Keplerian
orbital elements on the left (families indistinguishable) vs.
proper elements on the right (families visible).Strictly speaking, families and their membership are identified by analysing the
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 at least tens of millions of years, and perhaps longer.
Membership When the
orbital elements of
main belt asteroids are plotted (typically
inclination vs.
eccentricity, or vs.
semi-major axis), a number of distinct concentrations are seen against the rather uniform distribution of non-family
background asteroids. These concentrations are the asteroid families.
Interlopers are asteroids classified as family members based on their so-called
proper orbital elements but having spectroscopic properties distinct from the bulk of the family, suggesting that they, contrary to the true family members, did not originate from the same
parent body that once fragmented upon a collisional impact.
Family types As previously mentioned, families caused by an impact that did not disrupt the parent body but only ejected fragments are called
cratering families. Other terminology has been used to distinguish various types of groups which are less distinct or less statistically certain from the most prominent "nominal families" (or
clusters). The term
cluster is also used to describe a small asteroid family, such as the
Karin cluster.
Clumps are groupings which have relatively few members but are clearly distinct from the background (e.g. the
Juno clump).
Clans are groupings which merge very gradually into the background density and/or have a complex internal structure making it difficult to decide whether they are one complex group or several unrelated overlapping groups (e.g. the
Flora family has been called a clan).
Tribes are groups that are less certain to be statistically significant against the background either because of small density or large uncertainty in the orbital parameters of the members.
Hierarchical clustering method Present day computer-assisted searches have identified more than a hundred asteroid families (see below). 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 wavelet analysis, 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. == List ==