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INTRODUCTIONOn the night of July 23, 1995, astronomers AlanHale and Tom Bopp discovered Comet Hale-Boppfrom different locations. Hale and Bopp were thefirst people in more than 3000 years to view whatis now officially known as Comet Hale-Bopp(Comet H-B for short). This may have been thebiggest comet ever visible from Earth.

During this lesson, you will examine the orbitsof asteroids and comets within the solar systemand the possible effects of asteroid and cometimpact on the planets. What do you alreadyknow and want to know about asteroids, comets,and meteoroids? How are they alike or different?What are some of the possible effects of theirimpact on Earth and other planets? In this les-son, you will explore these and other questions.

17Asteroids, Comets,and Meteoroids

LESSON

OBJECTIVES FOR THIS LESSON

Analyze the position of the asteroid belt

using mathematical patterns.

Brainstorm what you know and want

to know about asteroids, comets,

and meteoroids; make comparisons

among them.

Analyze the ability of scientists to

forecast asteroid and comet impact,

and explore the challenges of making

such forecasts.

Read to learn more about Earth-

observing missions.

Comet Hale-Bopp. The white dust tail is composed of

large particles of dust and ice. The gas tail is blue.

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Getting Started

1. With your class, review your homeworkfrom Lesson 16, Student Sheet 16: Bode’s Law.

2. Read “Asteroids, Comets, and Meteoroids.”Carefully examine the photos in the read-ing selection. How are asteroids similar toor different from comets and meteoroids?Record a summary of your ideas in yourscience notebooks. Then discuss them as a class or within your group, as instructedby your teacher.

MATERIALS FORLESSON 17

For you1 completed copy of

Student Sheet 16:Bode’s Law

1 working copy ofStudent Sheet10.1c: PlanetaryChart

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INTRODUCTIONOn the night of July 23, 1995, astronomers AlanHale and Tom Bopp discovered Comet Hale-Boppfrom different locations. Hale and Bopp were thefirst people in more than 3000 years to view whatis now officially known as Comet Hale-Bopp(Comet H-B for short). This may have been thebiggest comet ever visible from Earth.

During this lesson, you will examine the orbitsof asteroids and comets within the solar systemand the possible effects of asteroid and cometimpact on the planets. What do you alreadyknow and want to know about asteroids, comets,and meteoroids? How are they alike or different?What are some of the possible effects of theirimpact on Earth and other planets? In this les-son, you will explore these and other questions.

17Asteroids, Comets,and Meteoroids

LESSON

OBJECTIVES FOR THIS LESSON

Analyze the position of the asteroid belt

using mathematical patterns.

Brainstorm what you know and want

to know about asteroids, comets,

and meteoroids; make comparisons

among them.

Analyze the ability of scientists to

forecast asteroid and comet impact,

and explore the challenges of making

such forecasts.

Read to learn more about Earth-

observing missions.

Comet Hale-Bopp. The white dust tail is composed of

large particles of dust and ice. The gas tail is blue.

© W

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AM

JAM

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Getting Started

1. With your class, review your homeworkfrom Lesson 16, Student Sheet 16: Bode’s Law.

2. Read “Asteroids, Comets, and Meteoroids.”Carefully examine the photos in the read-ing selection. How are asteroids similar toor different from comets and meteoroids?Record a summary of your ideas in yourscience notebooks. Then discuss them as a class or within your group, as instructedby your teacher.

MATERIALS FORLESSON 17

For you1 completed copy of

Student Sheet 16:Bode’s Law

1 working copy ofStudent Sheet10.1c: PlanetaryChart

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LESSON 17 AS T E R O I D S , CO M E T S, A N D ME T E O R O I D S

Inquiry 17.1Examining Asteroids

PROCEDURE

1. In the program Explore the Planets,review the Asteroids segment in the “Tourthe Planets” section. Discuss the conceptswith your class.

2. Make general observations about theasteroids shown on the program. Lookback at Lesson 12. How do you thinkBarringer (Meteor) Crater and the craterson the surface of asteroids Gaspra and Idawere formed? Discuss or record yourideas, as instructed by your teacher.

REFLECTING ON WHAT YOU’VE DONE

Answer the following questions in your sciencenotebook, and be prepared to discuss yourideas with the class:

A. How and when do scientists thinkasteroids may have formed?

B. Why do you think the belt of asteroidsexists between Jupiter and Mars?

C. How are the orbits of asteroids similarto, or different from, planetary orbits?

Inquiry 17.2Studying Asteroid Impact

PROCEDURE

1. Read “A Fiery Necklace.” How did Dr. Eugene Shoemaker contribute to theunderstanding of asteroid and cometimpacts? Record your ideas in your notebook.

