LESSON Asteroids, Comets, and Meteoroids

17LESSON
Asteroids, Comets,
and Meteoroids

© WILLIAM JAMES WARREN/CORBIS INTRODUCTION
On the night of July 23, 1995, astronomers Alan
Hale and Tom Bopp discovered Comet Hale-Bopp
from different locations. Hale and Bopp were the
first people in more than 3000 years to view what
is now officially known as Comet Hale-Bopp
(Comet H-B for short). This may have been the
biggest comet ever visible from Earth.

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

Comet Hale-Bopp. The white dust tail is composed of OBJECTIVES FOR THIS LESSON
large particles of dust and ice. The gas tail is blue.
Analyze the position of the asteroid belt
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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.

Getting Started MATERIALS FOR
LESSON 17
1. With your class, review your homework
from Lesson 16, Student Sheet 16: For you
Bode’s Law. 1 completed copy of
Student Sheet 16:
2. Read “Asteroids, Comets, and Meteoroids.” Bode’s Law
Carefully examine the photos in the read- 1 working copy of
ing selection. How are asteroids similar to Student Sheet
or different from comets and meteoroids? 10.1c: Planetary
Record a summary of your ideas in your Char t
science notebooks. Then discuss them as
a class or within your group, as instructed
by your teacher.

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Inquiry 17.1 Inquiry 17.2
Examining Asteroids Studying Asteroid Impact

PROCEDURE PROCEDURE

1. In the program Explore the Planets, 1. Read “A Fiery Necklace.” How did
review the Asteroids segment in the “Tour Dr. Eugene Shoemaker contribute to the
the Planets” section. Discuss the concepts understanding of asteroid and comet
with your class. impacts? Record your ideas in your
notebook.
2. Make general observations about the
asteroids shown on the program. Look REFLECTING ON WHAT YOU’VE DONE
back at Lesson 12. How do you think
Barringer (Meteor) Crater and the craters 1. In your notebook, record your ideas to
on the surface of asteroids Gaspra and Ida the following, and be prepared to discuss
were formed? Discuss or record your them with your class:
ideas, as instructed by your teacher.

REFLECTING ON WHAT YOU’VE DONE A. How has Earth’s history been influ-
enced by occasional natural catastrophes,
Answer the following questions in your science such as asteroid impacts?
notebook, and be prepared to discuss your
ideas with the class: B. An asteroid impact is considered a nat-
ural hazard on Earth, but it is not consid-
A. How and when do scientists think ered a natural hazard on any other planet
asteroids may have formed? or moon. Given this information, how
would you define “natural hazard?”

B. Why do you think the belt of asteroids C. What is the scientist’s role in forecast-
exists between Jupiter and Mars? ing asteroid and comet impacts?

C. How are the orbits of asteroids similar D. What challenges do scientists face when
to, or different from, planetary orbits? they forecast asteroid or comet impacts?

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

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

3. With your class, return to the Question J
folder for Lesson 1. Is there anything that
you would now change or add? Discuss
your ideas with the class.

4. Read “Mission: Earth.” Add any new
information about Earth to your working
copy of Student Sheet 10.1c: Planetary
Chart. Complete your planetary brochure.
You will present your planetary brochure
and your team’s mission design to the
class in Lesson 19.

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AastnedroMidest,eCoormoiedtss,

Among the planets in the solar system are the asteroid belt—a vast, doughnut-shaped
countless numbers of asteroids, comets, and ring located between the orbits of Mars and
meteoroids. Let’s take a look at how these solar Jupiter (see the illustration below). Gaspra
system objects are different from planets. and Ida, pictured on the next page, are two
asteroids found in the main belt. Some scien-
Asteroids tists theorize that asteroids may be pieces of
Asteroids are metallic, rocky objects in space. a planet that never formed because Jupiter’s
They have no atmospheres and move in inde- great mass exerted too much gravitational
pendent orbits around the Sun. Tens of thou- force to allow the pieces to combine into
sands of asteroids are found in an area called one planet.

Mars Jupiter

The asteroid belt is located between Jupiter and Mars.
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LESSON 17 A S T E R O I D S , C O M E T S , A N D M E T E O R O I D SNATIONAL AERONAUTICS AND SPACE ADMINISTRATION/JET PROPULSION LABORATORY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION/UNITED STATES GEOLOGICAL SURVEY
Because asteroids are too small to be classified
as 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 kilometers
across and one of the largest known asteroids.
Another asteroid, discovered in 2001, is about
half the size of Pluto (or 1150 kilometers). Most
asteroids are actually less than a kilometer wide.
If we could combine all of these asteroids, they
would be smaller than half the size of the Moon.

(continued)

Image of asteroid 951, Gaspra, taken by the
Galileo spacecraft

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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Comets Coma Nucleus Tail
A comet is a mass of frozen gas, cosmic dust,
and ice crystals. Comets are often described The parts of a comet
as “dirty icebergs.” They circle the Sun in
long, narrow orbits, mainly located in the dust, burning particles can be seen streaking
cold outer reaches of our solar system. They through the sky in a spectacular display called
orbit the Sun in the Kuiper Belt, which a “meteor shower.”
begins just past Neptune. A trillion more
comets may live even farther out in a cold
area called the “Oort Cloud.”

Some comets leave their orbits in the
Kuiper Belt or the Oort Cloud and journey
toward the Sun. When a comet flies near
the Sun, its ice begins to “boil” away. As it
vaporizes, a tail of glowing gases and dust
forms behind it, always pointing away from the
Sun. If Earth happens to pass through comet

© DENNIS DI CICCO/CORBIS

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.

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION/JOHNSON SPACE CENTER Meteoroids, Meteors,
and Meteorites
Meteoroids are pieces of rock and
metal dislodged from comets, planets,
asteroids, or moons. Most meteoroids
are made up of dust-sized particles.
When a meteoroid enters a planet’s
atmosphere, it burns up due to fric-
tion. As it burns, a meteoroid creates a
bright streak of light in the sky that we
call 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, after
which it is called a “meteorite.”

A meteorite stone discovered on Earth weighing 452.6 grams.
A 1-cm square cube is shown for scale.

COURTESY OF OFER GABZO

Meteors streak through the night sky.

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A FIERYNECKLACE

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 a small asteroid hit the Moon, forming the crater
planet. Until July 1994, only one comet strike that is now named Giordano Bruno.
had ever been observed. That was in 1178, when
five English monks reported seeing “a flaming Those 12th century monks weren’t equipped
torch” on the Moon “spewing out fire, hot coals, with cameras. But in 1994, astronomers all over
and sparks.” Modern astronomers have con- the world got a chance to see and photograph a
firmed that those monks had seen a comet or a similar event when Comet Shoemaker-Levy 9
collided with Jupiter.

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

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Big News
In 1993, astronomers Eugene and Carolyn
Shoemaker, a husband-and-wife team, and
David 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, but
this one made headlines. Shoemaker-Levy 9 was
a comet that had been ripped to pieces. Instead
of being a single ball, the comet was made up of
21 fragments, each one trailing a large cloud of
ice 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 about
21,000 kilometers of Jupiter. Jupiter’s gravita-
tional force (which is 2.36 times that of Earth’s)
had pulled the comet apart.
A NNAeSxAt Hcuabmblee eSvpeancebTieglgeescronpewims:agTehoef pcoiemceetsAoNfASA
Shoemaker-Levy 9 were on a collision course
with Jupiter. Astronomers predicted that
Jupiter’s gravitational force was about to grab
those fragments, once and for all. That set the
stage for one of the most photographed events
in astronomical history.

Ringing the Bell
Between July 16 and July 22, 1994, Shoemaker-
Levy 9’s fragments hit Jupiter’s upper atmos-
phere one by one. Virtually every large telescope
on Earth recorded the collisions. The Hubble
Space Telescope recorded the event as it orbited
Earth, and so did the Galileo spacecraft, which
was on its way to Jupiter.

