The asteroid belt is a region between the orbits of Mars and Jupiter where the majority of asteroids in the Solar System are to be found. The main asteroid belt extends from 2.15 to 3.3 astronomical units (255 to 600 million km) from the Sun – between Mars (1.5 AU) and Jupiter (5.2 AU) – and may contain over a million objects bigger than 1 km across. The largest body in the asteroid belt is Ceres (1,003 km in diameter), now classified as a dwarf planet, followed by Pallas (608 km) and Vesta (538 kilometers). Within the belt, the distribution of asteroids is non-uniform, with concentrations in asteroid groups and families, and also relatively empty zones known as Kirkwood gaps.
The belt appears to have originated as a system of perhaps 50 large bodies in the 100 to 1,000 km size range. These accreted during the formation of the Solar System and thereafter suffered collisions leading to fragmentation. This hypothesis suggests that only a few of the larger asteroids are still intact.