The Asteroid Belt .. The Asteroid Belt lies between the orbits of Mars and Jupiter. It is made up of millions of asteroids that range in size from asteroids like Ceres which has a diameter of 940 km to asteroids that are less than 1 km across. As they revolve around the Sun in elliptical orbits, their orbits are occasionally disrupted by Jupiter or Mars’s gravity, or by another larger asteroid. Such changes in orbit can send these objects hurtling into space, sometimes across the orbits of the planets. Mars’s 2 moons Phobos and Deimos may be asteroids that were captured by Mars’s gravity. In excess of 20,000 asteroids have been counted to date. Asteroids are made up of rock and metals such as nickel and iron. Scientists theorize that the Asteroid Belt are the pieces of a planet that never fully formed when the Solar System was very young. It is also believed that an asteroid which slammed into Mexico’s Yucatan peninsula 65 million years ago caused the extinction of the dinosaurs. Photo source (asteroid Gaspra): NASA'S Planetary PhotoJournal See credits for original photo information