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

The impact of a fragment of Comet Shoemaker-Levy 9
with Jupiter

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R. EVANS, J. TRAUGER, H. HAMMEL, AND THE HST COMET SCIENCE TEAM AND THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

LESSON 17 A S T E R O I D S , C O M E T S , A N D M E T E O R O I D S UNITED STATES GEOLOGICAL SURVEY

Tribute to Eugene Shoemaker

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

The following year, Shoemaker’s ashes were carried aboard the Lunar Prospector
spacecraft in a small capsule. In a fitting tribute, the spacecraft was deliberately
crashed 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

We have learned much about Earth’s neighbor- polar caps—like those on Mars—that cover
ing planets in the solar system. We’ve sent flyby Earth’s poles. Swirling clouds, flashes of light-
spacecraft to photograph them, put orbiters ning, and volcanic gases are evidence of an
around them for longer study, deposited landers active atmosphere.
on their surfaces, and flown probes through
their atmospheres. But what have we learned The presence of life on planet Earth is one of
about Earth as a planet using space technology? its unique features. Macro- and microscopic
organisms are teeming on land and in water.
Earth’s seven continents and vast oceans set One hint that life exists on Earth can be detected
it apart from the other planets. Liquid water from space—the electromagnetic noise caused
surrounds its continents, which are covered by by radios and TV broadcasts. But the presence
contrasting lush vegetation and desert land- of life is only one aspect of our world when con-
scapes. From space we can see frozen white sidered as a whole.

(continued)

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION/GODDARD SPACE FLIGHT CENTER/
IMAGE CREATED BY RETO STOCKLI, NAZMI EL SALEOUS, AND MARIT JENTOFT-NILSEN

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

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Earth System Enterprise planners, and, eventually, help predict how the
The key to a better understanding of Earth is to climate may change in the future.
explore how its systems of atmosphere (air),
geosphere (land), hydrosphere (water), and bio- Let’s look at a few examples of how the ESE
sphere (life) interact with each other. And the mission helps scientists and engineers observe
best way to study all of these systems together is Earth as a planet.
from space. Mission to Planet Earth—now called
Earth System Enterprise (ESE)—was established The Cooling and Warming Power of Clouds
in 1991 and is the foundation of NASA’s Earth- Clouds act to regulate Earth’s climate. Cirrus
observing program. clouds—high, wispy clouds—warm Earth by
trapping radiation from Earth’s surface.
This program has three main components: a Stratocumulus clouds—soft, gray clouds in glob-
series of Earth Observing System (EOS) satellites, ular patches or rolls—cool Earth’s surface by
a system to collect data, and teams of scientists reflecting incoming solar radiation back into
around the world who study the data. ESE uses space. Scientists who work with Earth-observing
satellites, such as Terra, and other tools to study satellites are studying how clouds affect Earth’s
Earth. Through ESE, NASA hopes to explain how climate. By using global cloud observations from
natural processes affect life on Earth—and how EOS satellites, scientists can determine to what
life on Earth is affecting Earth’s natural processes. extent warming or cooling caused by clouds has
Data from these studies may help improve weath- an impact on the global climate.
er forecasts, help manage agriculture and forests,
provide information to fishermen and local (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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Global Ice and Sea NATIONAL AERONAUTICS AND SPACE ADMINISTRATION/JOHNSON SPACE CENTER
Level Changes
Earth’s glacial ice contains This photo of the San Quintín Glacier in southern Chile was taken by the
more than 77 percent of crew of mission STS-068 in October 1994.
Earth’s fresh water. Over the
last century, many of the
world’s mountain glaciers and
ice caps have been retreating
(getting smaller). Melting
glaciers cause sea levels to
rise. One of the jobs of the
EOS scientists is to figure out
whether Greenland and
Antarctic ice sheets are grow-
ing or shrinking by studying
changes in sea level. Scientists
examine changes in sea level
by looking at satellite, laser,
and radar data.