REFLECTING ON WHAT YOU’VE DONE

1. In your notebook, record your ideas tothe following, and be prepared to discussthem with your class:

A. How has Earth’s history been influ-enced by occasional natural catastrophes,such as asteroid impacts?

B. An asteroid impact is considered a nat-ural hazard on Earth, but it is not consid-ered a natural hazard on any other planetor moon. Given this information, howwould you define “natural hazard?”

C. What is the scientist’s role in forecast-ing asteroid and comet impacts?

D. What challenges do scientists face whenthey forecast asteroid or comet impacts?

E. What is the scientist’s role in reducingthe risks of such an event?

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2. Return to your list of ideas about aster-oids, comets, and meteoroids. What newthings do you want to add to your list?Make your changes and additions now.

3. With your class, return to the Question Jfolder for Lesson 1. Is there anything thatyou would now change or add? Discussyour ideas with the class.

4. Read “Mission: Earth.” Add any newinformation about Earth to your workingcopy of Student Sheet 10.1c: PlanetaryChart. Complete your planetary brochure.You will present your planetary brochureand your team’s mission design to theclass in Lesson 19.

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Inquiry 17.1Examining Asteroids

PROCEDURE

1. In the program Explore the Planets,review the Asteroids segment in the “Tourthe Planets” section. Discuss the conceptswith your class.

2. Make general observations about theasteroids shown on the program. Lookback at Lesson 12. How do you thinkBarringer (Meteor) Crater and the craterson the surface of asteroids Gaspra and Idawere formed? Discuss or record yourideas, as instructed by your teacher.

REFLECTING ON WHAT YOU’VE DONE

Answer the following questions in your sciencenotebook, and be prepared to discuss yourideas with the class:

A. How and when do scientists thinkasteroids may have formed?

B. Why do you think the belt of asteroidsexists between Jupiter and Mars?

C. How are the orbits of asteroids similarto, or different from, planetary orbits?

Inquiry 17.2Studying Asteroid Impact

PROCEDURE

1. Read “A Fiery Necklace.” How did Dr. Eugene Shoemaker contribute to theunderstanding of asteroid and cometimpacts? Record your ideas in your notebook.

REFLECTING ON WHAT YOU’VE DONE

1. In your notebook, record your ideas tothe following, and be prepared to discussthem with your class:

A. How has Earth’s history been influ-enced by occasional natural catastrophes,such as asteroid impacts?

B. An asteroid impact is considered a nat-ural hazard on Earth, but it is not consid-ered a natural hazard on any other planetor moon. Given this information, howwould you define “natural hazard?”

C. What is the scientist’s role in forecast-ing asteroid and comet impacts?

D. What challenges do scientists face whenthey forecast asteroid or comet impacts?

E. What is the scientist’s role in reducingthe risks of such an event?

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2. Return to your list of ideas about aster-oids, comets, and meteoroids. What newthings do you want to add to your list?Make your changes and additions now.

3. With your class, return to the Question Jfolder for Lesson 1. Is there anything thatyou would now change or add? Discussyour ideas with the class.

4. Read “Mission: Earth.” Add any newinformation about Earth to your workingcopy of Student Sheet 10.1c: PlanetaryChart. Complete your planetary brochure.You will present your planetary brochureand your team’s mission design to theclass in Lesson 19.

the asteroid belt—a vast, doughnut-shapedring located between the orbits of Mars andJupiter (see the illustration below). Gaspra and Ida, pictured on the next page, are twoasteroids found in the main belt. Some scien-tists theorize that asteroids may be pieces of a planet that never formed because Jupiter’sgreat mass exerted too much gravitationalforce to allow the pieces to combine into one planet.

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Among the planets in the solar system arecountless numbers of asteroids, comets, andmeteoroids. Let’s take a look at how these solarsystem objects are different from planets.

AsteroidsAsteroids are metallic, rocky objects in space.They have no atmospheres and move in inde-pendent orbits around the Sun. Tens of thou-sands of asteroids are found in an area called

The asteroid belt is located between Jupiter and Mars.

Mars Jupiter

Asteroids, Comets, and Meteoroids

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Image of asteroid 951, Gaspra, taken by the

Galileo spacecraft

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Because asteroids are too small to be classifiedas planets, they are often called “minor planets.”Some asteroids are the size of a small building.Ceres was the first asteroid scientists observed.Discovered in 1801, it is about 1000 kilometersacross and one of the largest known asteroids.Another asteroid, discovered in 2001, is abouthalf the size of Pluto (or 1150 kilometers). Mostasteroids are actually less than a kilometer wide.If we could combine all of these asteroids, theywould be smaller than half the size of the Moon.

(continued)

the asteroid belt—a vast, doughnut-shapedring located between the orbits of Mars andJupiter (see the illustration below). Gaspra and Ida, pictured on the next page, are twoasteroids found in the main belt. Some scien-tists theorize that asteroids may be pieces of a planet that never formed because Jupiter’sgreat mass exerted too much gravitationalforce to allow the pieces to combine into one planet.

