How Does a Spacecraft Reach the Moon? (Science in the Real World) [1 ed.] 1604134704, 9781604134704

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How Does a Spacecraft Reach the Moon? by Barbara J. Davis Science and Curriculum Consultant: Debra Voege, M.A., Science Curriculum Resource Teacher

Science in the Real World: How Does a Spacecraft Reach the Moon? Copyright © 2010 by Infobase Publishing All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For information contact: Chelsea Clubhouse An imprint of Chelsea House Publishers 132 West 31st Street New York NY 10001 Library of Congress Cataloging-in-Publication Data Davis, Barbara J. How does a spacecraft reach the moon? / by Barbara J. Davis; science and curriculum consultant, Debra Voege. p. cm. — (Science in the real world) Includes index. ISBN 978-1-60413-470-4 1. Space flight—Juvenile literature. 2. Astronautics—Juvenile literature. 3. Aerospace engineering—Juvenile literature. I. Title. II. Series. TL793.D37376 2010 629.43’53—dc22 2009012925 Chelsea Clubhouse books are available at special discounts when purchased in bulk quantities for businesses, associations, institutions, or sales promotions. Please call our Special Sales Department in New York at (212) 967-8800 or (800) 322-8755. You can find Chelsea Clubhouse on the World Wide Web at http://www.chelseahouse.com Developed for Chelsea House by RJF Publishing LLC (www.RJFpublishing.com) Text and cover design by Tammy West/Westgraphix LLC Illustrations by Spectrum Creative Inc. Photo research by Edward A. Thomas Index by Nila Glikin Photo Credits: 4, 29: NASA; 8, 22, 24: NASA-HQ-GRIN; 14: Schiller Schiller/Photolibrary; 15: North Wind Picture Archives/Photolibrary; 18: ESA/CNES/Arianespace - Service optique CSG and European Space Agency; 20, 21: NASA-JSC; 23: © Chris Howes/Wild Places Photography/Alamy; 25: NASA-MSFC; 28: Getty Images. Printed and bound in the United States of America Bang RJF 10 9 8 7 6 5 4 3 2 1 This book is printed on acid-free paper. All links and Web addresses were checked and verified to be correct at the time of publication. Because of the dynamic nature of the Web, some addresses and links may have changed since publication and may no longer be valid.

Table of Contents Destination: The Moon............................................ 4 Motion Is Relative ................................................... 6 Speed ....................................................................... 8 Acceleration .......................................................... 10 Forces..................................................................... 12 Inertia .................................................................... 14 Gravity ................................................................... 16 Friction .................................................................. 18 Mass and Payload ................................................. 20 Action and Reaction ............................................. 22 Engines Big and Small .......................................... 24 Fueling Moon Flights............................................. 26 50 Years of Moon Shots ......................................... 28 Glossary ................................................................. 30 To Learn More ....................................................... 31 Index ...................................................................... 32 Words that are defined in the Glossary are in bold type the first time they appear in the text. 5

Destination: The Moon

The rocket being launched here sent American astronauts on their way to the Moon in 1969.

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he temperature is perfect. The surface on the half-pipe is smooth. There’s a nice breeze at your back. You’re thinking this is going to be a really cool run on your skateboard. As you zip along the ramps, you notice a big full Moon starting to rise above the skate park’s lights. You have a quick thought about how much fun it would be to skateboard to the Moon. But skateboards have nothing to do with the Moon, do they? Actually, they do. You and your skateboard rely on some of the same science principles as a spacecraft heading for the Moon. These

principles deal with motion and with gravity. They govern how things move. They also describe the effects of gravity—such as Earth’s gravity, which holds us all on the ground. Rocket Power Another set of science principles also plays a big role in how a spacecraft works. These have to do with what makes the spacecraft go. You yourself make your skateboard go, but spacecraft are powered by rockets. Most space rockets get their power by burning fuel. This involves a reaction between two or more chemicals. So along with the science of motion and gravity, the science of how chemicals react with each other is important in understanding how a heavy spacecraft can leave Earth and reach the Moon. DID YOU KNOW

