Minor planet groups/families

Last updated 2 December 2010

The following is an updated version of a list of asteroid groups and families that I posted on the Minor Planet Mailing List. I posted it in hopes of getting some corrections, and I got quite a few via the list and private mail.

Tim Spahr posted some useful data, including the following distinction between groups and families:

   "...[Groups are] loose dynamical associations. Families are
different and result from the catastrophic breakup of a large
parent asteroid sometime in the past. Prominent families are Eos
(a = 3.1, e = 0.1, i = 10) Themis (a = 3.1, e = 0.1, i = 1), and
Koronis (a = 2.87, e = 0.05, i = 1).   Notice on the MPC plot 
that groups are loose regions, families are very tight
groupings. And note that these are osculating orbital elements.
When proper elements are considered, the groups and families
change shape, in general the families become very tight clumps."

In addition to the MPC plots, this plot from a JPL site makes certain groups easy to distinguish. Also, Matthias Busch has some excellent plots of most minor planet groups as seen from above the solar system, made with his EasySky software. These plots make visualizing the layout of some groups (especially Jupiter Trojans and Hildas) much easier.

As far as I know, in the following list, Themis, Eos, and Koronis are for-real families, whereas the others are all groups.

Certain of the definitions appear to be a little fuzzy, especially those that correspond to arbitrary divisions rather than actual orbital characteristics. For groups from Amor to the Trojans, ranges in a, e, q, and i were supplied by Rob McNaught, from a FORTRAN snippet he sent me. He got the ranges from Clifford Cunningham's book. Past that, the ranges are reverse-engineered from MPC data.

Groups out to the orbit of Earth

The names of these first three groups are unofficial. The Minor Planet Center holds that the name for a group comes from the first asteroid in that group to be named (except for those in the Trojan and more distant groups.) So far, we aren't even close to having a named object for these groups, or even one that is unambiguously in the group. However, it would be surprising if any of these groups were really "empty".

Groups out to the orbit of Mars:

Groups out to the orbit of Jupiter

Several of the above distinctions are, to some extent, arbitrary. There are no orbital resonances dividing them. The opposite is usually true for the following groups. You'll see, for example, that some of the following are divided at places such as a = 2.5, where an object would be in a 1:3 resonance with Jupiter. The divisions I've figured out (a.k.a. "Kirkwood gaps") are:

   a = 1.9   (2:9 resonance)
   a = 2.06  (1:4 resonance)
   a = 2.25  (2:7 resonance)
   a = 2.5   (1:3 resonance... but see Alindas)
   a = 2.706 (3:8 resonance)
   a = 2.82  (2:5 resonance)
   a = 3.27  (1:2 resonance... but see Griquas)
   a = 3.7   (3:5 resonance)

This and other factors leads to the following zoo of groups between Mars and Jupiter:

Between the Hildas and the Trojans (roughly 4.05 < a < 5.0), there's a 'forbidden zone'. Aside from Thule and five objects in unstable-looking orbits, Jupiter has swept everything clean.

Groups past Jupiter: