Astronomy:Asteroid family

Short description: Population of asteroids that share similar proper orbital elements and orbital inclination
Asteroid families become visible as distinct concentrations when asteroids are plotted in the proper orbital element space (ip vs ap). Some prominent families are the Vesta, Eunomia, Koronis, Eos, and Themis family located in different (colorized) regions of the asteroid belt.

An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions. An asteroid family is a more specific term than asteroid group whose members, while sharing some broad orbital characteristics, may be otherwise unrelated to each other.

General properties

Plot of proper inclination vs. eccentricity for numbered asteroids

Large prominent families contain several hundred recognized asteroids (and many more smaller objects which may be either not-yet-analyzed, or not-yet-discovered). Small, compact families may have only about ten identified members. About 33% to 35% of asteroids in the main belt are family members.

There are about 20 to 30 reliably recognized families, with several tens of less certain groupings. Most asteroid families are found in the main asteroid belt, although several family-like groups such as the Pallas family, Hungaria family, and the Phocaea family lie at smaller semi-major axis or larger inclination than the main belt.

One family has been identified associated with the dwarf planet Haumea.[1] Some studies have tried to find evidence of collisional families among the trojan asteroids, but at present the evidence is inconclusive.

Origin and evolution

The families are thought to form as a result of collisions between asteroids. In many or most cases the parent body was shattered, but there are also several families which resulted from a large cratering event which did not disrupt the parent body (e.g. the Vesta, Pallas, Hygiea, and Massalia families). Such cratering families typically consist of a single large body and a swarm of asteroids that are much smaller. Some families (e.g. the Flora family) have complex internal structures which are not satisfactorily explained at the moment, but may be due to several collisions in the same region at different times.

Due to the method of origin, all the members have closely matching compositions for most families. Notable exceptions are those families (such as the Vesta family) which formed from a large differentiated parent body.

Asteroid families are thought to have lifetimes of the order of a billion years, depending on various factors (e.g. smaller asteroids are lost faster). This is significantly shorter than the Solar System's age, so few if any are relics of the early Solar System. Decay of families occurs both because of slow dissipation of the orbits due to perturbations from Jupiter or other large bodies, and because of collisions between asteroids which grind them down to small bodies. Such small asteroids then become subject to perturbations such as the Yarkovsky effect that can push them towards orbital resonances with Jupiter over time. Once there, they are relatively rapidly ejected from the asteroid belt. Tentative age estimates have been obtained for some families, ranging from hundreds of millions of years to less than several million years as for the compact Karin family. Old families are thought to contain few small members, and this is the basis of the age determinations.

It is supposed that many very old families have lost all the smaller and medium-sized members, leaving only a few of the largest intact. A suggested example of such old family remains are the 9 Metis and 113 Amalthea pair. Further evidence for a large number of past families (now dispersed) comes from analysis of chemical ratios in iron meteorites. These show that there must have once been at least 50 to 100 parent bodies large enough to be differentiated, that have since been shattered to expose their cores and produce the actual meteorites (Kelley & Gaffey 2000).

Identification of members, interlopers and background asteroids

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 (see above). 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.

Description

Comparison: osculating 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.

The Japan ese astronomer Kiyotsugu Hirayama (1874–1943) pioneered the estimation of proper elements for asteroids, and first identified several of the most prominent families in 1918. In his honor, asteroid families are sometimes called Hirayama families. This particularly applies to the five prominent groupings discovered by him.

Hierarchical clustering method

Present day computer-assisted searches have identified more than a hundred asteroid families. 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.

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).

Clusters, clumps, clans and tribes

The term cluster is also used to describe a small asteroid family, such as the Karin cluster.[2] 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.

List

Prominent families

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Distribution of the most prominent families, other families and background asteroids (up to number 398,000)[3]:23

Among the many asteroid families, the Eos, Eunomia, Flora, Hungaria, Hygiea, Koronis, Nysa, Themis and Vesta families are the most prominent ones in the asteroid belt. For a complete list, see § All families.

