/video/index.cfm?id=678, JPL Videotape Master Library no. AVC-2007-062-1/1 (5 Apr. 2007), accessed 18 May 2009. 6. Dennis Matson, London, interview by author, 22 June 2009. 7. Brent Buffington and John Smith, “Cassini Solstice Mission Tour Selection Project Science Group Meeting #47,” presentation at PSG #47, JPL (26-29 Jan. 2008); Discussions of the icy moon group, attended by the author, at the October 2008 Project Science Group #46 meeting held at JPL. 8. Jonathan Lunine, speaking in the NASA-JPL Web video, “The Tour Designers: Charting Cassini’s Next Moves,” http://www.jpl.nasa.gov/video/index.cfm?id=678, JPL Videotape Master Library no. AVC-2007-062-1/1 (5 Apr. 2007), accessed 18 May 2009. 9. John Smith, speaking in the NASA-JPL Web video, “The Tour Designers: Charting Cassini’s Next Moves,” http://www.jpl.nasa.gov/video/index.cfm?id=678, JPL Videotape Master Library no. AVC-2007-062-1/1 (5 Apr. 2007), accessed 18 May 2009. 10. Trina Ray telephone conversation and email with author, 10 January 2012.

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11. Not all teams ended up participating, however. According to Ralph D. Lorenz in a 30 Dec. 2010 email to the author, “in practice, CDA [the Cosmic Dust Analyzer team] never showed up as they basically have no Titan science.” 12. Bob Mitchell email to author, 19 Dec. 2009. 13. Trina Ray interview by author, 22 October 2008, JPL; Carolyn Porco email to author, 28 Dec. 2009; Candy Hansen email to author, 5 Jan. 2012. 14. Robert T. (Bob) Mitchell email to author, 6 July 2009; Michael Meltzer, Mission to Jupiter: A History of the Galileo Project, NASA SP-2007-4231 (2007). 15. Kevin Baines, London, interview by author, 25 June 2009. 16. Tariq Malik, “Cassini-Huygens: Knocking on Saturn’s Door,” 21 June 2004, SpaceNews (21 June 2004), NASA NHRC 18337 Cassini 2002-. 17. Robert D. Lange, “Cassini-Huygens Mission Overview and Recent Science Results,” Aerospace Conference, 2008 IEEE, Big Sky, MT (1-8 March 2008):1-10; Bob Mitchell review of manuscript, Feb. 2011. 18. Grande, “The Saturn Tour”; Bob Mitchell review of manuscript, Feb. 2011. 19. NASA-JPL, “Flybys,” http://saturn.jpl.nasa.gov/mission/flybys/, accessed 28 May 2009; Planetary Society, “Cassini’s Tour of the Saturn System,” http://www.planetary.org/explore/ topics/cassini_huygens/tour.html, accessed 28 May 2009; Planetary Society, “Mimas: That’s No Space Station,” http://www.planetary.org/explore/topics/saturn/mimas.html, accessed 28 May 2009. 20. Robert T. (Bob) Mitchell, 6 Jan. 2012 email to author. 21. Robert T. (Bob) Mitchell email to author, 6 July 2009, regarding the Cassini-Huygens tour design. 22. Robert T. (Bob) Mitchell, “Cassini-Huygens at Saturn and Titan,” Acta Astronautica 59 (2006):335-343. 23. Bob Mitchell review of manuscript, Feb. 2011. 24. Michelle Dougherty interview by author, London, 25 June 2009. 25. Imperial College, London, Space and Atmospheric Physics Group, “The Vector/Scalar Helium Magnetometer (V/SHM),” http://www3.imperial.ac.uk/spat/research/missions/space_missions/cassini/mag_instrument/v_shm, accessed 4 July 2009. 26. Michele Dougherty email to author, 1 Sep. 2010. 27. Michele Dougherty interview by author, London, 25 June 2009; Michele Dougherty email to author, 1 Sep. 2010. 28. Edward J. Smith, Michele K. Dougherty, Christopher T. Russell, and David J. Southwood, “Scalar Helium Magnetometer Observations at Cassini Earth Swing-By,” Journal OF Geophysical Research 106(A12) (1 Dec. 2001):30,129-30,139. 29. SpaceRef.com, “NASA Cassini Significant Events for 04/11/07 - 04/17/07,” http://www.spaceref.com/news/viewsr.html?pid=23964, 20 April 2007, accessed 4 July 2009. 30. NASA-JPL, “Mission Overview,” http://saturn.jpl.nasa.gov/mission/introduction/, Cassini Equinox Mission Web site, accessed 27 May 2009. 31. ESA, “Start of the Cassini Equinox Mission,” http://sci.esa.int/science-e/www/object/index. cfm?fobjectid=43032, ESA Science and Technology (30 June 2008), accessed 27 May 2009. 32. Carolina Martinez, “Cassini to Earth: ‘Mission Accomplished, but New Questions Await!’” http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20080627/, NASA-JPL (June 27,2008), accessed 27 May 2009; NASA-JPL, “Robert Pappalardo: Cassini Project Scientist,” http://science.jpl.nasa.gov/people/Pappalardo/, accessed 22 Apr. 2010. 33. Bob Pappalardo interview with author, London, 21 June 2009. 34. Bob Pappalardo interview.

References 259 35. Bob Mitchell telephone interview with author, 24 June 2011. 36. Bram Groen and Charles Hampden-Turner, The Titans of Saturn (Marshall Cavendish Business, 2005), p. 78. 37. Ibid., pp. 77-78. 38. Bob Mitchell telephone interview with author, 24 June 2011. 39. Emily Lakdawalla, “Titan: Callisto with Weather,” http://www.planetary.org/blog/article/00002391/, Planetary Society Blog (16 Mar. 2010). 40. A.J.S. Rayl, “A Conversation With Linda Spilker, Cassini Deputy Project Scientist Co-Investigator On Planetary Passions, Girl Power, and Striking a Balance,” http://www.planetary.org/news/2004/0914_A_Conversation_With_Linda_Spilker.html (14 Sep. 2004); Linda Spilker telephone interview with author (18 Feb. 2009). 41. Trina Ray telephone conversation with author, 10 January 2012. 42. Bob Mitchell telephone conversation with author, 10 January 2012. 43. A newton is the amount of force required to accelerate a mass of one kilogram at a rate of one meter per second per second. 44. Bob Mitchell email to author, 17 Jan. 2012. 45. Julie Webster, London, interview by author, 25 June 2009; Bob Mitchell review of manuscript, Feb. 2011. 46. Larry Gerstman, “Cassini-Huygens Mission Status Report – Feb. 2, 2009,” http://sites.google. com/site/larrygerstman/Home/unmanned-space-missions/cassini-mission-at-saturn. 47. John Spencer, “Cassini’s Proposed Extended-Extended Mission Tour,” http://www.planetary. org/blog/article/00001856, Planetary Society Web site (24 Feb. 2009), accessed 11 June 2009. 48. Guy Gugliotta, “A Saturn Spectacular, With Gravity’s Help,” http://www.nytimes. com/2010/04/20/science/space/20cassini.html?th&emc=th, NY Times (19 Apr. 2010). 49. Linda Spilker “White Paper for Solar System Decadal Survey 2013-2023: Cassini-Huygens Solstice Mission,” http://www.lpi.usra.edu/decadal/sbag/topical_wp/LindaJSpilker.pdf, Small Bodies Assessment Group (SBAG), Lunar and Planetary Institute (6 Oct. 2009). 50. John Spencer, “Cassini’s Proposed Extended-Extended Mission Tour,” http://www.planetary. org/blog/article/00001856/, Planetary Society Blog (24 Feb. 2009); Spilker “White Paper for Solar System Decadal Survey.” 51. Linda Spilker interview with author, JPL, 27 October 2010. 52. Spilker, “White Paper for Solar System Decadal Survey”; Bob Mitchell review of manuscript, January 2011.

Part IV

A great natural laboratory “Orbiting Saturn, Cassini is in the middle of the greatest natural laboratory accessible to us in space. With its rings, dozens of moons and magnetic environment, Saturn is like a mini-solar system, with Saturn as a stand-in for the Sun, and the moons and rings like planets in formation. Through Cassini and its instruments, we are making fundamental strides in understanding the physical processes that created and govern this and other solar systems.” – Dennis Matson, Cassini-Huygens Project Scientist1

This final part of the book features “the pot of gold at the end of the rainbow” – the wealth of Orbiter scientific discoveries that more than justified the mission’s enormous effort and expense. Of the many words that could be used to portray the Orbiter’s science return, the one that comes to mind is diversity. The Saturn system is incredibly complex and varied in its nature. Its icy moons, for instance, range from geologically very active, with interior temperatures high enough to melt the ice and spew water jets far into space, to other satellites that are dead lumps of rock and ice. Some of these moons coalesced into existence alongside the planet, whereas others may have wandered into Saturn’s gravity eons later from distant regions of the solar system. And the range of physical characteristics of these orbiting bodies! Some are extensively pock-marked by impacts, others pristine. One resembles a sponge; another is two-faced and reminiscent of a walnut. Most have deposits of the same black grainy material, suggesting that satellite-to-satellite material transport occurs. Certain satellites have a profound relationship with Saturn’s rings, helping to define their boundaries and also supplying the material from which some of them are made. The most surprising science return for some mission personnel came from the Orbiter’s continued observations of Titan, revealing a moon that looks startlingly like Earth, except with liquid hydrocarbons serving as water and ice standing in for silicate rocks.2 Other scientists were most fascinated by the complex interactions between the interplanetary solar wind and Saturn’s huge magnetosphere, its rings and satellites. The meteorology of the planet itself is the focus of another community of scientists who are trying to understand the nature of Saturn’s persistent storms and cloud structures. Then there is the amazingly dynamic moon Enceladus, whose jets of water and ice generate the E ring and whose interior may possibly harbor life. The Cassini Orbiter science data from the Saturn system tour greatly expanded our knowledge in five areas of inquiry: • • • • •

Saturn Magnetosphere Ring system Icy moons Titan.

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The main research results from each of these areas are detailed in the following four chapters.

REFERENCES 1. Carolina Martinez, “Cassini Mission to Saturn Celebrates 10 Years Since Launch,” JPL news release 2007–118, 11 October 2007. 2. Achim Schneider, “Titan Reveals Methane Rain and Rocks of Water,” http://www.nature.com/ news/2005/050117/full/news050117-14.html, Nature online (21 January 2005); Ralph Lorenz and Jacqueline Mitton, Titan Unveiled (New Jersey: Princeton University Press, 2008):20; Physorg.com, “Latest images of Titan Unveiled Today,” http://www.physorg.com/news2765. html (21 Jan. 2005).

11 The mother planet and its magnetosphere Planetary exploration opens a window into the natures of worlds other than Earth, and this can give us a better sense of our own position in the scheme of things. At times, we realize that long-held beliefs must be changed. For instance on Earth, the weather is thought of as highly changeable and inconstant. A storm refers to a short-lived atmospheric phenomenon, typically hours or days in length. Not so on Saturn. Comparing Cassini Orbiter with Voyager data demonstrates that certain Saturnian tempests have lasted for decades, at least.1 And while cloud formations in Earth’s atmosphere are notably transient, on Saturn they endure indefinitely. Exploring Saturn has helped us better understand our solar system, which formed from an enormous mass of gas and dust pulled together by gravity. Saturn contains much of the primordial gas not trapped by the Sun, as do our solar system’s other gas giant planets. Thus to study Saturn’s composition is to look back in time to the prehistory of our solar system. Also, Saturn and its satellites resemble a miniature solar system which we believe to have been formed by similar processes to those that shaped our own Sun-centric system. Saturn and its rings provide a model of the disk of gas and dust surrounding the early Sun and from which the planets formed. Studying the Saturnian system helps us better understand the process of planetary formation.2 # Because Saturn has a large radius and spins so rapidly – scientists believe its interior rotates with a period in the range of 10.6 to 10.8 hours – it experiences tremendous distortional forces which give rise to a marked flattening at its poles that can be seen even using small telescopes.3 Saturn differs from Earth in that no sharp distinction exists between atmosphere and planetary surface. Instead there is a gradual transition from gaseous atmosphere to liquid interior. Thus, the planet cannot be said to have a surface in the way that Earth does. Saturn is approximately 96% hydrogen and 4% helium, with traces of water, methane and ammonia, and its density is only 0.7 times that of water, which means that it would float in water.4 Pioneer 11 flew by Saturn on 1 September 1979, the first space vessel to closely inspect the planet. It found evidence that Saturn has an internal heat source which causes the © Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_11

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The mother planet and its magnetosphere

planet to radiate more energy than it receives from the Sun. Its imaging instruments showed that unlike Jupiter, the clouds of Saturn were rather bland, with little color variation.5 Voyager 1 encounter activities began on 22 August 1980, when the spacecraft was 109 million kilometers (68 million miles) away from Saturn. Closest approach to the planet occurred on 12 November 1980, when the vehicle skimmed 126,000 kilometers (78,000 miles) above its cloud tops. A principal objective of the mission was to compare the dynamics of Saturn’s colder atmosphere with that of Jupiter’s warmer one. One observed difference was that eastward wind speeds in Saturn’s near-equatorial clouds of up to 480 meters/second (1,100 miles per hour) were four times greater than the highest speeds measured in Jupiter’s clouds.6 To better understand this gas giant planet, the Cassini mission targeted several areas of inquiry, including: • • • • • • • •

11.1

Atmospheric structure, temperature, and gas composition Thermal energy sources driving planetary processes Wind velocities and storm characteristics Lightning discharges Ionospheric and other atmospheric features Radio emission characteristics Structure of the liquid and solid portions of the planet Internal and external rotational characteristics, including length of Saturn’s day.

ATMOSPHERIC STRUCTURE, TEMPERATURE, AND GAS COMPOSITION

The overall composition and chemistry of Saturn’s atmosphere directly influence its dynamics, horizontal and vertical structure, and visual appearance. Composition helps to determine thermal structure by influencing the balance between solar energy absorbed and infrared radiation emitted. The condensation characteristics of volatile materials in the atmosphere determine its cloud structure and appearance.7 11.1.1

Composition

As with the solar system’s other giant planets, hydrogen, helium, and methane make up the bulk of Saturn’s atmosphere. These three chemicals represent altogether more than 99.9% of the atmosphere by volume.8 Planetary scientists have estimated the individual percentages as 96.3% hydrogen, 3.25% helium, and 0.45% methane, with ammonia constituting approximately 0.01%. The interesting thing is that Saturn’s atmospheric helium fraction is considerably less than on Jupiter, where it is nearly 14%. Scientists consider it quite unlikely that this helium escaped in some way from the planet while hydrogen, which is lighter, remained behind. Jupiter and Saturn, with their large masses and low temperatures, should have held onto both of these elements. It is likely that during some period in Saturn’s past, the heavier helium sank toward the planet’s center. This reduced its abundance in at least the outer atmosphere and rendered that hydrogen-rich.9

11.1 11.1.2

Atmospheric structure, temperature, and gas composition 265

The troposphere and its cloud decks

The part of the atmosphere where weather occurs, the troposphere, has three regions where scientists think that cloud decks of particular types and compositions occur. The predicted locations of these cloud decks are based largely on the temperatures at which gases condense into droplets. For a given temperature profile, the altitude at which condensation occurs for the constituents of a certain type of cloud is where that cloud type should be found. According to thermal and chemical modeling of Saturn’s atmosphere, the highest visible cloud deck is made of ammonia (NH3) ice crystals and occurs near the top of the troposphere, or tropopause, at a temperature of about −250°C. The second cloud deck is ammonium hydrosulfide (NH4SH) at a temperature of −70°C, and the lowest cloud deck, composed of water clouds, occurs deeper in the troposphere where the temperature is approximately 0°C, the freezing point of water. However, the actual locations of these clouds are difficult to predict, because the abundances of the constituents which would condense to form them are not well known.10 Cassini cannot directly examine Saturn’s atmosphere below its water cloud level, although VIMS and RADAR were able to study the region underneath the ammonia cloud tops. Thermal emissions from Saturn itself silhouetted some ammonia clouds. VIMS thus saw a cloud as being relatively dark, surrounded by a brighter region (or in other words, a region of more intense infrared radiation), indicating a cloud-free area. VIMS also found a deep cloud layer where the ammonium hydrosulfide cloud deck had been predicted.11 Deeper in the troposphere, enormous gas pressures are expected to cause higher temperatures, possibly to over 700°C (1,300°F), but these cannot yet be measured directly because the depths at which they are thought to occur are far below those that can be probed by remote sensing techniques.12 11.1.3

Haze layers

Reflectivity measurements by Cassini and the Hubble Space Telescope indicate the presence of at least two haze layers of aerosol particles located above the ammonia cloud deck. The upper layer is thin, and in Saturn’s stratosphere. There is a thicker haze extending from the ammonia clouds down to the tropopause, which marks the top of the troposphere.13 11.1.4

Horizontal cloud bands

The defining visual feature of Saturn’s atmosphere is its system of horizontal yellow and gold cloud bands. These include powerful eastward and westward flowing wind jets. But in the visible light spectrum, the haze partially obscures the structures and colors of these bands, making them more muted in appearance than those of Jupiter. Infrared light of certain wavelengths, such as 5 microns, however, passes up through the haze after being emitted in the interior, allowing us to glimpse the deeper clouds underneath. What is revealed is an incredible amount of structure and detail in these lower clouds that is very reminiscent of Jupiter.14 Early in the mission, when the rings cast a shadow on the northern hemisphere of the planet, that hemisphere looked blue, reminiscent of Neptune’s atmosphere. One idea was

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The mother planet and its magnetosphere

that the haze layer had seasonally thinned and we were seeing deeper into the atmosphere. One goal of the Solstice Mission is to see if this also happens in the south as solstice approaches and the ring shadows move there.15 It is not obvious why Saturn has such noticeable haze layers whereas Jupiter does not. This may have something to do with Saturn’s atmosphere being less dynamic – there being less internal heat and hence less convection. The upper atmosphere does not constantly mix down into the interior. Its components have a longer time to cook in the Sun’s ultraviolet rays, which break apart hydrocarbon gas molecules, allowing the formation of molecules with longer and longer chains of carbon that eventually condense and create particles. These heavy hydrocarbon particles are what probably forms much of the haze.16 The amazing thing about both Saturn’s and Jupiter’s cloud bands and jet streams is their remarkable stability over decades and even centuries.17 The reason for this stability may lie in the parts of the clouds that we cannot see. Caltech meteorologist Andy Ingersoll commented, “We don’t know if what we see in the tops of the clouds goes way down deep in the atmosphere.”18 There are fluid dynamics reasons for believing that cloud bands and zonal jets around Saturn and Jupiter are the tops of a nested series of cylinders that descend deep into the atmosphere and spin around the planet’s rotation axis,19 as illustrated in Figure 11.1.

Figure 11.1 The cloud bands we see may be the tops of nested, rotating cylinders extending deep into Saturn’s atmosphere.

11.2

Wind and storm characteristics 267

According to Ingersoll, this theory is far from new; it is based in part on laboratory experiments performed a century ago with rapidly rotating containers of liquid. These experiments showed that under certain conditions, disturbing the fluid at one level caused a spinning cylinder to form all the way up and down in a line parallel to the rotation axis. At Saturn, such huge rotating cylinders would have vast inertia. This may be why cloud and wind patterns in the atmosphere persist for so long. Whether the wind jets associated with Saturn’s atmospheric bands are the tops of structures extending deep into the planet, or are confined to the upper cloud levels, the question still arises: What powers them? It is possible that convective storms – huge vortices as large as terrestrial hurricanes – power the wind jets. Imagine two jet streams flowing in opposite directions and a vortex spinning between them, pushing them along. Scientists have observed eddies that are at least capable of energizing jet streams. These eddies are, in turn, probably driven by thermal energy rising from the deeper interior.20 11.1.5

Stratosphere

A dominant feature of stratospheric processes is photolysis, light-induced chemical decomposition of methane molecules.21 Methane is a simple hydrocarbon, made up of one carbon atom and four hydrogens (CH4). It is transported by convection from the deep interior of Saturn and through the troposphere to stratospheric levels. The ultraviolet part of sunlight is largely responsible for photolyzing methane. Products of this process react to form the many complex hydrocarbons identified in Saturn’s stratosphere. These complex hydrocarbons, which are heavier than methane, diffuse downward through the troposphere. On eventually encountering high temperatures, they may be converted back into methane. There may therefore be a methane cycle operating.22 11.1.6

Ionosphere

Cassini radio occultation measurements revealed numerous areas in Saturn’s upper atmosphere where the plasma of the ionosphere is depleted. These are ionospheric holes. They were notably similar in size and shape to ion and electron depletions in Earth’s atmosphere caused by the introduction of spacecraft exhaust products, water in particular. Modeling of Saturn’s upper atmosphere has indicated that the plasma depletions may be caused by an influx of water.23

11.2

WIND AND STORM CHARACTERISTICS

Winds blow amazingly rapidly on Saturn, much faster than almost any other place in the solar system. Winds at the equator have been clocked at speeds as fast as 1,800 kilometers per hour (1,100 miles per hour). By comparison, the strongest hurricane-force winds on Earth rarely exceed 400 kilometers (250 miles) per hour. Sometimes, violent white storms have been observed breaking through Saturn’s cloud layers. These are so large, Earth could fit into one. While the origins of these storms have not been firmly established, the evidence suggests they are the result of sudden convective

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The mother planet and its magnetosphere

activity deep below the visible cloud tops. Such events trigger rapid vertical flows, resulting in the condensation of thick, bright, high ammonia clouds.24 Smaller storms appear as darker spots.25 The Imaging Science Subsystem (ISS) on the Cassini Orbiter observed an active, narrow, southern mid-latitude region in which dark ovals merged with each other and arose from the remnants of a white storm.26 One interesting phenomenon is the persistent brown spot at latitude about 43°N, within a region of wind shear. Discovered by the Voyager flybys, it is a long-lived anticyclonic vortex, a storm characteristic of an atmosphere around a high pressure center and involving a descending current of air. The brown spot may constitute an opening in Saturn’s upper cloud deck through which darker underlying clouds can be seen. The brown spot has been observed merging with smaller anticyclones.27 11.2.1

The eye of a Saturn cyclone

On 15 July 2008, Cassini acquired images of a monstrous vortex at Saturn’s south pole (Figure 11.2) that were ten times more detailed than any obtained previously and provided clues to the dynamics of the atmosphere. Earlier images showed an outer ring of high clouds surrounding a region that was thought to be mostly clear air, interspersed with a few

Figure 11.2 The south polar vortex.

11.2

Wind and storm characteristics 269

puffy clouds circulating around the center. But the new images revealed that the puffy clouds were vigorous convective storms (driven by temperature differences) and they formed yet another distinct, inner ring. The eye of this vast vortex was surrounded by an outer ring of high clouds 4,200 kilometers (2,600 miles) in diameter and, as measured from the shadows they cast, rising 40 to 70 kilometers (25 to 45 miles) above the clouds inside the ring. “It’s like seeing into the eye of a hurricane,” said Andy Ingersoll of Caltech and a member of the Cassini team. The strange thing is that this enormous storm is locked to the south pole, whereas a hurricane on Earth moves around.28 11.2.2

A hexagonal cyclone-related structure at the north pole

Another giant cyclone, this one at Saturn’s north pole, is surrounded by a strange honeycomb-shaped hexagon (Figure 11.3). It is amazing that the clouds and winds within it whip around at high speeds – in excess of 500 kilometers per hour (300 miles per hour) – without either disrupting the hexagonal structure or causing it to move. The two polar cyclones on Saturn seem to be powered by a different mechanism than Earth-bound hurricanes, which are driven by the ocean’s heat and water vapor. On Earth, condensing water releases significant thermal energy, which powers our hurricane vortices. Saturn’s cyclones, however, have no body of water at their bases to serve as their energy sources. Nevertheless, the features of the storms on the two planets look strikingly similar.

Figure 11.3 The strange hexagon surrounding Saturn’s north polar cyclone.

270 The mother planet and its magnetosphere Kevin Baines, a JPL scientist on the Visual and Infrared Mapping Spectrometer (VIMS) team, commented on the power sources that the mega-sized cyclones at Saturn’s two poles might use: “These are truly massive cyclones, hundreds of times stronger than the most giant hurricanes on Earth. Dozens of puffy, convectively formed cumulus clouds swirl around both poles, betraying the presence of giant thunderstorms lurking beneath. Thunderstorms are the likely engine for these giant weather systems.”29 The primary drivers for the winds in these Saturnian cyclones might be the heat that is released by the condensation of water in thunderstorms. 11.2.3

Jets

While both Saturn and Jupiter have a series of bright and dark bands encircling them and running east–west, the bands’ morphologies are different. On Saturn, a series of eastwardblowing jets occupy the dark bands, which are narrow and often bisected by a slim bright band. The much broader regions between successive eastward jets are generally brighter and include westward jets. On Jupiter, the bright and dark bands are comparable in width, each spanning about 5° of latitude.30 Jupiter’s wind patterns tend to be rather stable, but Saturn’s jets vary significantly over short time periods. For instance, Voyager observed an equatorial jet blowing at 450 meters per second that had slowed to only 250 meters per second by the time of the Cassini spacecraft’s arrival.31 The magnitudes of these Saturn wind velocities are somewhat uncertain because, as Cassini demonstrated, we do not know the rotation rate of the planet’s interior, and hence do not have a reliable reference frame from which to measure the winds. In fact, the preponderance of eastward winds that we see may be an artifact of using a rotation rate that is too slow.32 Rotation rate issues are further discussed later in this chapter.

11.3

LIGHTNING DISCHARGES

Cassini observations have established that Saturn has lightning storms, but for some reason they generally occur at “only one latitude and only one storm at a time at that latitude and sometimes there is no storm at all. It’s as if there was only one place on Earth that had a thunderstorm.”33 Saturn’s lightning storms appear at about 35°S, in what scientists have nicknamed Storm Alley. Like many of the planet’s atmospheric phenomena, such storms can be remarkably persistent. For instance, Cassini saw one lightning storm that began in January 2009 and raged through into October.34 Lightning storms can grow in a hurry on Saturn. On 5 December 2010, scientists captured an image of a small white spot, about 1,300 kilometers (800 miles ) north-to-south and 2,500 kilometers (1,600 miles) east-to-west. In less than three weeks, the storm grew almost eight times in north-to-south extent. By the end of January it had encircled Saturn in an east–west direction. It was shooting lightning bolts at the rate of ten per second. This kind of storm, known as a Great White Spot, commonly appears during the summer each Saturnian year. But this one, which arrived earlier than they usually do, was gargantuan, encompassing approximately 4 billion square kilometers of Saturn’s surface.35 It was by far the biggest, most active storm seen by Cassini.36

11.3

Lightning discharges 271

Until the middle of 2009, researchers never actually saw a visible lightning flash from a storm, but only strong evidence of it. What Cassini, and before that Voyager, actually received were radio waves generated by the lightning. The Cassini Orbiter’s Radio and Plasma Wave Science (RPWS) instrument measured Saturn Electrostatic Discharges (SED) – the radio signatures of lightning flashes – from various storms. In addition, pictures by ISS revealed cloud features whose occurrence, longitudinal drift rate, and brightness were strongly related to the SEDs.37 The data that Cassini collected from the 2009 storm indicated that the lightning flashes were 10,000 times stronger than similar discharges on Earth. Moreover, the dimension of the event – 1,850 miles (3,000 kilometers) across – was far larger than storms on our planet.38 The 2009 storm, though huge and long lived, was far from unique. It was the ninth similar occurrence measured since Cassini was inserted into orbit around Saturn in July 2004.39 Somewhere in Saturn’s atmosphere during a lightning storm, there is a visible flash but this is far more difficult to see than lightning on Earth. The location might be deep in the atmosphere, preventing the light from being observed. Also, sunlight reflecting off the ring system makes the night hemisphere of Saturn even brighter than Earth appears in full moonlight, so it is difficult to detect the flashes. Another complication is that we can’t look continuously for lightning. If it doesn’t happen when the camera is exposing an image, then it isn’t recorded.40 This situation changed temporarily in August 2009, during Saturn’s equinox. The ring system, being edge on to the Sun, did not illuminate the night hemisphere. Cassini scientists took advantage of this to look for lightning, and on 17 August the first visible images of Saturnian lightning were obtained (Figure 11.4). Georg Fischer of the Space Research Institute in Graz, Austria remarked, “This is the first time we have the visible lightning flash together with the radio data. Now that the

Figure 11.4 First visual images of lightning flashes in Saturn’s atmosphere, captured 17 August 2009. The storm that generated this lightning lasted from January to October 2009.

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radio and visible light data line up, we know for sure we are seeing powerful lightning storms.” Ulyana Dyudina, the Imaging Team member who was the first to see the flashes in the pictures, added: “The visible-light images tell us a lot about the lightning. Now we can begin to measure how powerful these storms are, where they form in the cloud layer and how the optical intensity relates to the total energy of the thunderstorms.”41 On Earth, latent heat given off by water as it condenses from vapor to liquid plays an important part in driving storms. Similar processes must almost certainly occur on Saturn, and latent heat from water condensation is probably vital for energizing that planet’s enormous thunderstorms.42

11.4

ORIGIN AND STRUCTURE OF SATURN’S LIQUID AND SOLID REGIONS

Scientists believe that Saturn, like other planets, formed about 4.6 billion years ago from the solar nebula, an enormous cloud of gas and dust which was perturbed in some way, possibly by colliding with another cloud or receiving a shock wave from a supernova, causing it to collapse. The huge pressures caused its center to become a protostar, an early stage in the process of star formation. This was surrounded by a spinning, flattened disk of material, the inner part of which contained many of the heavier elements and eventually formed the rocky terrestrial planets. The outer part of the disk was cold enough for ices to remain intact. These ices agglomerated and formed larger and larger planetesimals. As planetesimals collided with each other, some accreted to form planets. At one point early in Saturn’s life, a moon estimated at 300 kilometers (200 miles) across may have been torn apart, creating the planet’s ring system.43 Saturn is different from Earth in that, according to scientists, no sharp distinction exists between the gaseous atmosphere and a liquid planetary surface. The transition between these two phases is instead slow and gradual. Far below the visible clouds of Saturn, the hydrogen and helium gases that make up most of the atmosphere are subjected to enormous pressure due to all the atmospheric mass on top of them. This causes the hydrogen and helium to progressively transform into their liquid phases. According to widely held models, the hydrogen in the deep interior is subjected to such enormous pressure that it turns into a liquid metallic form which is a conductor of electricity. Many scientists believe that liquid helium is immiscible (incapable of remaining mixed) in liquid metallic hydrogen. This immiscibility may be a critical determinant of the interior structure. The denser liquid helium might form droplets that sink down through the metallic liquid hydrogen layer, resembling a helium rain that is gravitationally drawn towards the planet’s core. The friction that results as these droplets push and rub together heats up the helium considerably. This thermal energy then makes its way to the surface, likely through conduction and convection, and is radiated out into space. This mechanism may be responsible for Saturn giving off over twice the radiant energy that it receives from the Sun.44 Although many planetary scientists believe this is a likely explanation of Saturn’s interior processes, some new findings suggest that helium and hydrogen may be far more miscible than thought.45 The pressure deep in the planet could also transform helium into a metal, and that may make it miscible in metallic hydrogen, forming a liquid metal alloy of the two elements.46 If this were the case, then a rain of liquid helium droplets through

11.5

Internal and external rotational characteristics 273

the hydrogen layer might not occur, and the theory of why Saturn radiates more energy than it receives would have to be modified. Saturn does not contain as much metallic hydrogen as Jupiter. Because these two planets’ magnetospheres are believed to originate in their metallic hydrogen layers, where electrical conductivity is high, this may be a factor in why Saturn has a far smaller magnetosphere and less intense magnetic field than Jupiter.47 11.4.1

Interior composition

A priority objective of the Cassini mission was to determine the internal structure of Saturn. How does its composition vary with depth? What are the natures of the phase transitions within the planet? To what extent do hydrogen and helium in the interior mix or separate? What percentages of the interior are made up of ammonia, methane, water, and other materials? Scientists were eager to determine the particulars of the interactions between the gaseous, liquid, solid, and metallic phases. The answers to these questions would help with other issues, such as the planetary rotation rate and the details of how and where Saturn’s magnetic fields are generated.48 Saturn’s interior characteristics cannot be directly sensed, either by instruments on the Orbiter or elsewhere. Interior details instead have to be surmised from other observations. For instance, some aspects of Saturn’s interior mass profile can be deduced from measurement of the planet’s size and total mass, the distinctly oblate shape that results from its speedy rotation, and the details of its gravitational field. From the data available, scientists infer the existence of a massive core.49 Another barrier to modeling the interior of the planet is the absence of a unique rotation rate that can be assigned to its deep, metallic hydrogen regions. Additional uncertainties arise from zonal flows of unknown magnitudes occurring at unknown depths. Saturn’s interior temperature profile is also not known. Nevertheless, some features of the interior can be stated with a certain degree of confidence. At the very center of the planet, scientists believe there is a rocky and possibly icy core with a mass of 15 to 20 Earth masses (ME). Also, the equation of state of dense hydrogen-helium mixtures, which is extremely relevant to how these elements behave inside Saturn, has been worked out to a great extent using first-principle simulations – a powerful tool for studying the behavior of condensed matter. It is a sophisticated approach that applies quantum mechanics to determine the behaviors of chemical bonds and the electrons involved in them. But there remain major uncertainties in the natures and sizes of the mantle layers and core, as well as the details of helium separation within the planet.50

11.5

INTERNAL AND EXTERNAL ROTATIONAL CHARACTERISTICS

Measuring the rotation period from space of a rocky planet such as Earth is fairly straightforward. Unchanging landmarks on a solid surface provide reference points to show how fast the planet turns. But this technique cannot be applied to gaseous planets, which do not have a visible solid surface. The clouds that cover gas giants move around and do not provide dependable reference points for measuring rotation. Because gas giants are not as rigid as rocky planets, their rotation periods may vary with location. Saturn’s atmosphere,

274 The mother planet and its magnetosphere like all atmospheres, does not rotate as a solid body but includes several east and west flowing jet streams. The air at the equator circles the planet about once every 10 hours 12 minutes, but it can take 30 minutes longer at higher latitudes.51 Despite the planet’s fluid nature, scientists believe that electromagnetic forces in the electrically conducting interior, where the bulk of the planet’s mass resides, keep the rotation at a nearly constant value. How can the rotation of the planet’s interior be measured? Scientists have tried approaches such as measuring the repetition rate of particular radio signals, but the period of these signals changed markedly in the decades between the Voyager and Cassini measurements.52 The gas giant Jupiter has a feature that makes it far easier to measure its rotation period. The magnetic field axis of Jupiter is offset nearly 10o from its rotation axis, making it possible to track field characteristics as they rotate with the planet. Andy Ingersoll of Caltech explained it this way: “If you have a magnetic field that wobbles around … the magnetic field that is generated inside the planet … then you have a measure of the internal rate of rotation.”53 Offsets between the rotational and magnetic axes also occur on Neptune and on Uranus,54 but such is not the situation on Saturn, where these two axes are nearly coincident. This makes it difficult to define Saturn’s rate of rotation. Watching the magnetic field spin around is sort of like watching a featureless CD whirl around – there’s nothing on it to measure how fast it is turning. However, because of all the detailed studies that have been made of Saturn’s field, a small but regular periodic signature has been noted with a period of 10 hours 47 minutes 6 seconds, plus or minus 40 seconds. Finding this signature was like finding a small spot on the CD that facilitated a measurement of its spin rate. So far, scientists are hopeful that this signature period is actually indicative of Saturn’s true internal rotation rate.55 One encouraging sign is that the rotation rate measured from this magnetic signature has remained constant (as it should) since Cassini arrived in the system in 2004. Not all planetary scientists support the measurement of magnetic field variations as the best way to determine a rotation period. Andrew Prentice from the Centre for Stellar and Planetary Astrophysics in Australia has commented that “if you rely on magnetic fields then you are in for a rude shock since the period [measured from the magnetic field] seems to have lengthened by seven to eight minutes since the time of the Voyager missions.”56 The magnetic field thus does not appear to be locked onto the bulk rotation of the entire planet, for if it was, the lengthening of the magnetic signature’s period would indicate that Saturn itself had slowed down a great deal over just a few decades, an occurrence that would not make sense to scientists.57 Peter Read, a physicist at Oxford in the U.K., and his colleagues envisioned that Saturn’s rotation rate could be identified by Cassini on the basis of infrared radiation originating in the planet’s interior, along with measurements of jet streams, currents, and vortices in the planet’s atmosphere. Using both sets of data, a three-dimensional map of Saturn’s winds was constructed, giving information on how large waves and eddies developed in the atmosphere. From this, the underlying rotation of the planet was estimated. The length of a Saturnian day calculated using this approach was 10 hours 34 minutes 13 seconds, which was 13 minutes shorter than that measured by tracking a signature on the magnetic field.58 A shorter day has numerous implications for Saturn’s characteristics. For a start, it implies modifications to models of the deep interior and the evolution of the planet. A shorter day suggests a subtle change in the shape of the planet and the distribution of mass in the interior, and is consistent with a pattern of alternating jets. Finally, it implies

11.6

The magnetosphere

275

that the weather on Saturn is much more like that of Jupiter, indicating that the two planets may have more in common than previously thought.59 Other studies offer yet another picture of the planet’s rotational characteristics. For instance, radio emission data from the southern and northern auroral regions of Saturn suggest different planetary rotation rates south and north of the equator.60 Related to this strange phenomenon may be the fact that, due to the tilt of Saturn’s rotational axis relative to its orbital plane, there is a north–south difference in solar illumination falling on the two hemispheres. During the first five years of Cassini’s tour – 2004 to 2009 – the south polar region received more sunlight than the north. This might have caused differences in the magnetic field and plasma environment of the southern hemisphere versus the northern hemisphere that could account for the variance in their rotation rates.61

11.6

THE MAGNETOSPHERE

“The power in its equatorial electric current disk is ‘equivalent to 1,000,000 cities the size of Los Angeles.’” – Claudia Alexander, July 200962 Saturn’s magnetosphere is the region around the planet in which charged particles are more influenced by Saturn’s magnetic field than by the interplanetary magnetic field. It is sufficiently large to accommodate most of the planet’s moons. Inside the magnetosphere is a mixture of particles that includes electrons, various types of ions, neutral atoms and molecules, extremely energetic charged particles, and charged dust grains.63 As Pioneer 11 flew past Saturn on 1 September 1979, it measured a surprisingly intense magnetic field surrounding the planet, 500 times stronger than that of Earth and, unlike at our planet, almost exactly aligned with the rotational axis. The theory in vogue at the time of how a celestial body generated a magnetic field required that the rotational and magnetic axes not be aligned. For instance, at Earth the two axes differ in direction by 11.4° and at Jupiter by 9.6°.64 Voyager 1 detected a disk of plasma containing hydrogen and possibly oxygen ions that extended outward from Saturn almost to Titan’s orbit and co-rotated with Saturn’s magnetosphere. On the outbound leg of its flyby, Voyager 1 flew through the planet’s magnetic tail, which extended for about 80 Saturn radii. It saw copious fluxes of low-energy electrons within the tail.65 Voyager 2 also carried out many magnetospheric observation sequences, making use of its deeper penetration of the magnetic field, especially near the equatorial plane. But Pioneer and Voyager were flyby missions that could only sample the magnetosphere while passing through. It was not until the arrival of Cassini that an in depth, long-term analysis of the various sections of the magnetosphere and their particle populations could be performed.66 Cassini investigated characteristics of the planet’s magnetosphere that included: • • • • •

Electric currents Auroral phenomena Radio emissions Plasma wave phenomena Interactions with satellites and rings.

276 The mother planet and its magnetosphere 11.6.1

Electric currents in the magnetosphere

Magnetized plasmas that are in motion, such as in Saturn’s magnetosphere, create electric currents.67 One major phenomenon that Cassini studied was the equatorial ring current, a 10 million ampere flow of charged particles that circulates 600,000 kilometers (370,000 miles) above the planet, or over 100,000 kilometers outside of the E ring.68 The Magnetospheric Imaging Instrument (MIMI) revealed that this ring current was shaped like a warped disk. It resembled an old hat brim, crushed in front and tipped up at the rear.69 The ring current disk was tilted by the solar wind out of Saturn’s equatorial plane, and its vertical thickness varied by a factor of five in different locations. Data from six passes through the current disk showed that its half-thickness on the dayside was nearly constant at ∼1.5 Saturn radii (RS, 60,268 kilometers or 37,449 miles). In the inner region of the nightside there was a thin equatorial current layer with a half‐thickness of ∼0.5 RS. The central portion of the nightside current sheet had a half‐thickness of ∼2.5 RS, which declined rapidly to ∼0.4 RS in the outer region.70 Ring current phenomena are also found around Earth and Jupiter, arising when plasma gets trapped by magnetic fields. The aggregate motion of ions distributed around a planet’s equator generates the current. Scientists believe that Saturn’s ring current originates from material in the planet’s rings as well as from gas vented by geysers on Enceladus, all of which get ionized and accelerated. MIMI observations revealed that Saturn’s ring current is persistently asymmetric, unlike for Earth, and that the asymmetry rotates nearly in step with the planet. MIMI was essential for this kind of investigation, because its three sensors enabled scientists to “visualize the invisible”71 by showing the plasma and radiation belts in an image. 11.6.2

Auroral phenomena

In 2009 above Saturn’s northern hemisphere, Cassini observed the tallest known aurorae in our solar system. It put on “a dazzling show, shape-shifting rapidly and exposing curtains that we suspected were there, but hadn’t seen on Saturn before,” said Andrew Ingersoll of Caltech. The spacecraft took a video of the spectacle and captured flickering and rippling shapes reaching up 1,200 kilometers (750 miles). “Seeing these things on another planet helps us understand them a little better when we see them on Earth,” Ingersoll added.72 Aurorae generally appear in a planet’s high latitudes near its magnetic poles. As charged particles from the magnetosphere plunge into the upper atmosphere, they excite molecules residing there. When these molecules lose their excitation energy, they emit photons of light, and these are what cause the auroral glow. The curtain shapes of Saturn’s aurorae reveal the paths that the charged particles take as they flow along the magnetic field lines into the uppermost portion of the atmosphere. The height of Saturn’s auroral curtains indicates a key difference between that planet’s atmosphere and our own. While Earth’s atmosphere contains large amounts of oxygen and nitrogen, Saturn’s is primarily hydrogen. Because hydrogen is very light, the atmosphere and hence the aurorae reach farther out from Saturn. Earth’s aurorae typically flare at altitudes of only 100 to 500 kilometers (60 to 300 miles).