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Moon Shots The Moon is about 238,000 miles (383,000 kilometers) from Earth. Dozens of spacecraft have flown there. Some of them simply flew past the Moon. Some went into orbit around it—that is, they circled around it. Some landed on it. Several of these “Moon shots” included a return trip to Earth. Missions between 1968 and 1972 in the Apollo program of the United States carried people. All the rest of the flights had no crew and were controlled by signals from Earth and by onboard computers. Most Moon shots have been launched by just two countries: the United States and the Soviet Union (which is now split up into Russia and many other countries). 5

Motion Is Relative

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o understand how a spacecraft can reach the Moon, you first have to understand a basic fact about motion. Say you watch a bird fly away from a tree. What really happens? The bird changes its position. A scientist wanting to study this movement will measure how much and how quickly the bird’s position changes in relation to the tree. In other words, the bird’s motion is relative to the tree. The tree serves as what is called a reference point.

Earth is spinning around an imaginary line through its center called its axis. Earth is also going around, or orbiting, the Sun. At the same time, the Moon is orbiting Earth.

Earth and the Moon in Motion

Sun

Moon Earth 6

In fact, every example of motion you can find is relative. You see the motion in relation to some reference point or some frame of reference. Watch a kid on a skateboard, for instance. He seems to be moving relative to the ground. Everything’s Moving If you’re sitting at a desk reading this book and you use the objects around you as a frame of reference, it doesn’t seem like you are moving at all. Actually, Earth is constantly spinning. So the ground, you, and everything else on Earth’s surface are always moving relative to Earth’s center. Earth is moving in another important way as well, with the Sun as a reference point. It circles the Sun following a path called an orbit. It takes one year for Earth to go once around the Sun. DID YOU KNOW

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Moon Motion Several types of motion affect the path taken by a spacecraft going from Earth to the Moon. Scientists have to take these into account when planning the spacecraft’s course. One example is the upward movement, relative to the ground, of the rocket launching the spacecraft as it blasts off. Another important motion is the spinning of Earth, relative to its center, from west to east. A spacecraft launched from Earth’s surface shares this motion. Mission planners also have to pay attention to movement of the Moon as the spacecraft is in flight. The Moon is constantly traveling in an orbit around Earth. 7

Speed

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peed is one important way of describing the motion of a skateboard, a rocket, or any other object. Speed is the amount of distance something travels in a certain amount of time. It is talked about as a unit of distance divided by a unit of time, such as miles per hour (mph) or kilometers per hour (km/ hr). For example, a car moving at 50 mph (80 km/hr) will travel 50 miles (80 kilometers) in one hour. A rocket launched from Earth has to reach a speed of about 25,000 mph (40,300 km/hr) to escape the pull of Earth’s gravity. Changing Speed An object that always stays at the same speed is said to travel at constant speed. Most things do not travel at a constant speed, though. Their speed changes to fit the situation. For example, a runner might run quickly on level ground. Then, as she starts to go up a hill, her speed drops. Coming down the hill, her speed increases again. 8

Speed of Apollo 17 Spacecraft as It Headed for the Moon Time After Takeoff

Approximate Speed

0 seconds (resting on launch pad)

0 mph (0 km/hr)

2 minutes, 42 seconds

5,300 mph (8,500 km/hr)

9 minutes, 21 seconds

14,700 mph (23,700 km/hr)

11 minutes, 53 seconds

16,500 mph (26,600 km/hr)

3 hours, 18 minutes, 28.5 seconds

23,300 mph (37,500 km/hr)

When a spacecraft is launched to the Moon, it starts slowly on takeoff but quickly increases its speed. On many Moon missions, the spacecraft picks up enough speed to go into orbit around Earth. It has a certain speed in this orbit. When the time comes to head for the Moon, the spacecraft increases its speed in order to break out of orbit. As it comes near the Moon, the craft adjusts its speed in order to go into orbit around the Moon or land on it.

The table shows how the Apollo 17 spacecraft increased its speed in the first hours after launch. The craft’s three astronauts are shown shortly before liftoff.