Eos family
The Eos family (adj. Eoan; 9,789 members, named after 221 Eos)
Eunomia family
The Eunomia family (adj. Eunomian; 5,670 known members, named after 15 Eunomia) is a family of S-type asteroids. It is the most prominent family in the intermediate asteroid belt and the 6th-largest family with approximately 1.4% of all main belt asteroids.[3]:23
Flora family
The Flora family (adj. Florian; 13,786 members, named after 8 Flora) is the 3rd-largest family. Broad in extent, it has no clear boundary and gradually fades into the surrounding background population. Several distinct groupings within the family, possibly created by later, secondary collisions. It has also been described as an asteroid clan.
Hungaria family
The Hungaria family (adj. Hungarian; 2,965 members, named after 434 Hungaria)
Hygiea family
The Hygiea family (adj. Hygiean; 4,854 members, named after 10 Hygiea)
Koronis family
The Koronis family (adj. Koronian; 5,949 members, named after 158 Koronis)
Nysa family
The Nysa family (adj. Nysian; 19,073 members, named after 44 Nysa). Alternatively named Hertha family after 135 Hertha.
Themis family
The Themis family (adj. Themistian; 4,782 members, named after 24 Themis)
Vesta family
The Vesta family (adj. Vestian; 15,252 members, named after 4 Vesta)

All families

In 2015, a study identified 122 notable families with a total of approximately 100,000 member asteroids, based on the entire catalog of numbered minor planets, which consisted of almost 400,000 numbered bodies at the time (see catalog index for a current listing of numbered minor planets).[3]:23 The data has been made available at the "Small Bodies Data Ferret".[4] The first column of this table contains the family identification number or family identifier number (FIN), which is an attempt for a numerical labeling of identified families, independent of their currently used name, as a family's name may change with refined observations, leading to multiple names used in literature and to subsequent confusion.[3]:17

FIN Family Lbl # of Members Loc.
[note 1]
Taxonomy mean-
albedo
mean a mean e mean i Parent body · Notes Cat LoMP
006 unnamed family 006 7 trojan 0.06 5.204 0.049 31.75 9799 Thronium (Jupiter trojan) list
007 James Bond family[5] 007 1 A ASP 2.474 0.129 6.32 9007 James Bond list
010 unnamed family 010 13 trojan 0.09 5.204 0.041 24.23 (247341) 2001 UV209 (Jupiter trojan) list
417 unnamed family 417 9 A 2.465 0.153 3.93 (108138) 2001 GB11 list
526 unnamed family 526 58 C C 0.06 2.721 0.173 14.35 (53546) 2000 BY6 list
615 unnamed family 615 104 D CX 0.17 2.848 0.106 9.14 (18405) 1993 FY12 list
627 unnamed family 627 38 D CX 0.05 2.868 0.219 16.02 (15454) 1998 YB3 list
628 unnamed family 628 248 D S 0.10 2.850 0.081 5.12 (15477) 1999 CG1 list
629 unnamed family 629 58 D S 0.21 2.939 0.118 10.73 (36256) 1999 XT17 list
637 unnamed family 637 64 G CX 0.05 3.109 0.180 3.46 (106302) 2000 UJ87 list
640 Gibbs family GBS 8 E 3.004 0.023 10.34 331P/Gibbs "P/2012 F5 (Gibbs)". Other members include (20674), (140429), and (177075)

Other families or dynamical groups

Other asteroid families from miscellaneous sources (not listed in the above table), as well as non-asteroid families include:

