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.