There are about 20 to 30 reliably recognized families, with several tens of less certain groupings. Most asteroid families are found in the main asteroid belt, although several family-like groups such as the Pallas family, Hungaria family, and the Phocaea family lie at smaller semi-major axis or larger inclination than the main belt.
One family has been identified associated with the dwarf planet . Some studies have tried to find evidence of collisional families among the trojan asteroids, but at present the evidence is inconclusive.
Due to the method of origin, all the members have closely matching compositions for most families. Notable exceptions are those families (such as the Vesta family) which formed from a large differentiated parent body.
Asteroid families are thought to have lifetimes of the order of a billion years, depending on various factors (e.g. smaller asteroids are lost faster). This is significantly shorter than the Solar System's age, so few if any are relics of the early Solar System. Decay of families occurs both because of slow dissipation of the orbits due to perturbations from Jupiter or other large bodies, and because of collisions between asteroids which grind them down to small bodies. Such small asteroids then become subject to perturbations such as the Yarkovsky effect that can push them towards orbital resonances with Jupiter over time. Once there, they are relatively rapidly ejected from the asteroid belt. Tentative age estimates have been obtained for some families, ranging from hundreds of millions of years to less than several million years for e.g. the compact Karin family. Old families are thought to contain few small members, and this is the basis of the age determinations.
It is supposed that many very old families have lost all the smaller and medium-sized members, leaving only a few of the largest intact. A suggested example of such old family remains are the 9 Metis and 113 Amalthea pair. Further evidence for a large number of past families (now dispersed) comes from analysis of chemical ratios in iron meteorites. These show that there must have once been at least 50 to 100 parent bodies large enough to be differentiated, that have since been shattered to expose their cores and produce the actual meteorites (Kelley & Gaffey 2000).
Strictly speaking, families and their membership are identified by analysing the so-called proper orbital elements rather than the current osculating orbital elements, which regularly fluctuate on timescales of tens of thousands of years. The proper elements are related constants of motion that remain almost constant for times of at least tens of millions of years, and perhaps longer.
The Japanese astronomer Kiyotsugu Hirayama (1874-1943) pioneered the estimation of proper elements for asteroids, and first identified several of the most prominent families in 1918. In his honor, asteroid families are sometimes called Hirayama families. This particularly applies to the five prominent groupings discovered by him.
Present day computer-assisted searches have identified several tens of asteroid families. The most prominent algorithms have been the Hierarchical Clustering Method (HCM) which looks for groupings with small nearest-neighbour distances in orbital element space, and the Wavelet Analysis Method (WAM) which builds a density-of-asteroids map in orbital element space, and looks for density peaks.
The boundaries of the families are somewhat vague because at the edges they blend into the background density of asteroids in the main belt. For this reason the number of members even among discovered asteroids is usually only known approximately, and membership is uncertain for asteroids near the edges.
Additionally, some interlopers from the heterogeneous background asteroid population are expected even in the central regions of a family. Since the true family members caused by the collision are expected to have similar compositions, most such interlopers can in principle be recognised by spectral properties which do not match those of the bulk of family members. A prominent example is 1 Ceres, the largest asteroid, which is an interloper in the family once named after it (the Ceres family, now the Gefion family).
Spectral characteristics can also be used to determine the membership (or otherwise) of asteroids in the outer regions of a family, as has been used e.g. for the Vesta family, whose members have an unusual composition.
|Family Name||Named After||orbital elements||Size||Alternate Names|
|a (AU)||e''||i (°)||approx. % of asteroids|| members in Zappalà|
|The most prominent families within the main belt are:|
|Eos||221 Eos||2.99 to 3.03||0.01 to 0.13||8 to 12||480|
|Eunomia||15 Eunomia||2.53 to 2.72||0.08 to 0.22||11.1 to 15.8||5%||370|
|Flora||8 Flora||2.15 to 2.35||0.03 to 0.23||1.5 to 8.0||4-5%||590||Ariadne family after 43 Ariadne|
|Hygiea||10 Hygiea||3.06 to 3.24||0.09 to 0.19||3.5 to 6.8||1%||105|
|Koronis||158 Koronis||2.83 to 2.91||0 to 0.11||0 to 3.5||310|
|Maria||170 Maria||2.5 to 2.706||12 to 17||80|
|Nysa||44 Nysa||2.41 to 2.5||0.12 to 0.21||1.5 to 4.3||380||Hertha family after 135 Hertha|
|Themis||24 Themis||3.08 to 3.24||0.09 to 0.22||0 to 3||530|
|Vesta||4 Vesta||2.26 to 2.48||0.03 to 0.16||5.0 to 8.3||6%||240|
|Other notable main belt families:[C]|
|Bower||1639 Bower||13||Endymion family after 342 Endymion|
|Gefion||1272 Gefion||2.74 to 2.82||0.08 to 0.18||7.4 to 10.5||0.8%||89|| Ceres family after 1 Ceres and|
Minerva family after 93 Minerva
|Hilda||153 Hilda||3.7 to 4.2||>0.07||<20°||-|
|Massalia||20 Massalia||2.37 to 2.45||0.12 to 0.21||0.4 to 2.4||0.8%||47|
|Nemesis||128 Nemesis||29||Concordia family after 58 Concordia|
|Veritas||490 Veritas||29||Undina family after 92 Undina|
See also Asteroid groups and families, which names some less prominent and uncertain groupings.