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Among the planets in the solar system arecountless numbers of asteroids, comets, andmeteoroids. Let’s take a look at how these solarsystem objects are different from planets.

AsteroidsAsteroids are metallic, rocky objects in space.They have no atmospheres and move in inde-pendent orbits around the Sun. Tens of thou-sands of asteroids are found in an area called

The asteroid belt is located between Jupiter and Mars.

Mars Jupiter

Asteroids, Comets, and Meteoroids

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LESSON 17 AS T E R O I D S , CO M E T S, A N D ME T E O R O I D S

Image of asteroid 951, Gaspra, taken by the

Galileo spacecraft

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Image of the asteroid 243, Ida, and its small satellite, Dactyl. Dactyl is the small object to the right of Ida.

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Because asteroids are too small to be classifiedas planets, they are often called “minor planets.”Some asteroids are the size of a small building.Ceres was the first asteroid scientists observed.Discovered in 1801, it is about 1000 kilometersacross and one of the largest known asteroids.Another asteroid, discovered in 2001, is abouthalf the size of Pluto (or 1150 kilometers). Mostasteroids are actually less than a kilometer wide.If we could combine all of these asteroids, theywould be smaller than half the size of the Moon.

(continued)

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This photo shows two tails shed by Comet Hale-Bopp. The blue tail points directly away from the Sun. The white tail is cre-

ated by bits of grit that have come off the comet’s nucleus. They are being pushed away by the solar winds from the Sun.

Comets A comet is a mass of frozen gas, cosmic dust,and ice crystals. Comets are often describedas “dirty icebergs.” They circle the Sun inlong, narrow orbits, mainly located in thecold outer reaches of our solar system. Theyorbit the Sun in the Kuiper Belt, whichbegins just past Neptune. A trillion morecomets may live even farther out in a coldarea called the “Oort Cloud.”

Some comets leave their orbits in theKuiper Belt or the Oort Cloud and journeytoward the Sun. When a comet flies nearthe Sun, its ice begins to “boil” away. As itvaporizes, a tail of glowing gases and dustforms behind it, always pointing away from theSun. If Earth happens to pass through comet

dust, burning particles can be seen streakingthrough the sky in a spectacular display calleda “meteor shower.”

The parts of a comet

Nucleus TailComa

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A meteorite stone discovered on Earth weighing 452.6 grams.

A 1-cm square cube is shown for scale.

Meteors streak through the night sky.

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and MeteoritesMeteoroids are pieces of rock andmetal dislodged from comets, planets,asteroids, or moons. Most meteoroidsare made up of dust-sized particles.When a meteoroid enters a planet’satmosphere, it burns up due to fric-tion. As it burns, a meteoroid creates abright streak of light in the sky that wecall a “meteor.” Sometimes large mete-oroids do not burn up completely—one may make it all the way through a planet, moon, or asteroid’s atmos-phere and land on its surface, afterwhich it is called a “meteorite.” �

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This photo shows two tails shed by Comet Hale-Bopp. The blue tail points directly away from the Sun. The white tail is cre-

ated by bits of grit that have come off the comet’s nucleus. They are being pushed away by the solar winds from the Sun.

Comets A comet is a mass of frozen gas, cosmic dust,and ice crystals. Comets are often describedas “dirty icebergs.” They circle the Sun inlong, narrow orbits, mainly located in thecold outer reaches of our solar system. Theyorbit the Sun in the Kuiper Belt, whichbegins just past Neptune. A trillion morecomets may live even farther out in a coldarea called the “Oort Cloud.”

Some comets leave their orbits in theKuiper Belt or the Oort Cloud and journeytoward the Sun. When a comet flies nearthe Sun, its ice begins to “boil” away. As itvaporizes, a tail of glowing gases and dustforms behind it, always pointing away from theSun. If Earth happens to pass through comet

dust, burning particles can be seen streakingthrough the sky in a spectacular display calleda “meteor shower.”

The parts of a comet

Nucleus TailComa

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LESSON 17 AS T E R O I D S , CO M E T S, A N D ME T E O R O I D S

A meteorite stone discovered on Earth weighing 452.6 grams.

A 1-cm square cube is shown for scale.

Meteors streak through the night sky.

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and MeteoritesMeteoroids are pieces of rock andmetal dislodged from comets, planets,asteroids, or moons. Most meteoroidsare made up of dust-sized particles.When a meteoroid enters a planet’satmosphere, it burns up due to fric-tion. As it burns, a meteoroid creates abright streak of light in the sky that wecall a “meteor.” Sometimes large mete-oroids do not burn up completely—one may make it all the way through a planet, moon, or asteroid’s atmos-phere and land on its surface, afterwhich it is called a “meteorite.” �

A FIERY

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NECKLACE

A NASA Hubble Space Telescope image of comet Shoemaker-Levy 9, taken on May 17, 1994. When the comet was

observed, its train of 21 icy fragments stretched across 1.1 million km of space, or three times the distance

between Earth and the Moon.

