Minor planet groups/families

Last updated 25 March 2003

  • Groups out to the orbit of Earth
  • Groups out to the orbit of Mars
  • Groups out to the orbit of Jupiter
  • Groups past Jupiter

    • 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".

  • Vulcanoids: (roughly) aphelion < .4. This is the entirely hypothetical band of asteroids within the orbit of Mercury. Some searches have been conducted in this region, but there has been zero success so far.
  • Apoheles: aphelion < 1, i.e., the orbit is entirely inside that of the Earth. Other proposed names are IEOs (Inner-Earth Objects) and Anons (as in "Anonymous"). So far, this one is basically hypothetical, but it would be logical to expect that from time to time, Earth would slow down an NEA that would then be further perturbed by Venus, ending up inside our orbit. One probable object has been found in this group: 1998 DK36, by David Tholen. ("Apohele" is Hawaiian for "orbit", pronounced "ah-poe-hay-lay".)
  • Arjuna: Fuzzily defined to be "in orbits like that of Earth", meaning a near to 1, low eccentricity, and low inclination. Given that definition, objects in this group could also be Apoheles, Amors, Apollos, and/or Atens.
  • Earth Trojans: There have been a few small searches for objects at the Earth-Sun Trojan points, but nothing very thorough yet. Objects at these points would always be at about 60 degrees east and west of the Sun, and people tend to search for asteroids at much greater elongations. These objects could someday be of great practical value, though; after the Moon, they would be the most "accessible" objects in terms of the energy required to reach them, and the energy required to return materials from an Earth-Trojan orbit to the Earth is almost minimal.

    • Groups out to the orbit of Mars

  • Atens: a < 1
  • Apollo: q < 1.017, but a > 1
  • Amors: 1.017 < q < 1.3
    • (This seems to be a little fuzzy, with some preferring to say that the earth's orbit, rotated around its long axis, forms an ellipsoid; asteroids crossing this are     Apollos, those totally inside are Atens, those totally outside but with q < 1.3 are Amors. And some use 1 AU in place of 1.017 AU.)
  • Mars-crossers: either q < 1.52 and aphelion > 1.52, because Mars' a = 1.52; or use a similar ellipsoidal definition, rotating Mars' orbit around its long axis. Similar remarks apply to all other "planet-crossing" definitions. Also, some refer to q < 1.666 as a Mars-crosser.
  • Mars Trojans: Not much of a 'group', but there are six of them, (5261) Eureka, 1998 VF31, 1999 UJ7, 2001 DH47, 2001 FG24, and 2001 FR127. 1999 UJ7 is in the "leading" node, 60 degrees ahead of Mars; the other four are in the trailing node. The Minor Planet Center maintains a list of Mars Trojans.

    • 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:

  • Mars 1:2 Resonance ("Polanas"): Tabare Gallardo has made a good case that there are about a thousand objects with a=2.419 that are in a 1:2 resonance with Mars,

    the largest being (142) Polana. (He also mentions some objects in 1:2 and 2:5 resonance with Earth, and possible 1:2 resonance with Venus.) Details are at     this page. The objects all have semimajor axes near to 2.419 AU and eccentricities that are larger than usual, but the only way to determine if an object is really caught in this resonance is to integrate its motion and see if its long-term, average period is actually close to twice that of Mars.
  • Hungarias: 1.78 < a < 2, e < .18, 16 < i < 34. Very inner-main belt/just outside Mars objects of high inclination, such as (15964) Billgray. Possibly attracted by the 2:9 resonance?
  • Phocaeas: 2.25 < a < 2.5, e > .1, 18 < i < 32. Note that at present, MPC lumps Phocaeas in with Hungarias. The division is a real one, though, caused by the a=2.06 (1:4) resonance with Jupiter.
  • Floras: 2.1 < a < 2.3, i < 11.
  • Nysas: 2.41 < a < 2.5, e > .12, e < .21, 1.5 < i < 4.3
  • Main Belt I: 2.3 < a < 2.5, i < 18. I think this just means "everything in the inner main belt that doesn't happen to be a Nysa or Flora." The division made at a=2.3 appears to be an arbitrary one without physical significance.
  • Alinda: a = 2.5, .4 < e < .65 (very approximately!) These objects are held by the 1:3 resonance with Jupiter. If I understand what's happening here, an object that enters this resonance has its eccentricity steadily pumped up, until it eventually has a close encounter with an inner planet that breaks the resonance. (Or not; Sebastian Hönig has found possible cases of Alindas that have had their eccentricities pumped up to the point that they may fall into the sun.) Some Alindas, such as (4179) Toutatis, have perihelia very close to the earth's orbit; the result is a series of close passes at four-year intervals.
  • Pallas: 2.5 < a < 2.82, 33 < i < 38.
  • Marias: 2.5 < a < 2.706, 12 < i < 17.
  • Main Belt II: 2.5 < a < 2.706, i < 33.
  • Main Belt IIb: 2.706 < a < 2.82, i < 33.
  • Koronis: 2.83 < a < 2.91, e < .11, i < 3.5.
  • Eos: 2.99 < a < 3.03, .01 < e < .13, 8 < i < 12. Eos, Koronis, and Themis are families, each derived from a common ancestor object.
  • Main Belt IIIa: 2.82 < a < 3.03, e < .35, i < 30.
  • Themis: 3.08 < a < 3.24, .09 < e < .22, i < 3.
  • Griqua: 3.1 < a < 3.27, e > .35. These are in stable 2:1 libration with Jupiter, in high-inclination orbits. There are maybe 5 to 10 of these so far; (1362) Griqua and (8373) Stephengould are the most prominent.
  • Main Belt IIIb: 3.03 < a < 3.27, e < .35, i < 30.
  • Cybele: 3.27 < a < 3.7, e < .3, i < 25. This looks to be a cluster of objects around the 4:7 resonance with Jupiter.
  • Hildas: 3.7 < a < 4.2, e > .07, i < 20. Objects in a 2:3 resonance with Jupiter. As can be seen in this screenshot from EasySky, Hildas move such that their aphelia put them opposite Jupiter, or 60 degrees ahead of or behind Jupiter (i.e., at the Trojan points). Over three successive aphelia, they would occupy all three points. As seen from above the solar system, they would appear to form a big equilateral triangle pointing away from Jupiter.
  • Thule: This is even less of a group than the Mars Trojans. It appears to consist of a grand total of one object, (279) Thule, in a 3:4 resonance with Jupiter. It seems odd to me that there's this one pretty big, bright object, and nothing else... science fiction authors and X-Files fans, take note.

