3 6 comets asteroids (physical sciences)

Most travel along in the asteroid belt, an area of space between the orbits of Mars and Jupiter.. Asteroids that orbit closest to Jupiter are called outer-belt asteroids.. Ones that orbi

Genre Comprehension Skill Text Features Science Content

Nonfi ction Predict • Captions

• Labels

• Diagram

• Glossary

Objects in Space

Scott Foresman Science 3.6

ISBN 0-328-23535-0

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by Anne Cambal

Physical Sciences

Picture Credits

Illustrations

2 Paul Oglesby

Photographs

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ISBN: 0-328-23535-0

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Vocabulary

asteroid

binoculars

orbit

planet

solar system

star

telescope

Extended Vocabulary

asteroid belt coma

meteor meteorite meteoroid near-Earth asteroid nucleus

probe

by Anne Cambal

What You Already Know

A star is a giant ball of hot, glowing gases The Sun,

which is at the center of our solar system, is a star Our

solar system consists of the Sun, nine known planets and

their moons, and other objects

A planet is a large body of matter that orbits the Sun

An orbit is the path an object takes as it moves around a

planet or star

On most nights, the Moon can be seen from Earth

Sometimes it can be seen from Earth during daytime We

can see the Moon because sunlight refl ects off of it

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Mercury, Venus, Mars, Jupiter, and Saturn can be seen with only your eyes You need a telescope or binoculars

to see Uranus and Neptune

A telescope magnifi es distant objects Binoculars are double telescopes you hold in your hand They also magnify the things we see in the night sky

Comets and asteroids are much smaller than the Sun and its planets This makes them harder to observe But comets and asteroids play a major role in our solar system Keep reading to fi nd out more!

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All about Asteroids

Asteroids consist of carbon and metals such as nickel

and iron They also contain silicates, which are rocks

made mostly of the element silicon

The larger an asteroid is, the easier it is to spot One

of the largest known asteroids, Ceres, was also the fi rst

discovered Giuseppe Piazzi, working at the Palermo

Observatory in Palermo, Italy, observed Ceres in 1801

Ceres has a diameter of around 930 kilometers It is so

huge that it makes up somewhere between one-quarter

and one-third of the mass of all asteroids! Studies show

that Ceres might have frozen water

Asteroids come in many sizes, from the size of Ceres

to the size of a small garage An asteroid called 1991 BA

was spotted in 1991 It’s only about 20 feet across

Astronomer Giuseppe

Piazzi spotted Ceres over

200 years ago.

5

Asteroids orbit the Sun Most travel along in the asteroid belt, an area of space between the orbits of Mars and Jupiter These asteroids orbit the Sun at a distance of between 298 million and 596 million kilometers

Asteroids that orbit closest to Jupiter are called outer-belt asteroids Ones that orbit between Jupiter and Mars are called main-belt asteroids Main-belt asteroids are divided into various families Asteroids that orbit at nearly the same distance as Mars are called inner-belt asteroids

Finally, there are the asteroids that orbit inside of the asteroid belt, in the space between the orbits of Earth and Mars These asteroids, which are called near-Earth asteroids, will occasionally hit Earth Asteroid 1991 BA is

a near-Earth asteroid

Most asteroids orbit between Mars (the red planet close to the Sun) and Jupiter (in the bottom right-hand corner)

Hits and Misses

Asteroid 1991 BA may sound small But as a

near-Earth asteroid, it packs a big punch On January 18, 1991,

moving at about 21 kilometers a second, it came within

171,000 kilometers of Earth, or half the distance to the

Moon If it had hit Earth, 1991 BA would have created a

hole about two football fi elds long, with a depth equal to

the height of an eight-story building The hit would have

given off as much energy as a large atomic bomb

Scientists believe that 65 million years ago a large

asteroid struck Earth, forming Chicxulub Basin, a giant

crater located in southern Mexico Chicxulub Basin is

over 300 kilometers wide Some scientists think that the

asteroid that created it made the dinosaurs go extinct

Arizona’s Meteor Crater is 170 meters deep Scientists

think it was created 50,000 years ago by an asteroid.

