Asteroid family
A group of asteroids with similar orbits that are fragments of one larger body broken apart by a collision.
When a large asteroid is shattered by an impact, its fragments drift off with slightly different speeds but keep closely related orbits. Over millions of years they spread out, yet the clustering in their orbital properties remains. These groups are called asteroid families. The Japanese astronomer Kiyotsugu Hirayama first identified them in 1918. Well-known examples include the Koronis, Eos, Themis and Flora families, and the Vesta family of small 'Vestoids' blasted off Vesta.
Families let researchers study the insides of asteroids, because a break-up exposes interior material, and they are important sources of meteorites. Members tend to share a spectral class, and young families can be dated by tracing their orbits back to the moment of the break-up.
One remarkable case links a family-forming event to meteorites. Many L chondrites show their gases were driven out by heating about 470 million years ago, pointing to the destruction of their parent body. At about that time, fossil meteorites and grains of L-chondrite material became unusually common in sea-floor limestone now found in Sweden. Families are recognised only by statistical analysis of thousands of orbits, not visually.
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