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".
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.
Atiras: aphelion < 1, i.e., the orbit is entirely inside
that of the Earth. Named after (163693) Atira. Also known as
IEOs (Inner-Earth Objects). Several of these have been found,
though not many; you can only see them at elongations of less
than 90 degrees, where objects tend to be faint and not many
people are looking.
Arjuna: Fuzzily defined to be "in orbits like that of
Earth", meaning a near to 1, low eccentricity, and low inclination.
Almost all NEOs pass us with at least enough energy to reach Mars, because
that's basically how they came to us: from the Main Belt, kicked this
way by perturbations (mostly from Jupiter). If an object doesn't have
that much energy, resulting in a more earthlike orbit, you have to
wonder how that happened. Possibilities that have been suggested are
that these objects "aerobraked" (i.e., lost some energy relative to us
by plowing through some of the earth's atmosphere) or that they are
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're
actually rocks, or space junk. It's clear that 2006 RH120 is
an actual rock; it was observed via radar, and is affected by solar
radiation pressure in a manner consonant with a rock. 1991 VG and 2009
BD are almost certainly rocks; were they space junk, the effects of
solar radiation pressure would be observable. The other cases are less
clear; the observed arcs aren't long enough to really say for sure
one way or the other.
Earth Trojans: There have been a few small searches for
objects 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 (and Arjunas), 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
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 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
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
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
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.
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.
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