- Explain where comets come from and what causes their tails.
- Differentiate between meteors, meteoroids, and meteorites.
Introduction
When our solar system formed, most of the matter ended up in the Sun, the star at the
center of the system. Material spinning in a disk around the Sun clumped together into
larger and larger pieces, forming the eight planets and their moons. But some of the smaller
pieces of matter in the solar system never joined one of these larger bodies. In this lesson,
we will talk about some of these other objects in the solar system.
Asteroids
Asteroidsare very small, rocky bodies that orbit the Sun. “Asteroid” means “star-like,”
and in a telescope, asteroids look like points of light, just like stars. Asteroids are also
sometimes called planetoids or minor planets, because in some ways they are similar to
miniature planets. Unlike planets, though, asteroids are irregularly shaped because they
do not have enough gravity to become round like planets. They do not have atmospheres,
and they are not geologically active. The only geological changes to an asteroid are due
to collisions, which may break up the asteroid or create craters on the asteroid’s surface.
Figure25.31shows a typical asteroid.
The Asteroid Belt
Hundreds of thousands of asteroids have been discovered in our solar system. They are
still being discovered at a rate of about 5,000 new asteroids per month! The majority of
the asteroids are found in between the orbits of Mars and Jupiter, in a region called the
asteroid belt, as shown inFigure25.32. Although there are many thousands of asteroids
in the asteroid belt, their total mass adds up to only about 4% of Earth’s moon.
Scientists believe that the bodies in the asteroid belt formed there during the formation of
the solar system. However, they have never been able to form into a single planet because
the gravity of Jupiter, which is very massive, continually disrupts the asteroids.
Near-Earth Asteroids
Near-Earth asteroids are asteroids whose orbits cross Earth’s orbit. Any object whose orbit
crosses Earth can collide with Earth. There are over 4,500 known near-Earth asteroids;
between 500 and 1,000 of these are over 1 kilometer in diameter. Small asteroids do in fact
collide with Earth on a regular basis—asteroids 5–10 m in diameter hit Earth on average
about once per year. Evidence suggests that large asteroids hitting Earth in the past have