The Asteroid Belt
Between Mars and Jupiter lies the Asteroid Belt which is comprised of thousands of rocks left over from the formation of the solar system. These rocks vary in size from microscopic up to Ceres (950km diameter) which is classified as a dwarf planet. It is thought that Jupiter’s strong gravitational influence may have prevented the asteroids coalescing into larger objects such as a planet. Outside of the asteroid belt itself there are also three clusters of asteroids which are very much in Jupiter’s influence. These are the Trojan asteroids which orbit ahead and behind Jupiter and the Hilda Asteroids which orbit twice for every 3 Jupiter orbits. These asteroids are in Jupiter’s Lagrange points - areas of gravitational stability.
Although the asteroid belt has many bodies within it, it is still very thinly populated with many unmanned spacecraft having successfully passed through it without incident.