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Velocity and Speed Besides knowing the speed at which an object is traveling, it is also important to know the direction in which it is traveling. Velocity describes the speed of an object in a given direction. When scientists calculate the time it takes a spacecraft to reach its destination, they look at both the speed at which the spacecraft has to travel and the direction in which it needs to go. 9

Acceleration

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The powerful rockets that launch spacecraft to the Moon often have stages. As the fuel in each stage gets used up, that stage drops off and the next stage takes over the job of accelerating the spacecraft.

magine a track meet. The runners all line up at the starting line. At this point, their velocity is 0—they aren’t moving. Then, the starting gun goes off, and the runners push off. They begin to increase their speed. We say that they accelerate. To most people, acceleration means simply “speeding up.” In science, though, the word has a different meaning. It is the rate at which velocity changes. Remember that velocity involves the direction in which an object moves as well as its speed. So accelerating the object may involve changing its speed or changing its direction (or both).

A Two-Stage Rocket Spacecraft Going to the Moon Launch Rocket Second Stage

Launch Rocket First Stage

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Rocket Acceleration Scientists planning a space mission have to think carefully about acceleration. Spacecraft traveling to the Moon have rocket engines in them. Whenever the spacecraft needs to change its speed or change its direction during flight, it has to use its engines. Flight planners need to make sure the craft has the right amount of fuel and the right engines to follow the correct flight path to get it to the Moon. In order to get enough acceleration to get off the ground and out into space, a spacecraft going to the Moon is put on top of a big, very powerful rocket. Big rockets like those used to launch Moon shots usually have stages. Each stage carries fuel and engines. The stages are usually arranged in a stack. When the fuel in each stage is used up, that stage separates and drops away. (Often it falls into the ocean or onto an empty area of land.) Eventually, all of the stages of the launch rocket drop away, and only the spacecraft going to the Moon is left. DID YOU KNOW

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Booster Rockets Big rockets may also have booster rockets attached. Like stages, boosters carry engines and fuel, and they drop away when their job is finished. 11

Forces

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ow do you make something accelerate? How do you make it move, change its speed or direction, or stop? You give it a push or a pull. Pushes and pulls are examples of what scientists call forces. When you pull at the zipper on your jacket, you are using force. Scientists designing a Moon rocket have to take into account all the forces that will act on it. These include not only the forces generated by its engines to make it go, but also other forces that affect its motion, such as gravity. Force is described by the amount of push or pull. It is also described by the direction of the push or pull. When you push on a door, you are using force in one direction. When you pull on that same door, you are using force in the opposite direction. Balanced and Unbalanced Forces In a tug-of-war, two teams pull on a rope in opposite directions. The team that uses the most force pulls the other team across a line. This is an example of how motion is af-

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Thrust Lifts a Rocket fected by unbalanced force. The force of the pull from one team is greater than the force of the pull from the other team. Unbalanced forces acting on an object will change the obThrust ject’s motion. If the two tugof-war teams are evenly matched, however, the situation is different. The teams both pull as hard as they can, but the one force is exactly balanced by the other force. When balanced forces act on an object, they will not change that object’s motion.

Gravity

Rockets that launch spacecraft to the Moon produce a powerful upward force called thrust that is stronger than the force of gravity pulling down on the rocket and spacecraft.

DID YOU KNOW

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Measuring Force The strength of a force is often measured in units called newtons. One newton is not a very big force. It is about the force you would need to lift a normal-sized empty glass. The thrust, or upward force, produced by a Moon rocket’s engine is much greater. The engines in the first stage of the huge Saturn V rocket used to launch the Apollo spacecraft to the Moon could produce about 34.5 million newtons—enough to lift the colossal rocket, which weighed about 6.7 million pounds (3 million kilograms), off the launchpad. 13

Inertia

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he unit of measurement for force called the newton is named in honor of the English scientist and mathematician Isaac Newton. In the late 1600s, Newton discovered three basic laws, or principles, that describe how forces affect objects. Scientists still rely on these laws of motion when figuring out how to get a spacecraft to the Moon. Newton’s first law of motion deals both with objects that are at rest (that is, not moving at all) and with objects that are moving. It says that an object at rest will remain at rest unless it is acted upon by a force strong enough to make it move. The first law also says that an object in motion will move at a constant Seatbelts in a car stop the continued forward motion of the people when the car is stopped suddenly. In this photo, the strength of these seatbelts is being tested using crash dummies.