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It’s not often that you get to see a comet strike aplanet. Until July 1994, only one comet strikehad ever been observed. That was in 1178, whenfive English monks reported seeing “a flamingtorch” on the Moon “spewing out fire, hot coals,and sparks.” Modern astronomers have con-firmed that those monks had seen a comet or a

small asteroid hit the Moon, forming the craterthat is now named Giordano Bruno.

Those 12th century monks weren’t equippedwith cameras. But in 1994, astronomers all overthe world got a chance to see and photograph asimilar event when Comet Shoemaker-Levy 9collided with Jupiter.

A FIERY

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LESSON 17 AS T E R O I D S , CO M E T S, A N D ME T E O R O I D S

NECKLACE

A NASA Hubble Space Telescope image of comet Shoemaker-Levy 9, taken on May 17, 1994. When the comet was

observed, its train of 21 icy fragments stretched across 1.1 million km of space, or three times the distance

between Earth and the Moon.

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It’s not often that you get to see a comet strike aplanet. Until July 1994, only one comet strikehad ever been observed. That was in 1178, whenfive English monks reported seeing “a flamingtorch” on the Moon “spewing out fire, hot coals,and sparks.” Modern astronomers have con-firmed that those monks had seen a comet or a

small asteroid hit the Moon, forming the craterthat is now named Giordano Bruno.

Those 12th century monks weren’t equippedwith cameras. But in 1994, astronomers all overthe world got a chance to see and photograph asimilar event when Comet Shoemaker-Levy 9collided with Jupiter.

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Ringing the BellBetween July 16 and July 22, 1994, Shoemaker-Levy 9’s fragments hit Jupiter’s upper atmos-phere one by one. Virtually every large telescopeon Earth recorded the collisions. The HubbleSpace Telescope recorded the event as it orbitedEarth, and so did the Galileo spacecraft, whichwas on its way to Jupiter.

The 21 fragments hit Jupiter at speeds of morethan 60 kilometers per second. The impacts cre-ated plumes of hot gas that rose thousands ofkilometers high. They left marks on the planet’ssurface that lasted for nearly a year. The piecesplowed into the planet’s atmosphere with enor-mous energy. One astronomer described theimpacts as “ringing Jupiter like a bell.” �

The impact of a fragment of Comet Shoemaker-Levy 9

with Jupiter

Big NewsIn 1993, astronomers Eugene and CarolynShoemaker, a husband-and-wife team, andDavid Levy discovered the comet Shoemaker-Levy 9. The three astronomers were working at the Mt. Palomar Observatory in California.

New comets are discovered all the time, butthis one made headlines. Shoemaker-Levy 9 wasa comet that had been ripped to pieces. Insteadof being a single ball, the comet was made up of21 fragments, each one trailing a large cloud ofice and dust. It looked like a fiery necklace blaz-ing across the night sky. The astronomers calcu-lated that about 9 months before they spotted it, Shoemaker-Levy 9 had passed within about21,000 kilometers of Jupiter. Jupiter’s gravita-tional force (which is 2.36 times that of Earth’s)had pulled the comet apart. A NASA Hubble Space Telescope image of comet A NASANext came even bigger news: The pieces ofShoemaker-Levy 9 were on a collision coursewith Jupiter. Astronomers predicted thatJupiter’s gravitational force was about to grabthose fragments, once and for all. That set thestage for one of the most photographed eventsin astronomical history.

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Ringing the BellBetween July 16 and July 22, 1994, Shoemaker-Levy 9’s fragments hit Jupiter’s upper atmos-phere one by one. Virtually every large telescopeon Earth recorded the collisions. The HubbleSpace Telescope recorded the event as it orbitedEarth, and so did the Galileo spacecraft, whichwas on its way to Jupiter.

The 21 fragments hit Jupiter at speeds of morethan 60 kilometers per second. The impacts cre-ated plumes of hot gas that rose thousands ofkilometers high. They left marks on the planet’ssurface that lasted for nearly a year. The piecesplowed into the planet’s atmosphere with enor-mous energy. One astronomer described theimpacts as “ringing Jupiter like a bell.” �

The impact of a fragment of Comet Shoemaker-Levy 9

with Jupiter

Big NewsIn 1993, astronomers Eugene and CarolynShoemaker, a husband-and-wife team, andDavid Levy discovered the comet Shoemaker-Levy 9. The three astronomers were working at the Mt. Palomar Observatory in California.

