Hirayama family

A concentration of asteroids having similar orbits and thus located near each other in space. The existence of such groupings was first noted by Kiyotsugo Hirayama in 1918. More than a hundred have been identified. In many cases the members are of similar or related types, strongly suggesting that they were formed from the break-up of one parent body; some well-known examples are the Eos, Koronis and Themis families. About half of all asteroids are thought to belong to Hirayama families.