11.6

The magnetosphere

277

While Earth’s aurorae are controlled primarily by solar wind influences on our magnetosphere, the situation at Saturn is more complex. Data from Cassini and the Hubble Space Telescope indicate dramatic intensity changes in Saturn’s ultraviolet aurorae during times of solar wind pressure increases, which compress sections of the planet’s magnetosphere. But scientists also believe that Saturn’s rapid rotation plays an important role. Some auroral features appear at least to partially co-rotate with the planet. The rotation of Saturn and the presence of non-solar wind plasma sources from the rings and satellites (especially Enceladus) suggest that processes may be operating that are similar to those observed at Jupiter, where plasma that is diffusing outward interacts with the planet’s rotating magnetic field and generates strong electric currents in the ionosphere that are connected with auroral displays.73 11.6.3

Saturn kilometric radiation

The Voyagers established that Saturn is an intense radio wave emitter. The primary component of these emissions, Saturn kilometric radiation (SKR), is so-named due to its peak being in the kilometer wavelength range – typically at frequencies from 100 to 400 kilohertz (kHz). Since Voyager, space scientists have known that, like the aurorae, SKR is strongly influenced by the solar wind. During two analysis periods of 161 and 164 days, the radio emissions showed a periodicity of 25 days, identical to that observed in the solar wind at the time, and also consistent with the period of rotation of the Sun.74 SKR may be related to electrons in the solar wind interacting with the magnetic field at Saturn’s poles and generating aurorae.75 The correspondence between SKR and aurorae was investigated by Cassini using its Radio and Plasma Wave Science (RPWS) instrument and by the Hubble Space Telescope. The results showed SKR occurring at the same time as bright auroral features. In particular, bright aurorae in January 2005 were accompanied by intense SKR emissions that expanded to lower frequencies. This association of increasing SKR bandwidth with intense aurorae has been seen at Earth as well.76 As with the other giant planets, the intensity of SKR is modulated by the rotation of Saturn. During the Voyager flybys in 1980 to 1981, the SKR modulation period was found to be 10 hours 39 minutes 24 ±7 seconds. Because the charged particles responsible for radio emissions are controlled by the magnetic field, which is linked to the deep interior of the planet, and because of the difficulty in measuring Saturn’s rotation in other ways, the SKR period was widely adopted as the rotation period of the planet. However, later measurements have shown that the SKR-based estimates are not constant. Cassini’s RPWS observed SKR characteristics fairly continuously starting six months prior to Saturn Orbit Insertion in July 2004.77 By 2005 the SKR period had lengthened to 10 hours 46 minutes.78 As discussed above, scientists are seeking other methods for estimating Saturn’s rotation. 11.6.4

Plasma waves

Wave phenomena in plasmas involve an interconnected set of charged particle and electromagnetic field oscillations. Cassini picked up its first indications of plasma waves associated with Saturn during the approach to the planet. On 22 March 2004, several months prior to SOI, it detected a burst of electron plasma oscillations at a radial distance of 825 RS. Detection at this large distance was possible because the approach trajectory allowed

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The mother planet and its magnetosphere

electrons to stream along the solar wind magnetic field from Saturn’s bow shock to the spacecraft. The electron plasma oscillations became more common and more intense over the ensuing months. Finally, on 28 June 2004, several days before SOI, the spacecraft crossed the magnetosphere’s bow shock at a radial distance of 49 RS. The bow shock is the boundary at which the speed of the solar wind abruptly drops as a result of encountering the influence of the planet’s magnetosphere. The instrument “heard” the passage through the shock as an abrupt burst of electrostatic noise. Cassini noted several types of plasma waves in the outer magnetosphere, as had Voyager. But the strongest and most complex plasma waves detected were in the inner region of the magnetosphere, inside of 10 RS.79 11.6.5

Interactions with satellites and rings

One of the most fascinating aspects of the Saturn system is its interconnectedness. Rings, satellites, and magnetosphere exchange material and influence each other’s characteristics and behaviors. During the Voyager flybys, the average distance from Saturn of the magnetopause, the outer boundary of the planet’s magnetosphere, was 20 RS, or just about the orbit of Titan. However, a recent study by Cassini estimated it at 22 to 27 RS.80 This created an unusual situation: a body with a very substantial atmosphere, Titan, traveling through a planet’s magnetosphere. What was the result of Saturn’s magnetosphere interfacing directly with Titan’s neutral atmosphere and ionosphere? For one thing, it induced a magnetosphere surrounding the moon.81 Titan’s induced magnetosphere was certainly not the only one in our solar system. Like Titan, Venus has an ionosphere but no intrinsic magnetic field of its own, and the solar wind interfaces with the Venusian ionosphere in a way reminiscent of the Saturnian magnetosphere’s interaction with Titan. Similar situations occur between the solar wind and Mars, as well as comets.82 Pioneer data indicated that the satellite Mimas, whose intricate relationships with the ring system are discussed in Chapter 12, also interacts with particles flowing in the planet’s magnetosphere. Four separate University of Iowa detectors noted a one minute dip in electron intensity as Pioneer 11 crossed the orbit of Mimas. A detailed analysis suggested this was “plausibly attributable to the particle sweeping effect of Mimas.”83 The Voyager and Cassini missions also investigated processes by which this and other moons remove particles from the magnetosphere.84 Voyager 1 detected bursts of radio emissions that were modulated with a 2.7 day period. As this was the orbital period of the satellite Dione, this implied it interacts strongly with the magnetosphere and influences the radio emissions.85 The Cassini spacecraft found that the torus shaped clouds of particles around Dione and Tethys are important sources of the outward flowing plasma of Saturn’s magnetosphere.86 Saturn’s magnetic field may influence the unusual ring feature known as spokes. These markings fall across the B ring like the spokes of a wheel and may be caused by electrically charged particles captured by the magnetic field.87 Chapter 12 gives more details of this phenomenon. Particles ejected from Enceladus appear to interact with Saturn’s magnetosphere in an interesting way. Enceladus spews out water vapor and icy dust particles, and through various mechanisms a portion of them receive a charge. Some are ionized by way of electron

11.8 Looking beyond Saturn: Studies of the heliosphere 279 impacts, photoionization from ultraviolet radiation, and charge exchange, which involves capture of an electron from a slow neutral atom by a fast ion passing by.88 Tiny ice grains emitted from Enceladus’ interior may also receive a charge by triboelectric processes which occur when the particles bump together in the vent in the icy shell before they emerge into the plume.89 Triboelectric charging involves the emission of electrons from freshly exposed fractures and fissures in a particle, as might occur in volcanic vents where large numbers of flowing particles collide with one another.90 The plume of both neutral and ionized particles from Enceladus influences local properties of the plasma within Saturn’s magnetosphere, such as its mass density and flow patterns. The magnetic field lines behave as if frozen into this flowing plasma, and where the magnetosphere and its plasma encounter the charged particles around Enceladus the field lines bow out, following the diversion of the plasma flow. It was this bending of the field lines that enabled the magnetometer team to map the plume of water vapor and dust particles above the south polar region of Enceladus.91 The ions from Enceladus appear to add significantly to the particle population of Saturn’s magnetosphere. According to Geraint Jones of University College London, the “inner magnetosphere is awash with ionized components of water that probably originate at the E-ring and, as recently confirmed, ultimately at Enceladus itself.”92 This material significantly influences the inner magnetosphere and is analogous to the volcanic material from the satellite Io that helps shape Jupiter’s magnetosphere. The material originating from Enceladus in the magnetosphere also gives clues to the composition of that satellite. Using a model for the transport of magnetospheric ions, a small nitrogen ion component observed in the magnetospheric plasma was traceable back to Enceladus, suggesting the existence of nitrogen-bearing volatile components such as ammonia (NH3) in the interior.93 Ammonia inside Enceladus would be a significant discovery for, as will be discussed in Chapter 13, this lowers the melting point of water ice and may help explain why the satellite is able to spew geysers of water (in the form of vapor or ice) out into space, while its fellow moon Mimas, which probably experiences more tidal heating, exhibits no such behavior.

11.7

A LABORATORY FOR UNDERSTANDING STELLAR EXPLOSIONS

Mission data can often be used in unexpected, exciting ways. An example: a chance encounter of a strong blast of solar wind with Saturn’s magnetosphere led to Cassini detecting particles accelerated to ultra-high energies, rather like what occurs around distant supernovas. Saturn’s bow shock region, where the solar wind collides with the magnetosphere, provided an opportunity to study mechanisms of distant stellar explosions, but in a far closer “laboratory.”94

11.8

LOOKING BEYOND SATURN: STUDIES OF THE HELIOSPHERE

“It’s amazing how a single new observation can change an entire concept that most scientists had taken as true …” – Stamatios Krimigis95

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The mother planet and its magnetosphere

During its orbits around Saturn, the Cassini spacecraft was well positioned to image our heliosphere – the extensive region of the Sun’s influence that encompasses the entire solar system. Cassini was farther from the Sun than previous spacecraft that made observations. Also, Cassini swung sufficiently far from Saturn on some of its orbits to reduce the interference that had hampered other such observations.96 For the past 50 years, space scientists have largely believed that our heliosphere moved through space like a gargantuan comet with a flattened nose and elongated tail, and they assumed that this shape was sculpted by the collision of the solar wind with the interstellar medium. Images taken by the Orbiter’s Ion and Neutral Camera suggested something very different: our heliosphere travels through the galaxy more like a big, round bubble. According to Stamatios Krimigis and Don Mitchell of the Johns Hopkins Applied Physics Laboratory in Maryland, the heliosphere’s shape is more dependent than previously thought on the pressure from the solar wind’s hot population of charged particles as well as magnetic field energy density. As the solar wind flows out from our Sun, it carves out a spherical shape in the galaxy’s interstellar medium.97

REFERENCES 1. Saturn is not the only planet we know that displays such long lived phenomena. Observations of Jupiter also demonstrate long term persistence of many of its weather patterns. 2. “Cassini-Huygens Project,” http://www.tepapa.govt.nz/space/Cassini.htm#Saturn, Museum of New Zealand, accessed 10 May 2011. 3. John D. Anderson1 and Gerald Schubert, “Saturn’s Gravitational Field, Internal Rotation, and Interior Structure,” Science 317 (7 September 2007):1384–1387. 4. ESA, “Facts About Saturn,” http://www.esa.int/esaMI/Cassini-Huygens/SEMV75HHZTD_0. html, accessed 26 Sep. 2009. 5. National Air and Space Museum, “Pioneer Encounters Saturn,” http://www.nasm.si.edu/etp/ saturn/satpioneer.html (2002), accessed 26 Sep. 2009. 6. E. C. Stone et al., “Voyager 1 Encounter with the Saturnian System,” Science 212 (10 April 1981):159–163. 7. Thierry Fouchet et al., “Saturn: Composition and Chemistry,” chapter 5 in Michele K. Dougherty et al. (eds.), Saturn from Cassini-Huygens, (Springer, 2009), p. 83. 8. Michele K. Dougherty et al., “Overview,” chapter 1 in Michele K. Dougherty et al. (eds.), Saturn from Cassini-Huygens (Springer, 2009):10. 9. David R. Williams, “Saturn Fact Sheet,” http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html, NASA-GSFC, updated 17 November 2010. 10. Anthony D. Del Genio, “Saturn Atmospheric Structure and Dynamics,” chapter 6 in Michele K. Dougherty et al. (eds.), Saturn from Cassini-Huygens, (Springer, 2009):118; ESA, “Saturn’s Atmosphere,” http://www.esa.int/esaMI/Cassini-Huygens/SEMPQ6HHZTD_0.html, accessed 27 Sep. 2009. 11. Del Genio, “Saturn Atmospheric Structure and Dynamics.” 12. Fouchet, “Saturn: Composition and Chemistry.” 13. Del Genio 2009, p. 122. 14. Andy Ingersoll interview with author, 17 Dec. 2009, American Geophysical Union (AGU) conference, San Francisco. 15. Linda Spilker review of manuscript, March 2011.

References 281 16. Ingersoll interview, 17 Dec. 2009. 17. Anthony D. Del Genio et al., “Saturn Eddy Momentum Fluxes and Convection: First Estimates from Cassini Images,” Icarus 189 (2007):479–492; Andrew Ingersoll email to author, 21 Sep. 2010. 18. Andy Ingersoll interview with author, 17 Dec. 2009, American Geophysical Union (AGU) conference, San Francisco. 19. Andrew P. Ingersoll, “Jupiter and Saturn,” in J. Kelly Beatty et al. (eds.), The New Solar System, 2nd edition (Cambridge University Press and Sky Publishing Corporation, 1982), pp. 124–125. 20. Ingersoll interview, 17 Dec. 2009. 21. Photochemistry refers to the effects of light (in Saturn’s case, sunlight) on chemical systems. 22. D.F. Strobel, “The Photochemistry of Methane in the Jovian Atmosphere,” J. Atmos. Sci. 26 (1969):906–911. 23. Luke Moore and Michael Medill, “Are Plasma Depletions in Saturn’s Ionosphere a Signature of Time Dependent Water Input?” Geophys. Research Ltrs. 34 (2007):L12202; Luke Moore et al., “Cassini Radio Occultations of Saturn’s Ionosphere: Model Comparisons Using a Constant Water Flux,” Geophys. Research Ltrs. 33 (2006):L22202. 24. Patrick G. J. Irwin, “Giant Planets of Our Solar System: an Introduction,” Springer (21 March 2006). 25. ESA, “Saturn’s Atmosphere,” http://www.esa.int/esaMI/Cassini-Huygens/SEMPQ6HHZTD_0. html, accessed 27 Sep. 2009; NASA-JPL, “A Gas Giant with Super-Fast Winds,” http://www. nasa.gov/mission_pages/cassini/whycassini/planet.html, (26 May 2004). 26. C.C. Porco et al., “Cassini Imaging Science: Initial Results on Saturn’s Atmosphere,” Science 307 (25 Feb. 2005):1243–1247. 27. Enrique García-Melendo et al., “Numerical Models of Saturn’s Long-Lived Anticyclones,” Icarus 191 (2007):665–677; NASA, “Large Brown Spot in Saturn’s Atmosphere,” http://www.nasaimages.org/luna/servlet/detail/nasaNAS~4~4~13513~115739:Large-brown-spot-in-Saturn-satmosp, NASA Planetary Photo Journal Collection, image no. PIA0196, acquired August 1981 and added to collection on 28 Apr. 1999. 28. Carolyn Porco, “Cassini Sees Inner Workings in the Eye of a Saturn Cyclone,” cpcomments@ ciclops.org, Cassini Imaging Central Laboratory for Operations (CICLOPS), Space Science Institute, Boulder CO (13 October 2008); Ulyana A. Dyudina et al., “Saturn’s South Polar Vortex Compared to Other Large Vortices in the Solar System,” Icarus 202(1) (2009). 29. Carolina Martinez, “Giant Cyclones at Saturn’s Poles Create a Swirl of Mystery,” NASA News, image advisory 2008–192 (13 Oct. 2008). 30. Anthony D. Del Genio, “Saturn Atmospheric Structure and Dynamics,” chapter 6 in Michele K. Dougherty et al. (eds.), Saturn from Cassini-Huygens, (Springer, 2009), p. 122. 31. B. Marty et al., “Croons: Exploring the Depths of Saturn with Probes and Remote Sensing Through an International Mission” Exp. Astron. 23 (2009):952. 32. Andrew Ingersoll email to author, 21 Sep. 2010. 33. Andy Ingersoll interview with author, 17 Dec. 2009, American Geophysical Union (AGU) conference, San Francisco. 34. Jia-Rui C. Cook and J.D. Harrington, “Flash: NASA’s Cassini Sees Lightning on Saturn,” http://www.jpl.nasa.gov/news/news.cfm?release=2010- 129&cid=release_2010129&msource=2010129&tr=y&auid=6206677, JPL press release 2010–129 (14 Apr. 2010). 35. NASA, “Spotting Saturn’s Northern Storm,” http://www.nasa.gov/mission_pages/cassini/multimedia/pia12824.html, updated 6 July 2011; Tammy Plotner, “The Sights And Sounds of Saturn’s Super Storm,” http://www.universetoday.com/87269/the-sights-and-sounds-ofsaturns-super-storm/ (7 July 2011). 36. Linda Spilker review of manuscript, March 2011.

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37. Georg Fischer et al., “Atmospheric Electricity at Saturn,” Space Sci Rev. 137 (2008): 271–285. 38. John Roach, “Saturn Lightning Storm Breaks Solar System Record,” http://news.nationalgeographic.com/news/2009/09/090915-saturn-lightning-storms.html, National Geographic News (15 Sep. 2009). 39. Anita Heward and Eleni Chatzichristou, “Longest Lightning Storm on Saturn Breaks Solar System Record,” http://www.europlanet-eu.org/demo/index.php?option=com_content&task=v iew&id=141&Itemid=1, Europlanet Web site (accessed 29 Sep. 2009). 40. Bob Mitchell review of manuscript, Feb. 2011. 41. Both quotes in the paragraph are from Jia-Rui C. Cook and J.D. Harrington, “Flash: NASA’s Cassini Sees Lightning on Saturn,” http://www.jpl.nasa.gov/news/news.cfm?release=2010129&cid=release_2010-129&msource=2010129&tr=y&auid=6206677, JPL press release 2010–129 (14 Apr. 2010). 42. Andrew Ingersoll email to author, 21 Sep. 2010. 43. Fraser Cain, “Formation of Saturn,” http://www.universetoday.com/guide-to-space/saturn/formation-of-saturn/, Universe Today Web site (3 July 2008). 44. Cain, “Formation of Saturn.” 45. Tompa, “Jupiter and Saturn Full of Liquid Metal Helium.” 46. Rachel Tompa, “Jupiter and Saturn Full of Liquid Metal Helium,” http://berkeley.edu/news/ media/releases/2008/08/06_helium.shtml, press release, UC Berkeley News (6 Aug. 2008); Lars Stixrude and Raymond Jeanloz, “Fluid Helium at Conditions of Giant Planetary Interiors,” Proc. of Natl. Acad. of Sci. 105(32) (12 Aug. 2008):11071–11075. 47. William B. Hubbard et al., “The Interior of Saturn,” chapter 4 in Michele K. Dougherty et al. (eds.), Saturn from Cassini-Huygens, (Springer, 2009), p. 75; ESA, “Facts About Saturn,” http://www.esa. int/esaMI/Cassini-Huygens/SEMV75HHZTD_0.html, accessed 27 Sep. 2009; Windows to the Universe team, “The Composition of Saturn’s Interior,” http://www.windows.ucar.edu, Boulder, CO: © The Regents of the University of Michigan, (9 Apr. 1997). 48. Andrew Ingersoll, “Saturn Interior and Atmosphere,” http://saturn.jpl.nasa.gov/files/20090630_ CHARM_Ingersoll.pdf, CHARM Telecon presentation, 30 June 2009, originally presented at Cassini Solstice Mission Senior Review, 10 February 2009. 49. William B. Hubbard et al., “The Interior of Saturn,” Chapter 4 in Michele K. Dougherty et al. (eds.) Saturn from Cassini-Huygens (Netherlands: Springer, 2009), pp. 75, 80. 50. William B. Hubbard et al., “The Interior of Saturn,” in Saturn from Cassini-Huygens (Netherlands: Springer, 2009); Keith Refson, “FPS0: First Principles Simulation,” http://www. ccp5.ac.uk/SSCCP5/FPS/fps0.html, Collaborative Computational Project 5 - The Computer Simulation of Condensed Phases, Science & Technologies Facilities Council, U.K., lecture notes from the CASTEP workshop held in 2007. 51. Adam P. Showman, “Windy Clues to Saturn’s Spin,” Nature 460 (30 July 2009):582. 52. P. L. Read et al., “Saturn’s Rotation Period from its Atmospheric Planetary-Wave Configuration,” Nature 460 (30 July 2009):608; Particle Physics & Astronomy Research Council, “How Long Is A Day On Saturn?” ScienceDaily http://www.sciencedaily.com /releases/2006/05/ 060503202834.htm (3 May 2006), accessed 30 September 2009. 53. Both quotes in this paragraph are from Andy Ingersoll interview by author, Rome, Italy, 12 June 2008. 54. Geological Survey of Canada, “Geomagnetism: Earth’s Magnetic Field,” http://gsc.nrcan. gc.ca/geomag/field/index_e.php, last modified 16 Jan. 2008, accessed 12 Sept. 2008; Cambridge University Press, Cambridge University Press, “Book Resources,” http://www.cambridge.org/ resources/0521546206/678_s283b1f6_04.pdf, accessed 12 Sept. 08. 55. Particle Physics & Astronomy Research Council, “How Long Is A Day On Saturn?” ScienceDaily http://www.sciencedaily.com/releases/2006/05/060503202834.htm (3 May 2006), accessed 30 September 2009.

References 283 56. Amy Callaghan , “Day Length on Saturn Gets a Bit Shorter,” http://www.cosmosmagazine. com/news/2893/a-day-saturn-gets-a-bit-shorter?page=0%2C0, Cosmos (30 July 2009), accessed 30 Sep. 2009. 57. JPL, “Spacecraft: Cassini Orbiter Instruments - MAG.” 58. Callaghan , “Day Length on Saturn.” 59. P.L. Read et al., “Saturn’s Rotation Period from its Atmospheric Planetary-Wave Configuration,” Nature 460 (2009):608–610; Callaghan, “Day Length on Saturn.” 60. D. Gurnett et al, “Discovery of a North–south Asymmetry in Saturn’s Radio Rotation Period,” Geophys. Res. Letters 36 (2009):L16102. 61. NASA-JPL, “Two-Timing Saturn,” http://saturn.jpl.nasa.gov/news/cassiniscienceleague/science20090923/, Cassini Equinox Mission Web site (2009). 62. Claudia Alexander, “Saturn’s Magnetosphere: Five Times a CHARM,” http://saturn.jpl.nasa. gov/files/20090630_CHARM_Alexander_VerB.pdf, California Institute of Technology presentation (July 2009). 63. Linda J. Spilker (ed.), “Passage to a Ringed World,” NASA SP-533 (Washington D.C.: NASA, Oct. 1997), p. 67. 64. Spilker, “Passage to a Ringed World,” p. 69. 65. E. C. Stone et al., “Voyager 1 Encounter with the Saturnian System,” Science 212 (10 April 1981):159–163. 66. E. C. Stone et al., “Voyager 2 Encounter with the Saturnian System,” Science 215 (1982):499–504. 67. Margaret Galland Kivelson, “The Current Systems of the Jovian Magnetosphere and Ionosphere and Predictions for Saturn,” Space Science Reviews 116 (January 2005):299–318. 68. ESA, “Saturn’s Magnetosphere,” http://www.esa.int/esaMI/Cassini-Huygens/ SEMBJCHHZTD_0.html, accessed 3 Oct. 2009. 69. European Planetology Network, “Saturn’s Skewed Ring Current,” ScienceDaily (26 Aug. 2007); Johns Hopkins University, “Saturn’s Invisible Ring,” http://www.astronomy.com/asy/ default.aspx?c=a&id=6389#, Astronomy (13 Dec. 2007). 70. S. Kellett et al., “Thickness of Saturn’s Ring Current Determined from North‐South Cassini Passes Through the Current Layer,” J. Geophys. Res. 114 (23 April 2009):A04209. 71. European Planetology Network, “Saturn’s Skewed Ring Current.” 72. Joe Mason and Jia-Rui C. Cook, “Cassini Captures Ghostly Dance of Saturn’s Northern Lights,” http://ciclops.org/view.php?id=6012, Media Relations Office, Cassini Imaging Central Laboratory for Operations (CICLOPS), Space Science Institute, Boulder CO (24 Nov. 2009). 73. W.S. Kurth et al., “Auroral Processes,” chapter 12 in Michele K. Dougherty et al. (eds.), Saturn from Cassini-Huygens, (Springer, 2009), pp. 333–370; J.T. Clarke et al., “Morphological Differences Between Saturn’s Ultraviolet Aurorae and Those of Earth and Jupiter,” Nature 433 (17 Feb. 2005):717–719; J.T. Clarke et al., “Response of Jupiter’s and Saturn’s Auroral Activity to the Solar Wind,” Journal of Geophysical Research 114(A5) (1 May 2009); Andrew Ingersoll email to author, 15 Oct. 2010. 74. Michael D. Desch, “Evidence for Solar Wind Control of Saturn Radio Emission,” J. of Geo. Res. 87 (1 June 1982):4549–4554. 75. ESA, “Saturn’s Magnetosphere.” 76. W.S. Kurth et al., “An Earth-Like Correspondence Between Saturn’s Auroral Features and Radio Emission ,” Nature 433 (17 Feb. 2005):722–725; Europlanet Research Infrastructure, “Solar Wind influence on Saturn Kilometric Radiation,” http://europlanet-plasmanode.oeaw. ac.at/index.php?id=268, accessed 4 Oct. 2009; D.A. Gurnett et al., “The Cassini Radio and Plasma Wave Investigation,” Space Science Reviews 114 (2004):395–463. 77. H.O. Rucker et al., “Saturn Kilometric Radiation as a Monitor to the Solar Wind?” Advances in Space Research 42(1) (2008); L. Lamy et al., “Saturn Kilometric Radiation: Average and Statistical Properties” J. Geophys. Res. 113 (2008):A07201.

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78. D. A. Gurnett, “Radio and Plasma Wave Observations at Saturn from Cassini’s Approach and First Orbit,” Science 307 (25 February 2005):1255–1259. 79. D. A. Gurnett, “Radio and Plasma Wave Observations at Saturn from Cassini’s Approach and First Orbit,” Science 307 (25 February 2005):1255–1259. 80. N. Achilleos et al., “Large-Scale Dynamics of Saturn’s Magnetopause : Observations by Cassini,” Journal of Geophysical Research 113 (2008): A11209. 81. R.E. Hartle, “Interaction of Titan’s Atmosphere with Saturn’s Magnetosphere,” Advances in Space Research 5 (1985):321–332; S.A. Ledvina, “Titan’s Induced Magnetosphere,” Advances in Space Research 33 (2004):2092–2102. 82. S.A. Ledvina, “Titan’s Induced Magnetosphere,” Advances in Space Research 33 (2004):2092– 2102; Norbert Krupp, “Energetic Particles in the Magnetosphere of Saturn and a Comparison With Jupiter,” Space Science Reviews 116 (January 2005):345–369. 83. J.A. Van Allen et al., “The Energetic Charged Particle Absorption Signature of Mimas,” J. Geophys. Res. 85(A11) (1980):5709–5718. 84. C. Paranicas et al., “Sources and Losses of Energetic Protons in Saturn’s Magnetosphere,” Icarus 197 (2008):519–525. 85. E. C. Stone et al., “Voyager 1 Encounter with the Saturnian System,” Science 212 (10 April 1981):159–163; M.L. Kaiser et al., “Voyager Detection of Nonthermal Radio Emission from Saturn,” Science 209 (12 Sept. 1980):1238. 86. J. L. Burch, “Tethys and Dione as Sources of Outward-Flowing Plasma in Saturn’s Magnetosphere,” Nature 447 (14 June 2007):833–835. 87. ESA, “Saturn’s Magnetosphere,” http://www.esa.int/esaMI/Cassini-Huygens/ SEMBJCHHZTD_0.html, accessed 27 Sep. 2009. 88. R. L. Tokar, “The Interaction of the Atmosphere of Enceladus with Saturn’s Plasma,” Science 311 (10 March 2006):1409–1412. 89. Royal Astronomical Society, “Charged Dust From Inside Saturn’s Moon Enceladus,” http:// www.sciencedaily.com/releases/2009/04/090422085841.htm, ScienceDaily (25 Apr. 2009). 90. D. N. Baker et al., Solar Dynamics and Its Effects on the Heliosphere and Earth (Dordrecht, The Netherlands: Springer, 6 April 6 2007), pp. 123–124. 91. Margaret Galland Kivelson, et al., “Does Enceladus Govern Magnetospheric Dynamics at Saturn?” Science 311 (10 March 2006):1391–1392; Krishan Khurana email to author, 21 Oct. 2010. 92. G. H. Jones, “Enceladus’ Varying Imprint on the Magnetosphere of Saturn,” Science 311 (10 March 2006):1412–1415. 93. M. Bouhram et al., “The Enceladus Satellite as a Source of N+ ions in Saturn’s Magnetosphere,” Comptes Rendus Physique 6 (2005):1176–1181. 94. Linda Spilker email to author, 31 Dec. 2013. 95. Stamatios Krimigis of Johns Hopkins Applied Physics Laboratory, in NASA-JPL, “Cassini Data Help Redraw Shape of Solar System,” http://www.jpl.nasa.gov/news/features.cfm?featur e=2337&msource=f20091015&tr=y&auid=5467644 (15 Oct. 2009). 96. NASA-JPL, “Cassini’s Big Sky: The View from the Center of Our Solar System,” http://www. jpl.nasa.gov/news/features.cfm?feature=2370&msource=F20091119&tr=y&auid=5615216, JPL News & features Web site (19 Nov. 2009). 97. S. M. Krimigis et al., “Imaging the Interaction of the Heliosphere with the Interstellar Medium from Saturn with Cassini,” http://www.sciencemag.org/cgi/rapidpdf/1181079v1.pdf, Science Express Reports (15 October 2009); Krimigis, “Cassini Data Help Redraw Shape of Solar System.”

12 The ring system “It has been amazing to see the rings come to life before our very eyes, changing even as we watch, being colorful and taking on a tangible, 3-D nature. The rings were still a nearly unstructured object in even the best telescopes when I was a grad student, but Cassini has brought us an intimate familiarity with them.” – Jeff Cuzzi, Cassini Interdisciplinary Scientist

for Rings and Dust1

For four centuries, astronomers have gazed at Saturn’s rings in awe. Galileo Galilei first examined the Saturn system with a telescope in 1610, but his instrument was far too crude and weak to resolve the two peculiar protuberances that he saw on either side of the planet, and he guessed they must be two large moons. Two years later, he was astonished to see that they had apparently disappeared. After another two years, the protuberances were back again and he concluded that they were “arms” of some sort.2 Years later it was Christiaan Huygens who successfully determined the nature of the phenomenon girdling Saturn. In February 1656, using an improved telescope that he and his brother built, Huygens was able to resolve the protrusions, commonly referred to as ansae (handles). In a treatise published later in that year, he described Saturn as annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam inclinato (surrounded by a thin flat ring, nowhere touching, and inclined to the ecliptic).3 It is interesting that he published his description of the ring in the form of an anagram, a group of letters created by rearranging those Latin ones above. Huygens did not yet want to reveal his discovery, but wanted the ability to prove to anyone coming forth with the same theory that he had thought of it first.4 In his 1659 book Systema Saturnium, Huygens discussed why Saturn’s ring had appeared to Galileo to disappear. Huygens described in detail how the plane of the ring was tilted at a constant 20° to Saturn’s orbital plane. As the planet journeyed around the Sun, sometimes its ring was edge-on to Earth and thus nearly impossible to see with the instruments of the day. At other times the ring plane would be better aligned and readily observable. © Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_12

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Huygens was viciously attacked for his radical ring theory. Two of his critics, a telescope maker called Eustachio Divini and an astronomer in Rome named Honoré Fabri, claimed that four satellites orbited Saturn: two dark ones near the planet and two bright ones farther away, and that at times the darker satellites partially blocked the brighter ones, giving the appearance of handles on either side of the planet. But Huygens quickly counterattacked. In response, his critics modified their theory to involve six satellites. The elegance of Huygens’ concept attracted support, notably from the venerable Accademia del Cimento (Academy of Experiment) in Florence. By 1670, Huygens’ theory had become the generally accepted one, although it too proved to have flaws in it. Huygens’ concept only envisioned one solid ring girdling Saturn. It was Giovanni Cassini who proved otherwise, and by so doing, somewhat weakened Huygens’ argument. In 1675, Cassini discovered a division that was later named in his honor. This demonstrated there were at least two rings around Saturn, if not more. Cassini further theorized that the rings were not solid, but made up of a very large number of small satellites all orbiting Saturn. It would take centuries to prove this.5 Cassini’s vision of the many-satellite nature of the rings gained credence in 1785 when Pierre Simon, Marquis de Laplace mathematically demonstrated that solid rings orbiting around Saturn would be unstable. This was followed by James Clerk Maxwell’s 1857 mathematical proof that the only possible explanation for the rings was that they were constructed of a myriad of small particles orbiting the planet and closely packed enough to give the appearance of being solid. Then in 1895, James E. Keeler of the Allegheny Observatory in Pittsburgh made observations that proved Maxwell’s theory correct.6

12.1

MODERN RING SCIENTISTS

Scientists through the centuries have added their discoveries to our knowledge about Saturn’s rings. One current scientist who has devoted four decades of his life to the study of this beautiful phenomenon is Jeff Cuzzi of NASA’s Ames Research Center. The winner of the 2010 Gerard P. Kuiper Prize, a prestigious individual award in planetary sciences, Cuzzi was selected in large part for his research on the formation and evolution of planetary rings. Like Christiaan Huygens, he built up expertise in several technical disciplines. In his case, these included radiative transfer of energy, nebular dynamics, observational astronomy, and others. Saturn’s rings fascinated Cuzzi early in his career and he was drawn to studying their dynamics. But his first emotional connection to the planet came not from scientific study, but from a visual, subjective experience – seeing Saturn through a large telescope, namely the 24-inch instrument atop Mount Wilson near Los Angeles.7 Invited to join the Voyager mission’s imaging team in 1978, he became the leader of its rings subgroup and oversaw the planning of all Saturn, as well as Uranus and Neptune, ring encounters. In 1989, Cassini-Huygens mission management selected him as the interdisciplinary scientist for rings.8 Cuzzi says he is drawn to people who are motivated by the excitement of learning something new about the world. This excitement seems to reside within himself as well, for instance when he describes the days that the Voyager 1 spacecraft made its Saturn

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flyby, sending back the “beautiful, glowing images of the rings in a geometry no one had ever seen.”9 Linda Spilker, the current Cassini project scientist is an expert on Saturn’s rings and on the staff at JPL. Like Jeff Cuzzi, she served on the rings team of the Voyager mission, which she joined in 1977. In addition to developing her scientific career on Voyager, she also needed to find ways of fitting into its male-dominated culture. She became the only woman working on Voyager’s Infrared Interferometer Spectrometer and Radiometer (IRIS) team which took measurements of thermal radiation emitted from Saturn’s rings that helped identify their temperature profiles. She has said of this time that the “toughest thing is when you are in a meeting and you are the only woman in the room,” as she often was. In her early years, she experienced what she thought was subconscious stereotyping on the part of her team mates. If she spoke up in a meeting, the men would interrupt and talk over her. But as time went on, more women joined the space program and she found that if even one other woman was in the meeting, they could alter the dynamics. If one of them was interrupted, the other could reply, “wait a second, I want to hear what she has to say.”10 In this way they got their voices heard. Working with men who were a generation or two older also had its interesting moments. The head of one team Spilker was on was a scientist near retirement age. He just had certain ideas about how things had to work. For instance, he felt that he had to open every door for her. On the East Coast, where the weather can get very cold, there are often two doors close together that you have to go through to enter a building. The older man would open the first door and then Spilker, without giving it any thought, would naturally walk to the second door and expect to open it. At some point she realized it really frustrated the man if she did not stop and let him open the second door as well. It was how he had been raised. For a while, these interactions turned into a tug-of-war as to who would reach the second door first. Attitudes changed as the ratio of women to men on the mission grew. Today on the Cassini mission, Spilker believes that “gender blind” accurately describes the situation. Whoever gets to a door first usually opens it, especially with the younger generation of scientists. But the culture has changed in far more important ways than this. Spilker believes that women are pretty much just seen as people. Plus, women have moved up the management chain. Spilker’s tenure at JPL serves as an example of how an intense planetary science career can be balanced with family life. After the two Voyager spacecraft had flown by Saturn, there was a five year period from 1981 to 1986 until Voyager 2 reached Neptune. It was during this time of reduced activity that Spilker started her family, as did others on the mission. In fact, says Spilker, “you could sort of say there was like a baby boom on Voyager.”11 Later, she would inform her daughters that “their births were based on the alignment of the planets.”12 Spilker joined the Cassini-Huygens mission in 1988, during its planning stage and before it even had an official name. In 2004, when the spacecraft reached the Saturn system, her daughters were older, permitting her to “relinquish some of those mom duties that you have to do on a daily basis”13 and spend more time on the mission. “It was interesting,” Spilker has said, “to watch the cycles and how you sort of plan your life to the extent that you can around these key kinds of events.”14

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12.2

CHARACTERISTICS OF THE RING SYSTEM

When people think of Saturn they generally envision, as did Christiaan Huygens, the globe of the mother planet surrounded by a bright disk. As discussed above, the true nature of the ring system is rather more complex. After over four hundred years of telescope observations and more recent spacecraft surveillance, space scientists have identified eight distinctly different rings orbiting Saturn. Close examination of these rings reveals a mélange of different substructures such as ringlets, partially formed ring arcs, clumps, moonlets, divisions and gaps. Moreover, Saturn’s ring system is not a static entity, it is forever changing. The Voyagers and the Cassini spacecraft revealed this when they saw shifting ring patterns and even a moon spewing out icy particles that replenished those being lost from a ring.15 Astronomers gave seven of Saturn’s rings letter names from A to G. The letters do not indicate relative distance from the mother planet, but rather the sequence – more or less – in which the individual rings were discovered. Starting at the planet, the ordering is D, C, B, A, F, G, and E. There is also the enormous gossamer Phoebe ring that was discovered in 2009, with its outer edge at 207 Saturn radii (25 million kilometers).16 Figure 12.1 depicts Saturn’s A through G rings. Table 12.1 describes these rings, as well as the characteristics of various smaller structures in the ring system. From the solar reflection spectra of the rings, space scientists believe that they are predominantly composed of chunks of ice contaminated with a small amount of dust. Observations with infrared and radio waves indicated the ring material varies in size from the microscopic in the D and E rings, to sand- and pebble-sized particles in the C and F rings, to cobble- and boulder-sized particles in the A and B rings. Some of the particles in the B ring could in fact be much larger, perhaps up to several tens of meters across.17 The objects that comprise some rings are continually changing in size. Some are probably growing while others are breaking apart. The Cassini spacecraft revealed that collisions are routine, and the chunks of ice leave trails of debris in their wakes. Observations have also shown “how small moons play tug-of-war with ring material and how bits of rubble that would otherwise join together to become moons are ultimately ripped apart by the gravitational pull that Saturn exerts.”18

Figure 12.1 Saturn’s rings from A to G and the locations of various ring divisions and moons. Also shown is the location where the spacecraft crossed the ring plane during SOI.

12.2

Characteristics of the ring system

289

Table 12.1. Characteristics of rings and other structures. Name of Ring or Other Structure

Inner Edge

Outer Edge

D Ring C Ring

66,970 km/41,632 miles 74,490 km/46,285 miles

74,490 km/46,285 miles 91,980 km/57,154 miles

Width

7,500 km/4,660 miles 17,500 km/10,874 miles Colombo Gap 77,800 km/48,343 miles 100 km/62 miles Maxwell Gap 87,500 km/54,370 270 km/168 miles Bond Gap 88,690 km/55,109 miles 88,720 km/55,128 miles 30 km/19 miles Dawes Gap 90,200 km/56,048 miles 90,220 km/56,060 miles 20 km/12 miles B Ring 91,980 km/57,154 miles 117,580 km/73,061 miles 25,500 km/15,845 miles Cassini Division 117,500 km/73,011 miles 122,050 km/75,838 4,700 km/2,920 miles Huygens Gap 117,680 km/73,123 miles 285-440 km/177-273 miles Herschel Gap 118,183 km/73,436 miles 118,285 km/73,499 miles 102 km/63 miles Russell Gap 118,597 km/73,693 miles 118,630 km/73,713 miles 33 km/21 miles Jeffreys Gap 118,931 km/73,900 miles 118,969 km/73,924 miles 38 km/24 miles Kuiper Gap 119,403 km/74,194 miles 119,406 km/74,195 miles 3 km/2 miles Leplace Gap 119,848 km/74,470 miles 120,086 km/74,618 miles 238 km/148 miles Bessel Gap 120,236 km/74,754 miles 120,246 km/74,717 miles 10 km/6 miles Barnard Gap 120,305 km/74,754 miles 120,318 km/74,762 miles 13 km/8 miles A Ring 122,050 km/75,838 miles 136,770 km/84,985 miles 14,600 km/9,072 miles Encke Gap 133,570 km/82,997 miles 325 km/202 miles Keeler Gap 136,530 km/84,836 miles 35 km/22 miles Roche Division 136,770 km/84,836 miles 139,380 km/86,607 miles 2,600 km/1,616 miles F Ring 140,224 km/87,131 miles 30-500 km/19-311 miles G Ring 166,000 km/103,148 miles 174,000 km/108119 miles 8,000 km/4,971 miles E Ring 180,000 km/111,847 miles 480,000 km/298,258 300,000 km/186,411 miles miles Phoebe Ring 7,700,000 km/4,800,000 12,500,000 km/7,800,000 4,800,000 miles mi. km/3,000,000 mi.

Note: Divisions are typically large separations in the rings and differ from the much smaller openings called gaps. Distances are measured from the planet center to the feature of the ring system. Sources: 1. NASA, “Saturn: Rings,” http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn&Display=Ri ngs, Solar System Exploration Web site, accessed 8 July 2010 2. SETI, “Vital Statistics for Saturn’s Rings and Inner Satellites,” http://pds-rings.seti.org/saturn/saturn_tables.html, accessed 11 September 2009, material is from C.D. Murray and S. F. Dermott, “Solar System Dynamics” (Cambridge University Press: 1999) 3. International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN), “Gazetteer of Planetary Nomenclature,” http://planetarynames.wr.usgs.gov/Page/Rings, accessed 13 October 2010 4) SETI, “Vital Statistics for Saturn’s Rings and Inner Satellites,” http://pds-rings.seti.org/saturn/saturn_tables.html, accessed 15 October 2010.