Greenhouse Effect NATIONAL AERONAUTICS AND SPACE ADMINISTRATION/GODDARD SPACE FLIGHT
Scientists use the EOS satellites CENTER/MITI/ERSDAC/DARDS AND U.S./JAPAN ASTER SCIENCE TEAM.
to measure levels of greenhouse
gases such as carbon dioxide, This photo of the San Quintín Glacier was taken nearly 6 years later in May
methane, and chlorofluorocar- 2000. Like many glaciers worldwide, San Quintin appears to be retreating.
bons (CFCs) in our atmosphere.
Carbon dioxide is released into
the atmosphere when solid
waste, fossil fuels (oil, natural
gas, and coal), and wood and
wood products are burned.
CFCs are found in aerosol
sprays, in blowing agents for
foams and packing materials,
as solvents, and as refrigerants.
CFCs do not occur naturally;
therefore, their increase in the

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atmosphere is entirely the result of human imagery to study El Niño—an occurrence of
activity. The levels of these gases have been unusually warm surface water in the Pacific
increasing steadily. These gases trap heat within Ocean. EOS can help scientists investigate the
Earth’s atmosphere preventing it from escaping role Earth’s oceans play in regulating the amount
into space. of greenhouse gases in the atmosphere.

Ocean Processes The EOS satellite called Terra collects
Oceans cover more than 70 percent of Earth’s detailed measurements of the ocean’s surface
surface. These bodies of water transport heat and temperatures every day all over the globe. This
weather conditions around the globe. Satellites sensor acts like a sophisticated thermometer
can measure sea surface temperatures. These in space. It helps scientists understand how
temperatures are each assigned a color on the Earth’s oceans and atmosphere interact and
satellite image. A global view of Earth can show drive weather patterns. These patterns define
the locations of the warmest and coolest ocean our climate.
temperatures. Scientists can also use satellite
(continued)

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION/ GODDARD SPACE FLIGHT
CENTER. IMAGE BY JESSE ALLEN, SCIENCE SYSTEMS AND APPLICATIONS, INC.

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

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION/ GODDARD SPACE FLIGHT CENTER SCIENTIFIC VISUALIZATION STUDIO, IMAGES BY GREG SHIRAH LESSON 17 A S T E R O I D S , C O M E T S , A N D M E T E O R O I D S

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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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION IMAGE BY BRIAN MONTGOMERY,
ROBERT SIMMON, AND RETO STÖCKLI, BASED ON DATA PROVIDED BY THE MODIS SCIENCE TEAM.

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.

Ozone, Vegetation, and Snow monitors the rate of deforestation (the process
Both satellite and ground-based measurement of taking down trees) in the Brazilian rainforest.
tools have detected a hole in the ozone over Satellite images of Africa can show characteristics
the Antarctic. The ozone is a layer of O3 in the of vegetation. Instruments can measure how much
stratosphere, the second layer of the atmos- sunlight the leaves absorb.
phere above Earth’s surface. Decreased ozone
levels allow more ultraviolet radiation from the Satellites can monitor snow cover as well. A
Sun to reach Earth’s surface. Ultraviolet radia- decrease in the amount of snow cover may
tion can harm organisms, including humans. In indicate increased global temperatures.
New Zealand, for example, school children are
required to wear hats while outside, since expo- The Future of Earth as a Planet
sure to the Sun’s rays due to ozone depletion is In its natural state, Earth is in perfect balance.
particularly dangerous in that region. EOS ana- Water exists in all three states (liquid, gas, and
lyzes the natural and human activities on Earth solid). Its atmosphere is oxygen rich, and life is
that cause the decrease in the ozone. abundant. To maintain this balance, humans
must have an understanding of how Earth func-
Scientists at NASA also can evaluate processes tions as a system. The technology of the Earth
that directly affect Earth’s energy and water cycles. System Enterprise mission promotes the study
For example, satellite imagery continuously of Earth as an integrated system.

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EarthPLANETARY FACTS:
Earth: Quick Facts

Diameter 12,756 km Average temperature –55 °C to 70 °C
Length of sidereal day 23.93 hours
Average distance from the Sun 149,600,000 km 365.25 days
Length of year 1
Mass 597 × 1022 kg Number of observed moons
Surface gravity 1*

Relative size Crust
Mantle
Outer core
Inner core

Moon

Earth atmosphere

Nitrogen Oxygen Did You Know?
(78%) (21%)
• The oldest rocks on Earth date back 4 billion years.
Argon, carbon dioxide, • Only Earth has the temperature range that permits liquid water
and water vapor
(trace amounts) (1%) to exist, and only Earth has developed an oxygen-rich atmosphere.
These two factors enable Earth to support life.

Earth

* 9.78 m/s2
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