    • 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.

  • Trojans: 5.05 < a < 5.4, in elongated, banana-shaped regions 60 degrees ahead and behind of Jupiter. These can be considered the 'Greek' and 'Trojan' nodes respectively; with one exception apiece, objects in each node are named for members of that side of the conflict. (617) Patroclus in the Trojan node and (624) Hektor in the Greek node are "misplaced" in the enemy camps. This screen shot gives a good idea of the layout of Jupiter Trojans ahead of and behind Jupiter; in particular, it shows that there is a lot of "spread" around the ideal 60-degree nodes. The Minor Planet Center maintains a list of Jupiter Trojans.

    • Groups past Jupiter

  • Damocloid/"Oort cloud group": Named after the prototype object, (5335) Damocles. Very fuzzily defined to be objects that have "fallen in" from the Oort cloud, so their aphelia are generally still out past Uranus, but their perihelia are in the inner solar system. They therefore have high e, and sometimes high inclinations (including retrograde orbits). Click here for a list of these objects, created by Akimasa Nakamura and updated by Brian Skiff.
  • Centaurs: Fuzzily defined, but maybe 5.4 < a < 30? I think these are currently believed to be TNOs that 'fell in' after encounters with gas giants.
  • Neptune Trojans: Currently consists of the object 2001 QR322, but there are presumably others out there. (Incidentally, I've run across a claim that Uranian and Saturnian Trojans wouldn't be stable over billions of years, which would make some sense; such objects would be apt to get yanked out of their Trojan nodes by Jupiter.)
  • Trans-Neptunian Objects (TNOs): a.k.a. KBO (Kuiper-Belt Object) or EKO (Edgeworth-Kuiper Object.) Anything with a > 30, with some falling into the following sub-categories:
  • Plutinos: 2:3 resonance with Neptune, just like Pluto. The perihelion of such an object tends to be close to Neptune's orbit (much as happens with Pluto), but when the object comes to perihelion, Neptune alternates between being 90 degrees ahead of and 90 degrees behind of the object, so there's no chance of a collision. It appears to me that MPC defines any object with 39 < a < 40.5 to be a Plutino.
  • Cubewanos: Also known as "classical KBOs". The name comes from '1992 QB1', the first TNO ever found. These have 40.5 < a < 47, roughly. This appears to refer to objects in the Kuiper belt that didn't get scattered and didn't get locked into a resonance with Neptune.
  • "Hyperplutinos": (My own term of convenience) Objects in resonances with Neptune other than the 2:3 one occupied by Plutinos and the 1:1 occupied by Neptune Trojans. This MPEC mentions the several known objects in the 2:1 resonance, which have been christened "Twotinos" (Chiang and Jordan, AJ V124, I6, pp.3430-3444). These objects all have roughly a=48, e=.37. Also, there are several objects in the 2:5 resonance (a=55), which we could call "two-and-a-half-inos" or "tweenos". Then there are objects in the 4:5, 4:7, 3:5, and 3:4 resonances. (As far as I know, there is only one other example of a 3:4 resonance in the solar system: Saturn's satellite Hyperion is in such a resonance with Titan. Perhaps these could be called "Hyperinos?" Or "Hyperioninos"?)

    • To avoid seven different names for each resonance, I'd simply call them all "hyperplutinos".

  • Scattered-Disk Objects (SDOs): These objects generally have very large orbits of up to a few hundred AU. They are assumed to be objects that encountered Neptune and were "scattered" into long-period, very elliptical orbits with perihelia that are still not too far from Neptune's orbit.