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12.2.1

Moonlets and ringlets

Moonlets are small satellites less than 50 kilometers (30 miles) in diameter19 and typically irregular in shape. Little ones resembling rubble piles may be repeatedly forming and breaking into pieces in the rings. Cassini images of the F ring may have captured this process in action.20 The two Voyager spacecraft, which viewed the A, B, and C rings in some detail, revealed that, far from being smooth, unblemished expanses, they were composed of thousands of narrow ringlets, fine-scale radial structures that appeared in images like “grooves on a 270,000 kilometer wide phonograph record.”21 The Voyagers also found faint rings and “examples of all the types of ring systems you see in the rest of the solar system,”22 said imaging team member Carl Murray of the University of London, who was a postdoctoral fellow when the Voyagers sped past Saturn in 1980 and 1981.23 12.2.2

Gravitational interactions with ring particles

In the years between Voyager and Cassini, scientists carried out intensive theoretical modeling in order to better understand ring system dynamics. One of the interesting things they found was that gravitational interactions between the satellites and ring particles can yield amazing results. As an example, the gravity of the moon Mimas can give rise to waves that spiral both inward and outward like a phonograph needle riding the grooves of a record. And even stranger, gravity can make ring particles behave “as weirdly as any subatomic particle.”24 Gravity is usually regarded as an attractive force, but in the universe of Saturn’s ring system, it can essentially repel. A case in point is discussed in the section on the F ring. 12.2.3

Ring system formation models

How did Saturn’s rings form? To understand the various theories, it is necessary to realize the importance of the Roche limit. This is the closest distance that a satellite held together only by internal gravitational forces can approach its parent body – in this case Saturn – without being pulled apart by tidal forces. For Saturn, the Roche limit is 147,000 kilometers (92,000 miles). All of the rings out to the F ring orbit within this limit. By comparison, Earth’s Roche limit is 18,500 kilometers (11,500 miles). If our Moon were ever to venture closer than this, it would be torn apart by tidal forces and Earth would gain rings. The other gaseous planets – Jupiter, Uranus, and Neptune – all have their rings systems within their respective Roche limits. An occurrence not many years ago demonstrated what can happen when a body wanders inside a Roche limit. On 7 July 1992, Comet Shoemaker-Levy 9 made a close approach to Jupiter that was within the planet’s Roche limit. As a result, the comet broke apart into 21 pieces.25 Theories of the formation of Saturn’s ring system include primordial, satellite disruption, and comet disruption models. Primordial models suggest that the ring system’s material was left over from the original circumplanetary disk that accreted into the mother planet and its satellites. Tidal forces such as those within the Roche limit may have

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prevented this material from accreting. Satellite disruption models argue that a moon’s decaying orbit brought it within the Roche limit, where it was disrupted either by tidal forces or by collisions with the debris already there. Then again, if the moon was cohesive enough to exist within the Roche limit (if it was held together by more than just gravity), it might still have broken apart after an impact with another body. After this, tidal forces could have prevented it from re-accreting to form a new satellite.26 Finally, the comet disruption theory envisions that a large comet may have wandered close to Saturn and been destroyed by tidal forces.27 Voyager data intimated that the rings might not have been created during the primordial formative processes of Saturn. Studies of gravitationally generated wave structures in the ring system support this hypothesis. So does the belief that if the rings were primordial they ought to have been blackened by repeated sprinkles of dark interplanetary debris over millions or billions of years, which clearly has not occurred. Ring disruption phenomena such as meteoroid bombardment, ring spreading, and perturbations due to satellites result in ring particles being continually lost from the system. This supports the idea that the ring material which we observe today must be geologically young.28 Perhaps the ring system is itself ancient, but the material that it contains is constantly replenished by various sources to balance the particles being lost. Not all of the rings may have had the same origin, because some of the rings lie outside the Roche limit. Cassini may have discovered evidence that the ring system consists of a mix of old and young structures. It has found evidence that at least part of the ring system is more massive and longer lived than was believed from Voyager results. This realization has rejuvenated interest in primordial models.29 The B ring, for instance, may indeed have enough mass in it to be primordial, or in other words, to have lasted from the time of Saturn’s formation, even with various particle loss mechanisms operating. If Cassini discovers that the B ring is at least ten times more massive than originally estimated from Voyager data, this would lend support to its being of primordial origin. The C, D, and A rings may have been formed later by a comet or satellite migrating inside the Roche limit. At this point, we simply cannot make these determinations.30 However, we do know that at least some of the finer ring features appeared in the recent past. As an example, the Voyagers saw year-to-year changes in the F ring as it fragmented into strands that formed braids and then untangled,31 and Cassini saw significant changes in the D ring compared to during the Voyager flybys.32 12.2.4

The main and faint ring systems

Saturn’s rings are divided into two groups exhibiting somewhat different properties. The main rings – A, B, and C – are far denser and more easily seen than the others. A, B, and C are all within Saturn’s Roche limit and are made up of particles greater than about 1 centimeter (0.4 inch) across. Collisions between these particles play an important role in the dynamics of the main rings, as do the tidal effects of Saturn’s gravity. While Saturn’s main rings are far brighter and more readily seen than the others, they are astonishingly paper-thin, with many parts believed to be only 10 meters (30 feet) thick, or even less. In practical terms, this means that if an image is taken by a spacecraft traveling within the main rings, they can barely be seen, appearing as only a thin line.33

292

The ring system

Saturn’s faint rings include D, E, G, and the recently discovered Phoebe ring.34 Two of the faint rings – G and E – are made of micrometer-sized particles, orders of magnitude smaller than typical particles in the main rings. Due to the small size and mass of faint ring particles, forces such as radiation pressure (the pressure exerted by light and other electromagnetic waves) become important in ring dynamics. So does a related phenomenon called Poynting-Robertson drag. This is the gradual decrease in orbital velocity of a small particle owing to its absorption and reemission of solar radiation, causing the particle to slowly spiral inward.35 The G and E faint rings can barely be seen through Earth-based telescopes. Faint ring densities (the number of particles per unit volume) are much lower than for the main rings, and hence mutual collisions are fairly insignificant in the dynamics and evolution of the faint ring system. Tidal effects from Saturn also exert only minor influences because most of the faint rings lie outside the Roche limit.36 The D ring is between Saturn and the main rings. It is a complex band exhibiting noticeable brightness and structural changes over the last 25 years, as well as particle size characteristics that vary markedly with distance from Saturn.37 Saturn’s F ring – located between the A and G ring – has characteristics of both main and faint rings. The F ring lies near the Roche limit and contains a mixture of large bodies greater than 1 meter, and perhaps as large as 1 kilometer, but they are surrounded by micrometer-sized dust. The various ring characteristics are discussed in more detail in the sections below. Note that gaps occur entirely within a ring, whereas divisions, such as the Cassini and Roche Divisions, are typically larger and occur between rings. 12.2.5

The A ring

The A ring is the outermost of Saturn’s two brightest rings. It is not an undisturbed expanse of particles but contains gaps, all circular and centered on Saturn (Figure 12.2). The 325 kilometer (200 mile) Encke Gap is the widest feature in this ring. A moon called Pan, just 20 kilometers (12 miles) across, orbits within it and, planetary scientists believe, plays a vital role in keeping the gap clear by forcing ring particles into eccentric orbits.38 A satellite’s or particle’s orbital speed is not determined by its mass, but by its distance from Saturn. The closer an object’s orbit is to the planet, the faster it travels. Thus all particles orbiting inside the radius of Pan’s orbit pass it by, while particles whose orbits lie outside Pan’s lag behind the moon. Each time a particle passes or is passed by Pan, it experiences a gravitational tug and its orbital eccentricity increases. Once a particle starts traveling on an eccentric orbit, it will run into its neighboring particles whose orbits are circular. Most particles in orbits similar to that of Pan eventually get their paths modified. This is what formed and maintains the Encke Gap.39 The inner and outer edges of the Encke Gap exhibit gentle wave structures which are caused by Pan’s gravitational forces40 (Figure 12.3). It was the discovery of the scalloped edges of Encke Gap that suggested to scientists the existence of a nearby object large enough to provide the necessary gravitational tug to form those shapes, and eventually led to Pan’s discovery. Jeffrey Cuzzi and Jeffrey Scargle predicted such an object’s existence in 1985.41 The following year Mark Showalter et al.42 inferred its orbit and mass by

Figure 12.2 Details of Saturn’s outer A ring, including the large Encke Gap – the dark band on the left – and the Keeler Gap, the narrow dark band on the right. Note how the A ring brightens substantially to the right of the Keeler Gap.

Figure 12.3 The gentle scalloped edges of the Encke Gap, created by the moon Pan’s gravitational forces.

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The ring system

Figure 12.4 Wave structures along the edges of the Keeler Gap, sculpted by the gravitational pull of tiny Daphnis (7 kilometers, or 4.3 miles across).

modeling its gravitational effects. Pan was actually discovered at the predicted spot by Showalter in 1990 by reexamining Voyager 2 photos from 1981. Its existence was reconfirmed by Cassini in 2005.43 The same processes that operate in the Encke Gap occur elsewhere in the ring system. For instance, as depicted in Figure 12.4, the 35 kilometer (22 mile) Keeler Gap,44 a narrower space than the Encke Gap, lies near the outer boundary of the A ring. This was probably cleared out by the tiny moon Daphnis (7 kilometers, or 4.3 miles across) which orbits within it.45 The gravitational pull of this moonlet has also sculpted the wave phenomena observed on the edges of the Keeler Gap. Numerical models have been able to reproduce the Keeler Gap’s irregular and distinctly wavy edges, which arise in part because of Daphnis’ closer proximity to them than, for instance, Pan is to the Encke Gap’s ring edges. On the Encke Gap edges, only a gentle scalloping is observed. But Cassini’s mapping of Keeler Gap ring edges revealed that its particularly complex inner edge structure is also driven by resonances between ring particles, the satellite Prometheus, and possibly also the satellite Pandora. An orbital resonance occurs when two orbiting bodies, in this case a satellite and a ring particle, exert a regular, periodic gravitational influence on each other – for instance, each time one passes the other during its orbit. Over time, these repeated tugs alter the two bodies’ orbits, although the particle, with a mass so much smaller than the moon, will experience a far more dramatic orbital perturbation. As mentioned above, these gravitational tugs clear out gaps in Saturn’s rings as well as modify the edges of the gap.46

12.2.5.1

Ringlets

Pan appears to be maintaining at least two of the three or four ringlets in the Encke Gap, one of which appears coincident with the moon’s orbit (Figure 12.5). Ringlets may also be formed from particles orbiting between two nearby moonlets, such as is the case for the F ring.47 This “shepherding” of particles which travel between two nearby moons is discussed further in the section on the F ring.

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Figure 12.5 Ringlets in the A ring’s Encke Gap. The moon Pan is on the left and is casting a shadow on the ring. A background star can be seen nearby.

A ringlet is composed of particles held in proximity to each other by their mutual gravitation. Each particle of a ringlet orbits as a separate object, but all particles are constrained to approximately the same orbit. As with rings, ringlets are stable, long-lived structures even when located within Saturn’s Roche’s limit.48

12.2.5.2

Self-gravity wakes

In 2005 the Visual and Infrared Mapping Spectrometer (VIMS) monitored a series of occultations49 as the star omicron Ceti (Mira) passed behind Saturn’s rings from the point of view of the Cassini spacecraft. These occultations revealed significant variations in the optical depth (a measure of the transparency) of the A ring which were attributed to structures in the ring that have been called self-gravity wakes.50 These are clumpings of particles that arise from their mutual gravitational attraction, but such aggregation also depends in part on particle size. Numerical simulations indicate that centimeter-sized particles do not aggregate into self-gravity wakes as readily as meter-sized objects. Thus regions with more small particles tend to have more material between the wakes. The forms of the wakes arise because of a balance between particles’ mutual self-gravity and tidal forces originating from Saturn which tend to shear the structures apart.51 They develop far enough from the planet that its gravity is not very efficient at tearing them apart. Although they can grow to tens of meters in size they are ephemeral, sticking together for a while and then breaking up again. They show a degree of organization in their orientations, with their structures sometimes resembling “rows in a marching band.”52

296

The ring system

The Orbiter’s images and stellar and radio occultation data have shown that self-gravity wakes are ubiquitous throughout not only the A ring, but the B ring as well, although they have not yet been detected elsewhere in the ring system.53 Scientists now believe most of the A and B rings’ mass is contained in self gravity wakes.54 The clumping of ring particles in this manner may be analogous to the formation of planetesimals – solid objects larger than a kilometer – as well as full-sized planets. In protoplanetary disks, gravitational instabilities may have enabled these processes to form “vast ensembles of small particles.”55 However, a protoplanetary disk is a different environment than a planetary ring. The nebular gas strongly influences the processes in the protoplanetary disk and tidal forces have a lesser effect. In Saturn’s rings, the situation is reversed: tidal forces are very important and nebular gas is not present.56 Scientists have questioned whether local disruptions of self-gravity wakes are related to the characteristics of propellers, another structure found in the A ring.

12.2.5.3

Propellers

If enough particles clump together in a self-gravity wake, it may grow sufficiently large that it becomes difficult to break apart and then forms a structure known as a propeller.57 At the heart of a propeller structure is a solid chunk of material some 10 to 100 meters (30 to 300 feet) across that is trying to clear a gap through smaller particles in the ring, but is not massive enough. Instead, it ends up clearing a little space around itself which takes on the shape of a two-bladed propeller (Figure 12.6). These strange structures, unknown

Figure 12.6 Propellers occur mostly within three narrow bands in the mid-A ring and may derive from self-gravity wakes.

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before Cassini, occur mostly within three narrow bands in the mid-A ring, referred to as the propeller belt.58 They were first imaged by Cassini in July 2004, while flying just above the ring plane immediately after the orbital insertion maneuver. The propeller-shaped gaps may result from differences in the speeds of material orbiting in the rings. Gravitational attraction declines with increasing distance from Saturn, and orbital dynamics tells us that material circling closer to a planet moves faster than material orbiting farther away. As a result, small ring particles flanking the two sides of a moonlet would appear to be moving in opposite directions to a viewer standing on that moonlet. To better understand this, imagine three trains moving on parallel tracks but at different speeds. The train on the far left is moving fast; the middle train is moving slightly slower, and the train on the far right is moving slowest of all. If an observer in the middle train were to look out the window, the train on the left would appear to be moving forward while the train on the right would appear to be slipping behind. The moonlets in Saturn’s rings are like the middle train. The moonlets are massive enough for their gravity to disturb the movement of smaller ring particles on both sides of them, creating gaps to the moonlet’s left and right. Since ring particles on one side travel faster than the moonlet, a gap within them gets carried ahead, while a gap on the other side, with slower ring particles, gets left a bit behind. The result is a propeller-shaped disturbance with the blades pointing ahead and behind.59 The mass of a moonlet may be critical in determining whether it is able to make a propeller, open a gap within a ring (as described earlier), or indeed is unable to open any space at all in a ring. Small particles have very little gravitational influence on their neighbors, whereas moonlets like Pan and Daphnis – 20 and 7 kilometers (12 and 4 miles) wide, respectively – may be large enough to prevent the gaps that they have created from closing up again. Their size might achieve what smaller satellites cannot – make gaps of empty space that stretch around the entire planet.60 Finally, moonlets with diameters of 10 to 100 meters (about 30 to 300 feet) appear to have the right amount of mass and gravitational influence to create the propeller-shaped gaps observed in the mid-A ring. Another legion of propellers hundreds of times larger than those observed in the mid-A ring, but far rarer, occur farther out in the A ring. These can be thousands of miles long and several miles wide, and they “appear to kick up ring material as high as 1,600 feet (half a kilometer) above and below the ring plane,”61 which is much greater than the ring’s typical thickness of, scientists believe, 3 to 10 meters (10 to 30 feet).62 While the moonlets that generate these propellers are larger than the ones in the mid-A ring, Cassini was too distant from the moonlets to resolve them “amid the swirling ring material that surrounds them.”63 Scientists have estimated that the moonlets that generate the large propellers are approximately half a mile (about one kilometer) in diameter (Figure 12.7).64 The propellers within the A ring also tell us something about solar system history. According to Carolyn Porco, Cassini imaging team lead, “Observing the motions of these disk-embedded objects provides a rare opportunity to gauge how the planets grew from, and interacted with, the disk of material surrounding the early Sun.”65

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The ring system

Figure 12.7 A giant, sunlit propeller in the A ring.

12.2.6

The B ring

The innermost of Saturn’s two brightest rings, the B ring, is noticeably denser than the neighboring A ring. Although the A ring is diffuse enough to allow some light to pass through, B ring particles are numerous enough and packed so closely together in some locations as to render those parts of the ring completely opaque. The density of some of its regions makes the B ring harder to study using occultations in which light from the Sun or a star must pass through the ring to reach the instrument. For instance, ultraviolet light from the bright star 28 Sagittarii was successfully able to produce a profile of the inner and outer B ring during a pre-Cassini occultation, but was completely attenuated by its central region. Nevertheless, Cassini experiments using the Ultraviolet Imaging Spectrograph (UVIS), Visual and Infrared Mapping Spectrometer (VIMS), and Radio Science System (RSS) in which the rings occulted the Earth have greatly improved our knowledge of the fine structure of the ring, with wave structures in the inner B ring and regions that seem to be composed of opaque clumps separated by nearly transparent gaps.66 B ring particles crowd so closely together in some locations that they cannot avoid touching one other, and as a result, this ring behaves more like a fluid than a collection of individual particles. The ring’s particles appear to be organized into a myriad of ringlets, many of which are not circular, but are actually part of a spiral structure wrapping tightly around and around Saturn.67

12.2 12.2.6.1

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Spokes of the B ring

Voyager 1 and the Hubble Space Telescope observed features resembling the spokes of a bicycle wheel on the illuminated face of the B ring, but they seemed to have disappeared by 1998. The Cassini spacecraft did not see these spokes when it arrived at Saturn in 2004 during the planet’s southern hemisphere summer, but sighted them in September 2005, as Saturn’s seasons changed.68 The ring spokes can attain 6,000 miles (9,700 kilometers) in length and span 1,500 miles (2,400 kilometers) in width, but their role and formation within the ring plane are not completely understood. The spokes may appear more often while Saturn’s rings are edge-on to the Sun, but fade completely when the rings are tilted at maximum exposure.69 Figure 12.8 depicts spokes in the B ring. Gravitational forces alone cannot explain the existence of the spokes, although electrostatic repulsion between ring particles may play a role. C. J. Mitchell of the University of Colorado’s Laboratory for Atmospheric and Space Physics examined the theory that the spokes form when micrometer-sized particles of dust are levitated above the ring by electrostatic forces. Transient events such as meteorite impacts or high-energy auroral electron beams may be able to charge the grains to sufficiently high electric potentials for this to occur.70

Figure 12.8 The B ring spokes.

300 The ring system 12.2.6.2

Observations of spokes’ births

Ring spokes appear to be a rapidly evolving phenomenon. As an example, Voyager took images of fully formed spokes which were absent in images taken just minutes before. In a sequence of pictures taken 5 minutes apart, a new radial spoke 6,000 kilometers (3,700 miles) in length appeared in the middle of an existing pattern. One image showed no trace of the spoke, the next showed it but very faintly, then every following image in the sequence depicted it quite clearly.71 Spokes can form quite quickly over large radial distances. This would not be unexpected if they were being triggered by sheets of high-energy auroral electrons connecting Saturn’s ionosphere to its rings. Alternatively, if spoke formation was triggered by meteorite impact, the resulting transient plasma cloud would have to drift along the full radial extent of the spoke in a matter of minutes. Cassini can take images at a much higher rate than the Voyagers, and therefore may be able to resolve the spoke formation mechanism.72 12.2.7

The C ring

Next closest to Saturn from the B ring is the C ring. Far fainter and more transparent than either B or A, it appears to be constructed from many ringlets. Cassini observed the C ring to cast fine, thread-like shadows on Saturn. One of the best times to study this ring proved to be when the Sun illuminated it from the side opposite the viewer. In this geometry, fine dust particles in the ring scattered light forward and appeared far brighter than usual.73

12.2.7.1

Spiral corrugations

In 2009 Saturn went through its equinox with the Sun crossing the equatorial plane, which is also the plane of the ring system. When the rings are illuminated edge-on, this renders even small variations quite visible, and images revealed corrugations in the C ring that appeared as periodic brightness variations. A raised spiral in the D ring was discovered by Matt Hedman, a Cassini imaging team associate at Cornell University. Equinox observations showed it to be part of a much larger phenomenon – a spiral corrugation extending over 15,000 kilometers from its inner to outer edge that marched not only across the D ring, but right across the C ring as well. Some scientists speculated that a massive, very recent event – perhaps a cloud of comet debris plunging through the rings during the second half of 1983 – might have caused this spiral. Hedman commented about such occurrences, “What is cool, is we are finding evidence that a planet’s rings can be affected by specific, traceable events that happened in the last 30 years, rather than a hundred million years ago. The solar system is a much more dynamic place than we gave it credit for.”74 12.2.8

The D ring

Interior to the C ring is a very dim fuzz of material called the D ring. It is not visible in most Cassini or Voyager photographs; long exposures must be taken at favorable geometries to bring out detail. Cassini observations give testament to how dynamic and

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changeable the D ring is. The spacecraft discovered that although some features in the D ring were in the same places as Voyager saw them in the 1980s, the center of light of the brightest of the three ringlets had moved 200 kilometers (120 miles) closer to the planet. Also, the ringlet transformed from a narrow band less than 40 kilometers in width to a considerably broader and more diffuse 250 kilometer-wide feature.75 Cassini also found that the locations of other narrow features in the ring, as well as the configuration of the diffuse material in the ring, had changed since the 1980s. Furthermore, it detected ringlets and structures never seen by Voyager, including a sheet of material just interior to the inner edge of the C ring that was only observable at certain angles. Cassini’s high resolution instruments imaged fine-scale structures varying in time and perhaps longitude. One was the spiral corrugation discussed in the section on the C ring. Observations showed this to have a wavelength of about 30 kilometers (19 miles) and an amplitude in the D ring of ∼1 kilometer. Imagery obtained later showed that this corrugation had a lower amplitude in the C ring. It may have been observed in the D ring as far back as 1995 using the occultation of the star GSC5249-01240, at which time it had a wavelength of ∼60 kilometers.76 12.2.9

The F ring

Orbiting 3,000 kilometers beyond Saturn’s A ring and thus also lying outside the B, C, and D rings, is “a dusty band of rubble”77 called the F ring. It was discovered by Pioneer 11 in 1979. Although diffuse and faint, Earth-based telescopes can readily observe it.78 In November 1980, Voyager 1 took close-up images revealing what looked like kinks, clumps, strands and even braids.79 High resolution images from Voyager 2 as well as Cassini revealed the F ring to consist of a bright narrow ring, the core, with numerous dimmer strands just tens of kilometers wide on either side (Figures 12.9 and 12.10). While the core appears to be long-lived, its peripheral strands vary markedly on time scales of hours to decades. To begin with, scientists proposed models suggesting that the strands were either concentric ring segments or a collection of clumps of material traveling close to the core. But the Voyagers had been able to provide high resolution imagery only for short sections of the F ring. Cassini took 360° movie sequences of the entire ring that exposed a very different nature to the strands surrounding the F ring core – they were part of a single spiral ring that winds and winds around the planet like a gargantuan coiled spring.80 Each time the nearby moon Prometheus orbits by the F ring, the moon distorts the ring’s core by tens of kilometers (Figure 12.10). This is an analogous phenomenon to the perturbations produced by Pan and Daphnis that were discussed above, but is complicated by the large variations in closest approach distance that result from the orbital eccentricities (non-circularities) of both the F ring and Prometheus. During each of Prometheus’ 14.7 hour orbits, it approaches and retreats from the F ring. Its varying distance results in the moon’s gravity repeatedly pulling material out of the ring core to form streamers and excavate channels in the ring material. The strength of these perturbations peaks roughly every 19 years owing to differential precession between the F ring and the orbit of Prometheus, which alters how close the satellite approaches the ring.81 Some collections of channels excavated in the ring produce striking fan-like structures (Figure 12.11) which

302 The ring system

Figure 12.9 This image of the F ring (on the left) shows how narrow it is relative to the A ring. Also evident is the F ring’s structure of bright core and dimmer strands on each side. The moon Atlas (30 kilometers, or 19 miles, across) is visible between the F and A rings.

suggest the existence of additional objects in the F ring. Gravitational perturbations by a moonlet or clump of material could have created these fans.82 Planetary scientists have theorized that a narrow band such as the F ring can be made and maintained by shepherd moons such as Prometheus and Pandora, which orbit inside and outside the F ring, respectively.83 The dynamics of this interaction are discussed below.

12.2.9.1

Shepherd moons: When gravity repels

Gravity is almost always considered a force of attraction, drawing objects together. But strange things happen in Saturn’s ring system, and one of them is that in certain locations, gravity essentially repels. As an example, the 30 to 500 kilometer-wide F ring84 should have spread out long ago due to collisions.85 However, the satellites Prometheus and Pandora shepherd its particles back to keep the ring bunched up and slender (Figure 12.12). To explain how this happens, the satellite beyond the F ring, Pandora, travels a little slower than the ring particles circling Saturn just inside it, since an object’s orbital speed

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Figure 12.10 Closeup of the F ring’s bright core and the fainter strands that surround it. The kinks in the core are caused by the gravitational field of nearby moon Prometheus, which distorts the F ring each time it travels by.

is dependent on the radius of its orbit. As the inside particles pass the satellite, its gravity tugs on them and raises a bulge in the ring. After the bulge has passed by, the satellite pulls back on the bulge, taking kinetic energy away from the particles, causing them to slow down and fall back into the ring. Thus the shepherd guides particles back into the ring, keeping it bunched up. Meanwhile, the satellite orbiting interior to the ring exerts forces in the opposite manner but with the same effect. As a result of their combined actions, the F-ring is maintained in its narrow state.86 Saturn’s rings are a constantly changing phenomenon. In particular, some of the features in the F ring are of recent origin. The two Voyager flybys observed year-to-year

304 The ring system

Figure 12.11 Fan-like arrays of channels in the F ring.

Figure 12.12 The F ring shepherd moons Prometheus (interior to the ring) and Pandora keep the ring bunched up and slender.

changes such as, for instance, the F ring breaking into strands that formed into braids, then unbraided. Such rapid evolution in the rings could offer insight into the processes that operated in the primordial disk of gas and aggregating particles that gave rise to our solar system. In particular, ring dynamics might show how particles and objects in the

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preplanetary disk interacted. Studying Saturn’s complex rings is thus a vital objective for the Cassini Orbiter, which has huge advantages over the Voyager flybys. Besides using far more powerful and sensitive instruments, the Cassini craft is spending years in the Saturn system and thus can observe and study ring processes over much longer periods of time.87 Cassini gathered direct evidence of moonlets embedded in or close to the F ring’s core. The detection by the Ultraviolet Imaging Spectrograph (UVIS) and Visual and Infrared Mapping Spectrometer (VIMS) of thirteen opaque or nearly opaque objects during stellar occultations suggested the existence of objects with diameters ranging from 27 meters to 9 kilometers that lay within 10 kilometers of the core. At least one of these objects was sufficiently opaque to be classified as a moonlet, rather than a temporary clump of material. Most of the F ring’s morphology, or form, likely resulted from gravitational and collisional effects of small satellites often combined with perturbations originating from Prometheus. This moon plays several roles. In addition to shepherding the ring, its direct perturbations, discussed earlier, produce streamers and channel structures. But the same perturbations also excite embedded objects such as moonlets that then exert their own gravitational effects on surrounding material. The F-ring may be the only place in the solar system where significant collisional processes occur almost daily, with dramatic effects.88 # Outside the D, C, B, A, and F rings are three more rings of a very different character. In contrast to the super-flat main rings, the G and E rings and the Phoebe ring, the farthest from Saturn, are dispersed over large vertical distances. They are also many times more tenuous and transparent than the main rings. 12.2.10

The G ring

Discovered by Voyager 1 in 1980, the G ring is extremely diffuse and is most likely made of macroscopic particles – those that you can see without a microscope. As a result, it might be hazardous for spacecraft to fly through it. The Cassini mission shed light on a mystery regarding the very existence of the G ring. The dust sized particles mostly responsible for the G ring’s optical properties should rapidly erode away into Saturn’s magnetosphere. This means that the ring should disappear unless some source continually replenishes it with new particles. Yet, until Cassini, there was no direct evidence for such a source. The E ring has a close association with Enceladus, a satellite that directly supplies material to it, and the F ring has two satellites, Prometheus and Pandora, which confine particles into a tightly packed, narrow ring instead of letting them disperse. But what does the G ring have to maintain its existence? The G ring is located over 15,000 kilometers from what, before Cassini, was the nearest known satellite. The remote sensing and in situ instruments on the Orbiter, however, identified a structure that is probably the replenishment source of the G ring. On the ring’s inner edge is a bright arc – a partial ring – that extends 150,000 kilometers (90,000 miles), or one-sixth of the way around the ring’s circumference and contains centimeter to metersized icy particles. These relatively large particles are confined within the arc by

306 The ring system

Figure 12.13 Embedded in the G ring’s bright arc is at least one moonlet, seen here as a streak of light, that is a probable source for new particles replenishing the ring. The vertical streaks in the background are distant stars smeared by the long, 46 second exposure time of Cassini’s narrow-angle camera. But the highlighted moonlet streak is aligned with the G ring and moves along it, as expected for an object embedded in the ring.

gravitational effects arising from a resonance with the satellite Mimas. Scientists believe that micrometeoroids collide with these particles, generating dust grains that reflect sunlight and brighten the arc. Cassini’s cameras identified 1 to 10 micrometer (40 to 400 millionths of an inch) dust grains in this region. Plasma in Saturn’s magnetic field continuously sweeps through the arc and, it is believed, drags out fine dust grains that feed the G ring.89 On 15 August 2008, Cassini’s instruments detected a considerably larger body in the arc that probably contributes considerable dust to it as well as to the rest of the G ring. Scientists verified its presence by finding evidence of it in two earlier images. This moonlet is too small for an image to be resolved by Cassini’s cameras, so its size was not measured directly. It appeared instead as a small, moving streak of light within the ring arc (Figure 12.13). Its size has been estimated at half a kilometer (a third of mile) wide by comparing the brightness of the streak to that of another small Saturnian moon, Pallene. Carl Murray, a Cassini imaging team member from Queen Mary University of London has pointed out that this moonlet’s discovery, and the influence on its path by neighboring Mimas, are indicative of the close relationships and interplay between satellites and rings throughout the Saturn system.90 12.2.11

The E ring

The E ring was discovered telescopically in 1967 and its presence was confirmed by the Pioneer 11 flyby in 1979. It is a large donut-shaped structure composed of very fine material that starts at 3 Saturn radii and extends essentially all the way to Titan’s orbit at 20 Saturn radii.91 Its individual particles are typically only one micron (a millionth of a meter)

12.2

Characteristics of the ring system

307

Figure 12.14 A band of bright, icy material reaches tens of thousands of kilometers outward from Enceladus (the bright spot at the apex of the band) into the E ring.

across and have a distinct blue color. Five of the seven largest Saturnian moons are embedded within the E ring: Mimas, Enceladus, Tethys, Dione, and Rhea. The icy particles of the E ring are continually dispersing to the surrounding space. Andy Ingersoll of Caltech says that, over time, E ring particles are pushed outward by pressure exerted by radiation or by collisions between particles.92 For the ring to endure, there must be a replenishment source, and this would appear to be the moon Enceladus (Figure 12.14).

12.2.11.1

E ring particle properties and the characteristics of Enceladus

The Cassini Orbiter’s Cosmic Dust Analyzer (CDA) measured properties of nearly 300 impacting dust particles when the spacecraft crossed the E ring in October 2004. The particles were predominantly water ice with minor contributions from silicates, carbon dioxide, ammonia, molecular nitrogen, hydrocarbons, and possibly carbon monoxide. These chemicals, particularly ammonia and silicates, offered clues as to how the grains were formed, as well as to the composition of Enceladus, where it is believed the particles originated. The E ring particles could have been ejected from Enceladus by way of collision processes or by geysers spewing subsurface material into space. Processes within the moon might be melting some of the ice, then shooting plumes of it into space in the form of water vapor or frozen particles. Such geyser activity is more probable with ammonia-rich materials, rather than with pure water ice, as ammonia lowers ice’s melting point as anti-freeze does in a car’s cooling system. A lower melting point increases the chances that liquids exist within Enceladus’ interior. The problem is, E ring particles tend to be ammonia-poor. This would certainly be the case if the icy particles were generated by impacts with the surface

308 The ring system of the moon, which appears to be pure water ice. Alternatively, Enceladus’ ejected particles could have originally contained ammonia and lost it through mechanisms such as plasma sputtering – ions hitting the particles and causing some of their components to be removed – within the E ring. So the E ring’s particles could have originated in a subsurface body of ammonia-bearing water and been ejected by means of geysers.93 Evidence supporting the existence of a subsurface Enceladean sea was obtained through the detection of salts in E ring particles. Approximately 93% of the E ring spectra in one data set obtained between October 2004 and December 2005 showed a 0.5 to 2% by mass sodium salt presence in the particles. This suggested that those particles did indeed originate within an ocean that was in contact with a rocky part of the moon’s interior, where sodium would typically reside. The observed sodium salts included sodium chloride (table salt, NaCl, which is the main salt in Earth’s oceans), sodium bicarbonate (baking soda, NaHCO3, and present in small quantities in Earth’s seas), and/or sodium carbonate (washing soda, Na2CO3).94 But not everyone agrees that the icy particles ejected from Enceladus originate in a salty subsurface ocean. A group led by Nicholas Schneider of the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder, claims the material in the geysers does not have enough sodium in it to come from an ocean. A large amount of sodium would make it glow in the same yellow light that is emitted by sodium street lamps. But observations by several terrestrial telescopes appear to indicate that few if any sodium atoms existed in the plumes. One explanation for the contradictory results is that deep caverns may exist where water evaporates slowly. If the evaporation process is slow, the vapor would contain little sodium, just like water evaporating from our ocean. Only when the evaporation is more explosive would it contain more salt. The salts might not be contained in the water vapor but in solid particles, and the amount of salt in the geysers might depend on whether those solid particles were carried along in the geysers.95 12.2.12

The new supersize Phoebe ring

One constant thing about the state of our knowledge of the Saturn system is that it is always changing and expanding. In October 2009, NASA announced the discovery of yet another Saturn ring – this one by far the largest yet seen. It is approximately 100 times bigger than Saturn’s entire main ring system. It was initially spotted not by the Cassini spacecraft but by NASA’s Sun-orbiting Spitzer Space Telescope and it lies at the far boundary of the Saturnian system. Its orbit does not rest in the plane of the other rings, but is tilted at 27° and most of its material is not even encountered until six million kilometers (3.7 million miles) away from the planet. The bulk of it extends outward into space another 12 million kilometers (7.4 million miles). One billion Earths would be needed to fill the space within this enormous torus. This newest-discovered halo of Saturn is not only wide but thick as well, with a vertical height about 20 times the diameter of the planet. This matches the range of vertical motion of one of Saturn’s farthest moons, Phoebe, whose orbit lies within the ring. Phoebe is a probable source of most if not all of the ring’s particles, which were likely knocked loose from the satellite’s surface by impacts over many years from interplanetary meteoroids and particles circling Saturn.96

12.2

Characteristics of the ring system

309

The Phoebe ring consists of a highly tenuous collection of ice and dust particles that would be difficult to see with visible-light telescopes. In fact, the estimated density of this gossamer structure is only about 20 particles per cubic kilometer.97 According to Anne Verbiscer, an astronomer at the University of Virginia, “The particles are so far apart that if you were to stand in the ring, you wouldn’t even know it.” Indeed, Cassini passed through it on its way into the Saturnian system. It was Spitzer’s infrared sensors that spotted the glow of this cool dust. Hot and cold objects alike all shine with infrared (thermal) radiation – even a bowl of ice cream “is blazing with infrared light.”98 The discovery of the Phoebe ring seems to solve a mystery regarding another of Saturn’s moons. Iapetus has a bizarre appearance, with its trailing hemisphere high-albedo bright and its leading hemisphere very dark. The fact that the dark material is on the leading surface – the side of Iapetus that faces forward in its orbit – implies it is “sweeping up the dark material, plowing into it as it moves … around Saturn.”99 The astronomer Giovanni Cassini found this moon in 1671, years later realizing that it had a dark side, now called Cassini Regio. (This intriguing satellite is discussed in greater detail in Chapter 13.) Analysis of the Phoebe ring could explain how Cassini Regio was produced. Models suggest that ring particles smaller than centimeters in size slowly migrate inward. If this is so, then many of them would ultimately strike the leading face of Iapetus and be responsible for its shading.100 Phoebe ring particles orbit in a retrograde direction, as indeed does Phoebe. This is opposite to the predominant rotation of the Saturn system.101 If Phoebe originated elsewhere and was captured, as reasoned on the basis of Cassini’s observations, this would make sense. Iapetus, Saturn, and most of its other moons, as well as its other rings, orbit prograde. As the particles in the Phoebe ring and Iapetus are orbiting in opposite directions, their relative velocity is greater than if both orbited in the same direction. Iapetus’ leading face might thus be expected to sweep up the ring particles more efficiently. Furthermore, observations by the Cassini spacecraft have revealed spectral similarities between Phoebe and the dark material on Iapetus, suggesting a common origin.102 The dark-colored material drifting inward from Phoebe might be coating the leading face of Iapetus like billions of “bugs on a windshield,”103 giving that hemisphere its characteristic color. The relative locations of Phoebe, Iapetus, the Phoebe ring, and the remainder of the Saturn system are given in Figure 12.15. 12.2.13

Are there indications of other rings?

When Cassini scientists examined images of Saturn backlit by the Sun, they noticed evidence of dusty rings associated with various small moons, such as Pallene. They thought that these rings arose due to sputtering processes. Micrometeoroids striking the surfaces of these moons, and perhaps even electron bombardment, could result in dust being released, and this dust would tend to form a ring. It may actually be true that wherever there is a little moon, a ring will be created.104 12.2.14

Satellites sculpting and filling Saturn’s rings

To understand the Saturnian system, the effects of one part on another require to be studied. In particular, ring characteristics are heavily influenced by the motions and natures of other parts of the system. As discussed above, some moons and moonlets act

310

The ring system

Figure 12.15 Locations of Phoebe, Iapetus, Titan, and the Phoebe ring, as well as the relative size of Saturn and its other rings versus the Phoebe ring.

as patient sculptors, using their gravity fields to chisel out gaps in the rings and shape their boundaries. Moons can also add to, rather than carve away at the rings, supplying particles to them like Enceladus does to the E ring and Phoebe does to the Phoebe ring.

12.2.14.1

Mimas resonances and the Cassini Division

In between the A ring and the next one in toward Saturn, the B ring, lies the largest opening in the system – the Cassini Division, 4,800 kilometers (2,980 miles) wide, almost exactly the distance to drive from Los Angeles to Boston. The moon Mimas, which orbits Saturn every 22 hours 37 minutes,105 resonates in an interesting way with material in the Cassini Division. Mimas travels at just the right distance from the planet to complete one orbit for every two circuits made by objects at the inner edge of the Cassini Division. Those objects are thus said to have a 2:1 resonance with Mimas. The gravitational tug that Mimas gives those particles and chunks of rock when they pass closest to the moon is always in the same direction in space. In the same way as repeated pushes of a child’s swing at just the right time in its cycle propels the swing higher and higher, the repeated gravitational tugs on the orbiting objects eventually wrench most of them out of the Cassini Division, onto different paths. Consequently, the division is kept generally free of large objects, although it does contain some diaphanous ringlets of fine dust.106 Mimas affects other features in Saturn’s rings as well. The boundary between the B and C ring is a 3:1 resonance with Mimas, and the outer F ring shepherd satellite, Pandora, has a 3:2 resonance. Mimas is also in resonance with Dione and Enceladus in a manner that

12.2

Characteristics of the ring system

311

helps maintain them locked in their positions. Furthermore, Mimas strongly perturbs the tiny 3 kilometer (2 mile) diameter moonlet called Methone, the 4 kilometer (3 mile) moonlet Pallene, and the 2 kilometer (1 mile) moon Anthe, all of which orbit between Mimas and Enceladus, which is the next major satellite out from the planet. The greatly more massive Mimas causes Methone’s orbit to vary by as much as 20 kilometers (12 miles), and even more for tiny Anthe, but slightly less for Pallene.107

12.2.14.2

Other ring-moon interactions

There are other gaps and ringlets around Saturn in addition to those already noted. Tables 12.1 and 12.2 give some of the characteristics of the ring system’s myriad features. Table 12.1 presents data on rings, gaps, and divisions, while Table 12.2 calls

Table 12.2. Ringlets and narrow, faint rings or ring arcs associated with small moons. Feature

Ringlets: D Ring ringlets Titan Ringlet Maxwell Ringlet 1.470 Rs Ringlet 1.495 Rs Ringlet Huygens Ringlet Encke Gap Narrow, Faint Rings or Arcs Associated with Small Moons: Janus/Epimetheus Ring Methone Ring Anthe Ring Pallene Ring

Sources and comments

1. D ring contains narrow ringlets at 67,580 and 71,710 km. 1. A narrow, eccentric ringlet inside a gap in the C Ring. 1. A narrow, eccentric ringlet inside a gap in the C Ring. 1. A narrow, eccentric ringlet inside a gap in the C Ring. 1. A narrow, eccentric ringlet inside a gap in the C Ring. 1. A narrow, eccentric ringlet near the inner edge of the Cassini Division. 1. A gap in the A Ring maintained by the embedded moon Pan. Several faint ringlets are also present.

2. A narrow, very faint ring or arc in the region occupied by Janus and Epimetheus. 3. A narrow, very faint ring arc in the region occupied by Methone. 4. A narrow, very faint ring arc in the orbit of Anthe. 2. A narrow, very faint ring or arc in the region occupied by Pallene.

Note: SETI, “Vital Statistics for Saturn’s Rings and Inner Satellites” http://pds-rings.seti.org/saturn/saturn_tables.html Sources 1. Murray, C.D. and S.F. Dermott, “Solar System Dynamics”, Cambridge University Press, 1999. 2. Porco, C.C., and the Cassini Imaging Team, 2006. Rings of Saturn (R/2006 S 1, R/2006 S 2, R/2006 S 3, R/2006 S 4). IAU Circ. 8759 3. Roussos, E., Jones, G.H., Krupp, N., Paranicas, C., Mitchell, D.G., Krimigis, S.M., Woch, J., Lagg, A., Khurana, K., 2008. Energetic electron signatures of Saturn’s smaller moons: Evidence of an arc of material at Methone. Icarus 193, 455–464. 4. Porco, C.C., on behalf of the Cassini Imaging Team, 2008. R/2006 S 5 and R/2007 S 1. IAU Circ. 8970.

312 The ring system attention to various ringlets. Note that gaps are narrow phenomena within the ring system whereas divisions are considerably wider. For instance, the Columbo Gap is a mere 100 kilometers (60 miles) wide while the Cassini Division is 4,800 kilometers (2,980 miles) in width. Also found in the ring system are narrow, quite faint ring arcs and complete rings associated with various small satellites. Most of the planet’s small, inner moons may indeed orbit within ring arcs or complete rings. Cassini images reveal, for instance, ring arcs extending ahead of and behind Anthe and Methone in their orbits. Both of these satellites are in orbital resonances with the nearby larger moon Mimas and are perturbed by its gravity. This satellite provides varying gravitational tugs on Anthe and Methone, causing them to skip forward and backward within arc-shaped regions along their orbit. According to Nick Cooper of Queen Mary University of London and a member of Cassini’s imaging team, “When we realized that the Anthe and Methone ring arcs were very similar in appearance to the region in which the moons swing back and forth in their orbits due to their resonance with Mimas, we knew we had a possible cause-andeffect relationship.”108 Previous Cassini images revealed that faint rings are associated with other small moons such as Pan, Janus, Epimetheus, and Pallene (Table 12.2). The spacecraft also observed the previously discussed arc in the G ring, one of the fainter major rings. 12.2.15 12.2.15.1

The influence of Enceladus Creator of the E ring

As will be discussed in Chapter 13, the Cassini mission found active vents on the south pole of Enceladus which eject tiny particles of water ice. The location of the E ring’s highest density and smallest vertical extent is close to the orbit of this moon, which strongly suggests it is the principal source of the water ice within the ring. Motions of the moons and the magnetic field of Saturn probably spread the material out to form the broad donut shape of the E ring.109

12.2.15.2

Connection with the A ring

Enceladus orbits 100,000 kilometers (60,000 miles) outside of Saturn’s A ring. Scientists had thought the two bodies to be distinct entities, but Cassini revealed that Enceladus sends “a portion of its mass directly to the outer edge of the A-ring.”110 Some of the geyser material from the south pole of Enceladus becomes ionized particles which can travel, like little railroad cars, along magnetic field lines running near the moon. These mass-loaded field lines run both outward and inward from the moon, forming a donut-shaped collection of particles called a plasma torus. Cassini data demonstrated that some of this mass is unloaded at the inner edge of this plasma torus, which is coincident with the plasmaabsorbing A-ring.111

12.3 12.2.16

Saturn’s equinox 313

The ring system as an enormous Saturnian seismograph

While many patterns in Saturn’s rings result from the tug of moons, some patterns are triggered by Saturn itself. The planet naturally vibrates like a bell, with periods of several hours. The variation in gravity caused by these oscillations pulls on ring particles and instigates ripples. The rings thus behave like a seismograph, giving us information about Saturn’s shakes and tremors. Just as earthquakes and solar oscillations give us information about the interiors of the Earth and Sun, Saturn’s pulsations provide a new way to probe its internal activity and structure.112

12.3

SATURN’S EQUINOX: VIEWING RINGS EDGE-ON

“This famous adornment, impressed deep in the human mind for four centuries as a pure, two-dimensional form, has now, as if by trickery, sprung into the third dimension.” – Carolyn Porco, 21 September 2009113 On 11 August 2009, the rays from the Sun hit the rings of Saturn exactly edge-on, “performing a celestial magic trick that made them all but disappear.”114 This event occurs during the two equinoxes of Saturn’s year, which lasts for 29.5 Earth years. During an equinox, the Sun hangs directly above the planet’s equator, which in the case of Saturn is also in its ring plane. Figure 12.16 dramatically illustrates how thin Saturn’s rings are in comparison to their diameters. The A ring spans 274,000 kilometers (170,000 miles), but scientists estimate that it and the other main rings are on average just 3 to 10 meters (10 to 30 feet) thick,115 “no taller than two stories of a modern-day building”116 and possibly a lot less. But the exceptions to this are quite dramatic. Cassini’s equinox observations revealed “many places of vertical relief above and below the otherwise paper-thin rings,”117 such as bumps as lofty as the Rocky Mountains where particles were piled up in dramatic vertical formations in each of the rings. Corrugations rippled through several rings, and one ridge of icy particles that was generated by the gravitational pull of Daphnis, a satellite that travels through the plane of the rings, rose as high as 4 kilometers (2.5 miles). Reasons for these ultra-thin rings include Saturn’s dramatic oblateness – a sort of planetary midriff bulge. Scientists believe that the gravitational forces arising from this oblate region tend to pull back any ring particles that get too far out of the plane. In addition, the main rings are what is known as a dynamically cold system, which means that collisions occur very slowly, with relative velocities of millimeters per second. If it was a more dynamic system – for instance, like the cloud from which the solar system formed, where collisions occurred at relative velocities of perhaps centimeters or meters per second – then the rings would probably be considerably thicker. Instead, the very slow velocities of collisions helps to keep the particles pretty much in the ring plane.118

314

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Figure 12.16 Saturn’s main rings, so large in diameter, are thin enough to nearly disappear when viewed edge-on.

