The Solar System

CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 559

The Asteroid Belt

Th e fi rst asteroid was discovered on January 1, 1801 (the fi rst

night of the 19th century) by the Sicilian monk Giuseppe Piazzi.

It was later named Ceres after the Roman goddess of the harvest

(and source of our word cereal).

Astronomers were excited by Piazzi’s discovery because there

seemed to be a pattern to the location of planet orbits, except for

a wide gap between Mars and Jupiter where the pattern implied

a planet “ought” to exist at an average distance from the sun of

2.8 AU. Ceres fi t the pattern: Its average distance from the sun is

2.77 AU. But Ceres is much smaller than the planets, and three

even smaller objects—Pallas, Juno, and Vesta—were discovered

within a few years, all orbiting between Mars and Jupiter, so

astronomers decided that Ceres and the other asteroids should

not be considered true planets. As you learned in Chapter 24,

Ceres has now been re-classifi ed as a dwarf planet because it has

enough gravitational strength to squeeze itself into a spherical

shape but not enough to have absorbed or cleared away the rest

of the asteroids.

Today over 100,000 asteroids have well-charted orbits. Only

three are larger than 400 km in diameter (■ Figure 25-8), and

most are much smaller. Astronomers are sure that all the large

SCIENTIFIC ARGUMENT

How can you say that meteors come from comets, but meteorites

come from asteroids?

To begin with, remember the distinction between meteors and

meteorites. A meteor is the streak of light seen in the sky when a

particle from space is heated by friction with Earth’s atmosphere.

A meteorite is a piece of space material that actually reaches the

ground.

The distinction between comet and asteroid sources must take

into account two very strong effects that prevent you from fi nd-

ing meteorites that originated in comets. First, evidence that you

will learn more about later in this chapter indicates that cometary

material is physically weak, so comet particles vaporize in Earth’s

atmosphere easily. Very few ever reach the ground. Second, even if

a comet particle reached the ground, it would be so fragile that it

would weather away rapidly, and you would be unlikely to fi nd it

before it disappeared. Asteroidal particles, however, are made from

rock and metal and are stronger. They are more likely to survive

their plunge through the atmosphere and, afterward, more likely

to survive erosion on the ground. Every known meteorite is from

the asteroids—not a single meteorite is known to be cometary. In

contrast, meteor tracks show that most meteors you see come from

comets, and very few are from the asteroid belt.

Now build a new argument. What evidence suggests that mete-

orites were once part of larger bodies broken up by impacts?

Asteroids

Asteroids are distant objects too small to study in
detail with Earth-based telescopes. Astronomers never-
theless have learned a surprising amount about those
little worlds, using spacecraft and space telescopes.

Properties of Asteroids

Evidence from meteorites shows that the asteroids are
the last remains of the rocky planetesimals that built the
Terrestrial planets 4.6 billion years ago.
Study Observations of Asteroids on pages
560–561 and notice four important points:

Most asteroids are irregular in shape and battered

by impact cratering. Many asteroids seem to be

rubble piles of broken fragments.

Some asteroids are double objects or have small

moons in orbit around them. Th is is further evi-

dence that asteroids have suff ered collisions.

A few asteroids show signs of geological activity

that probably happened on their surfaces when

those asteroids were young.

Asteroids can be classifi ed by their albedos, colors, and spec-

tra to reveal clues to their compositions. Th is also allows

them to be compared to meteorites in labs on Earth.

25-2

1

2

3

4

3 Juno

704 Interamnia

511 Davida

1 Ceres

2 Pallas

4 Vesta

10 Hygiea

Earth’s

moon

■ Figure 25-8

The relative size and approximate shape of the larger asteroids are shown

here compared with the size of Earth’s moon. Smaller asteroids can be highly

irregular in shape.