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Asteroids Irregular (sometimes spherical) lumps of rock and metal that had never formed into planets during the formation of the solar system Several hundred thousand asteroids have been observed in the 10km to 100km size range, there maybe millions more of about 1km in size 26 are larger than 200km, the biggest is Ceres (933 km) and accounts for 25% of the mass of all the asteroids, in fact the total mass of all the asteroids is less than the mass of the Moon 75% are categorised as C-type (dark, carbon-rich material) 17% are S-type (bright, nickel-iron / magnesium silicate) Rest are M-type (very bright, pure nickel-iron) Asteroid Orbits Main Belt – covering 2-4 AU between orbits of Mars and Jupiter Near Earth Asteroids (NEA) – asteroids that have orbits that pass close to the Earth (including Amors, Atens, Apollos) Trojans – asteroids that orbit with Jupiter at the L4, and L5 Lagrangian points A crater on asteroid Eros – observed by the NEAR asteroid lander Asteroid Ida has its own moon, Dactyl. Observed by the Galileo space probe. 3D construction of asteroid Eros from NEAR observations Observing Asteroids Long exposure photographs reveal asteroid trails from which asteroid orbits can be calculated. Since asteroids are irregularly shaped their brightness will vary as they rotate.