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speed in a straight line unless acted upon by a force strong enough to make it change its speed or direction. The first law is sometimes called the law of inertia. Inertia is the tendency of an object to resist change in its motion. For example, the passengers in a moving car keep moving forward when the car stops suddenly. The passengers have inertia. The only way to stop inertia is to exert an opposite force. That is what seatbelts do. Mass and Inertia The inertia an object has depends on its mass. Mass is the amount of matter in the object. Think of two glass jars that are exactly alike in size and shape. Fill one jar with nickels. Fill the other jar with feathers. The mass of the jar with the nickels is much greater. You have to use more force to move the jar of nickels. It has greater inertia. DID YOU KNOW

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Isaac Newton (1642–1727) Isaac Newton lived from 1642 to 1727. He was one of the greatest scientists of all time. In addition to the three laws of motion, he made important discoveries about gravity, light, and color. He also developed new ideas about mathematics. 15

Gravity

G

ravity is a force that acts to pull objects straight down toward the center of Earth. It pulls you down to the ground when you fall off your skateboard. Actually, everything— not just Earth—has this sort of pull. Even you have a gravitational pull on things. The planet Jupiter, much larger than Earth, has more mass. Therefore, the pull of Jupiter’s gravity is stronger. For this reason, a person who weighs 100 pounds on Earth would weigh 236 pounds on Jupiter!

Weight and Mass People sometimes think the words weight and mass mean the same thing. But for scientists, they mean

Weight and Gravity 236 Pounds 100 Pounds

Earth

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Jupiter

different things. Weight is the force of gravity on a person or object at the surface of a planet. When you stand on a scale, the scale measures the force with which Earth pulls on you. Mass is something different. It is a measure of the amount of matter in an object. Far out in space, far from the pull of Earth’s gravity, your weight might go down to just about zero, but you would still have the same mass. The gravitational pull of an object depends on the amount of mass it has. The greater the mass, the stronger the pull. When you fall off your skateboard, you pull Earth to you at the same time Earth pulls you toward its center. But your mass is tiny compared to that of Earth. So the pull you exert on Earth is much, much weaker than the pull of Earth’s gravity on you. DID YOU KNOW

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Pulling Away from Earth…and from the Moon A spacecraft heading to the Moon has to deal with the force of gravity before it can get anywhere. The thrust of a powerful rocket is needed to overcome the pull of gravity, lift it up, and send it on its way. As the spacecraft gets farther from Earth, the pull of Earth’s gravity decreases. Of course, the Moon has its own gravity. As the spacecraft gets closer to the Moon, at a certain point it will be pulled toward the Moon by the Moon’s gravity. For a spacecraft that has landed on the Moon to return to Earth, it has to first pick up enough speed to escape from the Moon’s gravity. 17

Friction

L

ike gravity, friction is a force that can affect the motion of an object. Friction occurs when two surfaces rub together. Think of the wheels of a skateboard on pavement. It may seem that the wheels and the pavement are both smooth. But actually both have bumps and ridges. Friction is created when the bumps and ridges of the two surfaces come into contact with each other.

The launch rocket in the righthand photo has a streamlined shape to reduce friction as it travels through the air. The spacecraft in the left-hand photo does not need a streamlined shape. It travels in space, where there is no air. 18

Friction Opposes Motion If a moving object meets continuous friction, sooner or later it will be brought to a stop. Without friction, the object would keep moving at a constant speed forever. With friction, the only way the object can keep moving is if it gets a push (or a pull) from some other force. For the skateboard, you supply the push. How strong the force of friction will be depends on a couple of factors. One is the type of surfaces involved. For example, the rougher the surfaces, the greater the friction. Another factor is how hard the surfaces push together. There is more friction if you rub your hands together with some force than if you rub your hands together lightly. DID YOU KNOW

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Friction and Rockets Air can also be a source of friction. Air particles hit an object as it moves through the air. The faster the object goes, the more it is hit with air particles. The more air particles that collide with it, the greater the resistance it meets from the air. Designers of rockets have to pay attention to this fact. They want to make this air resistance, or “drag,” as small as possible. That is why rockets are given a streamlined shape. Such a shape reduces the surface hit by air particles, making it easier for the rocket to move smoothly through the air and lift its spacecraft. Of course, drag is not a problem for spacecraft that operate only in space, where there is no air. Such spacecraft do not need to be streamlined. 19

Mass and Payload

Astronaut Neil Armstrong was part of the payload of the spacecraft that took him to the Moon in 1969.