References 315 REFERENCES 1. Jia-Rui C. Cook and Rachel Prucey, “Cassini Shows Saturnian Roller Derby, Strange Weather,” JPL press release 2010–090 (18 Mar. 2010). 2. NASA Goddard Space Flight Center (GSFC), “Saturn: History,” http://huygensgcms.gsfc.nasa. gov/Shistory.htm, GSFC Web site, accessed 6 June 2011. 3. Christiaan Huygens, De Saturni Luna Observatio Nova (New Observation of a Moon of Saturn), (The Hague: 1656). The treatise was also published in Pierre Borel, De Vero Telescopii Inventore, Cum Brevi Omnium Conspiciliorum Historia-Observationum Microcospicarum Centuria (The Hague: Adriaan Vlacq, 1655–1656). 4. The anagram was, “a a a a a a a c c c c c d e e e e e h i i i i i i i l l l l m m n n n n n n n n n o o o o p p q r r s t t t t t u u u u u,” as reported in Ronald Brashear , “Christiaan Huygens: Systema Saturnium (1659),” http://www.sil.si.edu/DigitalCollections/HST/Huygens/huygens-introduction.htm, Special Collections Department, Smithsonian Institution Libraries (May 1999). 5. Ronald Brashear , “Christiaan Huygens: Systema Saturnium (1659),” http://www.sil.si.edu/ DigitalCollections/HST/Huygens/huygens-introduction.htm, Special Collections Department, Smithsonian Institution Libraries (May 1999); Planetary Society, “The Alphabet Soup of Saturn’s Rings,” http://www.planetary.org/explore/topics/saturn/rings.html (24 May 2005); “The New Science,” http://www.library.usyd.edu.au/libraries/rare/modernity/accademia.html, Rare Books and Special Collection Library, University of Sydney Web site, accessed 6 June 2011. 6. Brashear , “Christiaan Huygens.” 7. NASA, “Jeffrey Cuzzi,” http://solarsystem.nasa.gov/people/profile.cfm?Code=CuzziJ, last updated 12 Apr. 2011. 8. Ruth Dasso Marlaire, “NASA Ames Scientist Jeff Cuzzi Wins the Kuiper Prize,” http://www. nasa.gov/centers/ames/news/releases/2010/10-43AR.txt, Ames Research Center press release 10-43AR (27 May 2010). 9. NASA, “Jeffrey Cuzzi,” http://solarsystem.nasa.gov/people/profile.cfm?Code=CuzziJ, last updated 12 Apr. 2011. 10. Linda Spilker telephone interview with author, 18 February 2009. 11. Ibid. 12. Ibid. 13. Ibid. 14. Ibid. 15. Jeremy Hsu, “Saturn’s Rings Still Puzzle Scientists,” http://www.msnbc.msn.com/id/32542771, MSNBC Web site (24 Aug. 2009). 16. Anne J. Verbiscer et al., “Saturn’s Largest Ring,” Nature 461 (22 October 2009):1098–1100. 17. Matthew S. Tiscareno et al., “100-Metre-Diameter Moonlets in Saturn’s A Ring from Observations of ‘Propeller’ Structures,” Nature 440 (30 March 2006): 648–650; Planetary Society, “The Alphabet Soup of Saturn’s Rings.” 18. Jia-Rui C. Cook and Rachel Prucey, “Cassini Shows Saturnian Roller Derby, Strange Weather,” JPL press release 2010–090 (18 Mar. 2010). 19. Frank Spahn and Hanno Sponholz, “Existence of Moonlets in Saturn’s Rings Inferred from the Optical Depth Profile,” Nature 339 (22 June 1989):607. 20. Planetary Society, “The Alphabet Soup of Saturn’s Rings.” 21. Richard A. Kerr, “Saturn: The Unfinished Symphony,” Science 304 (28 May 2004):1231. 22. Kerr, “Saturn,” p. 1231. 23. NASA-JPL, “Saturn’s Magnificent Rings,” http://www.solarviews.com/eng/saturnrings.htmz (4 May 1990), accessed 23 Sep. 2009.

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tional conference on Modeling and Simulation, Montreal, Canada (2006):268–273; Planetary Society, “The Alphabet Soup of Saturn’s Rings.” M. Horanyi et al., “Diffuse Rings,” in chapter 16 of Michele K. Dougherty et al. (eds.), Saturn from Cassini-Huygens (Springer, 2009), p. 518; NASA/JPL/Space Science Institute, “PIA11502: Encke Ringlets,” http://pds-rings.seti.org/saturn/cassini/PIA11502.html, Planetary Photojournal image PIA11502; Jack J. Lissauer et al., “Moonlets in Saturn’s Rings,” Nature 292 (20 August 1981):707–711. William D. Ross, “Ringlets of the Major Planets Which May Consist of Coagulated Particles,” Earth, Moon, and Planets 42(3) (Netherlands: Springer, September, 1988):209–220. Occultation refers to the disappearance or obscuring of one celestial body behind another. Matthew M. Hedman et al., “Self-Gravity Wake Structures in Saturn’s A Ring Revealed by Cassini VIMS,” Astronomical Journal 133 (2007):2624–2629. Matthew S. Tiscareno et al., “An Analytic Parameterization of Self-Gravity Wakes,” Bulletin of the American Astronomical Society 40(21.06) (2008). Spilker interview, 14 Dec. 2009. Linda J. Spilker email to author, 25 Oct. 2010. Linda Spilker interview with author, JPL, 27 October 2010. Eugene Chiang et al., “Forming Planetesimals in Solar and Extrasolar Nebulae,” Annual Review of Earth and Planetary Sciences 38 (Apr. 2010). J. N. Cuzzi et al., “An Evolving View of Saturn’s Dynamic Rings,” Science 327 (19 March 2010):1470–1475. Spilker interview, 14 Dec. 2009. Matthew S. Tiscareno et al., “The Population of Propellers in Saturn’s A Ring,” Astronomical Journal 135 (14 Feb. 2008):1083–1091; NASA-JPL, “Propeller Swarm,” http://saturn.jpl.nasa. gov/photos/imagedetails/index.cfm?imageId=3596, Cassini Equinox Mission Web site (17 July 2009). Ker Than, “Cassini Data Aiding Theories on Origin of Saturn’s Rings,” Space News (17 April 2006), accessed 13 Sept. 2009. Than. Denise Chow, “Giant Propellers Discovered in Saturn’s Rings,” http://www.space.com/scienceastronomy/giant-propellers-saturn-rings-moons-100708.html (08 July 2010). Linda Spilker interview with author, 27 Oct. 2010. Ibid. Matthew S. Tiscareno et al., Physical Characteristics and Non-Keplerian Orbital Motion of ‘Propeller’ Moons Embedded in Saturn’s Rings,” Astrophysical Journal Letters 718(2) (1 Aug. 2010). Jia-Rui C. Cook, “Saturn Propellers Reflect Solar System Origins,” http://www.jpl.nasa.gov/ news/news.cfm?release=2010-227&cid=release_2010-227&msource=2010227&tr=y&a uid=6600177, NASA-JPL (8 July 2010). Phil Nicholson, “Through a Glass Darkly: Saturn’s Enigmatic B Ring,” http://www.cita.utoronto. ca/index.php/Events-Calendar/2007/Through-a-glass-darkly-Saturn-s-enigmatic-B-ring, Canadian Institute for Theoretical Astrophysics seminar (3 Dec. 2007), accessed 22 Sep. 2009; J.E. Colwell et al., “The Structure of Saturn’s Rings,” chapter 13 in Michele K. Dougherty et al. (eds.), Saturn from Cassini-Huygens, (Springer, 2009), p. 387; P. D. Nicholson et al., “Saturn’s Rings I, Optical Depth Profiles from the 28 Sgr Occultation,” Icarus 145 (2000):473–500, as reported in Colwell et al; NASA, “Saturn Occultation of 28 Sagittarius 1989,” http://starbrite.jpl. nasa.gov/pds/viewMissionProfile.jsp?MISSION_NAME=SATURN%20OCCULTATION%20 OF%2028%20SAGITTARIUS%201989, NASA Planetary Data System (1 Dec. 2009); J.E. Colwell et al., “Self-Gravity Wakes and Radial Structure of Saturn’s B Ring,” Icarus 190(1)

318

67. 68. 69. 70. 71. 72. 73. 74.

75. 76. 77. 78.

79. 80.

81. 82. 83. 84. 85. 86. 87. 88. 89.

The ring system (September 2007):127–144; NASA/JPL/Space Science Institute, “Ring Scan Spilling Secrets,” http://ciclops.org/view.php?id=5768 (21 Sept. 2009). Planetary Society, “The Alphabet Soup of Saturn’s Rings.” Planetary Society, “The Alphabet Soup of Saturn’s Rings.” Tariq Malik, “Saturn’s Ring Spokes Depend on Sun Angle, Study Says” http://www.space.com/ scienceastronomy/060316_cassini_spokes.html (16 March 2006). C.J. Mitchell et al., “Saturn’s Spokes: Lost and Found,” Science 311 (17 March 2006):1587–1589. Bradford A. Smith et al., “A New Look at the Saturn System: The Voyager 2 Images,” Science 215 (29 January 1982):504–537. Mitchell, “Saturn’s Spokes.” Planetary Society, “The Alphabet Soup of Saturn’s Rings.” M. M. Hedman et al., “Saturn’s Curiously Corrugated C Ring,” Sciencexpress (31 March 2011):1; Jia-Rui C. Cook and Michael Buckley, “Forensic Sleuthing Ties Ring Ripples to Impacts,” http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20110331.html, NASA news release 2011–102 (31 March 2011); Josh Colwell, “The Amazing Wound-Up C Ring,” http://www.planetary.org/blog/article/00002144/, Planetary Society Blog (7 Oct. 2009).. Matthew M. Hedman , “Saturn’s Dynamic D Ring,” Icarus 188 (2007): 89–107. Hedman , “Saturn’s Dynamic D Ring.” J.N. Cuzzi et al., “An Evolving View of Saturn’s Dynamic Rings,” Science 327 (19 March 2010):1470–1475. Windows to the Universe team, “Ring Structure of Saturn,” http://www.windows.ucar.edu/tour/ link=/saturn/ring_structure.html&edu=elem, University Corporation for Atmospheric Research (UCAR), Univ. of Michigan (7 Sept. 2002), accessed 22 Sept. 2009. Mark R. Showalter, “Saturn’s Strangest Ring Becomes Curiouser and Curiouser,” Science 310 (25 Nov. 2005): 1287–1288. S. Charnoz et al., “Cassini Discovers a Kinematic Spiral Ring Around Saturn,” Science 310 (25 Nov. 2005):1300; Mark R. Showalter, “Disentangling Saturn’s F Ring. I. Clump Orbits and Lifetimes,” Icarus 171 (October 2004):356–371; Preston Dyches , caption to “Brilliant F Ring,” http://www.ciclops.org/view.php?id=1402, NASA/JPL/Space Science Institute (6 Oct. 2005). Cuzzi, “An Evolving View of Saturn’s Dynamic Rings.” Carl Murray and Joe Mason, “‘Fan’ in the F Ring,” http://www.ciclops.org/view/6367/Fan_in_ the_F_Ring, NASA/JPL/Space Science Institute (20 July 2010). Astrobiology Magazine, “Shepherd’s Moon on Saturn,” http://euro.astrobio.net/pressrelease/934/shepherds-moon-on-saturn (21 Apr. 2004). Samantha Harvey, “Saturn: Rings,” http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sa turn&Display=Rings, NASA, (last updated 4 Mar 2010). Carl D. Murray et al., “The Determination of the Structure of Saturn’s F Ring by Nearby Moonlets,” Nature 453 (5 June 2008):739. Richard A. Kerr, “Saturn: The Unfinished Symphony,” Science 304 (28 May 2004):1231; Bob Mitchell review of manuscript, Feb. 2011. Kerr, “Saturn,” p. 1231. Carl D. Murray et al., “The Determination of the Structure of Saturn’s F Ring by Nearby Moonlets,” Nature 453 (5 June 2008):739–744. Matthew M. Hedman et al., “The Source of Saturn’s G Ring,” Science 317 (3 August 2007):653– 656; Todd J. Barber, “The Source of the G Ring,” http://saturn.jpl.nasa.gov/news/cassiniinsider/insider20070805/, NASA News & Features, Cassini Equinox Mission Web site (5 Aug. 2007); Whitney Clavin and Preston Dyches, “Cassini Finds Possible Origin of One of Saturn’s Rings,” http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20070802/, NASA news

References 319 release 2007–085 (2 Aug. 2007); Physorg.com, “Saturn has Small Moon Hidden in Ring,” http://www.physorg.com/news155318928.html (3 March 2009). 90. Matt Hedman and Joe Mason (caption of figure), “Tiny Moonlet Within G Ring Arc,” http:// ciclops.org/view.php?id=5493, NASA/JPL/Space Science Institute, NASA image PIA 11148 (3 March 3 2009); Carolina Martinez and Joe Mason, “Newfound Moon May Be Source of Outer Saturn Ring,” NASA news release 2009–035 (3 March 2009); Randy Showstack, “In Brief: Moonlet Discovered in Saturn’s G Ring,” Eos Trans. AGU 90(11) (17 March 2009):92. 91. Mihaly Horanyi et al., “The Dynamics of Saturn’s E Ring Particles,” Icarus 97 (June 1992):248– 259; Linda Spilker review of manuscript, March 2011. 92. Ron Cowen, “Saturn’s Moon May Host an Ocean,” http://www.sciencenews.org/view/generic/ id/44975/title/Saturn%E2%80%99s_moon_may_host_an_ocean, ScienceNews Web edition (24 June2009). 93. Jon K. Hillier et al., “The Composition of Saturn’s E Ring,” Mon. Not. R. Astron. Soc. 377 (2007):1588–1596. 94. F. Postberg et al., “Sodium Salts in E-Ring Ice Grains from an Ocean Below the Surface of Enceladus,” Nature 459 (25 June 2009):1098–1101; Linda J. Spilker email to author, 14 Oct. 2010. 95. Anne Minard, “Does Enceladus Harbor a Liquid Ocean? Reasonable Minds Disagree,” http:// www.universetoday.com/33334/does-enceladus-harbor-a-liquid-ocean-reasonable-mindsdisagree/, UnverseToday.com (24 June 2009); Bob Mitchell review of manuscript, Feb. 2011. 96. Anne J. Verbiscer et al., “Saturn’s Largest Ring,” Nature 461 (22 October 2009): 1098; Whitney Clavin and J.D. Harrington, “NASA Space Telescope Discovers Largest Ring Around Saturn,” NASA news release 2009–150 (6 Oct. 2009); Randy Russell, “The Phoebe Ring Around Saturn,” http://www.windows.ucar.edu/tour/link=/saturn/saturn_phoebe_ring.html, Windows to the Universe, University Corporation for Atmospheric Research (UCAR), University of Michigan (9 Oct. 2009). 97. Linda Spilker interview with author, JPL, 27 October 2010. 98. Both quotes in the paragraph are from Clavin and Harrington, “NASA Space Telescope Discovers Largest Ring Around Saturn.” 99. Emily Lakdawalla, “The Phoebe Ring,” http://www.planetary.org/blog/article/00002165/, Planetary Society Blog (14 Oct. 2009). 100. Verbiscer et al., “Saturn’s Largest Ring.” 101. Spilker interview, 14 Dec. 2009; Bob Mitchell review of manuscript, Feb. 2011; Linda Spilker review of manuscript, March 2011. 102. Verbiscer et al., “Saturn’s Largest Ring.” 103. Clavin and Harrington, “NASA Space Telescope Discovers Largest Ring Around Saturn.” 104. Linda Spilker interview with author, JPL, 27 October 2010. 105. Calvin J. Hamilton, “Mimas,” http://www.solarviews.com/eng/mimas.htm, accessed 22 Sept. 2009. 106. Planetary Society, “The Alphabet Soup of Saturn’s Rings?” Nature 292 (20 Aug. 1981):708. 107. NASA-JPL, “Mimas,” http://saturn.jpl.nasa.gov/science/moons/mimas/, Cassini Equinox Mission Web site, accessed 15 Sep. 2009; R.A. Jacobson et al., “The GM Values of Mimas and Tethys and the Libration of Methone,” Astronomical Journal 132 (2005): 711; R.A. Jacobson et al, “The Gravity Field of the Saturnian System from Satellite Observations and Spacecraft Tracking Data,” Astronomical Journal 132 (December 2006):2520–2526. 108. Carolina Martinez, Preston Dyches, and Julia Maddock, “Cassini Images Ring Arcs Among Saturn’s Moons,” http://www.jpl.nasa.gov/news/news.cfm?release=2008-172, NASA-JPL (5 Sep. 2008). 109. Frank Spahn et al., “Cassini Dust Measurements at Enceladus and Implications for the Origin of the E Ring,” Science 311 (10 March 2006):1416–1418.

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110. Bill Steigerwald, “Saturn Has a ‘Giant Sponge’,” NASA-JPL news (5 Feb. 2008). 111. W. M. Farrell et al., “Mass Unloading Along the Inner Edge of the Enceladus Plasma Torus,” Geophys. Res. Lett. 35 (23 Jan. 2008): L02203. 112. Linda Spilker email to author, 31 Dec. 2013. 113. Carolyn Porco, “Captain’s Log,” Cassini Imaging Central Laboratory for Operations, http:// ciclops.org/?js=1, 21 September 2009. 114. Jia-Rui C. Cook and Dwayne C. Brown, “Cassini Reveals New Ring Quirks, Shadows During Saturn Equinox,” NASA News release 2009–142 (21 Sept. 2009). 115. Linda Spilker interview with author, JPL, 27 October 2010. 116. Carolyn Porco comment in Cook and Brown, “Cassini Reveals New Ring Quirks.” 117. Linda Spilker comment in Cook and Brown, “Cassini Reveals New Ring Quirks.” 118. Linda Spilker interview with author, JPL, 27 October 2010.

13 The icy moons For Principal Investigator Michele Dougherty of the Magnetometer (MAG) team, the Cassini mission highlight occurred in February 2005. This was when she and her colleagues determined that Saturn’s magnetic field lines were unexpectedly bending as they went by the satellite Enceladus. This was possibly a sign that the tiny moon, even with its low gravity, had a localized, electrically conducting atmosphere. Not sure that her data was correct, Dougherty lobbied mission management for a closer Enceladus flyby than was planned, hoping to take some definitive measurements. John Spencer’s life became a lot more interesting when his instrument discovered that this small satellite did not fit the profile of a typical icy moon. In contrast to “well-behaved” Dione, Rhea, and Tethys, the warmest part of Enceladus was not at its equator, but at its south pole.1 As the Cassini spacecraft continued to make observations, it became apparent that this modestly sized satellite has a profound influence on Saturn’s magnetosphere and at least one of its enormous rings, and that its interior may contain an environment conducive to life. When daytime surface temperatures are plotted for Mimas, another icy moon in the same neighborhood, a map emerges that resembles Pac-Man about to eat a giant crater for lunch. Heat soaks into Mimas’ interior spectacularly faster in some regions than in others, but there are no tell-tale surface brightness features that give us clues as to why this is happening. Spencer’s thermal emission detector, the Composite Infrared Spectrometer (CIRS) also helped determine why another icy moon – Iapetus – looks so utterly peculiar, sort of like a cookie with one half covered in vanilla icing and the other in dark chocolate.2 The point of these examples is that Saturn’s icy moons behave in dramatically different ways and exhibit puzzling, often bizarre characteristics that Spencer and the other scientists on Cassini’s icy moons teams deciphered using some ingenious instruments and methodologies. #

© Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_13

321

322 The icy moons The term icy moons refers to all of Saturn’s satellites except Titan.3 The medium-sized icy moons that were studied in detail by the Orbiter have densities roughly that of water. The individual “personalities” of these icy moons and the particular ways that Cassini scientists have analyzed them are discussed below. These medium-sized icy moons include:4 • • • • • • • •

13.1

Iapetus, the chocolate-vanilla cookie moon Tethys, victim of a major collision at some time in its past Enceladus, a geologically dynamic moon that influences much of the Saturn system Hyperion, the smallest icy moon, with a radius of only 143 kilometers (89 miles), a strangely chaotic rotation, and hydrocarbon-filled craters Mimas, orbiting only 3 Saturn radii above the mother planet and dominated by a huge crater and a strange temperature profile Rhea, surrounded by strong, unexplained electromagnetic effects Dione, whose wispy, light-colored streaks have yet to be explained Phoebe, the wanderer from afar, orbiting at a distance of 215 Saturn radii.

THE SATELLITE ORBITER SCIENCE TEAM

The group of scientists and engineers who dealt with icy moon flyby planning was called the Satellite Orbiter Science Team (SOST). This team selected the particular moons to be visited, the orbits when this was to occur, the flyby altitudes, and the times that various research groups would be able to aim their instruments at a particular moon. Titan has ample ice in its composition, but was not included in SOST’s planning and is not referred to as one of the icy moons. Titan is much bigger than the other icy bodies and its phenomena put it in a class by itself. Flight planning issues for Titan were handled by the Titan Orbiter Science Team. Studies of shepherd satellites and other moonlets embedded in Saturn’s ring system were dealt with by another entity, the Rings Group.5 The dates and distances of the Cassini spacecraft’s flybys of the icy moons during the Prime Mission tour are given by Table 13.1, and during the Equinox Mission by Table 13.2. While all the icy moons have been visited, Enceladus’ unexpected and fascinating phenomena resulted in it getting additional attention. The spacecraft flew by it four times during the Prime Mission and paid many more visits to it throughout subsequent missions. As can be seen from Table 13.2, in fact, the icy moon flybys during the Equinox Mission were dominated by visits to Enceladus.

13.2

TWO-FACED IAPETUS

Iapetus, the highest-contrast body in the solar system, has a leading hemisphere that is as dark as asphalt and a trailing hemisphere that is bright, like Saturn’s other icy satellites (Figure 13.1). In addition, materials on the leading side have a substantially redder color than those on the trailing side. This two-faced aspect of Iapetus was not a recent discovery – Giovanni Cassini, who discovered the moon in 1671, deduced that one side had to be

13.2

Two-faced Iapetus

323

Table 13.1. Icy moon flybys during Cassini’s Prime Mission tour.6 Flyby

Distance (kilometers)

(miles)

Date

Phoebe Dione Iapetus Enceladus Enceladus Enceladus Mimas Tethys Hyperion Dione Rhea Tethys Rhea Iapetus Enceladus

2,068 81,400 122,647 1,179 500 175 46,912 1,500 1,010 1000 500 16,200 km 5,737 1,227 23

1,285 50,580 76,209 733 311 109 29,150 932 628 621 311 10,066 3,565 762 14

11 June 04 14 Dec. 04 31 Dec. 04 17 Feb. 05 9 Mar. 05 14 July 05 2 Aug. 05 24 Sep. 05 26 Sep. 05 11 Oct. 05 26 Nov. 05 27 June 07 30 Aug. 07 10 Sep. 07 12 Mar. 08

Table 13.2. Icy moon flybys during the Cassini Equinox Mission.7 Flyby

Distance (kilometers)

(miles)

Date

Enceladus Enceladus Enceladus Enceladus Enceladus Rhea Helene (see note) Dione Enceladus Enceladus Enceladus

25 197 103 1,606 100 1,800 500 103 438 2,554

16 122 64 998 62 1,131 311 64 272 1,587

11 Aug. 2008 9 Oct. 2008 31 Oct. 2008 2 Nov. 2009 21 Nov. 2009 2 Mar. 2010 3 Mar. 2010 7 Apr. 2010 28 Apr. 2010 18 May 2010 13 Aug. 2010

Note: Helene is not referred to as an icy moon, but rather as a Trojan moon because it is gravitationally tied to the much larger satellite Dione.

much darker than the other because the satellite appeared to vanish during part of each orbit around Saturn. He initially could only see the moon when it was on the western side of Saturn. Only years later, using a better telescope, was he finally able to observe it on the eastern side. He also correctly surmised that in order for Iapetus to constantly present the same face to Saturn, its rotation had to be tidally locked.8 A possible explanation for Iapetus’ dark side is that dust from the moon Phoebe was somehow transported inward through the Saturn system to deposit on one face of Iapetus. If this was indeed the darkening mechanism, then it would have had to steadily renew the dark color, as otherwise new impacts would have created fresh bright craters on the

324

The icy moons

Figure 13.1 These two global images show the extreme brightness dichotomy on Iapetus’ surface. The left-hand panel shows the moon’s leading hemisphere and the right-hand panel, the satellite’s trailing side. While low and mid latitudes of the leading side exhibit a surface almost as dark as charcoal, broad tracts of the trailing side are almost as bright as snow.

leading side, and very few have been detected. The puzzling thing, however, was that spectrometer measurements by the Cassini Orbiter linked the dark material on Phoebe more closely to the composition of the bright trailing side of Iapetus. An alternative theory was that icy volcanism might have spewed dark material such as hydrocarbons onto the surface. These could have darkened further after solar radiationinitiated chemical reactions. But again, the evidence was inconclusive. Other theories that have been considered by planetary scientists include: (1) Interplanetary micrometeoroid dust9 directly impacted Iapetus (2) The material came from a one-time, early major impact event on either Iapetus or a nearby moon (3) The dark material originated from a continuous inflow of reddish dust from outer Saturnian satellites created when they are struck by the interplanetary meteoroid flux, which includes both microscopic and macroscopic impacts. But if an interplanetary dust stream was responsible for darkening and reddening Iapetus’ surface as suggested in (1), then the other outlying moons, such as Phoebe, should have been similarly impacted and this was not the case. Phoebe, for instance, is not red. An early giant impact, as in (2), also appears to be an improbable causal mechanism. Images from the Cassini spacecraft reveal that Iapetus’ leading side is uniformly dark at low latitudes. A long-ago, one-time violent impact event would not explain why craters which formed much later should also have been darkened. Hypothesis (3) appears to have more merit. Solar-radiation pressure and Poynting-Robertson drag appear to be plausible mechanisms for moving dust particles from outer satellites inward towards Iapetus’ leading edge. Some of the potential sources of particles, however, orbit Saturn in a prograde10 direction. These are not likely candidates for producing Iapetus’ color dichotomy because the impacts of prograde dust on prograde Iapetus occur at relatively low velocities and in a fairly isotropic (invariant with respect to direction) manner. On the other hand, when

13.2

Two-faced Iapetus

325

dust particles orbiting retrograde meet Iapetus, they collide head-on at high velocity into Iapetus’ leading face. Saturn has a collection of retrograde satellites from which these dust particles may come. Furthermore, an enormous ring of particles located in the right place for this model to work has recently been identified. The Phoebe ring, which crosses the orbit of Iapetus, extends out far beyond that satellite to the fringes of interplanetary space, and scientists believe that this ring’s particles orbit in a retrograde direction. The Phoebe ring might indeed be splattering Iapetus with dark particles. This ring was described in more detail in Chapter 12.11 In December 2009, research results were published that supported and expanded upon another theory that explains Iapetus’ color dichotomy. This theory proposed a mechanism involving migrating ice triggered by reddish dust depositing on Iapetus’ leading surface. Scientists employed close-up images taken by Cassini’s Imaging Science Subsystem (ISS), thermal observations from the spacecraft’s Composite Infrared Spectrometer (CIRS), and the results of extensive modeling work to build up an understanding of the mechanism. Two researchers, John Spencer of Southwest Research Institute in Boulder, Colorado and Tilmann Denk of the Freie Universitat in Berlin, Germany proposed that infalling dust which was swept up by Iapetus did indeed darken the leading side of the satellite as suspected, but this was only part of the picture. CIRS observations taken in 2005 and 2007 found that because the dark material made the leading edge a better absorber of solar energy, it was able to reach temperatures sufficiently high to evaporate many meters of ice over billions of years. Iapetus’ very long rotation period, 79 days, contributed to these warm temperatures by allowing the Sun more time to warm the surface each day than on faster-rotating moons. The evaporating ice on Iapetus’ leading edge was not able to escape from the moon, however. Instead, it migrated and recondensed in the colder and brighter polar regions and on the trailing hemisphere. The loss of ice from the leading edge exposed additional dark material, inducing further warming and ice evaporation on the leading side. Simultaneously, the trailing edge and poles continued to brighten and chill due to ice condensation, until Iapetus ended up with the extreme contrasts in surface brightness that we see today. The low gravity of this relatively small moon, which is just 1,500 kilometers (900 miles) in diameter, would facilitate evaporated ice migrating from one hemisphere to the other.12 13.2.1

Orbital and rotational characteristics

Iapetus is relatively far from the mother planet. It orbits Saturn three times farther away than Titan does, although it is only one-fourth the distance of Phoebe. Despite this considerable distance, the planet has tidally locked the satellite so that it always presents the same face toward the planet. Iapetus also has an interesting relationship with the largest Saturnian moon, Titan. The two bodies speed up and slow down as they pass each other in a complex sort of dance. But since Iapetus is far smaller, its rotation and orbit are much more strongly affected by the interaction.13 Unlike most of the planet’s other moons, Iapetus has a significantly inclined orbit, tilted at about 15° relative to Saturn’s ring plane. This inclination takes the moon far above and

326

The icy moons

below the ring plane. Scientists speculate that Iapetus, like Phoebe, may have originated elsewhere and been captured when it flew nearby the Saturn system, rather than being formed from the primordial cloud of material that gave birth to the planet, most of its satellites, and at least some of its rings. The orbits of Iapetus and Phoebe around Saturn would therefore have been determined by the trajectories they had when they first approached the planet. This could explain why their orbits are so different from most other Saturnian moons.

13.2.1.1

Iapetus’ billions-of-years-old midriff bulge

In the words of JPL scientist Julie Castillo, “Iapetus spun fast, froze young, and left behind a body with lasting curves.”14 In its youth, Iapetus was still hot and pliable from its initial formation and rotated rapidly at an estimated rate of once every 5 to 16 hours. Centrifugal force – the outward force that a spinning body experiences – pushed the satellite’s equatorial regions outward and flattened its polar regions until it formed a distinct oblate spheroid shape (think of what a beachball would look like if you sat on it). Over time, the moon cooled and its outer region froze into a harder, less flexible crust. Its spin rate slowed due to gravitational interactions with Saturn that involved tides acting on the material of the moon, similar to the way that ocean tides on Earth occur. Eventually this slowed Iapetus’ rotation period to 79.33 days, the same as its orbital period, locking it into synchronous rotation. Cassini scientists developed a model of Iapetus’ history that took into account the details of its despinning and the preservation of its oblate shape. Key to developing the model was the identification of a source of heat that would have kept the satellite warm and pliable for just the right amount of time. Short-lived radioactive isotopes were the obvious choices for the heat sources. One likely candidate was the isotope aluminum-26, which was known to have been present in the early solar system and to have left its decay products in meteorites. Another possibility was iron-60.15 13.2.2

The equatorial ridge

Cassini’s images revealed the existence of a long chain of mountains on Iapetus that reached 20 kilometers (12 miles) in height.16 By way of comparison, Mount Everest on Earth stands not quite 6 miles tall. Iapetus’ dramatic mountain chain girdles most of its equator and gives the satellite the overall appearance of a walnut (Figure 13.2). One theory for this ridge’s formation is that it resulted from an upwelling caused by convection (heat-generated) currents inside the body. A variety of reasons for this convection have been advanced, including forces on the moon from its spin and orbit as well as from some sort of impact, all resulting in internal flows.17 Some scientists think the ridge was formed at a much earlier time, when Iapetus was rotating much more rapidly than it does today. Others say that the ridge was constructed of material left from the collapse of a ring that accreted onto the surface at the equator.18

13.3

Tethys 327

Figure 13.2 Iapetus’ equatorial ridge.

A theory supported by many planetary scientists states that as Iapetus’ rotation slowed down, the centrifugal forces that had originally forced it into an oblate shape decreased. But by then its surface material was cooling and becoming too rigid for the moon to flex smoothly back into a more spherical shape. At a certain point, the internal gravitational forces that no longer had a strong centrifugal force to balance them caused the oblate shape of the moon to collapse somewhat in its equatorial region. But the material falling inward forced excess material to pile up in a ridge at the equator. Still, it is curious why such a feature did not form on any other moon in the Saturnian system.19 While the equatorial ridge appears quite striking in imagery of Iapetus, it is rather small in comparison to the moon’s global equatorial bulge described in the previous section. The volume of the ridge is in fact less than 5% of the equatorial bulge.20

13.3

TETHYS: SIGNS OF A TUMULTUOUS PAST

Odysseus Crater (Figure 13.3), a huge gouge in Tethys’ surface, is a remnant of an impact so energetic that it could easily have shattered a satellite unable to flex and stretch under extreme stress. The fact that the impact did not do this suggests that the moon was still

328

The icy moons

Figure 13.3 Tethys’ Great Basin. Plunging cliffs and towering mountains characterize the gigantic impact structure, Odysseus Crater. Because many small craters are visible inside it, scientists believe that this is not a particularly young phenomenon.

partly molten and thereby pliable at the time. Nevertheless, whatever crashed into Tethys excavated a hole whose 450 kilometer (280 mile) diameter21 is nearly 40% that of the whole satellite. Today, Odysseus is fairly shallow with gently sloped boundaries, but other slopes on Tethys are considerably steeper and sharper. This is another clue that Tethys was a rather elastic body when the impact occurred, allowing Odysseus’s features to relax over subsequent years. Still more evidence for Tethys’ early pliability lies in the frequency of its craters. The surface of the satellite is scarred with such pockmarks, evidence that it has been thoroughly hammered by many impacts. But it is not as heavily cratered as its sister moons Dione and Rhea. A possible explanation for this is that Tethys’ proximity to Saturn resulted in more tidal warming than the other two moons experienced. Tidal warming is a phenomenon in which a planet’s gravitational field pulls and stretches a satellite, resulting in it warming up. The tidal warming of Tethys could have kept it partially molten for a longer period after its formation, erasing or dulling the features of its early cratering. Another dramatic feature of Tethys is the great rift called Ithaca Chasma (Figure 13.4) which runs diagonally for over 1,000 kilometers (600 miles) and spans a width of up to 100 kilometers (60 miles). Tidal heating is a plausible energy source for the formation of this long canyon.22 Tethys may once have been heated sufficiently to harbor internal oceans, and if so, then global surface cracking that likely occurred when the oceans froze may have created Ithaca Chasma.23 Tethys has a highly reflective surface, suggesting a composition that is largely water ice. Scientists think that the pervasive white color is from fresh ice particles continually falling on the moon from Saturn’s diffuse E-ring, particles that almost certainly originated inside the moon Enceladus (discussed below).

13.4

Enceladus 329

Figure 13.4 Powerful forces ripped apart the surface of Tethys at some time in its distant past, creating the Ithaca Chasma canyon system that slices diagonally across more than 1,000 kilometers (600 miles) of the moon’s icy surface. Many impact craters occur within Ithaca Chasma, indicating the rift’s great age.

The Cassini spacecraft made close flybys of Tethys on 24 September 2005 and on 27 June 2007, both during its Prime Mission tour. This moon was discovered in 1684 by Giovanni Cassini, who referred to it as one of the Sidera Lodoicea (Stars of Louis) after King Louis XIV. Its name comes from the Greek goddess Tethys, who was mother of the chief rivers and of three thousand daughters, the Oceanids, embodiments of the waters of the world.24

13.4

ENCELADUS: WATER JETS AND A POSSIBLE OCEAN

In Greek mythology, Enceladus was a giant. But this Saturnian satellite is so small that its diameter would fit inside France or New Mexico. Planetary scientists once believed that Enceladus was just too tiny a moon to harbor major geological activity. It should have lost its interior heat to the cold of space billions of years ago. Voyager observations suggested otherwise, however, giving an enticing view of terrain with very bright ice and a lack of craters over much of the surface. These were possible clues that geological processes had renewed the surface relatively recently, perhaps within the last one hundred million years.25

330 The icy moons It would be nearly a quarter of century after Voyager before Cassini arrived and carried out more definitive observations of the moon’s complex surface. Enceladus is indeed very bright. All regions of the satellite exhibit high albedos. This is typically the case with surfaces recently covered by snow, and that is why it is a clue that the entire moon might be coated with fresh material.26 New snowfalls tend to be composed of relatively small particles, resulting in more light reflection and a surface that appears brighter. But over time, when snow or ice particles touch other such particles under pressure, they melt and reform as larger, less reflective particles, and the surface grows dimmer. The dramatic brightness of Enceladus thus suggests a young surface. The amount of cratering on a satellite also is related to its age. The more craters, the older the surface tends to be, because if the surface were young there would not have been sufficient time for many craters to form.27 While scientists thought that some Enceladean geological activity was likely, they were unprepared for Cassini’s observations revealing the little moon to be “one of the most geologically dynamic objects in the solar system.”28 Among the surprises were watery jets shooting out plumes large enough to “drench the whole Saturn system,”29 as well as a south polar region and surrounding landscape being rapidly resurfaced by cryovolcanism and fresh snowfall. Cassini scientists on the Composite Infrared Spectrometer (CIRS) team were startled by their first thermal (infrared) radiation data of the south pole region. What they expected was that the south pole would be very cold, as shown in the left panel of Figure 13.5. But what they saw was a dramatic warm spot centered on the pole that was probably a sign of internal heat leaking from the interior, as shown in the right panel of Figure 13.5.30

Figure 13.5 Expected and actual Enceladus temperatures. Note that “absolute zero” on the Kelvin temperature scale is equivalent to minus 273.15 degrees on the Celsius scale.

13.4 13.4.1

Enceladus 331

Key flybys

In flybys conducted in February, March, and July 2005, the spacecraft descended to 1,000 and then 500 kilometers above Enceladus’s equator, and on the third flyby it passed a mere 168 kilometers (104 miles) above the south pole. Candy Hansen, a co-investigator on the Ultraviolet Imaging Spectrometer (UVIS) team, commented that it was the July 2005 flyby “that opened up all of the riches for everyone.”31 On this occasion the Orbiter flew right through an emergent plume of Enceladean material. Different instruments aboard the spacecraft sensed this plume in different ways. As the Cosmic Dust Analyzer (CDA) detected grains of ice spewing out of the moon, the ultraviolet sensor on UVIS mapped out the plume backlit by a blue-white giant star, Gamma Orionis. And Orbiter observations at infrared wavelengths revealed that “the south pole actually glows”32 with warmth, apparently due to a heat source located beneath a surface pattern of tiger stripes – parallel sets of linear trenches stained with dark bluish organic material, and hence very obvious against the surrounding icy surface (Figure 13.6). From these vents erupt water vapor, ice, and dust particles.

Figure 13.6 The warmest temperatures in Enceladus’ south polar region correspond to the prominent, bluish fractures dubbed tiger stripes.

332

The icy moons

Figure 13.7 A forest of water jets. Dramatic plumes spray water vapor and ice particles out from many locations along the tiger stripes near the south pole of Enceladus.

When Cassini flew through Enceladus’ plumes on 21 November 2009, it imaged a forest of individual jets – more than 30 of them. In excess of 20 of these had not previously been identified (Figure 13.7). The flyby also obtained particularly good images of the longest of the four tiger stripes, Baghdad Sulcus (Figure 13.8), with a length of 175 kilometers (109 miles).33 Cassini scientists have estimated that tiger stripe fractures like this one are typically 500 meters (1,600 feet) deep. The geyser-spewing regions they contain are narrow, perhaps only tens of meters across.34 The data obtained during this flyby included high resolution mapping of thermal energy leaking from Baghdad Sulcus, indicating a strong correlation between geologically youthful surface fractures and the anomalously warm temperatures of the south polar region. Swaths of heat previously measured by the Orbiter’s infrared spectrometer are now known to originate in a narrow, intense region no more than a kilometer (0.6 mile) wide along Baghdad Sulcus (Figure 13.8).35 13.4.2

Enceladus and the E ring

One of the most exciting aspects of the diminutive but vibrant body called Enceladus is its relationship with Saturn’s tenuous E ring, depicted in Figure 13.9 and made up of particles that peak in size somewhere between 0.3 and 3 micrometers (μm). These are small particles, for there are about 25,000 micrometers in a single inch. What is fascinating is that the E ring’s highest density and shallowest vertical extent both occur close to Enceladus’ orbit. This might be the case if the moon was actually the source of this huge, faint ring, as suggested in Figure 13.10. Farther away, the ring constituents would be expected to spread out vertically and horizontally, as is indeed the case. During the close flyby of Enceladus on 14 July 2005, the Orbiter’s Cosmic Dust Analyzer (CDA) observed tiny ice grains erupting from the south pole, replenishing the E ring. Because the vehicle passed only 168 kilometers above the satellite at this time, it was

13.4

Enceladus 333

Figure 13.8 Pockets of heat appear along the Baghdad Sulcus fracture, one of the tiger stripes that erupt with jets of water vapor and ice particles in Enceladus’ south polar region. This image layers temperature data (in color) atop of a visible-light image. The brightest colors in the map correspond to a combination of higher temperatures and larger areas of warm surface material. The image shows swaths of heat detected by the Orbiter’s infrared spectrometer confined to a narrow, intense region no more than a kilometer (half a mile) wide along the fracture.

334

The icy moons

Figure 13.9 Saturn’s E ring, far more diffuse than most of the planet’s other rings. Note that Enceladus is located within the E ring’s densest part.