New comets are discovered all the time, butthis one made headlines. Shoemaker-Levy 9 wasa comet that had been ripped to pieces. Insteadof being a single ball, the comet was made up of21 fragments, each one trailing a large cloud ofice and dust. It looked like a fiery necklace blaz-ing across the night sky. The astronomers calcu-lated that about 9 months before they spotted it, Shoemaker-Levy 9 had passed within about21,000 kilometers of Jupiter. Jupiter’s gravita-tional force (which is 2.36 times that of Earth’s)had pulled the comet apart. A NASA Hubble Space Telescope image of comet A NASANext came even bigger news: The pieces ofShoemaker-Levy 9 were on a collision coursewith Jupiter. Astronomers predicted thatJupiter’s gravitational force was about to grabthose fragments, once and for all. That set thestage for one of the most photographed eventsin astronomical history.

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Tribute to Eugene Shoemaker In July 1997, three years after Shoemaker-Levy 9 collided with Jupiter, EugeneShoemaker met his own end. He was killed in a car crash in Australia, where he wasstudying an ancient impact crater.

The following year, Shoemaker’s ashes were carried aboard the Lunar Prospectorspacecraft in a small capsule. In a fitting tribute, the spacecraft was deliberatelycrashed onto the Moon’s surface on July 31, 1999.

“I don’t think Gene ever dreamed his ashes would go to the Moon,” said his wife,Carolyn. “He would be thrilled.”

Eugene Shoemaker and wife, Carolyn

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Mission: Earth

polar caps—like those on Mars—that coverEarth’s poles. Swirling clouds, flashes of light-ning, and volcanic gases are evidence of anactive atmosphere.

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This image came from a single remote-sensing device flying more than 700 kilometers above the Earth on the Terra

Earth-observing satellite.

We have learned much about Earth’s neighbor-ing planets in the solar system. We’ve sent flybyspacecraft to photograph them, put orbitersaround them for longer study, deposited landerson their surfaces, and flown probes throughtheir atmospheres. But what have we learnedabout Earth as a planet using space technology?

Earth’s seven continents and vast oceans setit apart from the other planets. Liquid watersurrounds its continents, which are covered bycontrasting lush vegetation and desert land-scapes. From space we can see frozen white

The presence of life on planet Earth is one ofits unique features. Macro- and microscopicorganisms are teeming on land and in water. One hint that life exists on Earth can be detectedfrom space—the electromagnetic noise causedby radios and TV broadcasts. But the presence of life is only one aspect of our world when con-sidered as a whole.

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Tribute to Eugene Shoemaker In July 1997, three years after Shoemaker-Levy 9 collided with Jupiter, EugeneShoemaker met his own end. He was killed in a car crash in Australia, where he wasstudying an ancient impact crater.

The following year, Shoemaker’s ashes were carried aboard the Lunar Prospectorspacecraft in a small capsule. In a fitting tribute, the spacecraft was deliberatelycrashed onto the Moon’s surface on July 31, 1999.

“I don’t think Gene ever dreamed his ashes would go to the Moon,” said his wife,Carolyn. “He would be thrilled.”

Eugene Shoemaker and wife, Carolyn

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Mission: Earth

polar caps—like those on Mars—that coverEarth’s poles. Swirling clouds, flashes of light-ning, and volcanic gases are evidence of anactive atmosphere.

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This image came from a single remote-sensing device flying more than 700 kilometers above the Earth on the Terra

Earth-observing satellite.

We have learned much about Earth’s neighbor-ing planets in the solar system. We’ve sent flybyspacecraft to photograph them, put orbitersaround them for longer study, deposited landerson their surfaces, and flown probes throughtheir atmospheres. But what have we learnedabout Earth as a planet using space technology?

Earth’s seven continents and vast oceans setit apart from the other planets. Liquid watersurrounds its continents, which are covered bycontrasting lush vegetation and desert land-scapes. From space we can see frozen white

The presence of life on planet Earth is one ofits unique features. Macro- and microscopicorganisms are teeming on land and in water. One hint that life exists on Earth can be detectedfrom space—the electromagnetic noise causedby radios and TV broadcasts. But the presence of life is only one aspect of our world when con-sidered as a whole.

(continued)

Let’s look at a few examples of how the ESEmission helps scientists and engineers observeEarth as a planet.

The Cooling and Warming Power of Clouds Clouds act to regulate Earth’s climate. Cirrusclouds—high, wispy clouds—warm Earth bytrapping radiation from Earth’s surface.Stratocumulus clouds—soft, gray clouds in glob-ular patches or rolls—cool Earth’s surface byreflecting incoming solar radiation back intospace. Scientists who work with Earth-observingsatellites are studying how clouds affect Earth’sclimate. By using global cloud observations fromEOS satellites, scientists can determine to whatextent warming or cooling caused by clouds hasan impact on the global climate.

