This page tells all about the asteroid belt in between Mars and Jupiter.
The 36-mile-long asteroid Ida, with its moon (Dactyl). This is the first asteroid ever discovered with its own moon.
What is the Asteroid Belt?
The asteroid belt is a region located in between the orbits of the planets Mars and Jupiter occupied by irregular shaped asteroids (or minor planets). While the asteroids differ greatly in size, one, named Ceres, is considered a dwarf planet as its diameter extends nearly 575 miles. Three other asteroids have diameters of over 240 miles. Over half of the mass of the entire asteroid belt is contained within these four asteroids. Scientists have counted over 40,000 asteroids over a half of a mile in diameter. The asteroids within the belt are very thinly distributed, and flying spacecraft have generally had little difficulty in passing through it.
The asteroids within the belt orbit the sun between the distances of about 170,000,000 miles and 380,000,000 miles depending on their positioning within the belt. Those closer to Jupiter will take longer to orbit the sun. The gravitational force of Jupiter keeps the asteroids from leaving the belt and possibly colliding with the inner planets.