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magine an empty cardboard box. It has very little mass. It is very easy to push. Suppose you fill it with rocks. Now the mass is much greater, and you have to use a lot more force to push it. This fact is explained by Isaac Newton’s second law of motion. This science principle says that the amount of force needed to move an object—that is, change its speed or direction—depends on the size of the object’s mass. The greater the mass, the greater the amount of force required. The law also says that for a given mass, a greater force will produce a greater change in speed or direction.

The change in speed or in direction, by the way, will occur in the same direction as the force. The cardboard box will move in the direction you push. Tying Together Force, Mass, Acceleration Remember that a change in the speed or direction of motion of an object is known as acceleration. So Newton’s second law shows the connection between mass, force, and acceleration. If you want the box full of rocks to move quickly, you have to push hard. In other words, you have to use a lot of force to accelerate it to a fast speed. If you want the full box to move slowly, you use less force. According to the second law, however, there is a way you can make the box move quickly without pushing hard. You can reduce the box’s mass: Take some rocks out!

These rocks brought back from the Moon by Apollo program astronauts were part of the payload on the trip home.

DID YOU KNOW

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Payloads and the Second Law The payload of a spacecraft consists of the things it carries. Payload can include cargo, scientific instruments, and (for some missions) the crew. When scientists design spacecraft, they must take into consideration the mass of the payload. Many factors affect how big a payload a spacecraft can carry. However, in simple terms, if everything else is equal, you can make the spacecraft go faster simply by lightening the payload. 21

Action and Reaction

The downward force of the hot gas created when fuel is burned in a rocket produces the opposite upward force— the thrust—that lifts the rocket off the ground.

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he third of Newton’s three laws of motion says that forces are not “one-sided.” Whenever one object exerts a force on a second object, the second object also exerts a force back on the first object. The second force is equal in strength to the first force. However, that second force acts in the opposite direction from the first force. Newton’s third law is often stated in the following easy-to-remember

way: “For every action, there is an equal and opposite reaction.” Look Around You can see examples of the third law everywhere. When you walk, you push against the ground with your feet. The ground pushes back on your feet with an equal and opposite force. If you use a hammer to pound a nail into a piece of wood, the nail exerts an equal force back on the hammer. That’s why the act of hitting the nail with the hammer causes the hammer to stop moving. Bumper cars at amusement parks are so much fun because of Newton’s third law. If your car rams another car, your car bounces back. The other car pushes your car with a force equal to yours, but in the opposite direction.

When you ram another car on a bumper car ride, an equal and opposite force pushes your car back.

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Thrust and Newton’s Third Law Rockets lift spacecraft because of the principle of action and reaction. A rocket engine contains fuel inside it that produces hot gas that rushes out the rocket’s back end. The downward force of this stream of hot gas has an equal and opposite reaction: the upward thrust that lifts the rocket off the ground—and lifts the spacecraft the rocket carries. 23

Engines Big and Small

The small landing craft that brought Apollo 15 astronauts to the Moon’s surface in 1971 is on the left in this photo. The “rover” the astronauts used to ride around the landing area is on the right.

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ockets that launch spacecraft from Earth’s surface have a tough job. They have to produce enough thrust to overcome gravity and drag and to lift a heavy payload off the ground and into space. This is why the Apollo missions that took people to the Moon and back used the huge Saturn V rocket. The Saturn V stood about 360 feet (110 meters) tall. It had three stages, with five engines in each of the first two stages and one engine in the third stage.

Launching from Space The job is easier if you launch a spacecraft to the Moon from space— say, from orbit around Earth, where Earth’s gravity is weaker and there is no air resistance. The European Space Agency’s Smart-1 spacecraft was launched to the Moon from orbit in 2003. Of course, Smart-1 first had to be lifted into orbit by a large rocket. Perhaps someday it will be possible to put together spacecraft in orbit, using parts lifted into orbit by smaller rockets.

In this photo, the second stage of a huge Saturn V rocket is unloaded from its shipping container at the Kennedy Space Center in Florida.

