An asteroid is a minor planet—an object that is neither a true planet nor a comet—that orbits within the inner Solar System. They are rocky metallic or icy bodies with no atmosphere. The size and shape of asteroids vary significantly ranging from small rubble piles under a kilometer across to Ceres a dwarf planet almost 1000 km in diameter. Of the roughly one million known asteroids the greatest number are located between the orbits of Mars and Jupiter approximately 2 to 4 AU from the Sun in a region known as the main asteroid belt. Asteroids are generally classified to be of three types: C-type M-type and S-type. These describe asteroids with carbonaceous metallic and silicaceous compositions respectively. The size of asteroids varies greatly; the largest Ceres is almost 1000 km (600 mi) across and qualifies as a dwarf planet. The total mass of all the asteroids combined is only 3% that of Earth's Moon. The majority of main belt asteroids follow slightly elliptical stable orbits revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun.