Asteroids and The Asteroid Belt

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 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.

The Asteroid Belt 

The highest concentration of asteroids is in a region lying between the orbits of Mars and Jupiter called the asteroid belt that is illustrated schematically in the adjacent figure. Here is a more realistic distribution of semimajor axes for the minor planets. Some 7000 asteroids have been identified so far. Most, but not all, have average orbital radii lying in the region of the asteroid belt. It is likely that the origin of the asteroid belt lies in the gravitational perturbation of Jupiter, which kept these planetisimals from coelescing into larger bodies.