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Earth System Enterprise The key to a better understanding of Earth is toexplore how its systems of atmosphere (air),geosphere (land), hydrosphere (water), and bio-sphere (life) interact with each other. And thebest way to study all of these systems together isfrom space. Mission to Planet Earth—now calledEarth System Enterprise (ESE)—was establishedin 1991 and is the foundation of NASA’s Earth-observing program.

This program has three main components: aseries of Earth Observing System (EOS) satellites,a system to collect data, and teams of scientistsaround the world who study the data. ESE usessatellites, such as Terra, and other tools to studyEarth. Through ESE, NASA hopes to explain hownatural processes affect life on Earth—and howlife on Earth is affecting Earth’s natural processes.Data from these studies may help improve weath-er forecasts, help manage agriculture and forests,provide information to fishermen and local

planners, and, eventually, help predict how theclimate may change in the future.

(continued)

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BARBARA SUMMEY, NATIONAL AERONAUTICS AND SPACE ADMINISTRATION/GODDARD SPACE FLIGHT CENTER

Terra, an Earth-observing satellite

Let’s look at a few examples of how the ESEmission helps scientists and engineers observeEarth as a planet.

The Cooling and Warming Power of Clouds Clouds act to regulate Earth’s climate. Cirrusclouds—high, wispy clouds—warm Earth bytrapping radiation from Earth’s surface.Stratocumulus clouds—soft, gray clouds in glob-ular patches or rolls—cool Earth’s surface byreflecting incoming solar radiation back intospace. Scientists who work with Earth-observingsatellites are studying how clouds affect Earth’sclimate. By using global cloud observations fromEOS satellites, scientists can determine to whatextent warming or cooling caused by clouds hasan impact on the global climate.

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Earth System Enterprise The key to a better understanding of Earth is toexplore how its systems of atmosphere (air),geosphere (land), hydrosphere (water), and bio-sphere (life) interact with each other. And thebest way to study all of these systems together isfrom space. Mission to Planet Earth—now calledEarth System Enterprise (ESE)—was establishedin 1991 and is the foundation of NASA’s Earth-observing program.

This program has three main components: aseries of Earth Observing System (EOS) satellites,a system to collect data, and teams of scientistsaround the world who study the data. ESE usessatellites, such as Terra, and other tools to studyEarth. Through ESE, NASA hopes to explain hownatural processes affect life on Earth—and howlife on Earth is affecting Earth’s natural processes.Data from these studies may help improve weath-er forecasts, help manage agriculture and forests,provide information to fishermen and local

planners, and, eventually, help predict how theclimate may change in the future.

(continued)

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BARBARA SUMMEY, NATIONAL AERONAUTICS AND SPACE ADMINISTRATION/GODDARD SPACE FLIGHT CENTER

Terra, an Earth-observing satellite

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This photo of the San Quintín Glacier was taken nearly 6 years later in May

2000. Like many glaciers worldwide, San Quintin appears to be retreating.

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This photo of the San Quintín Glacier in southern Chile was taken by the

crew of mission STS-068 in October 1994.

Global Ice and Sea Level Changes Earth’s glacial ice containsmore than 77 percent ofEarth’s fresh water. Over thelast century, many of theworld’s mountain glaciers andice caps have been retreating(getting smaller). Melting glaciers cause sea levels torise. One of the jobs of theEOS scientists is to figure outwhether Greenland andAntarctic ice sheets are grow-ing or shrinking by studyingchanges in sea level. Scientistsexamine changes in sea levelby looking at satellite, laser,and radar data.

Greenhouse Effect Scientists use the EOS satellitesto measure levels of greenhousegases such as carbon dioxide,methane, and chlorofluorocar-bons (CFCs) in our atmosphere.Carbon dioxide is released intothe atmosphere when solidwaste, fossil fuels (oil, naturalgas, and coal), and wood andwood products are burned.CFCs are found in aerosolsprays, in blowing agents forfoams and packing materials, as solvents, and as refrigerants.CFCs do not occur naturally;therefore, their increase in the

imagery to study El Niño—an occurrence ofunusually warm surface water in the PacificOcean. EOS can help scientists investigate therole Earth’s oceans play in regulating the amountof greenhouse gases in the atmosphere.

atmosphere is entirely the result of humanactivity. The levels of these gases have beenincreasing steadily. These gases trap heat withinEarth’s atmosphere preventing it from escapinginto space.

Ocean Processes Oceans cover more than 70 percent of Earth’ssurface. These bodies of water transport heat andweather conditions around the globe. Satellitescan measure sea surface temperatures. Thesetemperatures are each assigned a color on thesatellite image. A global view of Earth can showthe locations of the warmest and coolest oceantemperatures. Scientists can also use satellite

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Sea surface temperatures: Cold waters are black and dark green. Blue, purple, red, yellow, and white represent

progressively warmer water.

