Asteroids are rocky or metallic objects, generally smaller than a planet but bigger than a meteoroid, that orbit the Sun or another star. Asteroids are also called minor planets. The largest asteroid, Ceres, however, has been classified as a dwarf planet. More than 20,000 asteroids have been given official designations, most of them in the asteroid belt between the orbits of Mars and Jupiter. Asteroids outside this belt include the Trojans, which share Jupiter's orbit, and the near-Earth asteroids. A number of different asteroid groups have been distinguished on the basis of their similar orbital characteristics. Asteroids range in size from a few meters to over 900 km across, and vary greatly in composition. The total mass of all the asteroids, most of which is concentrated in the main belt, is about one-twentieth that of the Moon.  READ MORE »