The asteroid belt between Mars and Jupiter is thought to be left over from a planet that was unable to form due to the gravitational influence of Jupiter or the remains of a planet that was destroyed in a collision. The belt is composed of small bodies of rock and ice of various sizes.
The asteroid belt encompasses an area between 2 and 4 astronomical units from the Sun. The belt was formed 4.6 million years ago, when either the gravitational stress of Jupiter or a collision likely destroyed a planet or protoplanet roughly half the size of the Moon. Most asteroids found in the belt have an orbital period of between three and six years. Objects in the belt range in size from pebbles to the asteroid Ceres, which is approximately 600 miles in diameter.
While most asteroids are located within the belt, they can be found scattered throughout the solar system. Asteroids that approach to within 1.3 astronomical units of the Sun or that have their orbits shifted outside the belt due to periodic collisions or the gravitational influence of Jupiter are called near earth asteroids. These asteroids may have the potential to one day collide with the Earth.