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MAZER
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Asteroids and comets orbit the Sun.
Objects called asteroids and comets formed along with the Sun, planets,
and moons. These objects still orbit the Sun at different distances.
Most of the objects are much smaller than planets and had too little
mass to become round. The objects that formed far from the Sun
are made mostly of ice, with some rock and metal. The objects that
formed closer to the Sun, where it was warmer, have little or no ice.
MAIN IDEA AND DETAILS
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to help you study later.
Asteroids
Small, solid, rocky bodies that orbit close to the Sun are called asteroids.
They range from almost 1000 kilometers (600 mi) in diameter down
to a kilometer or less. Except for the largest, their gravity
is too weak to pull them into round spheres. Therefore,
most asteroids have irregular shapes. Some asteroids are
the broken pieces of larger, rounded asteroids.
Most asteroids have paths that keep them between
the orbits of Mars and Jupiter. This huge region is called
the asteroid belt, and contains more than 10,000 asteroids.
However, the asteroids are so far apart that spacecraft
from Earth have passed completely through the belt
without danger of collision. The mass of all the asteroids
put together is estimated to be less than the mass of
our Moon.
large crater
The surfaces of asteroids are covered with craters, broken rock,
and dust. Even though asteroids are far apart, smaller objects do hit
them from time to time. Impacts from very long ago are still visible
because most asteroids are not massive enough to have formed cores,
mantles, and crusts. Therefore, they do not have volcanism or tectonics
to erase the craters. Most asteroids do not have atmospheres, so their
surfaces change only when impacts happen or when gravity pulls
material downhill.
check your reading
This asteroid is small compared with a planet, but
it is large compared with
a person. The large crater
at the bottom is about
the size of a small city.
Why do asteroids have craters?
Some asteroids have collided with Earth in the past. The collisions
left impact craters, some of which can still be seen today. Scientists have
found evidence that an asteroid 10 kilometers (6 mi) in diameter hit
Earth 65 million years ago. A cloud of dust from the collision spread
around the world and probably affected surface temperatures. Many
forms of life, including dinosaurs, died off at about that time, and the
impact may have been part or all of the reason. Today astronomers are
working to study all asteroids larger than 1 kilometer (0.6 mi) in
diameter to determine whether any could hit Earth.
Chapter 13: Our Solar System 437
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