Asteroids are rocky and metallic objects too small
to be considered planets. They are sometimes called
minor planets. They range in size from Ceres, with a
diameter of about 1000 km,
down to a few centimeters or less. The ajacent image shows the asteroid
Gaspra, as photographed by the Galileo space probe
(Ref).
The name asteroids, meaning "star-like",
derives from the fact that, compared with comets, they are
star-like in appearance because since they are rocky they do not emit the gases
and dust that give comets their fuzzy appearance.
Asteroids on a collision course with Earth are called meteoroids. If this meteoroid burns up because of frictional heating when it strikes our atmosphere, we term it a meteor (colloquially, a "shooting star"). If the meteoroid doesn't burn up completely and strikes the Earth we call it a meteorite. We shall discuss, meteoroids, meteors, and meteorites in the next section.