Enlargement Photograph of the asteroid Ida, which has a diameter of 30 kilometres. Ceres, the largest asteroid and the first to be discovered, is 933 kilometres long!
The asteroid belt is located in a wide zone between the planets Mars and Jupiter. The 100 000 asteroids in orbit around the Sun should have agglomerated to form a planet, but gravitational perturbations from Jupiter disrupted this process. Made up of a mixture of rock and metal, asteroids from the belt are the parent bodies of most meteorites. Collisions between asteroids hurl fragments into unstable zones where Jupiter's pull is greater: these zones are known as the Kirkwood gaps. The fragments are then catapulted into new orbits, making it possible to enter the inner regions of the solar system. They are then considered "near-Earth asteroids" (NEAs). Some of these asteroids' entire orbits are inside Earth's orbit (Aten asteroids); others cross the Earth's orbit (Apollo asteroids); still others orbit between Earth and Mars (Amor asteroids).
Enlargement Location of the asteroid belt between Mars and Jupiter.
Asteroids have different compositions depending on where they formed. In the part of the asteroid belt that is closest to the Sun, asteroids reach higher temperatures, and as a result have transformed and begun the process of differentiation. Asteroids that formed farther from the Sun have conserved more of their primitive composition-carbonaceous chondrites come from these distant regions.