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Apollo Engines The Apollo missions had a special spacecraft atop the Saturn V rocket. On some missions this spacecraft had to go into orbit around the Moon, send a small craft to the Moon’s surface, receive the craft back, and return to Earth. The little craft that visited the Moon’s surface had engines for landing and takeoff as well as more than a dozen small engines called thrusters for maneuvering. Only part of the Apollo spacecraft returned to Earth. Called the command module, it included the crew’s living space and had several thrusters. Another part of the Apollo spacecraft, called the service module, was linked to the command module for most of the mission. It had thrusters along with an engine for getting into and out of lunar orbit. The command module separated from the service module before reaching Earth. 25

Fueling Moon Flights

In a liquid-fuel rocket, the fuel and liquid oxygen are stored separately until the fuel is to be burned in another chamber. In a solid-fuel rocket, the fuel is stored in the chamber where it is burned.

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he chemical reaction produced by rocket fuels has to be very powerful. But it shouldn’t be too powerful. If it is too strong, and if the fuel burns too fast, the rocket engine will explode. Also, the reaction must occur at a more or less steady rate, so the engine can give a reliable amount of power for a certain period of time. Two Different Types of Fuel Some rocket engines use liquid fuel. Others use solid fuel. Liquid-fuel

Liquid-Fuel and Solid-Fuel Rockets Liquid-Fuel Rocket

Spacecraft

Solid-Fuel Rocket

Spacecraft Igniter

Liquid Oxygen

Fuel Chamber Where Fuel Burns

Pumps

Nozzle

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Chamber Where Fuel Burns

Nozzle

engines usually combine two chemicals to produce a reaction. The chemicals are stored in tanks. One chemical is the basic fuel. It is fed to the engine, where it burns when the other chemical, liquid oxygen or something similar, is added to it. Solid-fuel engines are something like giant fireworks. The solid fuel is kept in the engine. When it is ignited (lit), it usually continues to burn there once the reaction is started. Military missiles that launch weapons often use solid-fuel engines. Solid fuel, unlike liquid fuel, can be loaded well in advance, so the missile can be fired at a moment’s notice. Liquid-fuel engines are easier to control. For that reason, they are more often used in space rockets. Some big rockets use both solid-fuel and liquid-fuel engines. The European Space Agency’s Ariane 5 rocket has liquid-fuel main stages and solid-fuel boosters. DID YOU KNOW

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A Different Kind of Fuel Smart-1 used a third, and quite different, approach to getting to the Moon: a kind of electric engine! This engine produced a thrust that was very weak. The spacecraft had a very small mass, however, and it was launched from space. The engine also worked for a long time, so the thrust was enough to slowly move the craft toward the Moon. Smart-1 began to orbit the Moon more than a year after it was launched. 27

50 Years of Moon Shots

This rocket launched a Chinese spacecraft to the Moon in 2007.

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eople began sending spacecraft to study the Moon in 1959. The Soviet Union’s spacecraft Luna 1 flew past the Moon early that year. A couple of months later the U.S. spacecraft Pioneer 4 also did a flyby. The following years saw many Moon missions by both the United States and the Soviet Union, ending with the Soviet Union’s Luna 24 in 1976. Only the U.S. Apollo program sent people to the Moon. Apollo 8 orbited the Moon at the end of 1968. So did Apollo 10 in May 1969. Landings were made on the Moon by Apollo 11 and 12 in 1969, Apollo 14 and 15 in 1971, and Apollo 16 and 17 in 1972. Apollo 13 in 1970 was also supposed to land

on the Moon, but there was an explosion. The damaged craft with its three astronauts barely made it back to Earth. A few Soviet spacecraft without crews that landed on the Moon also returned to Earth. The U.S. and Soviet missions brought scientists valuable data and samples of Moon rocks. Back to the Moon After Luna 24 people focused on developing space stations and on sending space probes to other parts of the solar system. Eventually, space scientists again started sending spacecraft to study the Moon. In the past twenty years, Japan, China, India, the European Space Agency, and the United States have launched missions to the Moon, all without crews.

Astronaut Buzz Aldrin looks at the American flag he and Neil Armstrong placed on the Moon in 1969.

