section_path: Characteristics/Size distribution
section_name: Size distribution
paragraph_text: Size distribution
  
  Asteroids vary greatly in size, from almost 1000 km for the largest down to rocks just 1 meter across, below which an object is classified as a meteoroid. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered planetesimals or fragments of larger bodies.
  
  The dwarf planet Ceres is by far the largest asteroid, with a diameter of 940 km (580 mi). The next largest are 4 Vesta and 2 Pallas, both with diameters of just over 500 km (300 mi). Vesta is the brightest of the four main-belt asteroids that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.
  
  The mass of all the objects of the asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be (2394 ± 6) × 10 kg, ≈ 3.25% of the mass of the Moon. Of this, Ceres comprises 938 × 10 kg, about 40% of the total. Adding in the next three most massive objects, Vesta (11%), Pallas (8.5%), and Hygiea (3–4%), brings this figure up to a bit over 60%, whereas the next seven most-massive asteroids bring the total up to 70%. The number of asteroids increases rapidly as their individual masses decrease.
  
  The number of asteroids decreases markedly with increasing size. Although the size distribution generally follows a power law, there are 'bumps' at about 5 km and 100 km, where more asteroids than expected from such a curve are found. Most asteroids larger than approximately 120 km in diameter are primordial (surviving from the accretion epoch), whereas most smaller asteroids are products of fragmentation of primordial asteroids. The primordial population of the main belt was probably 200 times what it is today.