Figure 13.10 Enceladus, the little bright disc from which material seems to be spewing, is apparently resupplying Saturn’s E ring with ice particles. This Cassini image was obtained in visible light on 15 September 2006 at a distance of 2.1 million kilometers (1.3 million miles). Tethys is the bright dot to the left of Enceladus.

able to directly measure the distribution of freshly produced grains. The data showed that slow-moving dust grains cannot escape Enceladus’ gravity; they form a cloud around the moon. But the faster dust grains do escape and replenish the E ring.36 13.4.3

Enceladean eruptions

Beginning approximately 10 minutes before and ending 10 minutes after the moment of closest approach during the 14 July 2005 Enceladus flyby, Cassini’s Cosmic Dust Analyzer (CDA) noticed a significant increase in the count rate of particles, with the rate peaking at 4 particles per second 1 minute before closest approach. The Ion and Neutral Mass

13.4

Enceladus 335

Spectrometer (INMS) observed a similar profile, with the water vapor detection peaking 30 seconds before closest approach. This gas plume was also seen by the Ultraviolet Imaging Spectrograph (UVIS) and, during an earlier flyby, by the Magnetometer (MAG) instrument. These premature maxima – observations peaking before closest approach – are understandable if gas and dust were spewing out from Enceladus near its south pole. The spacecraft approached from the south, and if the moon had recently ejected material into space that subsequently spread out, then the craft would have passed through and observed this material before reaching its point of closest approach to Enceladus. Dust and gas clouds can also be generated by micrometeoroid impacts, as was observed by the Galileo spacecraft at Jupiter’s moons. In the vicinity of Enceladus, such micrometeoroids might originate either from the E-ring or from interplanetary dust. The typically large velocities of the particles relative to Enceladus ranged from several to tens of kilometers per second. Thus upon impact, these tiny projectiles would have sufficient kinetic energy to eject numerous particles from Enceladus’s surface and create a dust cloud. But simulations have shown that this phenomenon would result in a peak observation rate at the spacecraft’s point of closest approach to the satellite, rather than prematurely as Cassini observed. Also, the detection of anomalously high temperatures in Enceladus’ south polar region near the elongated fractures called tiger stripes supported the theory that the increase in dust particles and water gas was a result of cryovolcanism in the interior of the moon.37 13.4.4

Liquid water under the Enceladean surface?

The spacecraft’s CDA examined the compositions of tiny ice particles in Saturn’s E ring that had presumably come from Enceladus. The analyses revealed a potentially very important constituent in the particles – sodium salts. The most likely way that such substances got into the ice grains was for salty minerals deep inside Enceladus to have washed out from rock deposits at the bottom of a liquid layer,38 said Frank Postberg, a CDA scientist from the Max Planck Institute in Germany. The CDA data suggested that liquid water must be present, since this appeared to be the only way to dissolve the “significant amounts of minerals that would account for the levels of salt detected.”39 Sublimation, the process by which water vapor is released directly from solid ice such as in Enceladus’ crust, could not explain the presence of salt in the ice grains. The significance of detecting the salty ice grains is the inference that Enceladus harbors a reservoir of liquid water – perhaps even an ocean – underneath its surface. The existence of such an ocean has many implications, one of which is that it could be a suitable environment for the evolution of life precursors when coupled with organic compounds that the spacecraft found in the plumes and thermal energy that it measured near the south pole. What has not been determined, however, is the form of such a water reservoir. Is it trapped within pockets in the satellite’s thick ice crust, or is it connected to a large ocean that is in contact with the rocky core? 13.4.5

Why is Enceladus more active than Mimas?

It is difficult to identify a mechanism that would generate sufficient heat to drive the geological activity on Enceladus, especially when this little moon is compared with neighboring satellites that exhibit less activity. A clue lies in the orbital eccentricity of Enceladus,

336 The icy moons which is comparable to that of Jupiter’s Io – another extremely active satellite. In an eccentric orbit, the distance of the satellite from the mother planet is constantly changing, and thus so are the gravitational forces to which it is subjected. These changing gravitational forces may stretch and compress the satellite enough to cause substantial tidal heating, resulting in melting of internal ice and the generation of pressures sufficient to drive the water jets and the renewal of the Enceladean surface. But the absence of such internal activity on nearby Mimas is then very puzzling.40 Mimas is comparable in size to Enceladus, has a larger orbital eccentricity, and is closer to Saturn, so tidal heating in Mimas ought to be significantly greater than in Enceladus. Yet Mimas does not show evidence of tidal heating. One possible explanation suggested by Steven Squyres is that Enceladus might have more ammonia than Mimas, which would lower the melting temperature and thus the heating that is required for creating pockets of liquid within the moon. In addition, an ammoniawater solution could be less dense than ice, resulting in the observed spewing out of material from Enceladus’ interior, perhaps with sufficient energy for it to escape the moon’s gravity and create the E ring.41 Data collected by the Orbiter’s Ion and Neutral Mass Spectrometer (INMS) on Cassini during the 8 October 2008 Enceladus flyby showed the presence of many complex chemicals in the vapor and icy particles of a plume erupting from the moon. According to Hunter Waite, the INMS lead scientist from the Southwest Research Institute in San Antonio, Texas, “One of the chemicals definitively identified was ammonia.”42 13.4.6

What heats Enceladus?

A number of heating mechanisms for Enceladus have been proposed, including tides induced in the moon by Saturn as well as librations (wobbles and oscillations) of the satellite initiated by its rotational characteristics. And radioactive heat sources in the satellite’s rock fraction may also play a part.43 13.4.7

The effects of tidal forces

Tidal forces that may cause Enceladus’ interior to melt could also control the timing of eruptions from fissures in the satellite’s southern hemisphere. Terry Hurford of NASA’s Goddard Space Flight Center in Greenbelt, Maryland explained that when Enceladus flies closer to Saturn, the pull of Saturn’s gravity is stronger, creating a larger tide; when Enceladus is farther away, the weaker gravitational pull leads to a smaller tide. Owing to eccentricity of the moon’s orbit around Saturn, the position of the planet in Enceladus’ sky also changes slightly, moving the location of the tide on Enceladus’ surface from east to west and back again with each orbit. These two effects combine to produce changing stress on the satellite’s surface that may impact the south pole’s tiger stripes, the source of eruptions. According to Hurford, because of the orientations of these stripes, Saturn’s tidal forces pull most of the stripes open when the moon is farthest from the mother planet and closed when it is nearest to the planet.44 Scientists believe that tidal forces also twisted and buckled Enceladus’ surface ice into long ridges and fractures. Cassini’s imaging system captured indications of such activity, including lines of folded mountain ridges and cracked white ice plains.45

13.5 13.4.8

Hyperion 337

Enceladus and the A ring

There is evidence that Saturn’s E ring was created by the eruptions of particles and gas from Enceladus. But this small moon’s eruptions might accomplish more as well. The Cassini mission uncovered evidence that “Enceladus is actually delivering a portion of its mass directly to the outer edge of the A ring.”46 This is amazing, as the A ring lies 100,000 kilometers (60,000 miles) from Enceladus. Magnetic fields, however, could provide the pathway between these two bodies. Some of the material which spews from Enceladus becomes ionized by sunlight and collisions with other atoms and electrons, forming plasma – a gas of electrically charged particles. These electrically charged particles feel a force due to Saturn’s powerful magnetic field, which sweeps them into the space around the planet, where they bounce back and forth from pole to pole, riding magnetic field lines as if these were railroad tracks. Other field lines may carry charged particles inward toward the planet, including to its A ring, which absorbs them like an enormous sponge. Supplying material to the A ring may serve an important function, for according to William Farrell of NASA’s Goddard Space Flight Center, “Saturn’s rings mitigate the overall radiation environment around the planet, sponging up low- and high-energy particles.”47 By contrast, Jupiter has no dense rings to soak up high-energy particles, and therefore the extremely high radiation environment close to that planet persists.48 13.4.9

Enceladean astrobiological possibilities

It is conceivable that a liquid water environment beneath Enceladus’ south polar cap could be conducive to life. Microbial ecosystems have been identified on Earth that do not rely on sunlight, oxygen, or organic materials produced at the surface. These systems therefore constitute analogues for possible Enceladus ecosystems. Two of Earth’s ecosystems are deep in volcanic rock and involve methanogens – methane-generating bacteria often found in living systems. Terrestrial methanogens consume hydrogen generated from reactions of rock with water. Such reactions could occur in an underground, tidally heated Enceladean water reservoir. In support of this idea, methane has indeed been detected in Enceladus’ plume, and could be biological in origin. This is not to say that such an origin is likely, but it is within the realm of possibility.49

13.5

HYPERION: SPONGY AND SMALL

As Voyager 2 passed through the Saturnian system in 1981, it noticed that Hyperion possessed a chaotic spin and a distinctly irregular, non-spherical shape resembling a potato. This suggested that Hyperion was a remnant of a larger moon destroyed by a major impact many years ago. Hyperion has an average diameter of 270 kilometers (168 miles) and is heavily cratered in a manner that makes it resemble a sea sponge (Figure 13.11). Cassini scientists believe that Hyperion’s strange appearance can be attributed to its unusually low density for an object of its size, which gives it a weak surface gravity and high porosity. Hyperion and other moons such as Phoebe and Iapetus that are far from Saturn exhibit extensive cratering precisely because they are so distant and have therefore experienced little tidal warming to blur or erase earlier features as occurs on moons closer in.

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The icy moons

Figure 13.11 Tumbling, potato-shaped, sea sponge-like Hyperion.

Hyperion orbits at a mean distance of 1,481,100 kilometers (920,300 miles) from Saturn in an eccentric (non-circular) orbit. It has an interesting resonance with Titan, the largest moon, which orbits at 1,221,850 kilometers (759,200 miles). As the two objects pass each other, their mutual gravitational attraction gives rise to a complex dance in which they each speed up or slow down. As Hyperion is much less massive, its rotation and orbit are vastly more affected. It is Titan’s influence that appears to keep Hyperion’s orbit eccentric. Otherwise, its path might grow more circular over time. Due to its resonance with Titan and its large distance from Saturn, Hyperion is unable to be tidally locked into constantly presenting the same face to the planet, as various other moons in the system do. Indeed, although it takes 21 days to complete an orbit around Saturn, its axial rotation is chaotic (constantly changing in a random manner) with a period of about 13 days. Hyperion has a low albedo (reflectivity) similar to its neighboring distant moons, Phoebe and the dark side of Iapetus. But the low albedo of Hyperion distinguishes it from inner moons such as Rhea, Dione, and Enceladus. Hyperion’s low albedo could be caused by compounds made up of frozen carbon dioxide, water ice, and perhaps also hydrocarbons. Titan’s atmosphere might supply methane which gets stripped of its hydrogen by solar radiation to form carbon dust and is then somehow transported to Hyperion. A third theory is that dark material from Phoebe might drift through space to color both Hyperion and Iapetus.50

13.6

Mimas

339

Figure 13.12 Herschel Crater on Mimas.

13.6

MIMAS: THE BULL’S-EYE MOON

Looking like a massive bull’s-eye, Herschel Crater dominates Mimas (Figure 13.12). The crater is 130 kilometers (80 miles) wide,51 which wouldn’t seem enormous on a large planet, but is one-third the diameter of Mimas. If the object that excavated this crater had been larger or moving faster, the satellite would likely have shattered into pieces, then collapsed back into a new moon or become another ring of Saturn. The walls of Herschel are impressive: 5 kilometers (3 miles) high, with parts of its floor 10 kilometers (6 miles) deep and a central tower rising nearly 6 kilometers (4 miles) above the floor. To put this into perspective, a proportionally sized crater on Earth would be 4,200 kilometers (2,600 miles) in diameter. Much of the United States could fit in such a massive cavity. Mimas orbits quite close to its parent – about three Saturn radii away. Imagine if our Moon traveled only three Earth radii – roughly 12,000 miles – above the surface of our planet, instead of a quarter million miles away. How huge it would look as it passed overhead, and how enormous would be the tides it caused! Mimas is one of the most battered, pockmarked bodies in our solar system, and yet it would probably be in even worse shape if it hadn’t been positioned so close to Saturn

340

The icy moons

when the planet was young and very hot. During its early life, Mimas baked in Saturn’s intense thermal radiation and scientists believe that this kept the satellite soft and pliable. As a result, early impact craters largely faded away. Mimas has a density only 1.17 times that of liquid water, indicating it is formed mostly of water ice with just a small amount of rock. It appears to remain solidly frozen at –209°C (–344°F), and this is puzzling. Mimas is much closer to Saturn than Enceladus, and the Mimantean orbit is much more eccentric, or out of round than the Enceladean one. This is significant, because the gravitational force from Saturn is stronger the closer that a satellite gets; and the more eccentric the orbit, the greater difference in gravitational force the satellite experiences on different sections of its orbital path. This differential gravity stretches and contracts various portions of the moon as it makes its way around Saturn and the internal friction from such actions generates tidal heating. As previously discussed, liquid water is probably present in Enceladus’ interior and planetary scientists believe that tidal heating is largely responsible for this. But Mimas, whose orbit is closer and more eccentric, should by this line of reasoning experience more tidal heating. So why is it solidly frozen, while Enceladus is likely not? And why is Mimas so heavily cratered, suggesting a surface that has persisted for a very long time, while Enceladus’ heat-driven geological processes seem to be rapidly renewing its surface? Out of these paradoxes has come the Mimas test, which requires that a theory that explains the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.52 Mimas likely played an important part in forming Saturn’s ring system. Models have shown that by resonance interactions between Mimas and particles in Saturn’s early ring system, the satellite may well have cleared material out of part or all of what is now the 4,800 kilometer (3,000 mile) Cassini Division located between the A and B rings. Mimas probably controls the location of the B ring edge that borders the Cassini Division, keeping B ring particles from moving outward.53 13.6.1

The Pac-Man temperature map

After data from the 13 February 2010 flyby was analyzed, Cassini Project Scientist Linda Spilker commented that “other moons usually grab the spotlight, but it turns out Mimas is more bizarre than we thought.”54 A temperature map of the icy moon obtained by the Composite Infrared Spectrometer (CIRS) revealed unexpected “hot” regions (relatively speaking) that resembled Pac-Man about to eat Herschel Crater (Figure 13.13). Instead of smoothly varying temperatures, Mimas was divided into a warm part on the left at a typical temperature of –181°C (−294°F) and a cold part on the right at about −196°C (−320°F), with a sharp v-shaped boundary between them. Scientists speculated that the “cold” part was more frigid because its surface materials had greater thermal conductivity, which meant that absorbed solar energy was conducted more quickly into the subsurface instead of warming the surface. The “warm” part of Mimas in the temperature map, on the other hand, might have been more insulating, thus trapping the solar energy at the surface, causing it to warm up. The mystery is, why would conductivity vary so dramatically and abruptly across the surface of the moon?55 C. J. A Howett and colleagues discovered an interesting process that could help explain Pac-Man Mimas. The thermally anomalous region in Figure 13.13 happens to coincide in shape and location to a region of high-energy electrons from Saturn’s magnetosphere

13.7

Rhea

341

Figure 13.13 Bizarre temperature distribution on Mimas. Temperatures of the yellow section were typically 15° Celsius higher than those of the blue region. Temperatures dropped abruptly across the V-shaped boundary between regions.

impacting the surface. These electrons are able to penetrate into the surface to centimeter depths, about the same depths that experience the temperature variations. What may be happening is that these energetic electrons alter surface ice properties to sufficient depth to produce the observed effect on surface temperatures. The surface of an ice layer has been observed compacting when bombarded by high energy ions, perhaps because tiny pores in the ice are destroyed, causing the layer to collapse somewhat. Howett argues that on Mimas, molecules mobilized by electron bombardment increase the contact area between ice grains and essentially glue them together, resulting in a rise in heat conduction of the ice layer.56

13.7

RHEA

During a flyby of Saturn’s second-largest moon Rhea on 26 November 2005, the Orbiter’s Magnetospheric Imaging Instrument (MIMI) found that electron flow in the region of the satellite was being blocked. As the spacecraft approached the icy moon, MIMI detected a surprising drop in the quantity of energetic electrons, but only within a distance of approximately 6,000 kilometers (3,500 miles) from Rhea. This distance coincided with the satellite’s Hill sphere, the region of space in which Rhea’s gravitational force becomes stronger than that of Saturn. According to Geraint Jones, a scientist on both the MIMI and CAPS teams, “Around 70% of electrons in the key energy range disappear between the boundary of the Hill sphere and Rhea; we wouldn’t expect any to disappear.” A more distant flyby in August 2007 also observed electron depletion. These data suggested that some orbiting material that was gravitationally bound to Rhea was interfering with electron flow through the magnetosphere. Cassini’s instruments found no evidence of large amounts of newly ionized gas, which could theoretically scatter electrons. Hence neutral gas and dust populations were the top absorbing-medium candidates. Scientists expected the existence of some orbiting particulates because a rain of material constantly hits Saturn’s moons, including Rhea, and knocks particles into the regions of space around them. In particular, planetary scientists believe that that dust is removed

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The icy moons

from satellite surfaces by meteoroid bombardment and by sputtering from energetic electrons and ions.57 During the November 2005 flyby, the Cosmic Dust Analyzer (CDA) and the Radio and Plasma Wave Science (RPWS) instrument observed the environment around Rhea and three of the ship’s instruments directly sampled the dust in the region. The data suggested a particle cloud with an increase in the population of dust particles exceeding 1 micrometer, although the densities of this dust cloud were not high enough to explain the blockage of electron current. An additional electron-absorbing obstacle was thus required.58 A detailed look at the MIMI and CAPS data provided a clue as to what might be going on. These instruments saw a gradual drop in the number of electrons detected on either side of Rhea’s equator, which would have been the case if a much denser population of particles was orbiting above the moon’s equator. Such an equatorial disk of material – a dense ring – would act as an obstacle to the flow of electrons, preventing them from getting through to MIMI’s detector. As Jones put it, “Seeing almost the same signatures on either side of Rhea was the clincher. After ruling out many other possibilities, we said these are most likely rings. No one was expecting rings around a moon.”59 The Orbiter’s instruments could not directly determine whether this ring existed because the spacecraft did not fly through the equatorial region, which would have been required to obtain a sample. Nevertheless, excitement ran high that a ring might have been discovered surrounding a planetary satellite. This would have opened up an interesting new field of study. According to Jones, numerical simulations which took into account the satellite’s gravity indicated that indeed, a stable ring system could conceivably orbit near Rhea’s equatorial plane.60 But the excitement about a supposed Rhea ring was short lived. Analyses of Cassini images taken in 2008 and 2009 failed to supply any evidence of rings around the satellite.61 This led Cornell scientist Matthew Tiscareno to state, “We’re pretty confident that there is no solid material orbiting the moon.”62 This was based on investigation of 65 images of the moon, some taken with the Sun behind the spacecraft and others with it in front – a factor that was important because the two solar geometries would show particles of different sizes by the dissimilar ways they scattered light. Furthermore, according to Project Scientist Linda Spilker, a 2010 flyby of Rhea “provided very different fields and particles measurements”63 and the supposed ring signature64 seen earlier was absent in the most recent flyby. There are indeed “very strong and interesting and unexplained electromagnetic effects going on around Rhea,” according to Tiscareno, but it is apparently not because of solid material orbiting the moon.65

13.8

DIONE

This cracked and intensively cratered satellite of Saturn was discovered in 1684 by Giovanni Cassini. It is an icy body similar to Tethys and Rhea, with a density about 1.4 times that of water ice. This makes Dione the densest Saturn satellite other than Titan. Scientists believe that Dione has a rocky core that makes up one-third of its mass, surrounded by water ice. It is tidally locked, presenting the same face to Saturn throughout its orbit. In other words, the time it takes to turn once on its axis is the same as the time it takes to make one trip around Saturn. Tidal locking occurs when differential gravitational forces exerted by the planet on various parts of the satellite force one side of it always to face the planet, as in the case of Earth and its satellite. Or to be more specific, the planet’s

13.8

Dione 343

gravity pulls hardest on the closest point of the satellite, causing it to bulge. But because of internal friction, this point doesn’t immediately bulge – it takes some time. And by that time, the satellite has rotated a bit so that the bulge is no longer at the closest point to the planet. The planet thus pulls back on the bulge, exerting a torque on the satellite that eventually forces it into a position where the same side constantly faces the planet. In 1980, Voyager revealed bright, wispy features on Dione that at the time could not be explained. These wisps formed a highly reflective but thin layer that did not obscure the underlying features. After the Voyager flybys, scientists hypothesized that shortly after it formed, Dione was geologically active, with the bright material being caused by cryovolcanism. According to this theory, the wispy streaks formed from eruptions along cracks in Dione’s surface that fell back to the surface as snow or ash. Later, after the internal activity, cratering continued mainly on the leading hemisphere – the one that faced forward in the direction of Dione’s orbital motion – and wiped out the streak patterns there.66 During Cassini’s 14 December 2004 flyby, its Imaging Science Subsystem (ISS) established the wispy streaks to be a complex system of multiple braided fractures67 with bright scarps – steep slopes or cliff walls, typically at the edges of plateaus or ridges.68 An even closer flyby on 11 October 2005 that passed just 500 kilometers (300 miles) above the surface showed the scarps to be several hundred meters high. Scientists believe that the scarps are bright because darker material has slumped to expose light-colored water ice. These fractured cliffs provide a clue that Dione was subjected to tectonic activity in its past. The scarps might be a mature phase of the tiger stripes that are active today in the south polar region of Enceladus.69 Figure 13.14, taken in visible light by the wide-angle camera on 30 September 2007, is a panoramic view of Dione’s wispy system of fractures, and Figure 13.15, taken on 14 December 2004, shows some of the relief of the fractures and scarps. The spacecraft also observed diverse impact crater structures. The most heavily cratered terrain includes numerous craters greater than 100 kilometers (60 miles) in diameter. Their placement relative to the orbital direction of the moon is curious, for they are on the trailing hemisphere – the one that faces backwards away from the satellite’s direction of motion – with less cratered plains on the leading hemisphere. It is the opposite of what many scientists expected. Gene Shoemaker and R. F. Wolfe, for instance, had developed a cratering model for a tidally locked satellite with the highest cratering rates occurring on the leading hemisphere, which was flying into any foreign bodies, and the lowest cratering rates on the “protected” trailing hemisphere.70 The more frequent cratering on Dione’s trailing hemisphere suggests that during the time of the heaviest bombardment, the moon was tidally locked to Saturn in the opposite orientation. As Dione is relatively small, a large enough impact – estimated as one that would have formed a crater at least 35 kilometers (21 miles) in diameter – could have spun the satellite around to its present orientation. In fact, because there are many craters larger than this size, the satellite’s spin axis could have been altered many times. 13.8.1

Helene: Dione’s Trojan moon

Small, irregularly shaped Helene is not referred to as an icy moon, but as a Trojan moon due to the fact that it is gravitationally tied to a much larger satellite, in this case Dione.71

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The icy moons

Figure 13.14 Dione’s bright, wispy markings, taken with the Cassini Orbiter’s wide-angle camera.

Cassini flew by Helene at about 1,800 kilometers altitude the day after its March 2010 targeted flyby of Rhea.72 The ISS captured close-up views of Helene as VIMS took measurements to help analyze its surface composition and determine whether it is coated with particles from the E ring. Mission scientists were seeking answers to such questions as: How did Helene become gravitationally captured by Dione? And was a collision part of Helene’s past? 13.8.2

The 2010 doubleheader flyby

In April 2010, Cassini attempted a double flyby of Dione and Titan. This was made possible by a fortuitous alignment of the two satellites. The Titan flyby occurred on 5 April and the Dione encounter on 6 April, JPL time, with the spacecraft closing to within 500 kilometers (300 miles) of the icy moon’s surface. Observations by the Voyagers and the Cassini flyby in October 2005 hinted that the moon might be sending out a diffuse collection of charged particles into Saturn’s magnetic field and potentially contributing material to one of the planet’s rings. In its 2010 flyby, Cassini used

13.9 Summary of moons discovered to date

345

Figure 13.15 This visible light image captures some of the structure of Dione’s wispy, bright-colored system of surface fractures.

its magnetometer, fields and particles instruments, and thermal mapping by its Composite Infrared Spectrometer (CIRS) to seek evidence of such activity. In addition, the Visual and Infrared Mapping Spectrometer (VIMS) investigated the dark material on the satellite to try to identify its source.73 On 12 December 2011 Cassini again flew by Dione, this time at an altitude of only 99 kilometers (61 miles) to use its Ion and Neutral Mass Spectrometer (INMS) to study the immediate vicinity of the moon, and use the way the vehicle’s path was deflected by the extremely close pass to analyze the moon’s internal structure.74 Although the Dione environment is far from being understood, Sven Simon and his team have interpreted the available data as indicating the presence of a tenuous atmosphere surrounding the moon.75

13.9

SUMMARY OF MOONS DISCOVERED TO DATE

Table 13.3 is an overview of the Saturnian moons as currently known, with some of their characteristics. As the table illustrates, satellite radii range from very small: 6 kilometers for instance for Ijiraq, to nearly 2,600 kilometers for Titan. As extensive as Cassini’s activities have been, fuel and other limitations did not allow every moon to be visited.

Tethys

Dione

Rhea

Titan

Hyperion

Iapetus

Phoebe

Janus

III

IV

V

VI

VII

VIII

IX

X

XII

XI

Enceladus

II

Helene

S/1980 S6

S/1980 S1 Epimetheus S/1980 S3

Mimas

I

Moon

1980

1980

1966

1898

1671

1848

1655

1672

1684

1684

1789

1789

Date Discovered

J. Fountain, S. Larson, H. Reitsema, B. Smith/ Voyager 1 P. Laques and J. Lecacheux

A. Dollfus

W. Pickering

G.D. Cassini

W. Bond, W. Lassell

C. Huygens

G.D. Cassini

G.D. Cassini

G.D. Cassini

W. Herschel

W. Herschel

Discoverer

Helene

0.00076

GM (km3/sec2) Mimas 2.5026 ± 0.0006 Enceladus 7.2027 ± 0.0125 Tethys 41.2067 ± 0.0038 Dione 73.1146 ± 0.0015 Rhea 153.9426 ± 0.0037 Titan 8978.1382 ± 0.0020 Hyperion 0.3727 ± 0.0012 Iapetus 120.5038 ± 0.0080 Phoebe 0.5532 ± 0.0006 Janus 0.1263 ± 0.0087 Epimetheus 0.0351 ± 0.0047

Moon

17.6 ± 0.4

Mean radius (km) 198.20 ± 0.25 252.10 ± 0.10 533.00 ± 0.70 561.70 ± 0.45 764.30 ± 1.10 2574.73 ± 0.09 135.00 ± 4.00 735.60 ± 1.50 106.50 ± 0.70 89.5 ± 1.5 58.1 ± 1.8

0.5

Mean density (g/cm3) 1.150 ± 0.004 1.608 ± 0.003 0.973 ± 0.004 1.476 ± 0.004 1.233 ± 0.005 1.882 ± 0.001 0.544 ± 0.050 1.083 ± 0.007 1.638 ± 0.033 0.630 ± 0.030 0.640 ± 0.062

Table 13.3. Overview of Saturnian Moons [Courtesy of Linda Spilker and Bob Jacobson , JPL].

18.4

0.6

0.5

15.6

14.4

0.081 ± 0.002 0.6

0.6

0.3

0.2

0.6

0.6

0.8

1

0.6

Geometric Albedo

16.4

11

14.4

8.4

9.6

10.4

10.2

11.8

Magnitude V0 or R 12.8

346 The icy moons

Atlas

XIV

XV

XXV

XXIV

XXIII

XXII

XXI

XX

XIX

XVIII

XVII

XVI

Calypso

XIII

S/1980 S25

S/1980 S13

S/1980 S28 Prometheus S/1980 S27 Pandora S/1980 S26 Pan S/1981 S13 Ymir S/2000 S1 Paaliaq S/2000 S2 Tarvos S/2000 S4 Ijiraq S/2000 S6 Suttungr S/2000 S12 Kiviuq S/2000 S5 Mundilfari S/2000 S9

Moon Telesto

2000

2000

2000

2000

2000

2000

2000

1990

1980

1980

1980

1980

Date Discovered 1980 Discoverer B. Smith, H. Reitsema, S. Larson, J. Fountain/ Voyager 1 D. Pascu, K. Seidelmann, W. Baum, D. Currie Voyager Science Team Voyager Science Team Voyager Science Team M. Showalter/ Voyager 2 B. Gladman IAUC 7512 B. Gladman IAUC 7512 J.J. Kavelaars, IAUC B. Gladman 7513 J.J. Kavelaars, IAUC B. Gladman 7521 B. Gladman, IAUC J.J. Kavelaars 7548 B. Gladman IAUC 7521 B. Gladman, IAUC J.J. Kavelaars 7538 0.00017

GM 0.00027

0.000014

0.00022

Kiviuq Mundilfari

0.000014

0.00008

0.00018

0.00055

Suttungr

Ijiraq

Tarvos

Paaliaq

0.00044 ± 0.00015 Prometheus 0.01074 ± 0.00285 Pandora 0.00924 ± 0.00152 Pan 0.00033 ± 0.00015 Ymir 0.00033

Atlas

Calypso

Moon Telesto

3.5

8

3.5

6

7.5

11

15.1 ± 1.2 43.1 ± 2.7 40.7 ± 1.5 14.1 ± 1.3 9

10.7 ± 0.7

Mean radius 12.4 ± 0.4

2.3

2.3

2.3

2.3

2.3

2.3

0.460 ± 0.110 0.480 ± 0.090 0.490 ± 0.060 0.420 ± 0.150 2.3

0.5

Mean density 0.5

23.8R

22.1R

23.9R

22.6R

22.7R

21.1R

21.9R

19.4

16.4

15.8

19

18.7

Magnitude 18.5

(continued)

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.5

0.5

0.6

0.4

0.7

Geometric Albedo 1

13.9 Summary of moons discovered to date 347

Moon Albiorix

Skathi

Erriapus

Siarnaq

Thrymr

Narvi

Methone

Pallene

Polydeuces

Daphnis

Aegir

Bebhionn

XXVI

XXVII

XXVIII

XXIX

XXX

XXXI

XXXII

XXXIII

XXXIV

XXXV

XXXVI

XXXVII

S/2004 S11

S/2004 S10

S/2000 S11 S/2000 S8 S/2000 S10 S/2000 S3 S/2000 S7 S/2003 S1 S/2004 S1 S/2004 S2 S/2004 S5 S/2005 S1

2005

2005

2005

2004

2004

2004

2003

2000

2000

2000

2000

Date Discovered 2000

D. Jewitt, S. Sheppard, J. Kleyna D. Jewitt, S. Sheppard, J. Kleyna

Cassini Imaging Science Team Cassini Imaging Science Team Cassini Imaging Science Team Cassini Imaging Science Team

Discoverer M. Holman, T.B. Spahr J.J. Kavelaars, B. Gladman J.J. Kavelaars, B. Gladman B. Gladman, J.J. Kavelaars B. Gladman, J.J. Kavelaars S.S. Sheppard

IAUC 8523

IAUC 8523

IAUC 7545 IAUC 7538 IAUC 7539 IAUC 7513 IAUC 7538 IAUC 8116 IAUC 8389 IAUC 8389 IAUC 8432 IAUC 8524

0

Bebhionn

Aegir

0.0000052 ± 0.0000052 0

0.0000003

0.0000022

0.0000006

0.000023

0.000014

0.0026

0.000051

0.000021

GM 0.0014

Daphnis

Polydeuces

Pallene

Methone

Narvi

Thrymr

Siarnaq

Erriapus

Skathi

Moon Albiorix

Table 13.3 (continued)

3

3

1.6 ± 0.6 2.5 ± 0.6 1.3 ± 0.4 3.8 ± 0.8

3.5

3.5

20

5

4

Mean radius 16

2.3

2.3

0.340 ± 0.260

0.5

0.5

0.5

2.3

2.3

2.3

2.3

2.3

Mean density 2.3

24.1R

24.4R

?

?

?

?

23.8R

23.9R

19.9R

23.4R

23.6R

Magnitude 20.5R

0.04

0.04

?

?

?

?

0.04

0.06

0.06

0.06

0.06

Geometric Albedo 0.06

348 The icy moons

Bestla

Farbauti

Fenrir

Fornjot

Hati

Hyrrokkin

Kari

Loge

Skoll

Surtur

Anthe

XXXIX

XL

XLI

XLII

XLIII

XLIV

XLV

XLVI

XLVII

XLVIII

XLIX

Moon XXXVIII Bergelmir

S/2004 S19 S/2006 S2 S/2006 S5 S/2006 S8 S/2006 S7 S/2007 S4

S/2004 S14

S/2004 S8

S/2004 S16

S/2004 S9

S/2004 S18

S/2004 S15

2007

2006

2006

2006

2006

2006

2005

2005

2005

2005

2005

Date Discovered 2005 Discoverer D. Jewitt, S. Sheppard, J. Kleyna D. Jewitt, S. Sheppard, J. Kleyna D. Jewitt, S. Sheppard, J. Kleyna D. Jewitt, S. Sheppard, J. Kleyna D. Jewitt, S. Sheppard, J. Kleyna D. Jewitt, S. Sheppard, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna Cassini Imaging Science Team IAUC 8727 IAUC 8727 IAUC 8727 IAUC 8727 IAUC 8727 IAUC 8857

IAUC 8523

IAUC 8523

IAUC 8523

IAUC 8523

IAUC 8523

IAUC 8523

Anthe

Surtur

Skoll

Loge

Kari

Hyrrokkin

Hati

Fornjot

Fenrir

Farbauti

Bestla

Moon Bergelmir

0.0000001

0

0

0

0

0

0

0

0

0

0

GM 0

0.9

3

3

3

3

3

3

3

2

2.5

3.5

Mean radius 3

0.5

2.3

2.3

2.3

2.3

2.3

2.3

2.3

2.3

2.3

2.3

Mean density 2.3

?

24.8R

24.5R

24.6R

23.9R

23.5R

24.4R

24.6R

25.0R

24.7R

23.8R

Magnitude 24.2R

(continued)

?

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

Geometric Albedo 0.04

13.9 Summary of moons discovered to date 349

Moon Jarnsaxa

Greip

Tarqeq

Aegaeon

L

LI

LII

LIII

2005

2005

2005

2006

S/2004 S13

S/2004 S17

S/2006 S1

2005

2008

2007

2006

S/2004 S12

S/2006 S6 S/2006 S4 S/2007 S1 S/2008 S1 S/2004 S7

Date Discovered 2006 Discoverer S. Sheppard, D. Jewitt, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna Cassini Imaging Science Team D. Jewitt, S. Sheppard, J. Kleyna D. Jewitt, S. Sheppard, J. Kleyna D. Jewitt, S. Sheppard, J. Kleyna D. Jewitt, S. Sheppard, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna IAUC 8727

IAUC 8523

IAUC 8523

IAUC 8523

IAUC 8727 IAUC 8727 IAUC 8836 IAUC 9023 IAUC 8523 0

4E-09

0

0

GM 0

S/2006 S 1

0

S/2004 S 17 0

S/2004 S 13 0

S/2004 S 12 0

S/2004 S 7

Aegaeon

Tarqeq

Greip

Moon Jarnsaxs

Table 13.3 (continued)

3

2

3

2.5

3

0.3

3

3

Mean radius 3

2.3

2.3

2.3

2.3

2.3

0.5

2.3

2.3

Mean density 2.3

24.6R

25.2R

24.5R

24.8R

24.5R

?

23.9R

24.4R

Magnitude 24.7R

0.04

0.04

0.04

0.04

0.04

?

0.04

0.04

Geometric Albedo 0.04

350 The icy moons

S/2006 S3 S/2007 S2 S/2007 S3 S/2009 S1

2009

2007

2007

Date Discovered 2006 Discoverer S. Sheppard, D. Jewitt, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna S. Sheppard, D. Jewitt, J. Kleyna Cassini Imaging Science Team IAUC 8727 IAUC 8836 IAUC 8836 IAUC 9091 S/2007 S 3

S/2007 S 2

Moon S/2006 S 3

0

0

GM 0

2

3

Mean radius 2.5

2.3

2.3

Mean density 2.3

24.9R

24.4R

Magnitude 24.6R

0.04

0.04

Geometric Albedo 0.04

NOTES: 1. GM is the gravitational constant times the mass. It is the fundamental parameter in the gravity model used to compute the orbits of the satellites. 2. When the IAU officially recognizes a satellite it is given a name and a number. The number is by tradition expressed as a Roman numeral. 3. When a satellite is first discovered, the IAU assigns it a designation. S/2004 S7 is the designation for the 7th Saturnian satellite discovered in 2004. 4. D. Morrison et al., “Satellites of Saturn: Geological Perspective,” in T. Gehrels and M.S. Matthews (eds.) Saturn (Tucson: Univ. of Arizona Press, 1984):609–639 5. Simonelli et al., “Phoebe: Albedo Map and Photometric Properties”, Icarus 138 (1999):249–258 6. R.A. Jacobson et al., “Revised Orbits of Saturn’s Small Inner Satellites”, AJ 135 (2008):261–263 7. C. Porco et al., “Physical Characteristics and Possible Accretionary Origins for Saturn’s Small Satellites,” Presented at the 37th Lunar and Planetary Science Conference, League City, TX (2006) 8. T. Grav et al., “Photometric Survey of the Irregular Satellites,” Icarus 166 (2003):33–45 9. S.S. Sheppard, http://www.dtm.ciw.edu/users/sheppard/satellites/satsatdata.html (2010)

Moon

13.9 Summary of moons discovered to date 351

352 The icy moons REFERENCES 1. Todd J. Barber, “Insider’s Cassini: Dr. John Spencer and Unexpected Mimas Temperature Data,” http://saturn.jpl.nasa.gov/news/cassiniinsider/insider20100331/, JPL Cassini Insider (31 May 2010). 2. Ibid. 3. The Cassini team used to call the really small satellites the rocky moons, but the scientists later objected to that and that usage seems to be going away. Bob Mitchell, review of manuscript, Feb. 2011. 4. NASA-JPL, “Icy Satellites in the Cassini Tour of the Saturn System,” http://soc.jpl.nasa.gov/ resources/cassiniTelecon20040601.pdf (1 June 2004), accessed 1 June 2009. 5. Trina Ray interview by author, 22 October 2008, JPL; NASA-JPL, “Significant Event Report for Week Ending 4/26/2002,” http://saturn.jpl.nasa.gov/news/significantevents/sigevent20020426/, (April 26,2002), accessed 1 June 2009. 6. NASA-JPL, “About Saturn and Its Moons,” http://saturn.jpl.nasa.gov/science/moons/, Cassini Equinox Mission Web site, accessed 22 June 2010; NASA-JPL, “Rhea and Titan Flyby - Aug. 30 and 31, 2007,” http://saturn.jpl.nasa.gov/mission/flybys/t35rhea/ (Aug. 31, 2007); NASA-JPL, “PIA06150: First Flyby of Dione,” http://photojournal.jpl.nasa.gov/catalog/PIA06150 (13 Dec. 2004); NASA-JPL, “Dione D2 Encounter (Rev 129),” http://saturn.jpl.nasa.gov/files/20100407_ dione_mission_description.pdf; NASA-JPL, “Iapetus Flyby - Sept. 10, 2007,” http://saturn.jpl. nasa.gov/mission/flybys/iapetus/, accessed 22 June 2010; NASA, “Iapetus’ New Year’s Flyby,” http://www.nasaimages.org/luna/servlet/detail/NVA2~1~1~204~100242:Iapetus – New-Year-sFlyby , accessed 22 June 2010; NASA-JPL, “Anatomy of a Flyby,” http://www.nasa.gov/externalflash/enceladus_20080310/index.swf, accessed 22 June 2010.; NASA-JPL, “Enceladus Mission Description,” http://saturn.jpl.nasa.gov/multimedia/products/pdfs/20080312_enceladus_mission_description.pdf (Mar. 2008). 7. NASA-JPL, “About Saturn and Its Moons: Enceladus,” http://saturn.jpl.nasa.gov/science/ moons/enceladus/index.cfm?pageListID=1, accessed 23 June 2010; NASA-JPL, “Flybys,” http://saturn.jpl.nasa.gov/mission/flybys/?year=2008, http://saturn.jpl.nasa.gov/mission/ flybys/?year=2009, and http://saturn.jpl.nasa.gov/mission/flybys/?year=2010, accessed 23 June 2010; NASA-JPL, “Enceladus Flyby - August 11, 2008,” http://saturn.jpl.nasa.gov/mission/flybys/enceladus20080811/ (11 Aug. 2008); NASA-JPL, “Helene (Quasi-targeted) Flyby - March 3, 2010,” http://saturn.jpl.nasa.gov/mission/flybys/helene20100303/ (3 Mar. 2010); ESA, Enceladus Flyby – 11 August 2008,” http://sci.esa.int/cassini-huygens/43232-enceladus-flyby-11-08-08/, accessed 16 May 2014. 8. NASA-JPL, “Iapetus,” http://saturn.jpl.nasa.gov/science/moons/iapetus/, Cassini Equinox Mission Web site, accessed 11 June 2009. 9. Micrometeoroids are natural dust particles with masses between 10–21 and 10–9 kg (0.01–100 m). They originate from bodies in the solar system such comets, asteroids or planetary environments. Ralf Srama, “Micrometeoroids,” Encyclopedia of Aerospace Engineering (John Wiley & Sons, 2010). 10. A particle is in a prograde orbit if it is traveling counterclockwise, as seen from above Saturn’s north pole – the same direction as Saturn’s rotation. Particles traveling in the opposite direction are said to orbit in a retrograde manner. 11. Tilmann Denk et al., “Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging,” Sciencexpress (10 December 2009):1; A.J. Verbiscer, et al., “Saturn’s Largest Ring,” Nature 461 (22 Oct. 2009). 12. Joe Mason et al., “Cassini Closes In on the Centuries-Old Mystery of Saturn’s Moon Iapetus,” http://ciclops.org/view.php?id=6033, Media Relations Office, Cassini Imaging Central Laboratory for Operations (CICLOPS), Space Science Institute, Boulder CO (10 Dec. 2009).

References 353 13. NASA-JPL, “About Saturn & Its Moons: Iapetus,” http://saturn.jpl.nasa.gov/science/moons/ iapetus/, accessed 24 June 2010. 14. Carolina Martinez, “Saturn’s Old Moon Iapetus Retains Its Youthful Figure,” JPL news release 2007-079 (17 July 2007). 15. Martinez, “Saturn’s Old Moon Iapetus Retains Its Youthful Figure.” 16. C.C. Porco et al., “Cassini Imaging Science: Initial Results on Phoebe and Iapetus,” Science 307 (25 February 2005):1237-1242. 17. L. Czechowski and J. Leliwa-Kopystynski, “The Iapetus Ridge: Possible Explanations of Its Origin,” Advances in Space Research 42 (2008). 18. NASA-JPL, “About Saturn & Its Moons: Iapetus.” 19. Bob Mitchell interview with author, JPL, 26 October 2010; Richard A. Kerr, “How Saturn’s Icy Moons Get a (Geologic) Life,” Science 311 (6 January 2006):29. 20. J.C. Castillo-Rogez et al., “Iapetus’ Geophysics: Rotation Rate, Shape, and Equatorial Ridge,” Icarus 190 (2007):181. 21. Tilmann Denk and Preston Dyches, caption to the image “The Great Basin,” NASA/JPL/Space Science Institute, image no. PIA 07693 (2 Feb. 2006). 22. E.M. Chen and F. Nimmo, “Implications from the Ithaca Chasma for the Thermal and Orbital History of Tethys,” Geophys. Res. Letters 35 (2008); Bernd Giese et al., “Tethys: Lithospheric Thickness and Heat Flux from Flexurally Supported Topography at Ithaca Chasma,” Geophys. Res. Ltrs. 34 (2007): L21203; Preston Dyches, caption to image, “The Ancient Rift,” NASA/ JPL/Space Science Institute image PIA 10460 (2 Sep. 2008). 23. Robert Nemiroff and Jerry Bonnell, “Ice Moon Tethys from Saturn-Orbiting Cassini,” http:// antwrp.gsfc.nasa.gov/apod/ap091208.html, Astronomy Picture of the Day, (8 Dec. 2009). 24. Carolina Martinez, “Cassini Scores Closeup Flybys,” JPL Universe 35(20) (7 Oct. 2009), NASA NHRC 18337 Cassini 2002-; NASA-JPL, “Tethys,” http://saturn.jpl.nasa.gov/science/moons/ tethys/, accessed 2 June 2009. 25. Candy Hansen interview with author, JPL, 27 Oct. 2010; J. Wisdom, “Spin-Orbit Secondary Resonance Dynamics of Enceladus,” Astronomical Journal 128 (2004):484-491. 26. J. R. Spencer et al., “Cassini Encounters Enceladus: Background and the Discovery of a South Polar Hot Spot,” Science 311 (10 March 2006):1401 – 1405; B.J. Buratti, “Enceladus— Implications of its Unusual Photometri Properties,” Icarus 75 (July 1988):113-126. 27. Bob Nelson interview by author, London, 23 June 2009; “Surface Properties of the Moon ,” http://csep10.phys.utk.edu/astr161/lect/moon/moon_surface.html, class notes from Astronomy 161: The Solar System, University of Tennessee, accessed 29 Nov. 2010. 28. Jeffrey S. Kargel, “Enceladus: Cosmic Gymnast, Volatile Miniworld,” Science 311 (10 March 2006):1389 – 1391. 29. Joanne Baker, “Tiger, Tiger, Burning Bright,” Science 311 (10 March 2006):1388. 30. NASA-JPL, caption accompanying “Enceladus Temperature Map,” http://saturn.jpl.nasa.gov/ photos/imagedetails/index.cfm?imageId=1631, NASA image PIA 06432 (29 July 2005). 31. Candy Hansen interview with author, JPL, 27 Oct. 2010. 32. Baker, “Tiger, Tiger.” 33. Paul Helfenstein et al., “Baghdad Sulcus in 3-D,” http://www.ciclops.org/view.php?id=6016, caption for NASA image PIA 11687 (23 Feb. 2010). 34. Linda SPliker review of manuscript, March 2011. 35. Paul Helfenstein et al., “Enceladus’ Warm Baghdad Sulcus ,” http://www.ciclops.org/view/6154/ Enceladus_Warm_Baghdad_Sulcus, caption for NASA image PIA 11696 (23 Feb. 2010). 36. NASA-JPL, “Tiny Icy Moon ‘Feeds’ Giant Saturn Ring,” http://saturn.jpl.nasa.gov/science/ moons/enceladus/enceladusfeedring/, accessed 4 Apr. 2010.

354 The icy moons 37. Frank Spahn et al., “Cassini Dust Measurements at Enceladus and Implications for the Origin of the E Ring,” Science 311 (10 March 2006):1416 – 1418. 38. Dwayne Brown and D.C. Agle, “Salt Finding from NASA’s Cassini Hints at Ocean within Saturn Moon,” NASA news release 09-147 (24 June 2009). 39. Brown and Agle (24 June 2009). 40. J. R. Spencer et al., “Cassini Encounters Enceladus: Background and the Discovery of a South Polar Hot Spot,” Science 311 (10 March 2006):1401 – 1405; B.J. Buratti, “Enceladus— Implications of its Unusual Photometri Properties,” Icarus 75 (July 1988):113-126. 41. Steven W. Squyres et al., “The Evolution of Enceladus,” Icarus 53 (1983):319; C. C. Porco et al., “Cassini Observes the Active South Pole of Enceladus,” Science 311 (2006):1393; J. Wisdom, “Spin-Orbit Secondary Resonance Dynamics of Enceladus,” Astronomical Journal 128 (July 2004) :484–491. 42. D.C. Agle and Dwayne Brown, “Saturnian Moon Shows Evidence of Ammonia,” JPL press release (22 July 2009). 43. J. Wisdom, “Spin-Orbit Secondary Resonance Dynamics of Enceladus,” Astronomical Journal 128 (2004):484-491; Steven W. Squyres et al., “The Evolution of Enceladus,” Icarus 53 (1983):319; C. C. Porco et al., “Cassini Observes the Active South Pole of Enceladus,” Science 311 (2006):1393. 44. Bill Steigerwald, “Cracks on Enceladus Open and Close under Saturn’s Pull,” http://www.nasa. gov/mission_pages/cassini/media/enceladus_cracks_prt.htm, NASA news release (16 May 2007); C. J. Hansen et al., “Water Vapour Jets Inside the Plume of Gas Leaving Enceladus,” Nature 456 (27 November 2008):477-479. 45. Baker, “Tiger, Tiger.” 46. William Farrell of NASA’s Goddard Space Flight Center, as quoted in Fraser Cain, “Enceladus is Supplying Ice to Saturn’s A-Ring,” http://www.universetoday.com/2008/02/05/enceladus-issupplying-ice-to-saturns-a-ring/, Universe Today (5 Feb. 2008). 47. Bill Steigerwald, “Saturn Has a ‘Giant Sponge’,” http://www.jpl.nasa.gov/news/features. cfm?feature=1595, NASA-JPL news release (5 Feb. 2008). 48. W. M. Farrell et al., “Mass Unloading Along the Inner Edge of the Enceladus Plasma Torus,” Geophys. Res. Lett. 35 (2008):L02203. 49. Christopher P. McKay et al., “The Possible Origin and Persistence of Life on Enceladus and Detection of Biomarkers in the Plume,” Astrobiology 8(5) (2008):909. 50. NASA-JPL, “Hyperion,” http://saturn.jpl.nasa.gov/science/moons/hyperion/, Cassini Equinox Mission Web site, accessed 16 June 2009; Tilmann Denk and Preston Dyches, caption to “Odd World,” http://www.ciclops.org/view.php?id=1507, NASA/JPL/Space Science Institute image PIA 07740 (29 Sep. 2005). 51. Peter Thomas et al., caption to “Examining Herschel Crater,” http://www.ciclops.org/view. php?id = 6220, NASA/JPL/Space Science Institute image PIA 12568 (29 March 2010). 52. NASA-JPL, “Mimas,” http://saturn.jpl.nasa.gov/science/moons/mimas/, JPL Cassini Equinox Mission Web site, accessed 10 June 2009. 53. Peter Goldreich and Scott Tremaine, “The Formation of the Cassini Division in Saturn’s Rings,” Icarus 34 (May 1978):240-253; Linda Spilker interview with author, AGU, San Francisco, 14 Dec. 2009. 54. Jia-Rui C. Cook, “1980s Video Icon Glows on Saturn Moon,” JPL news release 2010-103 (29 Mar. 2010). 55. NASA, “PIA12867: Bizarre Temperatures on Mimas,” http://photojournal.jpl.nasa.gov/ catalog/?IDNumber=PIA12867 (29 Mar. 2010). 56. C.J.A. Howett et al., “A High-Amplitude Thermal Inertia Anomaly of Probable Magnetospheric Origin on Saturn’s moon Mimas,” Icarus 216 (2011):221–226.

References 355 57. Linda J. Spilker email to author, 19 Sep. 2010. 58. G.H. Jones et al., “The Dust Halo of Saturn’s Largest Icy Moon, Rhea,” Science 319 (7 March 2008):1380 - 1384; Emily Lakdawalla, “A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea,” http://www.planetarysociety.org/news/2008/0306_A_Ringed_Moon_of_ Saturn_Cassini.html, Planetary Society Web site (March 6, 2008), accessed 3 June 2009. 59. Dwayne Brown and Carolina Martinez, “Saturn’s Moon Rhea Also May Have Rings,” NASA News release 08-074 (6 March 2008). 60. Dwayne Brown and Carolina Martinez, “Saturn’s Moon Rhea Also May Have Rings,” NASA News release 08-074, 6 March 2008; ESA, “Saturn’s Moon Rhea May Also Have Rings,” http:// www.esa.int/esaMI/Cassini-Huygens/SEMY6NK26DF_0.html (7 March 2008), accessed 3 June 2009. 61. Richard A. Kerr, “The Moon Rings That Never Were,” http://news.sciencemag.org/sciencenow/2010/06/the-moon-rings-that-never-were.html, Science Now (25 June 2010). 62. Ker Than, “Saturn Moon Loses Its Ring, Gains a Mystery,” National Geographic Daily News (6 Aug. 2010). 63. Linda J. Spilker email to author, 19 Sep. 2010. 64. Ibid. 65. Lauren Gold, “No Rings Around Saturn’s Rhea,” http://www.spacedaily.com/reports/No_ Rings_Around_Saturn_Rhea_999.html, Space Daily (2 Aug. 2010). 66. E.P. Turtle, P. Helfenstein, P. Thomas, T. Denk, G. Neukum, et al., “Cassini ISS Observations of Saturn’s Icy Satellites Dione, Tethys, Rhea, Mimas, Hyperion and Phoebe,” Geological Society of America Abstracts with Programs 37(7) (2005): 237; Calvin J. Hamilton, “Dione,” in Views of the Solar System, Solar Views Web site, http://www.solarviews.com/eng/dione.htm, accessed 8 June 09. 67. NASA/JPL/Space Science Institute, “Dione Close-Up,” http://www.ciclops.org/view. php?id=673, NASA image PIA 06156 (16 Dec. 2004); NASA. “Captivating Dione,” http:// www.nasaimages.org/luna/servlet/detail/nasaNAS~4~4~13677~115993:Captivating-Dione, NASA Planetary Photo Journal Collection, accessed 28 June 2010. 68. Carolyn Porco, “Highest Resolution View of Dione,” http://www.ciclops.org/view.php?id=680, NASA/JPL/Space Science Institute image PIA 06163 (16 Dec. 2004); Preston Dyches (figure caption), “Scratches on Dione,” http://www.ciclops.org//view.php?id=3860, NASA/JPL/Space Science Institute image PIA 09764 (5 November 2007). 69. NASA-JPL, “About Saturn & Its Moons: Dione,” http://saturn.jpl.nasa.gov/science/moons/ dione/ 70. E.M. Shoemaker and R.F. Wolfe, “Cratering Time Scales for the Galilean satellites,” in D. Morrison (ed.), Satellites of Jupiter, University of Arizona Press, Tucson AZ (1982), pp. 277-339. 71. NASA-JPL, “Helene (Quasi-targeted) Flyby - March 3, 2010,” http://saturn.jpl.nasa.gov/mission/flybys/helene20100303/ (3 Mar. 2010). 72. Bob Mitchell review of manuscript, Feb. 2011. 73. Jia-Rui C. Cook, “Cassini Doubleheader: Flying By Titan and Dione,” JPL news release 2010110 (2 Apr. 2010); Jia-Rui C. Cook, “Cassini Finishes Saturnian Doubleheader,” ,” JPL news release 2010-124 (12 Apr. 2010). 74. NASA-JPL, “Dione Flyby (D-3) - Dec. 12, 2011,” http://saturn.jpl.nasa.gov/mission/flybys/ dione20111212/, Cassini Solstice Mission Web site (12 Dec. 2011). 75. Sven Simon, “Magnetic Signatures of a Tenuous Atmosphere at Dione,” Geophysical Research Letters. 38 (2011):L15102; Amanda Hendrix email to author, 10 Jan. 2012. doi:10.1029/201 1GL048454

14 Titan observations by the Cassini Orbiter “Unveiling Titan is like reading a mystery novel.” – JPL Director Charles Elachi1

14.1

THE VOYAGER LEGACY

“Voyager taught Cassini what instruments to take. Voyager flew by Titan and it saw an orange ball. The minute that happened, we knew we needed different instruments when we went back. We knew we needed a RADAR.” – Trina Ray, Science Lead for the Titan Orbiter Science Team2 Because Titan orbits a planet, it is officially a satellite. But it resembles a planet itself in some fascinating ways. Its size, for one thing: Titan is bigger than the planet Mercury. And it has an atmosphere even denser than Earth’s, which is most unusual for a moon.3 In fact, as mentioned in Chapter 9, if Titan had been a bit warmer, it might look a lot like Earth today. Toby Owens called Titan the Peter Pan moon, the one that never grew up. It has been frozen in its primeval state for billions of years. While Voyager took pictures as well as ultraviolet and infrared spectra of Titan, none of its instruments could penetrate the thick orange haze surrounding the moon. None of the instruments could see its surface, except in very limited ways. So NASA and ESA made sure that the Cassini mission included sensors that could pierce that thick hydrocarbon veil. Voyager did determine that Titan’s atmosphere was very cold, with temperatures low enough to turn its methane, which is a gas in the temperature range in which we live, into rain droplets. Scientists speculated about whether methane rain would form hydrocarbon lakes or even an ocean on Titan’s surface. Earth-based radar as well as the Hubble Space Telescope have been able to see to some extent through the haze, but beyond establishing that Titan’s surface was heterogeneous, these technologies could not discern much detail. That had to await the arrival of the Cassini-Huygens mission in the Saturnian system. So did obtaining further knowledge of the moon’s complex atmosphere.4

© Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_14

357

358

Titan observations by the Cassini Orbiter

14.2

STUDIES OF TITAN’S ATMOSPHERE

“It has 10 times the atmosphere that Earth has. Over every square centimeter on the surface of Titan there are 10 times more molecules above it.” – Kevin Baines, VIMS team5 When Christiaan Huygens first observed Saturn in 1655 using the telescope that he and his brother had built, he noticed a bright star close by the planet. He continued watching the star and after some weeks, realized that it was moving around Saturn. Huygens recognized that he had discovered a satellite. He kept his discovery secret until he was certain about what he had found, but to make sure no one else got credit for it, he sent an anagram to some friends which could be decoded to say, A moon revolves around Saturn in 16 days and 4 hours. Eventually named Titan, it turned out to be the planet’s largest satellite. More accurate observations later revealed that its true orbital period was slightly short of 16 days.6 In 1908, Catalan astronomer Jose Comas Solà claimed to have seen Titan limb darkening – the diminishing of brightness from the center of the moon’s disk to its edge, or “limb.” This phenomenon suggested a substantial atmosphere, a possibility that was confirmed by Gerald Kuiper, whose spectroscopic observations in 1943 and 1944 revealed significant methane around the moon. Voyager 1, besides determining that Titan was enveloped by an opaque layer of clouds and haze, also found that the atmosphere was dominated by nitrogen and methane.7 Titan’s gravity is weaker than Earth’s (more like that of our Moon) because the satellite has far less mass than our planet. As a result, Titan cannot hold onto its atmosphere as tightly and it extends to an altitude many times higher than Earth’s atmosphere. Titan’s atmospheric pressure at its surface is about 60% greater than Earth’s – roughly the same pressure found at the bottom of a swimming pool on our planet. With a thick, high atmosphere and low gravity, one day Titan might become the most desirable hang gliding site in our solar system. But the glider pilots better dress warmly. Titan’s surface temperature has been estimated at –178°C (–289°F).8 The Saturn system, including Titan, orbits more than nine times farther from the Sun than Earth. The intensity of sunlight hitting the top of Titan’s atmosphere is only about 1% the amount hitting Earth. Beneath Titan’s hazy skies, sunlight is far weaker, sort of like it would be during deep twilight on Earth.9 14.2.1

Titan’s haze

If Titan’s haze was less dense, the upper atmosphere would be somewhat colder. But the haze has a strong ability to absorb what sunlight there is, and so the hydrocarbon blanket plays an important role in warming the moon’s mainly nitrogen atmosphere, reducing condensation and losses due to precipitation.10 Haze characteristics may have been critical during the distant past in determining Titan’s climate. Many scientists believe that the Sun was 30% less luminous for the first billion years or so of its existence. At that time, atmospheric haze could have been key in

14.2

Studies of Titan’s atmosphere 359

warming the stratosphere sufficiently to prevent nitrogen collapse,11 in which atmospheric nitrogen becomes cold enough to condense onto the surface, beginning most probably at the poles. Such an occurrence would have resulted in falling atmospheric pressures. The reduced greenhouse effect and possibly reduced heat transport to the poles could then have led to continued nitrogen condensation and a dramatic contraction of the atmosphere. Nitrogen collapse might have occurred on Neptune’s moon Triton.12 Researchers believe that Titan’s haze is formed from complex, high molecular-weight organic molecules. They think that the process of haze generation begins at altitudes above 400 kilometers (250 miles), where ultraviolet light breaks down the methane and nitrogen molecules13 to produce the raw materials for the creation of more complex organic molecules containing carbon, hydrogen, and nitrogen. In due course, these combine to make the very small particles that constitute the haze. Planetary scientists seek to understand the chemistry of Titan’s atmosphere in part because, as mentioned above, it is similar to that of our own world four billion years ago. Spacecraft data suggests that Titan’s atmosphere might even contain prebiotic organic materials.14 In other words, the kind of chemistry occurring on Titan today may be similar to that which long ago created the conditions for the appearance of life on Earth. The complex organics in Titan’s atmosphere are probably not pure carbon and hydrogen, but incorporate some nitrogen, which is partly responsible for giving the haze its color. In addition, Hunter Waite of the Southwest Research Institute and his colleagues have identified the presence of benzene. Based on the observed chemical mix, a series of chemical reactions and physical processes are able to progress from simple molecules – methane and nitrogen – to larger, more complex molecules, and then to the massive molecules called tholins that make up the organic aerosols.15 Titan distinguishes itself from the solar system’s other satellites by having a massive atmosphere made up of approximately 95% nitrogen and 5% methane near the surface. However, the amount of methane decreases at higher altitudes to only a few percent.16 Solar ultraviolet radiation and collisions with charged particles from Saturn’s magnetosphere break up these gas molecules, initiating chemical reactions that readily produce simple hydrocarbons such as ethane and acetylene, as well as nitriles such as hydrogen cyanide and cyanogens. In time, more complex molecules such as propane, butane, polyacetylenes, and cyanoacetylene follow.17 Planetary scientists believe that Titan’s haze resulted from condensation of these and other molecules and polymers in processes similar to the formation of urban photochemical smog on Earth. Chemically, Titan’s haze is quite different from smog on Earth. Most Earth smog is associated with oxides of nitrogen and water. But both types of haze are the result of photochemistry, involve methane, and significantly influence the transparency of the atmosphere.18 Titan’s aerosols may act as nuclei for the condensation of clouds in the troposphere and may affect the compositions of those clouds.19 Different regions of Titan’s atmosphere play various parts in photochemical smog formation, resulting in distinct layers of haze (Figure 14.1). The haze in this figure is split into a main layer that resides below ~220 kilometers and a detached layer in the altitude range 300-350 kilometers. But the haze is seasonally variable. The detached layer may not always be present.20 The initial ionization of nitrogen and methane that begins the process of haze formation occurs high up in Titan’s atmosphere in its ionosphere-thermosphere

360 Titan observations by the Cassini Orbiter

Figure 14.1 Haze layers in Titan’s atmosphere. Note the detached layers of haze above the gap and the main layer below.

region (~1,000 kilometers altitude) and possibly also in the lower stratosphere (~200 kilometers). The hydrocarbon and nitrile products resulting from these reactions begin condensing into aerosols below ~200 kilometers, and this process continues down to the tropopause at ~40-50 kilometers. In the long term, the various aerosols precipitate out of the troposphere and fall to the surface, where they accumulate. 14.2.2

Benzene, PAHs, and smog

Our knowledge of Titan’s atmosphere expanded in 2003 when the European Space Agency’s Earth-orbiting Infrared Space Observatory tentatively detected parts per billion of benzene, a ring-like molecule of six carbons and six hydrogens (C6H6) that is involved in the creation of smog. Benzene is an essential step in the formation of polycyclic aromatic hydrocarbons (PAH). When the mass of a PAH reaches about 2,000 daltons (a dalton is an atomic mass unit equal to 1/12th the mass of a carbon atom) PAHs convert from the gaseous phase to

14.2

Studies of Titan’s atmosphere 361

particulates. Ultimately soot forms, sort of like the exhaust from a diesel engine. Models indicate that PAH polymers may be the largest contributors to Titan’s smoggy haze.21 14.2.3

Atmospheric circulation and wind patterns

The continuous monitoring of Titan’s atmosphere by the Cassini Orbiter since 2004 eventually started to reveal seasonal changes in atmospheric circulation, providing clues to the moon’s global climatology. In a study of over 10,000 images captured by the Visual and Infrared Mapping Spectrometer (VIMS) between July 2004 and December 2007, an international team led by Sebastien Rodriguez of the University of Nantes in France identified over 200 individual cloud activities. The goal was to analyze the spatial distribution, temporal variance, and spectral characteristics of the clouds. Identifying Titanian clouds was not an easy task because the moon’s thick stratospheric haze layer hid almost everything that lay below, including weather. The Nantes study was made possible however by the vast observational data the Orbiter accumulated over the years. But even with cutting-edge technologies, most Titanian clouds remain undetectable. So to start, scientists applied the parameters they knew about the moon – solar intensity, atmospheric composition, rotation rate, and orbital parameters – to predict what the weather might be. One thing that emerged from this modeling was that, with a day that is 16 times longer than that of Earth,22 the slow rotation rate of Titan would probably lead to atmospheric circulation patterns radically different to those on Earth. On our planet, solar radiation incident in equatorial regions causes heating that is transported away toward the poles through a major circulation system called Hadley cells, which start at the equator and extend to about 35° north and south. This leads to two symmetric circulation patterns, one in each hemisphere. One factor that influences atmospheric circulation patterns is the Coriolis effect. On a rotating body such as Titan or Earth, atmospheric flows tend to be deflected in a direction perpendicular to the flow, with the amount of deflection dependent on the rotation rate. On Earth, the strong Coriolis effect limits the extent of Hadley cell circulation. But the slow rotation rate on Titan weakens its Coriolis effect, and this should allow a single Hadley cell to straddle the equator and transport heat directly from the hemisphere experiencing summer to the one in which winter is occurring. In fact, modelers have predicted that Titan has but one gigantic pole-to-pole convection cell.23 Cloud observations have not necessarily supported this model, however. Cloud phenomena are discussed later in the section on Titan. 14.2.4

The north polar wind vortex and hood

From data taken during early flybys of Titan, the Composite Infrared Spectrometer (CIRS) team identified an isolated vortex of wind at Titan’s north pole similar to one that occurs at Earth’s south pole. CIRS data indicated that strong winds circulating the north pole of Titan tend to isolate the atmosphere there in winter, inhibiting the mixing of polar air with that in the surrounding regions during the long polar night. A series of steps may have led to this situation. Heavy organic molecules form naturally in Titan’s atmosphere, leading to a layer of orange haze that blankets the satellite. Because the stratospheric air over Titan’s winter pole is colder than in other regions, it sinks and

362 Titan observations by the Cassini Orbiter takes down with it the heavy organic compounds that formed higher up. This subsidence shields the heavy species from further break-up by solar radiation. If the air over Titan’s north pole remains isolated during the winter, the heavy organics would thus be expected to accumulate in the stratosphere, and this is just what the CIRS team observed – concentrations of several heavy organic species were highest there during winter.24 Another northern polar phenomenon is a sort of dark mantle known as the north polar hood that occurs during winter in the moon’s stratosphere. It is detectable in the visible and infrared as well as the near-ultraviolet parts of the spectrum, and was seen not only by Cassini but also by the Voyagers approximately one Saturn year, or roughly 30 Earth years, earlier. The polar hood of Titan is analogous to a feature on Earth: the Antarctic ozone hole, although their chemistries differ. On our planet, the south polar atmosphere is isolated for months during its winter in a similar manner to Titan’s northern polar region. This results in the formation of stratospheric clouds above our south pole. Normally inert chlorine compounds undergo chemical reactions on the crystals in these clouds, liberating molecular chlorine. Spring sunlight then decomposes these molecules of chlorine, leading to the annual ozone hole. Both our Antarctic ozone hole and Titan’s north polar hood involve downwelling from the upper atmosphere, bringing air enriched in certain compounds to locales where they are not normally found. These materials are usually colder than the rest of the atmosphere because they have been exposed to the long polar night. They may have condensed, whereas normally they might be in gaseous form. These materials may include compounds that would normally have been destroyed by solar ultraviolet light but have survived as a result of being in polar darkness. Also, both phenomena seem to typically break up with the onset of spring.25 Titan’s axis of rotation is tilted similarly to Earth’s, and this is why both bodies’ poles experience a long period without direct sunshine during winter. But Titan’s polar night is especially long because its winter is many Earth years in duration. 14.2.5

Atmospheric super-rotation

Images of Titan’s surface and lower atmosphere established that the moon’s winds blow significantly faster than the body rotates. Titan is similar in this way to Venus. Both bodies rotate slowly; Titan takes 16 Earth days to make one full rotation while Venus requires 243 days. And the atmospheres of both bodies move far faster than the solid surfaces do. This is called super-rotation, in that the atmosphere moves independently of, and at greater speed than, the solid surface. In contrast, the jet stream of Earth travels significantly slower than our planet’s surface.26 Near-surface winds may have an interesting origin on Titan. While solar heating is definitely a factor as it is on Earth, the Sun is far more distant from Titan and thus only 1% as intense. But there is another potential driver for these winds. Numerical circulation models that incorporate the portion of Titan’s atmospheric tides due to Saturn’s gravity, which is 400 times as strong as our Moon’s tidal effect on Earth, reveal that near-surface wind generation may indeed be dominated by Saturn rather than the Sun.27

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Studies of Titan’s atmosphere 363

Clouds

Even though Titan’s haze forms a shroud around the entire satellite, clouds are rare occurrences. Those whose motions were trackable were often small, typically 100 kilometers (60 miles) across and too faint to be seen from Earth. Identifying moving clouds required careful processing of images. Cassini scientists used special filters designed to help see through Titan’s ubiquitous haze, enabling them to detect some surface features as well as clouds. But it was a challenge to distinguish clouds from the satellite’s complex bright and dark surface. John Barbara of NASA’s Goddard Institute for Space Studies (GISS) in New York City was able to discriminate clouds from surface features by imaging the same region at different times and subtracting out common features on one image from those on another. When he did this, time-variable clouds stood out as regions of changing brightness.28 Wind speeds calculated from ten of these clouds were as high as 34 meters per second (approximately 75 miles per hour), which would be hurricane strength on Earth. These clouds were found in Titan’s middle and lower troposphere. Orbiter instruments also imaged much larger cloud streaks of 1,000 kilometers (600 miles), originating closer to the surface and moving at only a few meters per second. The locations of many of the clouds imaged in the University of Nantes global circulation study referred to above were consistent with a pole-to-pole atmospheric convection cell. For instance, clouds were seen where the ascending point of Hadley circulation should be in the southern mid-latitudes; cumulus clouds were observed at the south pole in summer, where the pole-to-pole circulation should drive methane storms; and other appropriate clouds were identified around the north polar region, where stratospheric ethane was expected to condense in the cold winter polar night. The pole-to-pole Hadley model implied that activity in the southern hemisphere should weaken and south polar clouds should disappear as the August 2009 equinox approached. This prediction came true when the clouds at the south pole completely disappeared just before equinox.29 Although some of Titan’s cloud phenomena appear related to global circulation patterns, others seem to have a more local driver. For instance, one group of clouds cluster above a particular region of the moon’s surface near 350°W and 40°S. Their presence cannot be explained by a seasonal shift in global circulation and thus may reflect local events on the surface such as geysering, cryovolcanism, or mountain ranges that could create orographic clouds (those that develop through forced lifting of air) as moist air rises to clear elevated terrain. Although they may occur only at specific locations, such events could have larger implications, such as for Titan’s methane cycle. For example, the amount of volatile chemicals that would have to be released in order to trigger the mid-latitude cloud formation might also supply enough methane to the atmosphere to balance its loss at high altitudes to photochemical processes.30 14.2.7

Titan’s methane cycle and rainfall

The methane in Titan’s atmosphere can exist as a gas, as ice crystals or as a liquid. It can form clouds, rain, and surface or subsurface liquid bodies. The methane cycle on Titan resembles the hydrological cycle on Earth in some ways, albeit with a different chemical.31

364

Titan observations by the Cassini Orbiter

Methane accounts for 1.5 to 5% of Titan’s predominantly nitrogen atmosphere. Like water vapor on Earth, the methane percentage varies with time as well as with altitude. Titan is the only other place in our solar system besides Earth where rain is believed to fall. But due to surface tension and aerodynamic forces, a falling drop of liquid methane may be huge – fully a centimeter in diameter, over 100 times larger than raindrops on Earth. And such gargantuan drops would fall far slower owing to the thick atmosphere and low gravity. On Titan a raindrop would descend rather like “fluffy snowflakes do on Earth.”32 Rain which falls on Earth is subjected to ample solar energy that can eventually evaporate it back into the atmosphere, completing our planet’s hydrological cycle. But due to Titan’s great distance from the Sun and its atmospheric haze, only one-thousandth as much sunlight gets to Titan’s surface as to Earth’s surface, greatly reducing its evaporative effect. The average Titan rainfall is estimated at only one centimeter (0.4 inch) per Earth year versus 1 meter (3 feet) on Earth.33 Titan’s rainfall is not constant. The moon’s riverbedlike erosion patterns suggest that when rain does occur, it may well be monumental. Titan likely has a climate typified by centuries of drought with brief periods of spectacular storms causing massive floods and significant erosion. Because photolytic chemistry – decomposition and other reactions induced by sunlight – has been estimated to occur at a rate that would deplete the methane in Titan’s atmosphere within 107 to 108 years, the persistent presence of methane in the atmosphere indicates that some replenishment source must exist on or beneath the surface.34 Otherwise Titan’s atmospheric methane would have been lost by now. But Titan’s lakes are not deep enough to be the main reservoirs providing a long-term resupply of methane to the atmosphere. To do this, the lakes would have to be tens of kilometers deep. Alternatively, the extensive methane or methane-ethane equivalent of a terrestrial aquifer system might replenish the atmospheric methane. An aquifer that extends tens of kilometers down through the water ice crust could possibly have the same effect as a global surface layer of pure methane a kilometer thick – the amount required to feed enough methane into the atmosphere to balance against photolytic losses over geological time. But at present, there is insufficient evidence in the Cassini data to rule either for or against such an aquifer system.35 A more likely source of methane is Titan’s deep interior. Perhaps cryovolcanoes bring water with entrained methane to the surface. Although there is some evidence of cryovolcanism, this interpretation of the data is disputed. We will return to this topic later in this chapter.

14.3

EXPLORING TITAN’S SURFACE FROM ORBIT

The remote sensing instruments on the Cassini Orbiter’s obtained evidence of a wide variety of geological features on Titan: including impact cratering; river or stream erosion patterns; shallow lakes; aeolian (wind-related) structures formed by dynamic meteorological processes; cryovolcanic and tectonic features driven by energy from the interior; and a surface veneer built up by a rain of complex organic particles. The surface exhibits a certain amount of topography, with the relief ranging from several tens to several hundreds of meters near where the Huygens Probe landed, to mountains elsewhere that rise several thousand meters. A general trend observed on Titan is that bright features are topographically high, while darker features tend to be low.36

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RADAR imaging

“Titan went from this orange haze-covered ball that we saw in the Voyager data to a world that looks hauntingly familiar, very reminiscent of the Earth.” – Linda Spilker, Project Scientist37 During the many Titan flybys of Cassini’s tour of the Saturnian system, its RADAR has proved invaluable for mapping and observing swath after swath of the satellite’s surface, slowly exposing a world that is tantalizingly like our own, yet dramatically different. The first RADAR images revealed an extremely complex and fascinating geology with many surprises, and confirmed the requirement for a wide range of sensors to thoroughly study this moon. Project Scientist Dennis Matson pointed this out when he expressed the difficulty of the investigation that lay ahead, commenting: “We are glad that we have a full complement of instruments on this spacecraft because it is going to take all of them to reveal the story of Titan.”38 14.3.2

The RADAR instrument

The Orbiter’s RADAR imaged Titan by bouncing microwave radio signals off its surface and timing their return. This approach could be compared to standing on the edge of a canyon, shouting, timing how long it takes for an echo to return, and then using our knowledge of the speed of sound waves to estimate how wide the canyon is. RADAR conducted millions of these echo bounces, but it used radio instead of sound waves. From the returning echoes, the instrument painstakingly assembled a three-dimensional map of Titan’s surface. The instrument also provided indications of surface composition. Early RADAR data, for instance, suggested that Titan is covered with carbon based compounds. The system used the same radio dish – the high-gain antenna – that the spacecraft employed for communicating with Earth. As explained in Chapter 3, the instrument could operate in three modes: radiometry, in which the antenna pointed at a target and “listened” for radio energy, indicative of the body’s temperature; altimetry, in which the antenna bounced a radio signal off the surface and calculated the distance to that part of the surface by when the echo returned; and synthetic aperture radar (SAR), in which the instrument built up images.39 Because radio wavelengths are far longer than those of visible light, SAR cannot resolve features as small as visible light can. RADAR’s SAR resolution capability “gets down to about 350 meters, but that is about it. Compare that with [visible light pictures of] Mars, where we can almost see the ants on the ground, if there were any …”40 Since neither visual nor infrared imaging can access Titan’s surface that well, SAR images are the sharpest the Orbiter can achieve, with a horizontal resolution of 350 to 1,700 meters (1,100 to 5,600 feet). To collect this data, the Orbiter looked off to one side of the ground track – the path it flew over – instead of straight down as it did for altimetry. The high-gain antenna sent a signal to the moon, then listened for the echo and measured the delay time – the period between signal transmission and return of the echo. But the echo was not exactly like the signal originally sent. It got stretched out over time because the signal returned first from the ground closest to the spacecraft, and later from the ground farther away. Also, the echo was Doppler shifted to slightly different wavelengths depending on whether it had bounced off the ground ahead of the Orbiter – which, due to the spacecraft’s velocity,

366 Titan observations by the Cassini Orbiter appeared to be moving toward the craft, shifting the wavelength shorter – or off the ground behind, which appeared to be retreating and thus shifted the wavelength longer. To better understand how SAR “saw” an object on Titan’s surface, think of a flash camera versus one without a flash. A camera without flash uses natural light to illuminate an object, but a flash camera emits electromagnetic waves to illuminate the object. The light reflected back to the camera forms the image. On Cassini, the RADAR emitted electromagnetic waves in the radio range, with wavelengths far longer than visible light. Beyond this similarity, flash cameras and radar imagers operate quite differently. Visual digital cameras build up images from the pixels they photograph and place them in an image based on how far off the center of the lens each pixel resides. The Orbiter RADAR placed the pixels based partly on their range – how far away each one was. Each pixel was located at a spot along the vertical axis that depended on how long it took the reflected signal to come back. The pixels were placed along the horizontal axis based on their Doppler shift – the frequency changes of the reflected signals.41 Computer processing built up a three-dimensional characterization of the surface from these two measurements of each point on the surface – the distance to the point determined by the time it took for radar signals to return, and the Doppler shift in the returned signal.42 14.3.3

RADAR’s first close look at Titan’s surface

During the Orbiter’s initial close flyby on 26 October 2004, which had a minimum range of 1,174 kilometers (729 miles), the RADAR managed to map approximately 1% of Titan’s surface at a resolution of 500 meters, which meant the system “saw” objects of that size or greater. The RADAR also mapped larger areas of the body in lower resolution modes. The images revealed a complex terrain, with areas of low relief and a variety of geological features suggesting dome-like volcanic structures, flows, and sinuous channels. One of the most notable results was “the lack of unambiguous impact craters.”43 Objects producing craters smaller than 20 kilometers (14 miles) would have been screened by the atmosphere, meaning that the objects would have been destroyed during atmospheric entry. The surfaces of some of Saturn’s other larger satellites, however, contain heavily cratered terrains that have typically 200 to 400 craters per million square kilometers that are larger than 20 kilometers. If the swath of Titan mapped by the RADAR were an ancient surface, 100 to 200 craters should have been seen. Instead, no fresh craters and only a few degraded possible craters were observed. Cassini scientists therefore concluded that the resurfacing rate in this first Titan area studied must be considerably higher than the impact cratering rate. Or to put it another way, this part of Titan’s surface was geologically young. 14.3.4

VIMS and ISS: Other instruments that imaged the surface

When Voyager 1 made its close flyby of Titan in 1980, one of the disappointments was “all we saw was smog – L.A. smog. We didn’t see anything of the surface.”44 So at that point, planetary scientists realized that to explore Titan a radar would be required capable of seeing through the haze. Hence one was included on the Cassini Orbiter. What most scientists did not realize at the time was that infrared instruments would also be able to yield valuable surface data, as Titan’s haze was transparent in a number of infrared frequency ranges.

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Cassini capitalized on this opportunity: the Visual and Infrared Mapping Spectrometer (VIMS) and Imaging Science Subsystem (ISS) were able to see some of the surface features and also collect data concerning its surface composition. For instance, VIMS confirmed the presence of “dirty” water ice and, in some of Titan’s distinctly bright regions that could be cryovolcanic flows, suggested the presence of carbon dioxide frost. This was interesting because carbon dioxide was not a likely component of cryomagma; it was probably condensed out of the atmosphere.45 Excellent correlations were found between the RADAR and VIMS data in their observations of many surface features. This was especially so for many (although not all) areas appearing bright in RADAR observations and thought to be elevated terrain. Jointly analyzing Titan features with both RADAR and VIMS proved quite useful for testing hypotheses about the nature of the surface, especially since each instrument measured different characteristics. VIMS data provided information on surface topography, composition, and grain size, but penetrated the surface only to depths of several tens of microns, a tiny fraction of an inch. Operating in SAR mode, the RADAR penetrated to meter-scale depths and provided information on slopes, roughness, and composition.46 14.3.5

Pre-Huygens RADAR, VIMS, and ISS data from Titan’s surface

Some of the Cassini Orbiter’s early haze-piercing images of Titan, taken at visible and infrared wavelengths in October 2004 before the Huygens Probe was released, showed “a strange, striated landscape that both thrilled – and mystified – planetary scientists.”47 The images revealed sharply defined bright and dark regions, possibly blanketed by a thin transparent or translucent layer. Within the dark expanses were white, island-like areas. The data also showed linear streaks that were indicative of active geological processes. But these preliminary pictures may have raised more questions than answers. Imaging team leader Carolyn Porco commented that “there isn’t much we are absolutely, definitively confident about right now.”48 What was not seen was perhaps more interesting than what was imaged. No large craters were apparent, suggesting relatively recent resurfacing by tectonic, volcanic or depositional processes. No proof of liquid hydrocarbon lakes showed up during these early flybys either, even though many scientists believed such lakes must be present, given the satellite’s ultra-low temperature, high atmospheric pressure, and organic chemistry. Orbiter as well as Probe data of Titan’s surface revealed significant geological variations as well as the effects of a variety of processes, including possible fluvial erosion, impacts, and cryovolcanism. What was suggested from early data was the possibility of aeolian transport – dust and sand particles distributed by the wind. Ralph Lorenz and his colleagues on the RADAR team argued for one form of this mechanism: saltation processes (from the Latin for “leaping”) that move small particles around through a series of jumps or skips. On Earth, and possibly on Titan, saltating particles returning to the ground and hitting other particles may bump them up into the air to carry on the process. At high wind speeds, saltation can be a continuous process.49 Large-scale near-infrared data from the Imaging Science Subsystem (ISS) on the 26 October 2004 flyby revealed dark streaks suggesting a net eastward transport of material,

368 Titan observations by the Cassini Orbiter possibly by the wind.50 RADAR imaging data from the 15 February 2005 flyby found numerous distinct linear features spaced 1 to 2 kilometers (0.6 to 1 mile) apart that were many tens of kilometers long and typically oriented in an east-west direction; these were nicknamed “cat scratches.” 14.3.6

The search for hydrocarbon lakes

When the Orbiter passed over Titan’s cloudiest known region in June 2005, its ISS saw something which fascinated team members: a 234 by 73 kilometer (145 by 45 mile) dark spot about the size of Lake Ontario which had a perimeter “intriguingly reminiscent of the shorelines of lakes on Earth”51 (Figure 14.2). The south polar region containing this potential lake was a likely spot for methane rainfall to occur. According to imaging team member Tony DelGenio of NASA’s Goddard Institute for Space Studies in New York City, “It’s possible that some of the storms in this region are strong enough to make methane rain that reaches the surface.”52 Tempting as it was to call this feature a liquid lake, planetary scientists required more evidence. A past lake that dried up, for instance, might have left behind dark deposits that

Figure 14.2 Titan’s south polar region with a dark feature that may be a lake of liquid hydrocarbons.

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looked like what had been observed. Meanwhile, numerous new lake candidates were found. The 22 July 2006 flyby of Titan’s high latitudes saw well-defined dark patches in RADAR images. Scientists were fairly confident that at the frigid –179°C temperature of Titan’s surface, the liquids in any such lakes were most likely to be methane or a combination of methane and ethane. But proof that there were bodies of liquid remained elusive. The Orbiter had not passed over this area of Titan before. During the flyby, its RADAR spotted several dozen lake candidates as small as 0.6 miles wide, with some nearly 20 miles wide. The largest was 62 miles long. Dark regions in RADAR images generally imply smoother terrain that reflects less signal back to the instrument. Rough terrain with surfaces lying at a wide range of angles, reflects more signal back and therefore appears brighter. Some of the new RADAR images were completely black, which meant they were extremely smooth features. This was consistent with the surface of a liquid. What is more, the images showed channels that might have been carved by liquids leading into or out of the dark patches. If these dark areas did turn out to be lakes, they would be potential sources for the high concentrations of methane and other hydrocarbons in Titan’s atmosphere.53 Evidence for liquid bodies on Titan continued to mount with the sighting in 2007 of dark features that were larger than any of North America’s Great Lakes (Figure 14.3). The Orbiter’s RADAR imaged one dark feature near Titan’s north pole that stretched for more than 1,000 kilometers (600 miles). If the entire dark area was a lake, it would be almost the size of Earth’s Caspian Sea and would have covered a greater fraction of Titan than the largest inland body of water on Earth, the Black Sea.54 In July 2008, the VIMS instrument, which identified the chemical composition of objects by the way in which they reflected light, examined a Titan dark area named Ontario Lacus and detected the hydrocarbon ethane. Models predicted that this was in liquid solution with methane, nitrogen and other hydrocarbons of low molecular weight. Principal Investigator Robert Brown of the University of Arizona’s Lunar and Planetary Laboratory commented, “This is the first observation that really pins down that Titan has a surface lake filled with liquid.”55

Figure 14.3 A body of liquid on Titan exceeding the size of Lake Superior, one of Earth’s largest lakes. The Titan lake is at least 100,000 square kilometers (40,000 square miles) in extent.

370

Titan observations by the Cassini Orbiter

The VIMS data at a wavelength of 2 microns confirmed that the lake held ethane because the reflected signal dipped at the precise wavelength that ethane absorbs infrared light. Furthermore, the team decided that Ontario Lacus had to be liquid because it reflected so little light. According to Brown, “It was hard for us to accept the fact that the feature was so black when we first saw it. More than 99.9% of the light that reaches the lake never gets out again.”56 To be that dark, it had to be so quiescent and mirror-smooth that it simply could not have represented a naturally occurring solid surface.57 14.3.7

Why are the lakes distributed unevenly between Titan’s northern and southern polar regions?