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One Small Step… People in the United States and around the world were proud and excited when Apollo 11 astronauts Neil Armstrong and Buzz Aldrin became the first people to walk on the Moon on July 20, 1969. Armstrong got out of the spacecraft first. When he set foot on the Moon, he said: “That’s one small step for man, one giant leap for mankind.” 29

Glossary acceleration—A change in the velocity of an object. booster—A rocket that is attached to a main rocket and helps provide the thrust it needs. force—A push or pull. Forces are what cause, accelerate, or stop the motion of objects. friction—A force that resists the movement of one surface over another when the two surfaces are in contact. gravity—A force that pulls objects toward the center of a body. Its strength depends on the mass of the body. inertia—The tendency of objects to resist a change in their motion. lunar—Having to do with the Moon. mass—The amount of matter in an object. newton—A unit of measurement for force. It is the amount of force that can give a mass of 1 kilogram an acceleration of 1 meter per second per second. orbit—The path followed by a body as it circles around another body because of the pull of the second body’s gravity.

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payload—The things carried by a rocket or other vehicle, such as a spacecraft with scientific instruments and crew. reaction—(1) A process in which substances act on each other. Some reactions can produce energy. (2) A force opposite to another force. rocket—An engine or vehicle that carries all its own fuel and is powered by the reaction force created by hot gas rushing out of the back end. stage—A part of a rocket that contains one or more engines and separates away when its fuel is used up. Usually stages are arranged in a stack. thrust—The reaction force that pushes a rocket forward. velocity—The rate at which an object changes position. In the strict sense, velocity involves both the speed and the direction of an object’s motion. weight—The force of gravity on an object, especially as measured at the surface of a planet, such as Earth.

To Learn More Read these books: Green, Carl R. Apollo 11 Rockets to First Moon Landing. Berkeley Heights, N.J.: MyReportLinks.com, 2004. Jedicke, Peter. Great Moments in Space Exploration. New York: Chelsea House, 2007. Miller, Ron. Rockets. Minneapolis: Twenty-First Century Books, 2007. Thimmesh, Catherine. Team Moon: How 400,000 People Landed Apollo 11 on the Moon. Boston: Houghton Mifflin, 2006.

Look up these Web sites: European Space Agency http://www.esa.int/esaCP/SEM2S8WJD1E_index_0.html Fear of Physics: Learn about Position, Velocity, and Acceleration http://www.fearofphysics.com/Xva/xva.html NASA: Solar System Exploration http://solarsystem.nasa.gov/multimedia/gallery.cfm?Category=Spacecraft Yahoo! Kids: Spacecraft http://kids.yahoo.com/science/space/article/spacecraft

Key Internet search terms: Apollo Program, Moon, NASA, Isaac Newton, rocket, Smart-1

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Index Acceleration 10–11 Air resistance see Drag Aldrin, Buzz 29 Apollo program 5, 9, 24, 25, 28, 29 Armstrong, Neil 20, 29 Axis (Earth) 6 Balanced force 13 Booster rockets 11 Chemical reaction 5, 26–27 Command module 25 Computers (on spacecraft) 5 Constant speed 8 Course of spacecraft 7, 9 Design of rocket 12 Drag 19, 24 Earth (planet) 5, 6, 7, 17 Engines see Rocket engines European Space Agency 25, 27, 29 First law of motion 14–15 Forces 12–13 Friction 18–19 Fuel for rockets 5, 11, 22, 23, 26–27 Gravity 5, 16–17 History of Moon shots 5, 28–29 Inertia 15 Jupiter (planet) 16 Liquid-fuel engines 26–27

Mass 15, 17, 20 Motion 5, 6–7, 14–15, 20–21, 22–23 Newton, Isaac 14–15 Newtons (unit of measure) 13, 14 Orbit 5, 6, 7 Payload 20, 21 Planning of space mission 7, 11 Rocket engines 5, 11, 13, 23, 24–25, 26–27 Saturn V rocket 13, 24, 25 Seatbelts 14 Second law of motion 20–21 Shape of rockets 18, 19 Skateboard 4, 8, 17, 18 Smart-1 (spacecraft) 25, 27 Solid-fuel engines 26–27 Soviet Union 5, 28, 29 Speed 8–9, 10 Speed of rockets 8 Stages of rockets 11, 24, 25 Sun 6, 7 Takeoff 9, 25 Third law of motion 22–23 Thrust 13, 22, 23 Two-stage rockets 10 Unbalanced force 13 Velocity 9, 10 Weight 16–17

About the Author Barbara J. Davis has written books on science topics for kids for more than fifteen years. She has published books on ecosystems and biomes, as well as earth science subjects. 32