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Minor planet groups/families

Last updated 2 December 2010

• 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 andfamilies that I posted on the MinorPlanet Mailing List. I posted it in hopes of getting somecorrections, and I got quite a few via the list and private mail.

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

   "...[Groups are] loose dynamical associations. Families aredifferent and result from the catastrophic breakup of a largeparent 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), andKoronis (a = 2.87, e = 0.05, i = 1).   Notice on theMPC plotthat groups are loose regions, families are very tightgroupings. And note that these are osculating orbital elements.When proper elements are considered, the groups and familieschange 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, MatthiasBusch has some excellentplots of most minor planet groups as seen from above the solar system,made with his EasySkysoftware. 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 arefor-real families, whereas the others are all groups.

Certain of the definitions appear to be a little fuzzy, especiallythose that correspond to arbitrary divisions rather than actual orbitalcharacteristics. For groups from Amor to the Trojans, ranges in a, e, q, and i weresupplied by Rob McNaught, from a FORTRAN snippet he sent me. He gotthe ranges from Clifford Cunningham's book. Past that, the ranges arereverse-engineered from MPC data.

Groups out to the orbit of Earth

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

• Vulcanoids: (roughly) aphelion< .4. This is the entirelyhypothetical band of asteroids within the orbit of Mercury. Somesearches have been conducted in this region, but there has beenzero success so far.
• Atiras: aphelion< 1, i.e., the orbit is entirely insidethat of the Earth. Named after (163693) Atira. Also known asIEOs (Inner-Earth Objects). Several of these have been found,though not many; you can only see them at elongations of lessthan 90 degrees, where objects tend to be faint and not manypeople are looking.
• Arjuna: Fuzzily defined to be "in orbits like that ofEarth", meaning a near to 1, low eccentricity, and low inclination.Almost all NEOs pass us with at least enough energy to reach Mars, becausethat's basically how they came to us: from the Main Belt, kicked thisway by perturbations (mostly from Jupiter). If an object doesn't havethat much energy, resulting in a more earthlike orbit, you have towonder how that happened. Possibilities that have been suggested arethat these objects "aerobraked" (i.e., lost some energy relative to usby plowing through some of the earth's atmosphere) or that they arelunar ejecta.
  Objects in this group, thus far, are 1991 VG, 2000 SG₃₄₄,2006 RH₁₂₀, 2009 BD, 2010 UE₅₁, and 2010 VQ₉₈.One problem with these objects is that it can be hard to tell if they'reactually rocks, or space junk. It's clear that 2006 RH₁₂₀ isan actual rock; it was observed via radar, and is affected by solarradiation pressure in a manner consonant with a rock. 1991 VG and 2009BD are almost certainly rocks; were they space junk, the effects ofsolar radiation pressure would be observable. The other cases are lessclear; the observed arcs aren't long enough to really say for sureone way or the other.
• Earth Trojans: There have been a few small searches forobjects at the Earth-Sun Trojan points, but nothing very thorough yet.So far, one such object, 2010 TK₇, has been found.Such objects could conceivably be of great practical value someday,though; after the Moon (andArjunas), they wouldbe the most "accessible" objects in terms of the energyrequired to reach them, and the energy required to return materialsfrom 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 four of them, (5261) Eureka, (101429) 1998 VF₃₁, (121514) 1999 UJ₇, and 2007 NS₂. 1999 UJ₇ is in the (L4) "leading" node, 60 degrees ahead of Mars; the other four are in the (L5) 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 usuallytrue for the following groups. You'll see, for example, that some ofthe following are divided at places such as a = 2.5, where an objectwould be in a 1:3 resonance with Jupiter. The divisions I've figuredout (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 seeAlindas)   a = 2.706 (3:8 resonance)   a = 2.82  (2:5 resonance)   a = 3.27  (1:2 resonance... but seeGriquas)   a = 3.7   (3:5 resonance)

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

• Mars 1:2 Resonance ("Polanas"): TabareGallardo has made a goodcase that there are about a thousand objects with a=2.419 that arein 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 atthis page. The objects all have semimajor axes near to 2.419 AUand eccentricities that are larger than usual, but the only way todetermine if an object is really caught in this resonance is tointegrate its motion and see if its long-term, average period isactually 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!) Theseobjects are held by the 1:3 resonance with Jupiter. If I understand what'shappening here, an object that enters this resonance has its eccentricitysteadily pumped up, until it eventually has a close encounter with aninner planet that breaks the resonance. (Or not; Sebastian Hönighas foundpossible cases of Alindas that have had their eccentricities pumpedup to the point that they may fall into the sun.) Some Alindas,such as (4179) Toutatis, have perihelia very close to the earth'sorbit; 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. For a long time, only one member was known, (279) Thule, in a 3:4 resonance with Jupiter. Since then, (186024) 2001 QG₂₀₇ and (185290) 2006 UB₂₁₉ have brought the total up to three.

Between the Hildas and the Trojans (roughly 4.05< a< 5.0), there'sa 'forbidden zone'. Aside from Thule and five objects in unstable-lookingorbits, 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: This list shows (as of this writing) six Neptune Trojans, all in the "leading" (L4) node. (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 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.
• "Mystery distant" objects: There are a few objects (most notably Sedna and Eris) whose perihelia lie far beyond Neptune. How they got into such orbits is, at present, a mystery. One leading guess is that another star passed close enough to the Sun to exchange planets and disrupt the orbits of sufficiently distant objects.