Asteroid Ceres and Earth. Computer artwork showing the comparative sizes of the Earth (left), Ceres (middle) and the Moon (right). Ceres is the largest member of the asteroid belt, a large group of rocky and icy bodies orbiting in space between the orbits of Mars and Jupiter. Ceres, with a diameter of 950 kilometres, is classified as a dwarf planet. It accounts for around a third of the mass of the asteroid belt. While Ceres is the most massive single object in the asteroid belt, it is quite small when compared with the Earth and moon.