The EOS satellite called Terra collectsdetailed measurements of the ocean’s surfacetemperatures every day all over the globe. Thissensor acts like a sophisticated thermometer in space. It helps scientists understand howEarth’s oceans and atmosphere interact anddrive weather patterns. These patterns defineour climate.

(continued)

284 STC/MS™ EA R T H I N SPA C E

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This photo of the San Quintín Glacier was taken nearly 6 years later in May

2000. Like many glaciers worldwide, San Quintin appears to be retreating.

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This photo of the San Quintín Glacier in southern Chile was taken by the

crew of mission STS-068 in October 1994.

Global Ice and Sea Level Changes Earth’s glacial ice containsmore than 77 percent ofEarth’s fresh water. Over thelast century, many of theworld’s mountain glaciers andice caps have been retreating(getting smaller). Melting glaciers cause sea levels torise. One of the jobs of theEOS scientists is to figure outwhether Greenland andAntarctic ice sheets are grow-ing or shrinking by studyingchanges in sea level. Scientistsexamine changes in sea levelby looking at satellite, laser,and radar data.

Greenhouse Effect Scientists use the EOS satellitesto measure levels of greenhousegases such as carbon dioxide,methane, and chlorofluorocar-bons (CFCs) in our atmosphere.Carbon dioxide is released intothe atmosphere when solidwaste, fossil fuels (oil, naturalgas, and coal), and wood andwood products are burned.CFCs are found in aerosolsprays, in blowing agents forfoams and packing materials, as solvents, and as refrigerants.CFCs do not occur naturally;therefore, their increase in the

imagery to study El Niño—an occurrence ofunusually warm surface water in the PacificOcean. EOS can help scientists investigate therole Earth’s oceans play in regulating the amountof greenhouse gases in the atmosphere.

atmosphere is entirely the result of humanactivity. The levels of these gases have beenincreasing steadily. These gases trap heat withinEarth’s atmosphere preventing it from escapinginto space.

Ocean Processes Oceans cover more than 70 percent of Earth’ssurface. These bodies of water transport heat andweather conditions around the globe. Satellitescan measure sea surface temperatures. Thesetemperatures are each assigned a color on thesatellite image. A global view of Earth can showthe locations of the warmest and coolest oceantemperatures. Scientists can also use satellite

STC/MS™ EA R T H I N SPA C E 285

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Sea surface temperatures: Cold waters are black and dark green. Blue, purple, red, yellow, and white represent

progressively warmer water.

The EOS satellite called Terra collectsdetailed measurements of the ocean’s surfacetemperatures every day all over the globe. Thissensor acts like a sophisticated thermometer in space. It helps scientists understand howEarth’s oceans and atmosphere interact anddrive weather patterns. These patterns defineour climate.

(continued)

286 STC/MS™ EA R T H I N SPA C E

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These images clearly show that since 1979, the protective ozone layer had declined in concentration and area. In fact, the

ozone hole had grown so much over the years that in 1999, it was about the size of the entire Antarctic continent.

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Satellites can monitor snow cover as well. Adecrease in the amount of snow cover may indicate increased global temperatures.

The Future of Earth as a Planet In its natural state, Earth is in perfect balance.Water exists in all three states (liquid, gas, andsolid). Its atmosphere is oxygen rich, and life isabundant. To maintain this balance, humansmust have an understanding of how Earth func-tions as a system. The technology of the EarthSystem Enterprise mission promotes the studyof Earth as an integrated system. �

Ozone, Vegetation, and SnowBoth satellite and ground-based measurementtools have detected a hole in the ozone over the Antarctic. The ozone is a layer of O3 in thestratosphere, the second layer of the atmos-phere above Earth’s surface. Decreased ozonelevels allow more ultraviolet radiation from theSun to reach Earth’s surface. Ultraviolet radia-tion can harm organisms, including humans. InNew Zealand, for example, school children arerequired to wear hats while outside, since expo-sure to the Sun’s rays due to ozone depletion isparticularly dangerous in that region. EOS ana-lyzes the natural and human activities on Earththat cause the decrease in the ozone.

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Madagascar was once covered in lush green vegetation. Today, an estimated 80

percent of its forests have been destroyed. The reddish-brown exposed terrain

can be seen in this true-color image of northern Madagascar taken in May 2000.

monitors the rate of deforestation (the processof taking down trees) in the Brazilian rainforest.Satellite images of Africa can show characteristicsof vegetation. Instruments can measure how muchsunlight the leaves absorb.

Scientists at NASA also can evaluate processesthat directly affect Earth’s energy and water cycles.For example, satellite imagery continuously

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ozone hole had grown so much over the years that in 1999, it was about the size of the entire Antarctic continent.

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Satellites can monitor snow cover as well. Adecrease in the amount of snow cover may indicate increased global temperatures.

The Future of Earth as a Planet In its natural state, Earth is in perfect balance.Water exists in all three states (liquid, gas, andsolid). Its atmosphere is oxygen rich, and life isabundant. To maintain this balance, humansmust have an understanding of how Earth func-tions as a system. The technology of the EarthSystem Enterprise mission promotes the studyof Earth as an integrated system. �

Ozone, Vegetation, and SnowBoth satellite and ground-based measurementtools have detected a hole in the ozone over the Antarctic. The ozone is a layer of O3 in thestratosphere, the second layer of the atmos-phere above Earth’s surface. Decreased ozonelevels allow more ultraviolet radiation from theSun to reach Earth’s surface. Ultraviolet radia-tion can harm organisms, including humans. InNew Zealand, for example, school children arerequired to wear hats while outside, since expo-sure to the Sun’s rays due to ozone depletion isparticularly dangerous in that region. EOS ana-lyzes the natural and human activities on Earththat cause the decrease in the ozone.

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Madagascar was once covered in lush green vegetation. Today, an estimated 80

percent of its forests have been destroyed. The reddish-brown exposed terrain

can be seen in this true-color image of northern Madagascar taken in May 2000.

monitors the rate of deforestation (the processof taking down trees) in the Brazilian rainforest.Satellite images of Africa can show characteristicsof vegetation. Instruments can measure how muchsunlight the leaves absorb.

Scientists at NASA also can evaluate processesthat directly affect Earth’s energy and water cycles.For example, satellite imagery continuously

288 STC/MS™ EA R T H I N SPA C E

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Earth: Quick Facts

Diameter 12,756 km

Average distance from the Sun 149,600,000 km

Mass 597 × 1022 kg

Surface gravity 1*

Average temperature –55 °C to 70 °C

Length of sidereal day 23.93 hours

Length of year 365.25 days

Number of observed moons 1

Did You Know?• The oldest rocks on Earth date back 4 billion years. • Only Earth has the temperature range that permits liquid water

to exist, and only Earth has developed an oxygen-rich atmosphere.These two factors enable Earth to support life.

Inner core

Relative size

Earth atmosphere

Nitrogen(78%)

Argon, carbon dioxide,and water vapor (trace amounts) (1%)

Oxygen(21%)

Earth

Outer core

Mantle

Crust

* 9.78 m/s2

PLANETARY FACTS: Earth

Moon

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Q: How often are cometsdiscovered? Are they namedthe same way as asteroids?A: Many new comets are dis-covered every month. They are generally named for theirdiscoverers. Comet Halley was named after astronomerEdmund Halley, who was thefirst to predict the return of thisparticular comet. But like aster-oids, comets are given codesthat reflect their discovery date.

Q: What about stars?A: A few bright stars easily seen from Earthhave ancient, traditional Arabic names, such asSirius. We have identified hundreds of millionsof stars. To study them, we need to be able tofind them, so they are simply known by catalognumbers. By looking up its number in a hugecatalog, we can find a star’s precise coordinates,or position in the sky.

Q: Is it true that people can pay to have a star or a planet named after them? A: Some companies claim to offer such servicesfor a fee. However, those names are completelyinvalid. As an international scientific organization,the IAU has nothing to do with the commercialpractice of “selling” fictitious names of stars,planets, moons, or any other space “real estate.”

If you’re interested in stars and space, go toyour nearest planetarium or local observatory.Have someone show you real stars through atelescope. You also may want to join a localastronomy club. Someday you may discover anew asteroid or comet that could be namedafter you! �

Dr. Michael F. A’Hearn knows his way aroundspace. Dr. A’Hearn, a professor of astronomy at the University of Maryland, is also an office-holder in the International AstronomicalUnion (IAU), the organization responsible fornaming celestial bodies. Scientists, space agencies, and authorities around the worldrecognize and use IAU’s names. Here, Dr. A’Hearn answers some common questionsabout the space “name game,” or how astro-nomical bodies get their names.

Q: How are asteroids named?A: First, an asteroid is given a set of numbersand letters that tells when it was first discovered.Once the asteroid’s orbit is well known, a perma-nent number is assigned—in numerical order.After that, a name is assigned. The discoverer ofthe asteroid can suggest a name, but the IAU has final approval.

For example, an asteroid discovered by P. Wildon March 5, 1973, was given the designation“1973 EB.” This means that the asteroid wasidentified in 1973 in the first half of March (E)and was the second (B) asteroid discovered inthe first half of that month. Once we under-stood the orbit of 1973 EB, we gave it the per-manent number of 2001 because that’s howmany asteroids had been discovered by then.This asteroid was named Einstein in memory of Albert Einstein, the greatest scientist of the20th century.

Q: How often are asteroids discovered? A: New asteroids are discovered nearly every day!However, people tend not to search during the fullMoon because the background light interferes toomuch. Most discoveries are made around the newMoon, when our cameras can “see.”

Dr. Michael F. A’Hearn

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The Space Name Game