The RADAR imaging data revealed the puzzling fact that the hydrocarbon lakes in Titan’s northern high latitudes covered 20 times more area than those in the southern high latitudes. In addition, there were more partially filled and empty lake basins in the north. A number of explanations for this difference were proposed. Could the perceived asymmetry be a statistical fluke rather than real? Scientists considered this unlikely because of the large amount of data collected over the years. Was there something inherently different about the topography of the north polar region versus the south, perhaps causing more rain or quicker draining or infiltration into the ground in one hemisphere versus another? This last idea seemed unlikely, as no known differences of this type had been found between the two regions. Alternatively, the mechanism responsible for regional asymmetry might have been related to Titan’s seasons. One year on Titan lasts 29.5 Earth years, and approximately every 15 Earth years Titan’s seasons reverse. Scientists know that methane rainfall and evaporation vary with the seasons, and this perhaps resulted in the filling of lakes during part of the year and drying them out during another. But this idea too was shot down because dry seasons would only account for decreases of about one meter per year in the depths of lakes. Since Titan’s lakes are a few hundred meters deep on average, they wouldn’t drain (or fill) during a 15 year season. In addition, seasonal variations do not explain the puzzling difference between hemispheres in the number of empty lake basins. The north polar region has around three times as many dried-up lake basins as the south and seven times as many partially filled lake basins. According to Oded Aharonson of Caltech, a more plausible explanation for the hemispheric asymmetries is related to the eccentricity of Saturn’s path around the Sun. Saturn’s orbit, like that of Earth and other planets, is not perfectly circular. It is slightly elliptical (eccentric) and the planet approaches 12% closer to the Sun during Titan’s southern hemisphere summer than during its northern summer. This orbital characteristic results in northern summers being longer and more subdued, whereas southern summers are shorter and more intense.58 The heart of Aharonson’s logic is that, for Titan, the difference between evaporation and precipitation is not the same during opposite seasons, and this leads to a net transport of methane from south to north. Over time, more methane, and hence more lakes, accumulate in the northern hemisphere.59 It is important to note that this net transport of methane is not permanent. Over tens of thousands of years, Saturn’s orbital parameters change. At times during this cycle, Titan travels closer to the Sun during its northern summers and farther away during southern summers. This should reverse the direction of net methane transport and produce a

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build-up of hydrocarbons and an abundance of lakes in the southern hemisphere. On Earth, a similar phenomenon results in global redistribution of water in the form of glaciers that may be related to ice-age cycles. 14.3.8

Fluvial activity

The locale around where Huygens landed exhibits complex systems of channels that have been carved into relatively bright, possibly highland materials. These channels drain into a darker and smoother area that might well be a large, dry lake basin. This possible lake bed appears too large to have been fed by the observed channels in the area, so it might be a remnant from either a considerably wetter period in the moon’s history or a catastrophic flooding event. In general, the Huygens landing site region suggests that significant fluvial erosion and deposition processes can occur.60 Fluvial features (those related to the flow of liquids) were observed in other areas as well, some of which are much larger than the Huygens site, several hundreds of kilometers in length. Some, predominantly at low- and mid-latitudes, are braided and resemble desert washes on Earth. Others at high latitudes drain into or connect polar lakes.61 Most of the fluvial features so far observed (mainly by the Cassini Orbiter, but also to a smaller extent by the Huygens Probe) are dendritic,62 meaning that they have branching structures somewhat like a tree, and suggest an origin in rainfall.63 But some of the channel networks possess short, stubby features and may be spring fed. These channels are typically wider, often relatively straight, and can begin or end in dark circular areas, possibly ponds or pits. Both of the above fluvial features are illustrated in Figures 14.4 and 14.5. 14.3.9

Can terrestrial models of fluvial erosion be applied to Titan?

Fluvial channels on Titan, so Earth-like in their appearance, offer an opportunity to determine if models of bedrock erosion designed for similar structures on Earth are exportable to the radically different environment of Saturn’s largest moon. In one laboratory experiment conducted to help shed light on this issue, erosion models based on strength properties of silicate bedrock on Earth were examined for their suitability to Titan, where the –179°C “rock” is mostly water-ice and hydrocarbon liquids flow through the fluvial channels.64 To determine if the erosional properties of cryogenic water ice resembled those of Earth-type bedrock, the dependencies on temperature of certain water ice properties were measured. For instance, ice’s tensile strength was determined as a function of temperatures that ranged from near melting levels to as cold as Titan’s surface. The experiments showed that ice’s tensile strength increased significantly as temperature decreased. Fracture toughness was also tested and it, too, increased as temperatures plummeted to approximately –23°C, but below this value it appeared insensitive to temperature. In other experiments, water ice at cryogenic temperatures was shown to have a percussive strength comparable with that of soft terrestrial rocks.65 While the results from these experiments yielded some evidence that geological erosion models used on Earth may be exportable to Titan environments, there were notable differences in the behavior of ice versus rock as a function of temperature. For instance, the fracture toughness of artificial sandstone was strongly correlated with its tensile

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Titan observations by the Cassini Orbiter

Figure 14.4 Dendritic fluvial features observed by the Orbiter RADAR.

Figure 14.5 Two short, stubby, relatively wide and straight fluvial channels on Titan, indicated by arrows at the ends.

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strength at various temperatures. But there did not appear to be such a correlation between the fracture toughness and tensile strength of ice. Future work on the erosion-related properties of Titan surface materials will hopefully shed more light on the applicability of terrestrial geological modeling to Titan’s surface.66 14.3.10

The case for cryovolcanism

Most of us think of volcanoes as cone-shaped mountains which have smoke and lava spewing from their summits. But rather different types of volcanoes can form on the frigid moons of the outer solar system, from which ice, water, and chemicals such as ammonia or methane erupt. These are cryovolcanoes, and they may help explain the orange-brown blanket of smog that veils Titan. Models of Titan’s smog layer envisioned that it was produced by photochemical reactions of methane, nitrogen, and their dissociation products. The reactions were triggered by solar ultraviolet light, and led primarily to the formation of ethane and heavier hydrocarbons. But there was a problem with this theory. The production of ethane, and the erosion of elevated terrain by rainfall as the ethane condensed out, should have created a global ocean with a depth of about 1 kilometer, according to planetary scientists. However, the Cassini-Huygens mission did not find any such ocean. Cryovolcanism can potentially produce other structures capable of sequestering ethane, and thus the existence of cryovolcanoes could help support the models of Titan’s haze and hydrocarbon cycles. If cryomagma that was rich in water ice and ammonia regularly flowed across Titan’s surface and cooled, liquid hydrocarbons such as ethane could have percolated down through the magma’s pores, accumulate in the pores of the flows’ subsurface layers, and remain isolated from the surface to form subsurface reservoirs that could sequester huge amounts of hydrocarbons. For example, even if subsurface cryomagmas had porosities of only 10%, a 2,300 meter thick layer could hold all of Titan’s liquid hydrocarbons generated over the entire lifetime of the solar system.67 Cryovolcanoes are not found on Earth, but may be common throughout the outer solar system. Imagine, for a moment, what would have happened if major eruptions on Earth were cryovolcanic in nature. When Mount Vesuvius erupted in the year 79, its hot gas, ash, and rock engulfed the Roman cities of Pompeii and Herculaneum and killed 10,000 to 25,000 people. But if Mount Vesuvius had been a cryovolcano, according to JPL RADAR team scientist Rosaly Lopes, “its lava would have frozen the residents of Pompeii.”68 Cassini observations have revealed relatively few craters on Titan, indicating a young surface. This result is consistent with cryovolcanism possibly renewing the surface on a regular basis. Studies by Robert Nelson, a senior research scientist at JPL, along with many co-workers, uncovered compelling evidence that the moon’s surface might indeed be replenished by active cryovolcanic processes. VIMS data showed significant changes in the infrared reflectance of a 73,000 square kilometer bright region called Hotei Reggio, also known as Hotei Arcus, between July 2004 and March 200669 (Figure 14.6). Its reflectance increased by a factor of two, then returned to its former level, then became even brighter before fading once again. The area of this bright region roughly mirrored the reflectance variations, doubling then falling to nearly its initial value, then rising and falling again.

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Titan observations by the Cassini Orbiter

Figure 14.6 Active cryovolcanism on Titan? This is an image of Hotei Reggio viewed in infrared. Brightness variations, combined with a flow-like morphology, suggest that Hotei Reggio may be the site of current activity and is possibly an ice volcano. This image was taken at a distance of 29,000 kilometers (18,000 miles) on 19 November 2008.

Spectral analyses on eight different flybys suggested that the reflectance changes occurred at or very close to the surface, and ruled out differences between the region and its surroundings due to the distribution of materials such as ices of water, carbon dioxide, or methane. The changes were consistent with the deposition and removal of ammonia frost over a water ice substrate, which was interesting because planetary scientists proposed ammonia as a constituent of Titan’s interior. An ammonia frost could be explained by episodic cryovolcanism in which it was brought up from the subsurface. Localized geysering from ammonia-rich deposits in the crust could also have accomplished this. The reflectance changes probably were not explicable by precipitation of ammonia from the atmosphere, where it has a very low abundance. Also, if the brightening was formed by precipitation, we would have expected to see ammonia polar caps and deposits at high latitudes. These have not been observed.70 Certain morphologies strengthened the case for cryovolcanism,71 such as lobate flows whose leading edges resemble the shapes of ear lobes (Figure 14.7). In 2009, Cassini scientist Randy Kirk of the U.S. Geological Survey used overlapping swaths of RADAR imagery taken on different flybys to produce a three-dimensional map of the Hotei Reggio area showing several lobate structures, each of which was 100 to 200 meters high and had shapes and thicknesses consistent with highly viscous, lava-like material.72 Even more dramatically, when VIMS and RADAR images of Hotei Arcus were superimposed, the data showed a bright area that looked like it could be a circular plug of magma in a structure resembling a volcano.73 Furthermore, fluvial channels appeared to be truncated by the bright unit in Hotei Reggio, as might have occurred if, after the channels had formed – perhaps by liquid streams of methane in the wake of numerous rain storms – a cryovolcanic event had released magma that flowed across and blocked the channels.74 Observations prior to the May 2007 flyby suggested that the cryovolcanic features were older than Titan’s dunes. However, more recent VIMS observations of spectral changes “indicate that volcanism on Titan is … possibly still ongoing.”75 Features such as Hotei Reggio might indeed be actively dispersing and outgassing material.76 Theories of recent

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Figure 14.7 Taken 22 February 2008 by the Cassini Radar Mapper, this image of a region just north of Hotei Reggio shows lobate, flow-like features consistent with cryovolcanic activity. The region is approximately 400 kilometers (250 miles) across.

cryovolcanic activity in the Hotei Reggio area are supported not only by VIMS, but also by RADAR and ISS data. Another explanation posited for the observed changes in appearance was that they were caused by transitory atmospheric processes such as clouds. However, when this possibility was analyzed for two regions, planetary scientists found that the changes could not be explained by clouds high above the surface. They were due to processes occurring at or very near the surface. The changes could conceivably have been due to clouds very near the ground – for instance, a fog bank, but if fog was the reason, the appearance of the region should have changed in shape and size as a function of wind activity and this was not the case. Furthermore, the regions in question were at relatively low latitude where ground fog seemed to be unlikely. Finally, during 2006, a region that had exhibited changes in appearance was closely tracked. The behavior expected if fog had been the cause never occurred. Alternatively, precipitation could either have deposited a condensate or washed away an existing surface deposit, thus altering the appearance of the surface. There is also the possibility that the observed changes were due to a single instance of aeolian (windrelated) deposition such as a sandstorm. But if this were the case, then dune-like deposits might be expected at the location in question, and no such morphology was found.77 In December 2010, Randy Kirk stated, “We think we have found the strongest case yet for an ice volcano on Titan.”78 He was talking about the Titan formation called Sotra Facula, which included a tall mountain, a deep crater, and lobate flows – all clues that this might really be a cryovolcano. While some cryovolcanoes, such as the tiger stripes on the moon Enceladus, do not look at all like terrestrial volcanoes, Sotra Facula has a certain similarity to Mt. Etna in Italy and Laki in Iceland (Figure 14.8).79 Although the data strongly suggested cryovolcanism, this interpretation has been disputed. William McKinnon of Washington University in St. Louis, Missouri, for instance, says the data is ambiguous and it was “much more logical … that wind or weather does

376 Titan observations by the Cassini Orbiter

Figure 14.8 Sotra Facula, a possible cryovolcano. The flyover shows two peaks more than 1,000 meters (3,000 feet) tall and multiple craters as deep as 1,500 meters (5,000 feet). These land features suggest (but do not prove) cryovolcanism.

this.”80 Jeffrey Moore and Robert Pappalardo have claimed that all identifiable landforms on Titan can be explained by exogenic processes, by which they mean those which take place on the surface, such as fluvial activity or impact cratering.81 As regards Sotra Facula, however, JPL senior scientist Rosaly Lopes and colleagues have noted that the region was “totally devoid of fluvial channels, making a fluvial origin”82 improbable. Furthermore, “The fact that the depressions [associated with Sotra] are not circular makes an impact origin unlikely.”83 Hence it would appear that cryovolcanism is at least as likely an explanation for the observed surface changes as other mechanisms. 14.3.11

The dunes of Titan

Before the Cassini-Huygens spacecraft reached the Saturn system in 2004, planetary scientists theorized that the dark regions seen around Titan’s equator might be liquid ethane oceans. But the Orbiter’s RADAR, and to some extent its VIMS instrument, revealed a very different environment. Confined within 30° of Titan’s equator were many thousands of giant, linear sand dunes, estimated at covering 20% of the total surface.84 The dunes were typically 30 to 50 kilometers (20 to 30 miles) in length, and aligned east-west. Surprisingly, they were similar in morphology (shape and structure), size and spacing (1 to 3 kilometers) to many large linear dunes on Earth, in particular the longitudinal dunes of Africa’s Namib and Sahara Deserts as well as the Australian Desert. Longitudinal dunes are rare, however, in the Americas. These dunes are characteristically formed by winds that fluctuate around a mean direction, rather than winds of highly variable directions. As sand is blown one way and then the other by the fluctuations, a long narrow mound tends to arise in the mean wind direction.85

14.3

Exploring Titan’s surface from orbit 377

Are the winds of Titan strong enough to have formed its dune systems? Models indicated that atmospheric tides created by Saturn can lead to surface winds on the satellite that reach about one mile per hour (a half-meter per second). On Earth, such weak winds probably would not have led to the large longitudinal dunes which exist in various parts of our world. But Titan has a thick, buoyant atmosphere and, even more important, low gravity. Simulations indicate that even such gentle winds are enough to move the loose grains about on the surface of Titan and eventually work them into dunes.86 While the forms of Titan’s dunes may be similar to some of those on our planet, their materials are fundamentally different. The sand from which Titan’s dunes are constructed is not made from the typical rocky materials of terrestrial sands, but from icy or organic substances.87 It has been difficult to determine the particular mechanisms that produced Titan’s sand, but two possibilities have been identified: liquid methane rain may have eroded particles from ice bedrock; or photochemical reactions in Titan’s atmosphere could have generated particles of organic solids. The dunes typically appeared dark in RADAR images compared to the interdune regions. This helped identify a dune’s start and end points. The dark color possibly indicated materials containing organic substances rather than ice. 14.3.12

Could Titan’s methane indicate life?

Methane is a substance often related to living things, at least on Earth and possibly on Mars. In Earth’s atmosphere, 90 to 95% of the methane is biological in origin – largely the result of bacterial processes in the guts of grass-eating ungulates such as cows and goats, as well as in swamps and rice paddies. These particular methane-related terrestrial environments do not exist on Titan. But some types of bacteria could still be present on the moon. Planetary scientists do not claim this as a likely occurrence, but the possibility exists. Although astronomers knew about the presence of methane on Titan as early as 1944, it was not until the discovery of nitrogen 36 years later that interest developed regarding the biological possibilities of the satellite. Nitrogen is a key ingredient of certain liferelated molecules that include amino acids and nucleic acids. Planetary scientists reasoned that a body with methane and nitrogen in its atmosphere, where the gas pressure at the surface is comparable to that of Earth, might be a place with the molecular precursors to life, and perhaps even life itself.88 The methane may not have been manufactured on Titan, but been included in the primordial cloud out of which the moon formed. But in that case, scientists believe, it would have been accompanied by the heavy noble gases xenon and krypton. The apparent absence of these gases in Titan’s atmosphere argues for the methane having been formed in some way on the moon. This could have occurred through geological or biological processes. Methanogens are anaerobic microorganisms which produce methane gas. They would require a source of nutrients, but according to Christopher McKay of NASA’s Ames Research Center and others, acetylene and hydrogen could serve that function. What is more, although Titan is far from the Sun, it still receives sufficient solar energy to turn nitrogen and methane into the precursor molecules of life. And a brine of water, ammonia, and other hydrocarbons, if it exists beneath the surface, could be an environment friendly to the production of complex molecules and even living microorganisms.89

378

Titan observations by the Cassini Orbiter

14.3.13

The interior

Before Cassini-Huygens arrived at Saturn in 2004, planetary scientists knew little about Titan’s interior – in particular, its origin and evolution. The Orbiter flyby data shed light on these issues, indicating that Titan may have evolved in quite a different manner than inner planets such as Earth or icy moons such as Jupiter’s Ganymede, whose interiors have separated into distinct layers. Finding out about the apparently different formative processes of Titan could be “fundamental to understanding the history of moons of the outer solar system,”90 according to former Cassini Project Scientist Bob Pappalardo. Planetary scientists knew that Titan was roughly half ice and half rock, but they needed accurate profiles of the moon’s gravitational field in order to determine the distribution of the various interior materials. They obtained this gravity map in an interesting way, by tracking minute changes in the Orbiter’s range rate – the rate at which the distance from the spacecraft to Earth-based measuring equipment changed with respect to time – as the craft made four close flybys of Titan between February 2006 and July 2008, skimming 1,300 to 1,900 kilometers (800 to 1,200 miles) above the satellite’s surface.91 According to Luciano Iess of the Sapienza University of Rome and a member of the Cassini Radio Science team, “The ripples of Titan’s gravity gently push and pull Cassini along its orbit as it passes by the moon and all these changes were accurately recorded by the antennas of the Deep Space Network.”92 The precision with which the DSN could measure spacecraft range rate was rather amazing in that despite the spacecraft being over a billion kilometers (600 million miles) away, it was possible to distinguish speed differences within 5 thousandths of a millimeter per second (0.2 thousandths of an inch per second). From these flybys, scientists surmised that Titan’s interior below 500 kilometers (300 miles) resembled “a sorbet of ice studded with rocks.”93 According to David Stevenson of Caltech, the lack of ice/rock separation suggested that the satellite was never heated up beyond a relatively lukewarm temperature that was too cool to melt a substantial portion of the primordial ice-rock mixture, which would have led to greater separation of ice from rock94 (Figure 14.9). The incomplete separation of ice and rock indicated that Titan was less like the Jovian moon Ganymede, in which ice and rock have fully separated, and possibly more like Callisto, another of Jupiter’s moons, that is also thought to have a mixed ice and rock interior. Although these three satellites reside in the outer solar system and have roughly the same primordial ice-rock fraction and mean density and are of similar size,95 they evidently were not all formed under the same conditions. Titan, for instance, may have been constructed rather slowly for a moon, over a period of a million years or so, soon after the solar system was formed. 14.3.14

A subsurface ocean?

Cassini data indicates that Titan likely harbors a layer of liquid water under its ice shell. Titan experiences considerable stretching and squeezing as it orbits Saturn. If Titan were composed entirely of rigid rock, Saturn’s gravitational forces would only cause bulges, or solid “tides” on the moon about 3 feet (1 meter) in height. Cassini data indicated solid

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Exploring Titan’s surface from orbit 379

Figure 14.9 This artist’s conception depicts Titan’s possible interior structure deduced from gravity field data. Radio science data makes a strong case for a global subsurface ocean, sitting above a subsurface layer of high-pressure ice and a water-infused silicate core.

380

Titan observations by the Cassini Orbiter

tides of 30 feet (10 meters), which suggests that the moon is not entirely solid rocky material. Such a large response requires that Titan’s interior is deformable “in a way that is consistent with a global ocean at depth.”96 The search for liquid water is a central goal in our exploration of the solar system, and now we may have found “another place where it is abundant.”97

14.4

TITAN: A MODEL FOR THE FUTURE EARTH?

Far in the future, our Sun is expected to swell. This will be accompanied by a huge increase in its luminosity, the rate at which it radiates energy. The resulting rise in Earth’s temperature that this will cause may result in a runaway greenhouse effect. In this grim scenario, liquid water evaporating from the surface would continuously increase the water vapor content of the troposphere, trapping infrared radiation and leading to a further increase in temperature. Under some models, water molecules would begin to rise into the stratosphere in large quantities, where they would be ionized by ultraviolet light. The end result may be that one distant day hundreds of millions or billions of years from now, Earth will begin to resemble Titan, with dry equatorial regions and remnant liquid at the poles.98 However, this is only one of a variety of doomsday scenarios for our Earth. A scenario explored by Ken Caldeira and James Kasting envisioned that our plant-based biosphere might survive for at least another 0.9 to 1.5 billion years; within an additional billion years, Earth might then lose its water to space and become hot and uninhabitable like our sister planet Venus.99 14.4.1

Titan’s distant future

Interestingly, the effect of our Sun’s eventual swelling and increase in luminosity may make Titan more rather than less habitable for some forms of life. Perhaps 6 billion years from now, a window of several hundred million years may open in which liquid waterammonia solutions form oceans on the surface and react with the organic compounds. While insolation levels would be far less at Titan than at Earth, the presence of ammonia would have an antifreeze-like effect that could permit the existence of liquid water oceans. The several hundred million year duration of these conditions exceeds estimates of the time it took for life to develop on Earth, thus it is conceivable that a similar set of events might occur on Titan.100

REFERENCES 1. Carolina Martinez, “Titan Flyby Reveals Complex Features,” JPL Universe 34(22) (5 Nov. 2004), NASA NHRC 18337, Cassini 2002-. 2. Trina Ray interview by author, 22 October 2008, JPL. 3. Ira Flatow, “Analysis: Landing of the Huygens Probe on the Surface of Titan, Saturn’s largest Moon,” From NPR Talk of the Nation Science Friday (14 Jan. 2005). 4. Paul R. Mahaffy, “Intensive Titan Exploration Begins,” Science 308 (13 May 2005): 969 – 970.

References 381 5. Kevin Baines, VIMS scientist, interview by author, London, 24 June 2009. 6. NASA, “Huygens Discovers Luna Saturni,” http://apod.nasa.gov/apod/ap050325.html, NASA Astronomy Picture of the Day (25 March 2005); Ronald Brashear, “Christiaan Huygens: Systema Saturnium (1659),” http://www.sil.si.edu/DigitalCollections/HST/Huygens/huygensintroduction.htm, Special Collections Department, Smithsonian Institution Libraries (May 1999); Carl Koppeschaar, “350 Years Ago – March 25, 1655: Christiaan Huygens Discovers Saturn’s Moon Titan Using This Telescope Lens,” http://carlkop.home.xs4all.nl/huyglens.html, Astronet Web site, accessed 11 June 2011. 7. Athena Coustenis et al., “Earth-Based Perspective and Pre-Cassini-Huygens Knowledge of Titan,” Chapter 2 in Titan from Cassini-Huygens, Robert H. Brown et al. (eds), (Springer, 2009), p. 9; Andrew Dominic Fortes,”Pre-Voyager Work,” http://www.es.ucl.ac.uk/research/ planetary/undergraduate/dom/weathering_titan/chap1.htm, in Surface Processes on Titan: A Review of the Literature, University College London (Aug. 1997). 8. NASA, “Titan: Overview,” http://solarsystem.nasa.gov/planets/profile.cfm?Object=Titan, Solar System Exploration Web site (last updated 1 April 2011); Bob Mitchell review of manuscript, Feb. 2011. 9. NASA-JPL, “Titan – Atmosphere,” http://saturn.jpl.nasa.gov/science/index.cfm?Science PageID=75, Cassini Equinox Mission Web site, accessed 8 Apr. 2010. 10. Sushil Atreya, “Titan’s Organic Factory,” Science 316 (11 May 2007): 843-845; Athena Coustenis et al., “Titan’s Best Look Before Cassini,” Icarus 161 (2003). 11. Ralph Lorenz interview with author, AGU, San Francisco, 16 Dec. 2009. 12. Ralph D. Lorenz, “Photochemically Driven Collapse of Titan’s Atmosphere,” Science 275 (31 Jan. 1997):642-644; Ralph Lorenz email to author, 4 Jan. 2010. 13. BBC News, “Cassini Peers into Titan’s Haze,” http://news.bbc.co.uk/2/hi/science/ nature/3528616.stm (2 Aug. 2004). 14. C. Sagan et al., in T. Gehrels (ed.), Saturn (Univ. Arizona Press, Tucson, 1984), pp. 788-807, as reported in Atreya, May 2007. 15. J. H. Waite, Jr. et al., “The Process of Tholin Formation in Titan’s Upper Atmosphere,” Science 316 (11 May 2007): 870 – 875. 16. Linda Spilker review of manuscript, March 2011. 17. Y. L. Yung et al., Astrophys. J. Suppl. 55, 465 (1984) 18. Lorenz interview, 16 Dec. 2009. 19. E.H. Wilson and R.A. West, “Examination of the Sources, Characteristics, and Effects of Titan Haze,” Bulletin of the American Astronomical Society 36 (Nov. 2004):1114. 20. E.H. Wilson and S.K. Atreya, “Chemical Sources of Haze Formation in Titan’s Atmosphere,” Planetary and Space Science 51 (2003):1017-1033; Linda Spilker review of manuscript, March 2011. 21. Atreya, May 2007 22. NASA, “Titan: Overview,” http://solarsystem.nasa.gov/planets/profile.cfm?Object=Titan, Solar System Exploration Web site (last updated 1 April 2011); Bob Mitchell review of manuscript, Feb. 2011. 23. Kunio M. Sayanagi, “Springtime on Titan? Cassini Mission Sees Seasonal Changes,” http:// arstechnica.com/science/news/2009/06/springtime-on-titan-cassini-mission-sees-seasonalchanges.ars, Ars Technica Web site (June 2009). 24. F.M. Flasar et al., “Titan’s Atmospheric Temperatures, Winds, and Composition,” Science 308 (2005):975; Y. L. Yung, Icarus 72, 468 (1987), as reported in Flasar et al., 2005; Bill Steigerwald, Goddard Space Flight Center, “Titan’s Atmosphere Revealed by New NASA Observation,” http://www.nasa.gov/mission_pages/cassini/media/Titan_Atmosphere.html (13 May 2005). 25. Ralph Lorenz interview with author, AGU, San Francisco, 16 Dec. 2009.

382 Titan observations by the Cassini Orbiter 26. CICLOPS/Space Science Institute, “Cassini Images Discover A Windy, Wavy Titan Atmosphere,” http://www.sciencedaily.com/releases/2005/03/050326003701.htm, adapted by ScienceDaily (30 Mar. 2005). 27. R. D. Lorenz et al., “The Sand Seas of Titan: Cassini RADAR Observations of Longitudinal Dunes,” Science 312 (5 May 2006):724-727. 28. CICLOPS/Space Science Institute, “Cassini Images Discover A Windy, Wavy Titan Atmosphere,” http://www.sciencedaily.com/releases/2005/03/050326003701.htm, adapted by ScienceDaily (30 Mar. 2005). 29. Space Daily, “Spring On Titan Brings Sunshine And Patchy Cloud,” http://www.spacedaily. com/reports/Spring_On_Titan_Brings_Sunshine_And_Patchy_Cloud_999.html#pantalla3 (23 Sep. 2010). 30. Henry G. Roe et al., “Geographic Control of Titan’s Mid-Latitude Clouds,” Science 310 (2005): 477; C.A. Griffith et al., “The Evolution of Titan’s Mid-Latitude Clouds,” Science 310 (21 October 2005):474 -477. 31. C. A. Griffith, “The Evolution of Titan’s Mid-Latitude Clouds,” Science 310 (21 October 2005): 474 – 477. 32. Ralph D. Lorenz, “The Changing Face of Titan,” Physics Today (Aug. 2008):35. 33. Ralph D. Lorenz, “The Weather on Titan,” Science 290 (20 October 2000): 467-468. 34. Roe et al., “Geographic Control.” 35. Jonathan I. Lunine and Sushil K. Atreya, “The Methane Cycle on Titan,” Nature Geoscience 1 (March 2008):159-164. 36. Robert H. Brown et al., “Overview,” chapter 1 in Robert H. Brown et al. (eds.), Titan from Cassini-Huygens, (Springer, 2009), p. 2. 37. Linda Spilker interview with author, JPL, 27 October 2010. 38. Martinez, “Titan Flyby Reveals Complex Features.” 39. Planetary Society, “Cassini RADAR,” http://www.planetary.org/explore/topics/cassini_huygens/instrument_radar.html, accessed 8 July 2009. 40. Rosaly Lopes, “Titan from Cassini RADAR,” http://beyondthecradle.wordpress. com/2009/04/14/titan-from-cassini-radar-%E2%80%93-rosaly-lopes/, accessed 8 July 2009 (14 Apr. 2009). 41. Steve Wall, deputy team leader for the Cassini RADAR, interview by author, London, 24 June 2009. 42. NASA-JPL, “Magellan: The Unveiling of Venus ,” http://history.nasa.gov/JPL-400-345/text. htm, JPL 400-345 3/89 (1989, updated 6 Aug. 2004). 43. C. Elachi et al., “Cassini Radar Views the Surface of Titan,” Science 308 (May 2005):970-974. 44. Baines, 24 June 2009. 45. P. Hayne, T. B. McCord, C. Sotin, M. Barmatz, R. Mielke, J-Ph. Combe, and G. B. Hansen, “Titan’s Surface Composition: Constraints from Laboratory Experiments and Cassin/VIMS Observations,” http://www.lpi.usra.edu/meetings/scssi2008/pdf/9093.pdf, Lunar and Planetary Institute meeting—Science of Solar System Ices: A Cross-Disciplinary Workshop, Oxnard, CA, paper no. 9093 (5-8 May 2008). 46. Alexander Hayes et al., “Joint Analysis Of Titan’s Surface Using The Cassini VIMS And RADAR Instruments,” Bulletin of the American Astronomical Society 40 (Sep. 2008):457. 47. William Harwood, “Scientists Elated by Cassini’s Titan Observations,” http://spaceflightnow. com/cassini/041027science.html, Spaceflight Now (27 Oct. 2004). 48. Ibid.

References 383 49. R. D. Lorenz et al., “The Sand Seas of Titan: Cassini RADAR Observations of Longitudinal Dunes,” Science 312 (May 2006): 724 – 727; Keith C. Heidorn, “Aeolian Transport,” Weather Doctor Web site, http://www.islandnet.com/~see/weather/elements/aeolian.htm, (1 June 2002). 50. C. C. Porco, “Imaging of Titan from the Cassini Spacecraft,” Nature 434 (2005):159-168; William Harwood, “Scientists Elated by Cassini’s Titan Observations,” Spaceflight Now Web site, http://spaceflightnow.com/cassini/041027science.html (27 October 2004). 51. Elizabeth Turtle of the University of Arizona, quoted in Science Daily, “NASA’s Cassini Reveals Lake-Like Feature On Titan,” http://www.sciencedaily.com/ releases/2005/06/050629071302.htm (29 June 2005). 52. Science Daily, “NASA’s Cassini Reveals Lake-Like Feature On Titan.” 53. Science Daily, “Cassini Finds Lakes On Titan’s Arctic Region,” http://www.sciencedaily.com/ releases/2006/07/060728103452.htm (28 July 2006) 54. Science Daily, “Seas Discovered On Saturn’s Moon Titan,” http://www.sciencedaily.com/ releases/2007/03/070314080639.htm (Mar. 14, 2007). 55. Science Daily, “Saturn’s Moon Titan Has Liquid Surface Lake,” http://www.sciencedaily.com/ releases/2008/07/080730140726.htm, (July 30, 2008). 56. Science Daily, “Saturn’s Moon Titan Has Liquid Surface Lake.” 57. R. H. Brown et al., “The Identification of Liquid Ethane in Titan’s Ontario Lacus,” Nature 454 (31 July 2008): 607-610. 58. O. Aharonson , A. G. Hayes , J. I. Lunine , R. D. Lorenz , M. D. Allison & C. Elachi, “An Asymmetric Distribution of Lakes on Titan as a Possible Consequence of Orbital Forcing,” Nature Geoscience 2 (1 Dec. 2009):851-854. 59. Aharonson 2009; Jia-Rui C. Cook and Stephen Cole, “Scientists Explain Puzzling Lake Asymmetry on Titan,” JPL press release 2009-180 (30 Nov. 2009). 60. Robert H. Brown et al., “Overview,” chapter 1 in Robert H. Brown et al. (eds.), Titan from Cassini-Huygens, (Springer, 2009), p. 3. 61. Ralph D. Lorenz et al., “Fluvial Channels on Titan: Initial Cassini RADAR Observations,” Planetary and Space Science 56(8) (June 2008):1132-1144. 62. Dendritic fluvial features have branching structures some like a tree does. 63. Jonathan I. Lunine and Ralph D. Lorenz, “Rivers, Lakes, Dunes, and Rain: Crustal Processes in Titan’s Methane Cycle,” Ann. Rvw. Earth Planet Sky 37 (2009):299-320. 64. Beth Rachel Zygielbaum, “Temperature Effects on Ice Strength Properties: Implications for Erosion Resistance on Titan,” Master of Science in Geoscience thesis, San Francisco State University (July 2009); Leonard S. Sklar and William E. Dietrich, “Sediment and Rock Strength Controls on River Incision into Bedrock,” Geology 29 (Dec. 2001):1087-1090; M.P. Lamb et al., “A Model for Fluvial Bedrock Incision by Impacting Suspended and Bed Load Sediment,” J. Geophys. Res. 113 (2008):F03025. 65. Geoffrey C. Collins, “Relative Rates of Fluvial Bedrock Incision on Titan and Earth,” Geophys. Res. Lett. 32 (2005):L22202. 66. Jonathan I. Lunine and Ralph D. Lorenz, “Rivers, Lakes, Dunes, and Rain: Crustal Processes in Titan’s Methane Cycle,” Annu. Rev. Earth Planet. Sci. 37 (2009):299-320; Zygielbaum, “Temperature Effects on Ice Strength.” 67. Olivier Mousis et al., “Sequestration of Ethane in the Cryovolcanic Subsurface of Titan,” Astrophysical Journal 677 (2008):L67-L70. 68. Carolina Martinez, “Titan’s Volcanoes Give NASA Spacecraft Chilly Reception,” JPL news release 2008-237 (15 Dec. 2008). 69. Robert M. Nelson email to author, 29 Oct. 2010.

384 Titan observations by the Cassini Orbiter 70. R. M. Nelson et al., “Saturn’s Titan: Surface Change, Ammonia, and Implications for Atmospheric and Tectonic Activity,” Icarus 199 (February 2009):429-441. 71. NASA-JPL, “PIA11839: Hotei Arcus in Infrared,” http://photojournal.jpl.nasa.gov/catalog/ PIA11839, (3 Mar. 2009). 72. Eric Hand “Titan May Boast Ice-Spewing Volcanoes,” Nature online news 2009.190 (25 March 2009). 73. Robert Nelson interview by author, London, 23 June 2009. 74. S. D. Wall et al., “Cassini RADAR images at Hotei Arcus and Western Xanadu, Titan: Evidence for Geologically Recent Cryovolcanic Activity,” Geophysical Research Letters 36 (24 Feb. 2009):L04203; Robert M. Nelson email to author, 23 June 2009. 75. R.M.C. Lopes et al., “Distribution and Interplay of Geologic Processes on Titan from Cassini Radar Data,” Icarus 205 (2010) 540–558. 76. Todd J. Barber, “Insider’s Cassini: Dr. Rosaly Lopes and Cryovolcanism on Titan,” http://saturn.jpl.nasa.gov/news/cassiniinsider/insider20100119/, JPL Cassini Equinox Mission Web site (14 Jan. 2010). 77. R.M. Nelson et al., “Photometric Changes on Saturn’s Titan: Evidence for Active Cryovolcanism,” Geophys. Res. Lett. 36 (2009):L04202. 78. Eric Betz, “Giant Ice Volcano Candidate Found on Saturn Moon Titan,” http://www.physorg. com/news/2010-12-giant-ice-volcano-candidate-saturn.html, Physorg.com (14 Dec. 2010). 79. Jia-Rui Cook, Dwayne C. Brown, and Paul Laustsen, “Cassini Spots Potential Ice Volcano on Saturn Moon,” http://www.jpl.nasa.gov/news/news.cfm?release=2010-416, JPL news release 2010-416 (14 Dec. 2010). 80. Hand (March 2009). 81. Jeffrey M. Moore and Robert T. Pappalardo, “Titan: An Exogenic World?” Icarus 212 (2011):790-806. 82. R. M.C. Lopes et al., “Cryovolcanism on Titan: A Re-assessment in Light of New Data from Cassini RADAR and VIMS,” EPSC Abstracts 6, EPSC-DPS2011-303 (2011). 83. Ibid. 84. R.D. Lorenz et al., “Huygens Boundary Layer Data Explain the ~3km Spacing of Titan’s Dunes,” paper 2014, Lunar and Planetary Institute, Second International Planetary Dunes Workshop, Alamosa CO (18-21 May 2010). 85. Ralph Lorenz and Jacqueline Mitton, Titan Unveiled, Princeton University Press (2008), p. 192. 86. University of Arizona, “Titan’s Seas Are Sand, Cassini’s Images of Saturn’s Moon Show,” ScienceDaily, accessed 22 July 2009 (8 May 2006); R. D. Lorenz et al., “The Sand Seas of Titan: Cassini RADAR Observations of Longitudinal Dunes,” Science 312 (5 May 2006):724-727. 87. Ralph D. Lorenz et al., “Global Pattern of Titan’s Dunes: Radar Survey from the Cassini Prime Mission,” Geophys. Res. Letters 36 (11 February 2009): L03202. 88. Sushil K. Atreya, “The Mystery of Methane on Mars and Titan,” Scientific American 296 (May 2007):43-51. 89. Ibid. 90. Jia-Rui C. Cook, “Cassini Data Show Ice and Rock Mixture Inside Titan,” JPL news release 2010-084 (11 Mar. 2010). 91. Luciano Iess et al., “Gravity Field, Shape, and Moment of Inertia of Titan,” Science 327 (12 March 2010):1367-1369. 92. Ibid. 93. Ibid. 94. Frank Sohl, “Revealing Titan’s Interior,” Science 327 (12 March 2010):1338-1339. 95. Frank Sohl, “Revealing Titan’s Interior,” Science 327 (12 March 2010):1338-1339.

References 385 96. Luciano Iess et al. “The Tides of Titan,” Science 337 (27 July 2012):457-459. 97. Jia-Rui C. Cook and Dwayne Brown, “Cassini Finds Likely Subsurface Ocean on Saturn Moon,” JPL News Release 20120628 (28 June 2012). 98. T. Pujol and G. R. North, “Runaway Greenhouse Effect in a Semigray Radiative–Convective Model,” Journal of the Atmospheric Sciences 59 (October 2002):2801–2810; Lunine and Atreya, 2008. 99. Ken Caldeira and James F. Kasting, “The Life Span of the Biosphere Revisited,” Nature 360 (31 December 1992):721-723. 100. Ralph D. Lorenz et al., “Titan Under a Red Giant Sun: A New Kind of Habitable Moon,” Geophys. Res. Ltrs. 24 (15 Nov. 1997):2905-2908.

15 Conclusions “Saturn and its environment … is likely the most rewarding, most exciting, most fruitful place in our solar system to go exploring.” – Bob Mitchell, Cassini-Huygens Program Manager1

The Cassini-Huygens spacecraft has been an amazingly prolific vessel, transmitting to Earth “about a gigabit per day of science data for the last six-plus years”2 and essentially rewriting our textbooks on Saturn, its moons, and its spectacular rings. According to Project Scientist Linda Spilker, Cassini-Huygens “has revolutionized our understanding of the Saturn system compared to what we thought we knew with Voyager.”3 The ringed planetary system turned out to be a far stranger place than imagined or previous evidence suggested. For example, Voyager’s images of Titan basically showed an orange hazecovered ball, its surface hidden. Cassini-Huygens looked through this haze and revealed a world that appeared “hauntingly familiar”4 with lakes, fluvial channels, and dunes very reminiscent of those on our own planet. Voyager gave us tantalizing clues that the little moon Enceladus might have some interesting geological processes going on, but it was Cassini-Huygens that identified the watery plumes shooting far out into space and possible evidence of an ocean of water beneath the icy surface. Now we see this small, icy moon as a potential home for living organisms. And then there is the ubiquitous E ring. Many mission scientists believe that we will find this ring’s particles, its little bits of water ice, popping up on all of Saturn’s moons. The Cassini spacecraft found compelling evidence that the vast majority of these fartraveling E ring particles come from Enceladus. Cassini-Huygens showed us just how different weather phenomena are on Saturn versus Earth. The spacecraft revealed a strange circulation pattern at Saturn’s north pole that has no parallel on Earth, or possibly any other world in our solar system. A bizarre, hexagon-shaped storm has held its unexplained shape for years.

© Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9_15

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388

Conclusions

The mission observed satellites sculpting the edges of Saturn’s rings into scallops and waves and other shapes, and showed that Saturn’s A ring, so enormous in two of its dimensions, is as little as 3 meters in extent along its third dimension. We discovered a towering ridge twice the height of Mount Everest along Iapetus’ equator that makes the satellite resemble an enormous walnut, an electric current in the equator of Saturn’s magnetosphere equivalent to that generated by one million Los Angeles’s, and many other singular features of the Saturn system. However, as impressive as the CassiniHuygens mission has been, certain of its discoveries made scientists wish that they had built greater capabilities into some of their instruments or included additional instruments in their arsenal. For instance, we now know that Enceladus might harbor biotic activity, but if fragments of microorganisms emerged from the moon, could the Orbiter’s bank of instruments have identified them for what they were? A mass spectrometer able to analyze much larger molecules than the present instruments would be invaluable for identifying biotic or prebiotic material in the plumes of Enceladus. Such a spectrometer would be useful on Titan as well, to better understand the basis of its haze particles. If the RADAR instrument operated in additional wavelength ranges, more data about the icy satellites and Saturn could have been obtained. Similar statements could be made of the Composite Infrared Spectrometer (CIRS) and the Ultraviolet Imaging Spectrograph (UVIS).5 Also, if an additional Huygens Probe had been included, one could have been sent to Titan’s equatorial region and the other to the polar lakes. This would have greatly augmented our in situ data about this moon. A nearly universal comment from mission staff was that the project would have run more smoothly and more efficiently, and allowed scientists additional time to do research, if the scan platforms had not been eliminated from the spacecraft design. Their inclusion would have allowed simultaneous operation of more instruments and less time spent in meetings planning which teams would be able to take observations at which times. And spacecraft reliability has also been a subject of discussion, even though the vessel’s performance has been exceptional up until now. Continued operation of the reaction wheels – the flywheels used for adjusting the vehicle’s attitude – has been a source of concern for mission staff, because one of the reaction wheels has already been replaced with the only spare onboard. This concern would have been allayed to a great extent if the spacecraft had been constructed with two or three spare reaction wheels. But this would have had the downside of adding considerable mass, since each reaction wheel assembly weighs roughly 20 kilograms.6

15.1

THEMES RUNNING THROUGH THE BOOK

In this telling of the story of the Cassini-Huygens mission, several themes emerge, including: • • • •

Issues of risk The influence of previous missions Adaptability to unforeseen problems Cross-border challenges.