7

It is easy to confuse meteors, meteorites, and asteroids

Here is how to remember which is which An asteroid

is an object that travels through space, beyond Earth’s atmosphere If an asteroid or a fragment of one enters Earth’s atmosphere, it becomes a meteoroid Meteoroids slow down rapidly as they enter the atmosphere

They generate friction that creates light and heat A meteoroid’s light produces a meteor, or shooting star

Almost all meteoroids burn up as a result of the intense heat and friction created by their entry into Earth’s atmosphere However, a tiny number hit Earth’s surface These are called meteorites Objects such as the one that created the Chicxulub Basin start as asteroids or comets, and end up as meteorites

An asteroid such as this one may have caused the dinosaurs to go extinct.

Comets

Comets are made up of gas, ice, dust, and rock Most

comets revolve around the Sun in an oval orbit

A comet’s main part, the nucleus, consists of rock,

dust, ice, methane, carbon dioxide, and water It is

sometimes called a “dirty snowball.” The coma, which

surrounds the nucleus, is made up of gas escaping from

the nucleus It can grow to be thousands of kilometers

wide Finally, there is the comet’s tail, consisting of dust,

ions, and electrons The tail can stream out for millions of

kilometers It is produced by the Sun’s heat and energy

Most of the time comets are too distant from the Sun

to develop tails and comas But as they draw closer, their

tails and comas grow enormously

This photo of Halley’s comet, taken in 1986,

clearly shows its nucleus, coma, and tail.

nucleus

coma

9

Halley’s comet is named in honor of the 17th century British astronomer Edmond Halley It is a large comet that has been spotted from Earth many times

In the years 1531, 1607, and 1682, there were comet sightings all over Earth Each time, people thought they were seeing a different comet Then, in 1705, Edmond Halley made some calculations Borrowing from studies done by his friend and fellow scientist Isaac Newton, he predicted that a comet would appear in 1758 It did!

Halley’s work proved that the comet sightings of

1531, 1607, and 1682 were different sightings of the same comet Last seen in 1986, Halley’s comet should arrive again around the year 2061, as it does every 76 years

tail

In 1705, Edmond Halley (right) correctly predicted that a comet would appear in 1758.

Space Missions

In 1985, fi ve spacecraft were sent to study Halley’s

comet The space probe Giotto took photos of the comet.

The Deep Impact space mission was launched in

2000 It was designed to send a probe into the comet

Tempel I and send the fi ndings back to Earth The

spacecraft released the probe on July 2, 2005, when it was

still 880,670 kilometers away from the comet The next

day, the probe collided with Tempel I Data from the

probe will help us learn more about comets

The Hayabusa probe was launched by Japan in 2003

Hayabusa was designed to be the fi rst probe to bring

pieces of an asteroid back to Earth It was set to land on

the asteroid Itokawa, which orbits at distances of between

143 million and 251 million kilometers from the Sun

In 1985, the rocket Ariane (right) carried

Giotto (below) into space to take photos

of Halley’s comet

11

Using computers, space probes, telescopes, and other modern technology, today’s astronomers have been spotting near-Earth asteroids at a rate of 10 to 20

a year The SOHO spacecraft, controlled by NASA and the European Space Agency, identifi ed 100 new comets between December 1995 and February 2000

Modern technology allows astronomers to quickly detect objects orbiting in our solar system It also allows astronomers to make highly detailed observations of comets’ and asteroids’ characteristics Now is an exciting time to be studying comets and asteroids!

The Deep Impact space mission was sent

to study the comet Tempel I (above and

to the right) in 2000.

asteroid belt the area of the solar system where most

asteroids travel

coma a cloud of dust and gas around the

center of a comet

meteor the streak of light created by a

meteoroid as it enters Earth’s atmosphere; a shooting star

meteorite a meteoroid that lands on Earth without

fully burning up in Earth’s atmosphere

meteoroid an asteroid or piece of an asteroid that

enters Earth’s atmosphere

near-Earth an asteroid that can hit Earth

asteroid

nucleus the core of a comet, made up of solid

(or frozen) methane, carbon dioxide, and water

probe a spacecraft carrying scientifi c devices

to record and report information

1 Which types of asteroids get split into families?

2 What do some scientists think produced Mexico’s

Chicxulub Basin?

3 How long does it take for Halley’s comet to return to

Earth?

Ceres and its discovery Be sure to include a topic sentence and supporting details.

about comet Tempel I from the Deep Impact mission?