Asteroids are mostly small solar system bodies that are composed in significant part of rocky and metallic non-volatile minerals. The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be the remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter. Asteroids range in size from hundreds of kilometers to as small as dust. All asteroids save the largest, Ceres, are classified as small solar system bodies; however, a number of other asteroids, such as Vesta and Hygeia, could potentially be reclassed as dwarf planets if it can be conclusively shown that they have achieved hydrostatic equilibrium. The asteroid belt contains tens of thousands - and potentially millions - of objects over one kilometre in diameter.[29] However, despite their large numbers, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.[30] In contrast to its various depictions in science fiction, the main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with a diameter of less than 50 m are called meteoroids. Ceres CeresCeres (2.77 AU) is the largest astronomical body in the asteroid belt and the only known dwarf planet in this region. It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the nineteenth century, but was reclassified as an asteroid as further observation revealed additional asteroids.[31] It has since been again reclassified as a dwarf planet. Asteroid groups Asteroids in the main belt are subdivided into asteroid groups and families based on their specific orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets[32] which may have been the source of Earth's water. Trojan asteroids are located in either of Jupiter's L4 or L5 points, (gravitationally stable regions leading and trailing a planet in its orbit) though the term is also sometimes used for asteroids in any other planetary Lagrange point as well. Hilda asteroids are those Trojans whose orbits are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits. The inner solar system is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.