asteroid

The term asteroid is used for minor planets in the asteroid belt, out to Jupiter's orbit. It is also used more generally: often to include minor planets closer to the Sun, such as those that are near-Earth objects, and sometimes to include centaurs, i.e., minor planets further out. Vulcanoids are a theoretical population of asteroids within the orbit of Mercury (which would be difficult to detect).

The first discovered asteroids were Ceres and Pallas (now designated 1 Ceres and 2 Pallas) in 1801, with the 15th discovered in 1851. The term asteroid means star-like: they appeared as points (unresolved) though they move like planets. Now a million have been identified, many by computer, and the population is estimated to be multiple millions. Among the largest are Juno (3 Juno), Vesta (4 Vesta) and Hygiea (10 Hygiea).

A diameter of 10 meters is often (but not always) used as the threshold between asteroid and meteoroid. Ceres, the largest in the asteroid belt, is near 1,000 km in diameter, the next largest with less than 600 km. Asteroids are generally classified by type of orbit and by spectrum.

Asteroids are useful for studying the solar system's history and planets: they are thought to be shards due to collisions of un-fully-formed planets. They show varying composition, including iron that suggests a piece of a rocky planet's core, as well as various other materials including complex molecules. In addition to studying them by telescope and space probe, they are studied through the meteorites that are evidently asteroid material. A number of space missions have studied asteroids during flybys, in some cases by a mission primarily to visit a planet. A few missions have been specifically to study asteroids and a few of those have carried material back to Earth for study.