main asteroid belt
The main 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 kilometers) from the Sun – between Mars (1.5 AU) and Jupiter (5.2 AU) – and may contain over a million objects bigger than 1 kilometer across, the largest being Ceres (1,003 kilometers), Pallas (608 kilometers), 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-kilometer 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.
Beyond the orbit of Neptune lies an even large and more populous region of minor bodies known as the Kuiper belt.