15.1 15.1.1

Themes running through the book 389

Issues of risk

We have seen that various issues of risk had significant influences on the conduct and development of the mission. Different forms of risk were addressed. There was risk to the success of the mission, as well as risk to the health and safety of humans and ecosystems. 15.1.1.1

Risk of mission failure

A key factor in determining acceptable levels of risk, and one that Administrator Dan Goldin was very concerned about, was the relationship between science return and the risk of mission failure. Goldin believed that smaller, cheaper spacecraft that were developed more quickly and flown more frequently would deliver a superior science return, although at a higher risk of failure, than large flagship missions such as Cassini. Smaller, faster-to-build missions were much better able to take advantage of the 1990s explosion of new technologies, and hence were preferred by Goldin, although not apparently by the media and the general public, who reacted quite negatively to the mission failures of that decade. Goldin perceived a “suffocatingly risk averse” tendency of the Agency that was linked to the large size of its missions. With so much invested in each major mission, NASA could not tolerate losing even one of them. This dictated the use of ultra-reliable but dated technology over more cutting edge designs. Goldin feared that science would suffer employing such a conservative approach. It is interesting that while Goldin accepted and expected a higher risk of failure for his faster, better, cheaper missions, he viewed the Agency’s large missions as presenting too much risk. Missions such as Cassini-Huygens put so much expense, time, and instrumentation into one spacecraft that its failure would severely damage NASA. In his opinion, the Agency should not risk that much on one mission. 15.1.1.2

Component failure dangers

Some missions have had to severely reduce their scope due to the failure of vital spacecraft components. For instance, the failure of Galileo’s high-gain antenna to open severely curtailed the science return of that mission. This book discusses the many actions taken by the Cassini-Huygens design team to reduce such risks, the most pervasive of which was to implement the principle of redundancy. Backup components of all kinds were integrated into the spacecraft to take over should the primary component fail. Redundant engines, computers, transmitters, and a myriad of other parts enabled the spacecraft to be one-fault tolerant to many different kinds of components failures. 15.1.1.3

Failure modes

Another risk reduction approach was to eliminate certain common failure modes. Electric switches possessing moving parts, for instance, could wear out and operate intermittently or cease to function. The solid-state switches employing application-specific integrated circuits (ASIC) that were used throughout the Cassini-Huygens spacecraft did not have

390

Conclusions

moving parts and were not subject to this particular failure mode. Nor were the hemispherical resonator gyros (HRG) or the digital recorder that the craft used.

15.1.1.4

Risk to humans and the environment

The RTGs and RHUs aboard the spacecraft that generated electricity and heat were fueled with plutonium 238. Many people in the U.S., as well as in other countries, believed that the risk to health and the environment from these devices was far too great for them to be used. While plutonium 238 is extremely dangerous if it enters the human body through, for instance, inhalation, the U.S. Department of Energy, which constructed the devices, took great pains to securely contain the radioactive material within the RTGs and RHUs, even in the event of a crash. NASA and DOE went to heroic efforts to protect the devices during transport and while they were in storage at Kennedy Space Center. NASA undertook far-reaching public awareness campaigns that gave details on the extensive safety features that had been built into the devices to greatly minimize any risks they presented, as well as on the in-depth risk analysis study that had been performed. However, its public awareness efforts were only partially successful. International movements still believed that the use of plutonium was too risky and lobbied and protested vigorously to stop the launch of the spacecraft. In fact, the protest movement’s efforts introduced a new type of threat: the risk of potential damage to the Cassini-Huygens spacecraft if the protesters were able to get to it. This physical threat influenced NASA’s decision to transport the space vehicle in secrecy to KSC in April 1997, and only after securely storing it did the Agency announce its arrival. Such secrecy was also in place when KSC personnel moved the spacecraft from a protected NASA building to a heavily guarded launch pad, then mounted it on the launch vehicle. The plutonium fuel itself remained highly secured until just days before the launch, when it was installed in the spacecraft. 15.1.2

Influence of previous missions

This book tracks ways in which the development of Cassini-Huygens was guided by lessons of previous missions, especially the Galileo and Voyager expeditions to the outer solar system. The component reliability problems encountered on Galileo were particularly influential on the design of the Cassini-Huygens spacecraft, for instance in its dramatic reduction of components containing moving parts. The discovery by the Voyagers that the surface of Titan was obscured by a smoggy atmosphere, which their instruments were unable to penetrate, dictated the need for a radar on the next expedition to the Saturn system. 15.1.3

Adaptability to unforeseen problems

Cassini-Huygens’ longevity and success was related to its team’s ability to adapt quickly and effectively to serious and unanticipated setbacks arising from equipment failures and communication breakdowns. The one-fault tolerance of its design was also essential in keeping the vehicle operating and the scientific data flowing back to Earth. Examples of adaptability include the Doppler shift communications-link problem that threatened to limit data flow from the Huygens Titan Probe, the malfunction in the

15.2

The remaining years of the mission 391

communication channel that was to yield data for the Doppler Wind Experiment as the Probe descended on its parachute through Titan’s atmosphere, difficulties with the Orbiter’s thrusters, and the loss of one of its reaction wheels. All these situations were satisfactorily resolved through the ingenuity and efforts of the mission team as well as by the use of many redundant components and modes of operation. 15.1.4

Cross-border challenges

Woven through this book are discussions of serious issues that arose from multiple scientific and industry teams from many different countries, with various levels of restriction imposed upon their data, trying to work effectively with each other. The communications link and Doppler Wind Experiment issues mentioned above were examples of problems that can arise from complicated operations involving many different cultures and business practices, often without the necessary transparency. Sharing of new technology such as the latest software tools is an important element in an effective cross-border partnership, but such sharing of U.S. tools with Europe was often made very difficult owing to International Traffic in Arms Regulations (ITAR) restrictions, which considered software that monitored a spacecraft to be a sensitive defense-related article. Similar restrictions applied in the reverse direction. For instance, France would not fully share with the U.S. the details of a missile nose cone material that was used in the Huygens Probe. Business-sensitive information also was sometimes held too closely and not sufficiently shared, resulting in serious communication deficits between countries, such as with the Huygens Probe Doppler transmission link. Also discussed were the many ways in which potential communication issues were circumvented, such as regularly holding the pan-mission science meetings in Europe as well as in the U.S., and typically including scientists from both locales in the instrument research groups, regardless of whether the instrument was developed in Europe or the U.S.

15.2

THE REMAINING YEARS OF THE MISSION

If the spacecraft continues to function well, the Cassini-Huygens mission to Saturn will continue until 2017. While this book reports on many of the important events and discoveries that occurred from conception of the project through the start of its Solstice Mission, there will probably be much more to report in the years ahead. Among the most important targets are the moons Enceladus and Titan, which undoubtedly have many more secrets to reveal. Questions surrounding Enceladus’ astrobiological potential are at the heart of future investigations. Scientists also hope to catch signs of seasonal climate change on Titan, such as storms, floods, and lake level variations, as well as seeking further evidence of cryovolcanism. Other satellites will also be carefully studied. Multiple flybys are envisioned of the mysterious bright and dark surfaces of Dione and Rhea in order to compare their geological and cratering histories with those of other moons. Evidence of a possible tenuous atmosphere on Dione must be investigated, and the unique thermal features recently discovered on Mimas deserve further study.

392

Conclusions

Towards the end of the mission, the Orbiter will carry out closer studies of the mother planet and its rings and magnetosphere. During these close encounters, it will analyze the internal structure of Saturn, its meteorology and magnetic fluctuations, and the mass and internal dynamics of its rings. A particularly intriguing study will be the role that water ice from Enceladus plays in Saturn’s magnetosphere, possibly influencing its radio and auroral activity, and even causing changes in the rotation of the magnetic field itself.7 Subsequent editions of this book will include an Epilogue that will bring readers up-todate on mission discoveries. # Cassini-Huygens has been one of the most ambitious space ventures ever launched. It is not possible to encapsulate its wealth of scientific results in a book of this size. And the spacecraft continues to make exciting discoveries nearly every week during its second mission extension. The Cassini-Huygens experience was a tremendous opportunity for many nations to take on an incredible challenge in interplanetary exploration. The mission was an undertaking whose scope and cost could not have been borne by any single nation. It was made possible by shared investment and complex cooperation activities between many countries. This expedition to explore the Saturnian system has inspired young and old to learn more about the power of science, engineering, and technology, in order to provide insights into profound matters ranging from the origin of worlds to the beginning of life. It has been an honor to tell the story of this memorable voyage of discovery.8

Figure 15.1 Raging storm on Saturn. A northern hemisphere tempest larger than Earth soon expanded completely around the planet. The rings are seen nearly edge-on as a thin horizontal line. The warped dark bands below the rings are the shadows of the rings cast onto the cloud tops by the Sun to the upper left. A source of radio noise from lightning, this intense storm may relate to seasonal changes as spring slowly emerges in the north of Saturn.

References 393 REFERENCES 1. 2. 3. 4. 5.

Bob Mitchell interview by author, JPL, 26 October 2010. Ibid. Linda Spilker interview by author, JPL, 27 October 2010. Ibid. Linda Spilker interview with author, JPL, 27 Oct. 2010; Candy Hansen interview with author, 27 Oct. 2010. 6. Bob Mitchell interview with author, 26 Oct. 2010. 7. NASA-JPL, “Mission Overview,” http://saturn.jpl.nasa.gov/mission/introduction/, Cassini Solstice Mission Web site, accessed 11 Jan. 2012. 8. NASA/JPL, “Cassini’s Earthly Benefits,” http://saturn.jpl.nasa.gov/multimedia/products/pdfs/ earthly.pdf (May 1995).

Appendix Breakdown of mission costs On 16 November 1990, President George Bush signed a bill that gave the joint CRAF/ Cassini-Huygens project a budget of $1,600 million for development of the spacecraft and their scientific experiments, conducting the launches, and 30 days of operations. The budget grew considerably over the lifetime of the project, even after the cancellation of the CRAF mission. The total Cassini-Huygens missions costs in 2010, two decades after President Bush’s approval of the project, are listed in Table A.11 and discussed below.

3.1

NASA COSTS

The total cost to NASA of designing and developing the Cassini-Huygens spacecraft and getting it ready to launch was $1,422 million. This work was conducted during fiscal years 1990 through 1997. The cost of the launch itself was $422 million. Note that NASA’s development and launch costs of $1,844 million exceeded the initial project budget intended to develop and launch both Cassini-Huygens and CRAF. Operating the spacecraft on its voyage from Earth to Saturn in fiscal years 1997 through 2004 cost $425 million. Operating the spacecraft during its Prime Mission at Saturn, in fiscal years 2004 through 2008, cost $315 million. The NASA budget for the Extended Mission from 1 July 2008 to 30 September 2010 was approximately $80 million per year, or total of $180 million for Cassini operations and science. That figure included an engineering team to operate the 12-instrument Orbiter, a navigation team to keep it on course, and 125 NASA-selected U.S. scientists and their staffs working mostly part time on the program.

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396

Appendix Table A.1. Cassini-Huygens mission costs (in real-year $ million)

Item NASA costs Design, development Launch Earth-Saturn cruise Prime Mission Extended Mission Total NASA costs Foreign contributions ESA for Huygens Probe Italy for antenna & science components Total foreign contributions Total mission costs through fiscal year 2010

3.2

$ million

Fiscal years

$1,422 $422 $425 $315 $180 $2,764

1990 - 1997 1997 1997 - 2004 2004 - 2008 2009 - 2010

$500 $160 $660 $3,424

FOREIGN CONTRIBUTIONS

Foreign contributions from ESA for the Huygens Probe mission were $500 million. The Italian Space Agency contributed $160 million for the antenna, radio and radar subsystems, and various other science instrument components. REFERENCES 1. Robert T. Mitchell email, to author, 9 Mar. 2010 and 29 Mar. 2010; Mark R. Dahl email to author, 18 Mar. 2010; Donald Savage and Guy Webster, “Jupiter Millennium Mission,” http://www.jpl. nasa.gov/news/press_kits/jupiterflyby.pdf, NASA Press Kit (Oct. 2000); Linda Spilker, “Cassini Extended Missions,” http://www.lpi.usra.edu/opag/march_08_meeting/presentations/spilker. pdf, presentation (1 Apr. 2008); Debra Werner, “Cassini Team Pushes for 7-Year Extended Mission at Saturn,” http://www.space.com/news/090127-cassini-mission-extension.html, Space. com (27 Jan. 2009); NASA-JPL, “Mission Overview: Quick Facts,” http://saturn.jpl.nasa.gov/ mission/quickfacts/, accessed 28 Mar. 2010.

About the author Michael Meltzer began writing about science and technology in the 1980s. He has written books and articles about interplanetary voyages throughout the solar system, the protection of planetary environments, designing solar energy systems, and the history of North American commercial fishing. He has also published two science fiction works with environmental themes, one of which won first place in the Writers of the Future contest. Michael has degrees in physics, geophysics, and environmental science and engineering from the University of California. He was an engineer at Lawrence Livermore National Laboratory for 15 years, where he helped start a pollution prevention program. He has worked on environmental projects in Indonesia, Ecuador, and the United Kingdom. He lives in Oakland, California with his wife and daughter.

© Springer International Publishing Switzerland 2015 M. Meltzer, The Cassini-Huygens Visit to Saturn, Springer Praxis Books, DOI 10.1007/978-3-319-07608-9

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Image credits 3.1 3.2 3.3 3.4 3.5a 3.5b 3.5c 3.6a 3.6b 3.7a 3.7b 3.8 3.9 3.10 3.11 4.1 4.2 4.3 4.4 4.5 4.6 4.7 5.1 5.2 5.3 5.4 5.5

NASA/JPL NASA/JPL NASA/JPL NASA/LASP-University of Colorado Boulder Cassini Imaging Team-ISS/JPL/ESA/NASA NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/LASP-University of Colorado Boulder NASA/LASP-University of Colorado Boulder NASA/JPL/University of Arizona/DLR NASA/JPL/University of Arizona NASA/JPL/University of Stuttgart NASA/JPL/John Hopkins University NASA/JPL NASA/JPL ESA/D. Ducros ESA/H. Hassan & J.C. Jones ESA/H. Hassan & J.C. Jones ESA/H. Hassan & J.C. Jones ESA/H. Hassan & J.C. Jones ESA/J.C. Zarnecki, “The Huygens Surface Science Package,” sci.esa.int/science-e/ www/object/doc.cfm?fobjectid=35106, 16 April 2004 ESA/J.C. Zarnecki, “The Huygens Surface Science Package,” sci.esa.int/science-e/ www/object/doc.cfm?fobjectid=35106, 16 April 2004 NASA/JPL NASA/JPL NASA/JPL Warren Moore, Cassini Spacecraft Assembly, Test and Launch Operations (ATLO) Final Report I (NASA-JPL 699-209, JPL D-15701, 15 May 1998) Warren Moore, Cassini Spacecraft Assembly, Test and Launch Operations (ATLO) Final Report I (NASA-JPL 699-209, JPL D-15701, 15 May 1998)

(continued)

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400

Image Credits

(continued) 6.1 6.2 7.1 7.2 7.3 7.4a 7.4b 7.4c 9.1 9.2 9.3 9.4 10.1 10.2 10.3 11.1 11.2 11.3 11.4 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10 12.11 12.12 12.13 12.14 12.15 12.16 13.1 13.2 13.3 13.4 13.5

Department of Energy Facts: Radioisotope Heater Units (RHU), December 1998 “Typical Onboard Systems” in Basics of Space Flight (NASA/JPL, 2010) NASA-JPL, “VVEJGA Trajectory,” JPL 400-856D 9/99, LS-1999-08-003-JPL Michael Meltzer, Mission to Jupiter, A History of the Galileo Project (Washington DC: NASA SP-2007-4231, 2007) NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute ESA/Obs. De Paris Meudon/CETP-IPSL ESA/Obs. De Paris Meudon/CETP-IPSL NASA/JPL/ESA/University of Arizona NASA/JPL/ESA/University of Arizona NASA/JPL/Space Science Institute Cassini-Huygens Saturn Arrival Press Kit (NASA, June 2004) Courtesy of Dave Seal, supplied by Linda Spilker from her Plenary Session talk at Cassini Project Science Group Meeting #52, JPL, 25 October 2010 Courtesy of Andy Ingersoll and Cambridge University Press NASA/JPL/Space Science Institute/University of Arizona NASA/JPL/University of Arizona NASA/JPL-Caltech/Space Science Institute NASA/JPL NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/GSFC

(continued)

Image Credits 401 (continued) 13.6 13.7 13.8 13.9 13.10 13.11 13.12 13.13 13.14 13.15 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 A.1

NASA/JPL/GSFC/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/GSFC/SWRI/SSI NASA/JPL NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/GSFC/SWRI/SSI NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL/Space Science Institute NASA/JPL-Caltech/GSFC NASA/JPL-Caltech/ASI NASA/Cassini RADAR Titan Mapper, courtesy of Devon Marjorie Burr of the University of Tennessee, Knoxville NASA/JPL/University of Arizona NASA/JPL-Caltech/ASI NASA/JPL-Caltech/ASI/USGS/University of Arizona NASA/JPL-Caltech NASA/JPL-Caltech/Space Science Institute

Index

A A Strategy for Exploration of the Outer Planets: 1986–1996, 35 ablative material, 114, 117 Advanced Solid Rock Motor (ASRM), 30, 31 aeolian, 364, 367, 375 Aerosol Collector and Pyrolyzer (ACP), 120–122, 124, 225, 229 Aerospatiale, 109, 110, 131 Aharonson, Oded, 370 Air Force, 29, 30, 34, 139, 147, 149–151, 168, 169, 221 Alan Shepard, xi albedo, 309, 330, 338 Alenia Spazio, 109, 213, 216, 217 Alexander, Claudia, 275 American Geophysical Union (AGU), 171 Ames Research Center (ARC), xii, 5, 8, 14, 81, 165, 286, 377 ammonia, 229, 232, 263–265, 268, 273, 279, 307, 308, 336, 373, 374, 377, 380 Announcement of Opportunity [AO] for the Cassini Mission: Saturn Orbiter, 72 Anthe, 311, 312, 349 Apollo, 54, 157, 164 application-specific integrated circuit (ASIC), ix, 70, 71, 389 A-ring, viii, 310, 312, 337 Arthur C. Clarke, 4 ASI. See Italian Space Agency (ASI) ASIC. See application-specific integrated circuit (ASIC) Asmar, Sami, 228 Assembly, Test, and Launch Operations (ATLO), 139, 142, 143

asteroid, 13, 15, 17, 30, 53–55, 139, 196, 200, 202, 206, 213 asteroid belt, 196, 213 attitude and articulation control subsystem (AACS), 61, 64–66, 140, 186 aurora, 76, 79, 198–199, 255, 256, 275–277, 299, 300, 392

B Baikonour, 18 Baines, Kevin, 246, 270, 358 bipropellant engine, 56 Bonnet, Roger, 17, 19 bow shock, 191, 192, 194, 278, 279 B-ring, 278, 288, 291, 296, 298–300, 310, 340 Brown, Robert, 369, 370 Bryan, G.H., 56, 71 Buffington, Brent, 255 Burch, James, 32 Burns, Joseph A., 35, 40 Bush, President George, 21, 27, 30–32, 36, 49, 139, 161, 162, 395

C Callisto, 199, 378 Cape Canaveral, vii, 165–167, 181–183, 213 Cape Canaveral Air Force Station, 139, 149, 151, 168, 221 Casani, John, 39, 52 Cassini Division, vi, 286, 292, 310–312, 340 Cassini, Giovanni, vi, 286, 309, 322, 329, 342 Cassini Plasma Spectrometer (CAPS), 81–83, 85, 146, 185, 191, 194, 195, 341, 342

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404 Index Cassini Project Science Group (PSG), 72 Cassini Regio, 309 Cassini/Huygens Project Implementation Plan (CHPIP), 27 Centaur, xiv, 10–11, 60, 132, 139–153, 181, 182, 245 Challenger, vi, vii, xii–xiv, 10–13, 19, 31–33, 36, 42, 94, 111, 150, 151, 164, 221, 245, 252, 363, 388, 391, 392 Chandra, 33 charge coupled device (CCD), 65, 76, 122 Clementine, 36 coast phase, 118, 222–224 Colombo, Umberto, 29 Columbo Gap, 312 Comet Rendezvous Asteroid Flyby (CRAF) mission, 13, 19–21, 30–33, 35, 38, 39, 49, 53, 58, 139, 395 Comet Shoemaker-Levy, 290 command and data subsystem (CDS), 61, 64–65, 119, 140 Committee on Planetary and Lunar Exploration (COMPLEX), 5, 32, 33, 35, 40–41, 72 Composite Infrared Spectrometer (CIRS), 35, 68, 74–75, 78, 95, 146, 185, 321, 325, 330, 340, 345, 361, 362, 388 Congress, vii, x–xiii, 3, 10, 12, 14, 18–21, 27–28, 30–36, 38, 40, 42, 50, 139, 150, 162, 170–172, 245, 252 Cordova, Frances, 39–41 Cosmic Dust Analyzer (CDA), 81, 83–85, 96, 146, 307, 331, 332, 334, 335, 342 Coughlin, William, xi Cranston, Senator Alan, 20 Credland, John, 217, 218 C-ring, 290, 300, 301, 310 cruise phase, 115, 117, 145, 182–193, 195, 222, 244, 250 cryovolcanism, 206, 233, 247, 330, 335, 343, 363, 364, 367, 373–376 cryovolcano, 81, 364, 367, 373, 375, 376 Cuzzi, Jeff, 165, 286, 287

D Dahl, Mark, 34, 61 Daimler-Benz Aerospace, 124, 131–132 d’Allest, Frederic, 18 Daphnis, 294, 297, 301, 313 Davis, Derek, 42 Deep Space Network (DSN), 93, 182, 185, 186, 195, 200, 214, 215, 378 density waves, 79, 80 Department of Defense (DoD), 32, 50, 62, 162, 166

Department of Energy (DOE), xiii, 160, 161, 164, 169–173, 390 Descent Imager and Spectral Radiometer (DISR), 120, 122–123, 225, 228, 233, 234 Dione, vi, ix, 278, 307, 310–311, 321, 322, 328, 338, 342–345, 391 discipline team, 244 Doppler shift, xii, 201, 213–219, 227, 228, 243, 249, 365, 366, 390 Doppler Wind Experiment (DWE), xiv, 120, 123–124, 227–229, 390–391 Dougherty, Michele, 87, 95, 321 Draper, Ron, 39, 54 D-ring, 255, 256, 291, 292, 300–301 dunes, 374–377, 387 Dyudina, Ulyana, 272

E Earth flyby, 191, 193, 194, 250 Edwards Air Force Base, 147 Elachi, Charles, 94, 357 Enceladus, viii, ix, xiv, 77, 81, 86, 87, 242, 251, 252, 256, 261, 276–279, 305, 307–308, 310–313, 321, 322, 328–338, 340, 343, 375, 387, 388, 391, 392 Encke Gap, 204, 292–295 Environmental Impact Statement, 168 Environmental Protection Agency (EPA), xiii, 165 equatorial plane, 202, 243, 247, 248, 275, 276, 300, 342 Equinox Mission, xiv, 241, 248, 251–254, 256, 257, 322, 323 E-ring, 83, 261, 276, 279, 288, 305–308, 310, 312, 328, 332–337, 344, 387 ESA. See European Space Agency (ESA) Esposito, Larry, 40, 78, 80 Europa, 86, 197, 199, 200, 253 European Science Foundation (ESF), 10 European Space Agency (ESA), vii, x, xii-xiv, 7–10, 12, 15, 17–20, 27–30, 42, 50, 51, 58, 72, 75, 87, 88, 95, 107, 109–111, 116–117, 119, 120, 122, 129, 131, 132, 139, 141, 142, 147–149, 151, 159, 195, 213, 214, 216–218, 221–223, 226, 228, 230, 235, 250, 357, 360, 396 European Space Operations Centre (ESOC), 214–216, 226, 227 extremely low frequency (ELF) radio wave, 231–233

F facility instrument, 94–95 faint rings, 290–292, 311, 312, 332

Index faster, better, cheaper, 37, 38, 40, 41, 389 Fechtig, Hugo, 10 fields, particles, and waves instruments, 74, 81–90, 202 Fischer, Georg, 271 Fisk, Lennard, 17, 33–35, 38, 40, 42 Flamini, Enrico, 59 Fletcher, Administrator James, 12, 14 Florida Coalition for Peace and Justice, 165, 167 fluvial, 225, 234, 235, 367, 371–374, 376, 387 Foale, Michael, xi Fowler, Senator Wyche, Jr., 19 F-ring, 76–77, 243, 248, 255, 288, 290–292, 294, 301–305, 310

G Gagarin, Yuri, 130 Gagnon, Bruce, 164, 167 Galileo Galilei, v–vii, xii–xiv, 5, 6, 8, 10, 11, 13, 17, 35, 39, 50–55, 57–60, 66, 68, 70–72, 80, 86, 117, 124, 126, 146, 157, 158, 163, 168, 190, 192, 197–202, 221, 241, 245–246, 250, 285, 335, 389, 390 Ganymede, 197, 199, 378 Gary Flandro, 4 Gas Chromatograph Mass Spectrometer (GCMS), 120, 122, 124–125, 225, 226, 229, 230 Gautier, Daniel, 7–10, 14, 18–19, 72, 121 Gavin, Tom, 182, 183 Geological Survey, 10, 14, 81, 374 George P. Miller, xi Giotto, 9, 18, 51–52, 122 Glenn Research Center (GRC), xii, xiv, 37, 150, 151 Global Network Against Weapons and Nuclear Power in Space, 164 Goddard Space Flight Center (GSFC), 14, 75, 83, 86, 90, 95, 124–125, 336, 337 Goldin, Administrator Dan, x, xiii, 27, 35–42, 168, 389 Goldstone, 214–216 Gorbachev, Mikhail, 18 Gore, Senator Al, Jr., 20, 21, 38, 42 gravity assist, xiv, 4, 30–32, 165, 181–183, 187–189, 193, 195, 197, 200–201, 206, 219, 222, 241, 246–248 gravity waves, 201 Green Bank Telescope, 228 Griffin, Administrator Mike, 170 Griffin, Michael, v Grigg-Skjellerup Comet, 9, 122 G-ring, 248, 256, 288, 292, 305–306, 312 Guerriero, Luciano, 29, 30 Gurnett, Donald, 90, 190

405

H Hadley cell, 361 Halley’s Comet, 9, 11, 18, 51, 83, 122 Hansen, Candy, 171, 331 Helene, 343–344 heliosphere, 279–280 hemispherical resonator gyro (HRG), 55–56, 65, 71–72, 390 Herschel Crater, 339, 340 hexagon-shaped storm, 81, 387 HGA. See high-gain antenna (HGA) high-gain antenna (HGA), x, 29, 34, 51, 57–60, 64, 65, 91, 92, 143, 145–148, 181, 185, 189, 196, 223, 224, 226, 244–246, 365, 389 Hill sphere, 341 Hollings, Senator Fritz, 21 Horizon2000, 7 Hotei Reggio, 373–375 House of Representatives, xi, 19–21, 30, 170, 171 Hubble Space Telescope, 19, 36, 199, 265, 277, 299, 357 Huntress, Wes, 42 Huygens Atmosphere Structure Instrument (HASI), 120, 125–126, 188, 225, 230–232, 234 Huygens, Christiaan, vi, 285, 286, 288, 358 Huygens Recovery Task Force, 218 Huygens Science Working Team (HSWT), 72 hypergolic, 56, 67 Hyperion, 322, 337–338

I Iapetus, vi, ix, 243, 248, 309, 310, 321–327, 337, 338, 388 icy moons, vi, xiv, 34, 75, 83, 89, 243, 250–252, 261, 321–351, 378, 387 Imaging Science Subsystem (ISS), 74–78, 95, 193, 194, 197, 199, 200, 245, 268, 271, 325, 343, 344, 366–368, 375 inertial reference unit (IRU), ix, 65, 71 Ingersoll, Andy, 6, 39, 198, 266, 267, 269, 274, 307 interdisciplinary scientist (IDS), 72, 121, 202, 226, 232, 243, 244, 285, 286 International Solar Polar Mission (ISPM), 8–9, 19, 28 International Traffic in Arms Regulations (ITAR), 391 Io, 197–200, 279, 335–336 Ion and Neutral Mass Spectrometer (INMS), 81, 85–86, 95, 146, 244, 334–336, 345 Ip, Wing, 8–10, 14, 18, 72 Italian Space Agency (ASI), vii, x, xii–xiv, 15, 27–30, 57, 59, 92, 139, 201, 213, 396 Ithaca Chasma, 328, 329

406

Index

J Japan, 12, 28, 80 Jet Propulsion Laboratory (JPL), x–xiii, 4, 5, 8, 10, 11, 14, 27, 34, 39, 41, 50–55, 58, 63, 66, 81, 87, 88, 92–95, 132, 139, 141–147, 151, 153, 163, 170, 182, 183, 195, 202, 216, 217, 246, 252–254, 270, 287, 326, 344, 346, 373, 376 Johnson, Torrence, 9, 39, 50, 52, 202 Joint Science Working Group (JSWG), 14, 72, 120, 121 Joint Working Group (JWG), 10 Jones, Geraint, 279, 341 JPL. See Jet Propulsion Laboratory (JPL) Jupiter, vi, vii, xii, xiv, 4–6, 8, 15, 17, 30–32, 35, 50–53, 73, 86, 87, 90, 159, 163, 183, 195–201, 213, 241, 245–247, 253, 264–266, 270, 273–277, 279, 290, 335, 337, 378 Jupiter flyby, 196–201 juste retour, 110–111

K Karth, Joseph, xi Keeler Gap, 293, 294 Keeler, James E., 286 Kennedy, President John F., 8, 13 Kennedy Space Center (KSC), 132, 139, 140, 142, 144, 146–147, 149, 151, 167, 168, 390 Kerry, Senator John, 19 Keyworth, George, 8 Khurana, Krishan, 35 Kirk, Randy, 374, 375 Krimigis, Stamatios, 89, 280 KSC. See Kennedy Space Center (KSC)

L lakes, 7, 81, 82, 128, 194, 206, 235, 357, 364, 367–371, 387, 388, 391 Lanzerotti, Louis J., 33, 35, 40 launch vehicle, xii, xiv, 12, 15, 21, 30, 40, 60, 139–153, 163, 164, 168, 169, 181, 183, 187, 390 Launius, Roger, 28 Lebreton, Jean-Pierre, 111, 195, 215, 252 Lew Allen, 11, 93 Lewis Research Center (LRC), 144, 151 lightning, vii, 69, 76, 90, 120, 125, 130–132, 186–187, 190, 192, 231, 264, 270–272, 392 liquid water, viii, 232, 242, 256, 335, 337, 340, 378, 380 lobate, 374, 375

Lockheed Martin Astronautics Company, 58 Lockheed Martin Space Systems, 253 Logsdon, John, 170 Lopes, Rosaly, 373, 376 Lorenz, Ralph, 91, 235, 367 Lunine, Jonathan, 121, 202, 243 Luton, Jean-Marie, 28, 42

M Magellan, viii, 11, 19, 187 Magnetometer (MAG), 35, 53, 58, 81, 85–88, 95, 192, 194, 250–251, 279, 321, 335, 345 magnetopause, 195, 278 magnetosphere, vi, xiv, xv, 5, 8, 9, 15, 17, 34, 50, 72, 83, 85, 88–90, 96, 120, 191, 194, 197–200, 206, 230, 244, 248, 251, 261, 263–280, 305, 321, 340, 341, 359, 388, 392 Magnetospheric Imaging Instrument (MIMI), 81, 85, 88–89, 146, 185, 191, 194, 200, 276, 341, 342 magnetotail, 194 main engine, 53, 56, 67, 69, 96, 145, 147, 185, 188, 189, 193, 196, 205, 224 main rings, 199, 205, 247, 291, 292, 305, 308, 313, 314 Mariner, 39, 53, 57, 80, 151 Mariner Mark II, 11, 13, 18, 39, 53, 54, 57 Mars, vi, vii, xi, 4, 5, 10, 17, 32, 39, 40, 53, 55, 67, 90, 115, 157, 167, 170, 196, 278, 365, 377 Marshall Space Flight Center (MSFC), 95 Martin Marietta, 6, 122 Masursky, Hal, 10 Matson, Dennis, 33, 244, 252, 365 Max Planck Institute, 8, 14, 122, 335 Maxwell, James, vi, 286 Mercury, v, xi, 7, 17, 39, 50, 53, 357 Mercury Redstone, xi metallic hydrogen, 86, 272, 273 methane cycle, 267, 363–364 methanogens, 337, 377 Methone, 311, 312 micrometeoroid, 63, 67–69, 251, 306, 309, 324, 335 microwave remote sensing instruments, 74, 91–93 Mimas, 248, 278, 279, 290, 306, 307, 310–312, 321, 322, 335–336, 339–341, 391 Minovitch, Michael, 4 mission costs, 30, 53, 57, 59, 218, 395–396 Mitchell, Bob, 60, 158, 196, 217, 252, 253 Mitterrand, Francois, 18 moonlet, viii, 72, 288, 290, 294, 297, 302, 305, 306, 309, 311, 322

Index N Nagy, Andrew, 40 NASA Administrator, vi, xi–xiii, 12, 14, 28, 31, 33, 36–38, 168, 170 NASA Historical Reference Collection (NHRC), xii NASA Leadership and America’s Future in Space, 12 National Academy of Sciences (NAS), 5, 10, 31, 40, 72, 172 National Center for Space Exploration/Studies (CNES), 8, 18 National Environmental Policy Act, 168 National Space Council, 36, 38 Nelson, Robert, 373 Newburn, Ray, Jr., 4 nitrogen collapse, 359 Noonan, Norine, 32 north polar hood, 362

O Obama, President Barack, 170–172 Observatoire de Paris, 8, 14, 90, 95, 121, 125 occultation, 80, 93, 250, 255, 267, 295, 296, 298, 301, 305 Office of Management and Budget (OMB), 17, 32, 35, 42 Office of Science and Technology Policy (OSTP), 8, 42, 168 Officine Galileo, 66, 126 one-fault tolerant, 61, 389, 390 Ontario Lacus, 369, 370 optical remote sensing instruments, 74–81, 194 optics rotation sensors (FORS), 55 Owen, Tobias (Toby), 7–10, 14, 18, 72, 357

P Paine Commission, 11–12, 14, 19 Paine, Thomas, 11 Pallene, 306, 309, 311, 312 Pan, 110, 204, 292, 294, 295, 297, 301, 312, 391 Pandora, 294, 302, 304, 305, 310 Pappalardo, Bob, 252, 253, 378 Peter Pan moon, 357 Phase A study, 10, 12, 14–16, 72, 109, 120 Phobos, vii Phoebe, ix, xiv, 183, 197, 202–203, 308–310, 322–326, 337, 338 Phoebe Ring, 288, 292, 305, 308–310, 325 PI instruments, 94–95, 129 Pioneer, v, xii, xv, 4–6, 11, 13, 14, 28, 35, 50, 51, 80, 157, 190, 263, 275, 278, 301, 306 Planetary Observer, 54 planetary protection, xiv, 241, 255–257

407

planetesimal, 17, 230, 272, 296 plasma waves, 58, 90, 185, 189, 192, 194, 275, 277–278 plutonium, vi, vii, xi, xiii, xiv, 60, 66–68, 118, 144, 146, 148, 153, 157–173, 193, 223, 256, 390 pointing time, 242, 244–246 Porco, Carolyn, v, 75, 77, 203, 297, 367 prebiotic organic material, 359 Prime Mission, xiv, 60, 206, 218, 241, 243–246, 248–252, 322, 323, 329, 395 Project Science Group (PSG), 72, 243 Prometheus, 294, 301–305 propeller, 296–298 propulsion module, 58, 143, 146 protostar, 272

Q quadrupole mass spectrometer, 124 Quayle, Vice President Dan, 36, 38

R Radar (RADAR), viii, 8, 10, 11, 13, 29, 34, 35, 53, 59, 60, 74, 91–93, 95, 125, 127, 143, 187, 192, 194, 200, 206, 225, 242, 244, 265, 357, 365–370, 372–377, 388, 390 radiation hardening, 61–64 Radio and Plasma Wave Science (RPWS), 74, 81, 85, 89–90, 190, 192, 194, 271, 277, 342 radio science subsystem (RSS), 29, 91–93, 95, 201, 298 Radioisotope Heater Unit (RHU), xiv, 67, 118, 148, 151, 159–161, 168, 223, 390 Radioisotope Thermoelectric Generator (RTG), xiv, 40, 60, 66–68, 88, 96, 139–140, 144, 146–149, 153, 157–166, 169–173, 193, 256, 390 Ranger, xi, 57, 151 Raulin, François, 121, 226 Ray, Trina, 221, 241, 253, 357 reaction wheel, 57, 58, 66, 202, 253–254, 388, 391 Reagan, President Ronald, 8, 13, 28, 36, 54, 245 redundancy, xii, 61–65, 67, 83, 87, 88, 115, 119, 225, 227, 228, 253, 389, 391 Rhea, vi, 77, 307, 321, 322, 328, 338, 341–342, 344, 391 RHU. See Radioisotope Heater Unit (RHU) Ride, Sally, 12–15, 19 Riegle, Senator Donald, 19 ringlet, 288, 290, 294–295, 298, 300, 301, 310–312 riverbeds, 233, 364 Roche Division, 292

408

Index

Roche limit, 290–292, 295 RTG. See Radioisotope Thermoelectric Generator (RTG)

S safe mode, 71, 188 safing, 65, 186, 188 saltation, 367 Satellite Orbiter Science Team (SOST), 322 Saturn kilometric radiation (SKR), 16, 73, 277 Saturn Orbit Insertion (SOI), 67, 183, 197, 201, 202, 205, 206, 218, 222, 248, 249, 277, 278, 288 Saturn rotation rate, 86–87, 274 scan platform, 34, 35, 54, 85, 159, 242, 245, 388 Schiff, Representative Adam, 170–172 self-gravity wake, 295–296 Senate, 19–21, 111, 162, 170–172 separation phase, 222–223 shepherd moon, 302–305 Shoemaker, Gene, 343 Showalter, Mark, 292, 294 Simon, Pierre, 286 Skinner, Samuel, 33 Smeds, Boris, 214–216 Smith, John, 242, 243 Smith, Peter G., 29 solar conjunction, 201–202 solar nebula, 17, 230, 272 Solar System Exploration Committee (SSEC), 8, 10, 13, 54, 72, 221 solar wind, 9, 16, 61–62, 73, 83, 89–90, 189–192, 194, 195, 198–199, 202, 206, 230, 261, 276–280 Solid Rocket Booster (SRB), 31, 181 Solid Rocket Motor Upgrade (SRMU), 30–34, 150 solid-state power switch (SSPS), ix, 60–61, 70 solid-state recorder (SSR), ix, 57, 61, 70, 140–141 Sollazzo, Claudio, 214, 215 Solstice Mission, xiv, 241, 248, 254–257, 266, 391 Sotra Facula, 375, 376 South Coast Air Quality Management District (AQMD), ix Southwest Research Institute, 32, 75, 83, 86, 325, 336, 359 Southwood, David, 95, 226 Soviet Union, vii Space Science Advisory Committee (SSAC), 12, 18 Space Science and Applications Advisory Committee (SSAAC), 32 Space Science Board (SSB), 5, 10, 72 Space Science Committee, 10 Space Shuttle, 8–10, 12, 21, 33, 60, 150, 164, 245 Space Studies Board (SSB), 31–33, 40–41

Spacecraft Assembly Facility (SAF), 139–140, 142, 143 Spehalski, Richard, 34, 165 Spilker, Linda, 246, 253, 287, 340, 342, 346, 365, 387 spiral corrugations, 300, 301 Spitzer Space Telescope, 308 spokes, 7, 40, 255, 278, 287, 299, 300 Squyres, Steven, 336 stellar reference unit (SRU), 65, 66, 186, 188 Stofan, Andrew, 28–29 Strategic Defense Initiative (SDI), 36 subsidiarity, 56, 111, 252 subsurface ocean, 232, 233, 308, 378–380 Sununu, John, 36 super-rotation, 228, 362 Surface Science Package (SSP), 120, 121, 126–130, 225, 234–236 synchronous rotation, 326 synthetic aperture radar (SAR), 53, 60, 91, 92, 365–367 Systema Saturnium, 285

T Tethys, vi, 278, 307, 321, 322, 327–329, 334, 342 The Titans of Saturn, 38, 252 thruster, 57, 58, 66–67, 96, 185, 186, 189, 202, 253–254, 391 tidally locked, 323, 325, 338, 342, 343 tiger stripes, 331–333, 335, 336, 343, 375 Titan, 4, 28, 50, 107, 139, 181, 213, 221, 241, 275, 306, 322, 357, 389 Titan 4, xiv, 28, 34, 40, 132, 144, 149–151, 169, 181, 230 Titan Orbiter Science Team (TOST), 221, 241, 244, 245, 253, 322, 357 Titan’s haze, 60, 122, 358–361, 363, 366, 373 tour designer, 242–243, 245–247, 250, 255 tour engine, 246–248 trajectory correction maneuver (TCM), xiv, 183–189, 193, 195–197, 202 Trojan moon, ix, 323, 343–344 troposphere, 122, 125, 131, 190, 229, 265, 267, 359, 360, 363, 380 Truly, Administrator Richard, 28, 31, 33, 36 TRW, 10, 14, 36–38

U UCLA, 35 Ultraviolet Imaging Spectrograph (UVIS), 74, 78–80, 122, 146, 188, 192, 194, 200, 298, 305, 331, 335, 388

Index Ulysses, 8, 9, 11, 19, 51–52, 87, 168, 250 USSR, 12, 18, 19, 121

V VeGa, 11, 18, 121 Venus, vi–viii, xiv, 4, 11, 17, 19, 30, 59, 118, 121, 145, 182, 183, 186–193, 200, 246, 247, 278, 362, 380 Venus flyby, 185–187, 189 Verbiscer, Anne, 309 Vesta, 17, 18 Visual and Infrared Mapping Spectrometer (VIMS), 29, 35, 68, 74, 80–82, 95, 188, 192, 194, 199, 202, 244, 265, 270, 295, 298, 305, 344, 345, 358, 361, 366–370, 373–376 Voyager, v, vii, viii, xii, xiv, xv, 4–7, 11, 13, 14, 17, 35, 39, 50, 51, 55, 57, 58, 67, 90, 116, 151, 157, 168, 169, 172, 197, 221, 230,

409

231, 253, 263, 264, 268, 270, 271, 274, 275, 277, 278, 286–288, 290, 291, 294, 299–301, 303–305, 329, 330, 337, 343, 344, 357, 358, 362, 365, 366, 390, 392 VVEJGA trajectory, 400

W Waite, Hunter, 86, 336, 359 water jets, 206, 261, 329–337 Webb, James, xi White House, 3, 11, 17–19, 31–33, 35, 36, 38, 42, 162, 168, 169, 245 Whitten, Representative Jamie, 31 Wright, Howard T., 30, 31

Z Zarnecki, John, 129, 130, 236