Asteroid
Asteroid, one of the many small or minor rocky planetoids that are members of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter. SIZES AND ORBITS The largest representatives are 1 Ceres, with a diameter of about 1,003 km (about 623 mi), and 2 Pallas and 4 Vesta, with diameters of about 550 km (about 340 mi). The naming of asteroids is governed by the International Astronomical Union (IAU). After an astronomer observes a possible unknown asteroid, other astronomers confirm the discovery by observing the body over a period of several orbits and comparing the asteroid’s position and orbit to those of known asteroids. If the asteroid is indeed a newly discovered object, the IAU gives it a number according to its order of discovery, and the astronomer who discovered it chooses a name. Asteroids are usually referred to by both number and name. About 200 asteroids have diameters of more than 97 km (60 mi), and thousands of smaller ones exist