Family Parent Cat Description
Aemilia family 159 Aemilia MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 62 members.
Anius family 8060 Anius MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 31 members.
Ashkova family 3460 Ashkova MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 59 members.
Ausonia family 63 Ausonia Single member. Unsourced. Member of the Vesta family according to AstDyS-2 and Nesvorný (2014).[3]
Bontekoe family 10654 Bontekoe MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 13 members.
Brokoff family 6769 Brokoff MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 58 members.
Bower family 1639 Bower Micro-family with 10 members as per Zappalà (1995). Adj. Bowerian. Alternative name Endymion (Endymionian) family after 342 Endymion.[C] All members: (1639), (3815), (8832), (14306), (15666), (22286), (32637), (85133), (120446) and (145685).[8] This family corresponds in large parts with the König family by Nesvorný (2014).[3]
Cindygraber family 7605 Cindygraber MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 19 members.
Devine family 3561 Devine MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 19 members.
Duponta family 1338 Duponta MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 133 members.
Epeios family 2148 Epeios Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.[9]
Eumelos family 5436 Eumelos Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.[9]
Euryalos family 4007 Euryalos Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.[9]
Hanskya family 1118 Hanskya MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 116 members.
Helio family 895 Helio MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 50 members.
Higson family 3025 Higson MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 17 members.
Hippasos family 17492 Hippasos MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 7 members.
Huberta family 260 Huberta MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 26 members. Nesvorný moved family to candidate status.[3]:19
Kalchas family 4138 Kalchas Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.[9]
Levin family 2076 Levin MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 1534 members.
Makhaon family 3063 Makhaon Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.[9]
Marsili family 40134 Marsili MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 16 members.
Matterania family 883 Matterania MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 169 members.
Mecklenburg family 6124 Mecklenburg MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 78 members.
Melanthios family 12973 Melanthios Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.[9]
Menelaus family 1647 Menelaus Jupiter trojan family according Milani (1993).[10] Part of the Menelaus clan according to Roig and Gil-Hutton (2008).[9]
Nele family 1547 Nele MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 344 members.
Nocturna family 1298 Nocturna MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 186 members.
Podarkes family 13062 Podarkes Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.[9]
Prokne family 194 Prokne MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 379 members.
Sinden family 10369 Sinden MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 24 members.
Takehiro family 8737 Takehiro MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 57 members. Nesvorný moved family to candidate status.[3]:19
Telamon family 1749 Telamon Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.[9]
Traversa family 5651 Traversa MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 56 members.
Univermoscow family 6355 Univermoscow MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 13 members.
Zhvanetskij family 5931 Zhvanetskij MBA-family (AstDys) according to Milani and Knežević (2014).[6][7] Total of 23 members.
Legend:

Notes

1. "close" refers to asteroids inside the 9:2 resonance, "inner" refers to asteroids between the 9:2 and 4:1 resonance. A refers to between 4:1 and 3:1, B is 3:1 to 8:3, C is 8:3 to 5:2, D is 5:2 to 7:3, E is 7:3 to 9:4, F is 9:4 to 11:5, G is 11:5 to 2:1, "outer" refers to asteroids between the 2:1 and 11:6 resonance, and "rim" refers to asteroids beyond the 11:6 resonance.

References

1. Michael E. Brown, Kristina M. Barkume, Darin Ragozzine & Emily L. Schaller, A collisional family of icy objects in the Kuiper belt, Nature, 446, (March 2007), pp 294-296.
2. David Nesvorný, Brian L. Enke, William F. Bottke, Daniel D. Durda, Erik Ashaug & Derek C. Richardson Karin cluster formation by asteroid impact, Icarus 183, (2006) pp 296-311.
3. Nesvorný, D.; Broz, M.; Carruba, V. (December 2014). Identification and Dynamical Properties of Asteroid Families. 297–321. doi:10.2458/azu_uapress_9780816532131-ch016. ISBN 9780816532131. Bibcode2015aste.book..297N.
4. "Small Bodies Data Ferret". Nesvorny HCM Asteroid Families V3.0.
5. This is a joke by Nesvorný et al. In their Table 2 the reference is to the 1995 film, GoldenEye.
6. Milani, Andrea; Cellino, Alberto; Knezevic, Zoran; Novakovic, Bojan; Spoto, Federica; Paolicchi, Paolo (September 2014). "Asteroid families classification: Exploiting very large datasets". Icarus 239: 46–73. doi:10.1016/j.icarus.2014.05.039. Bibcode2014Icar..239...46M.
7. Knezevic, Zoran; Milani, Andrea; Cellino, Alberto; Novakovic, Bojan; Spoto, Federica; Paolicchi, Paolo (July 2014). "Automated Classification of Asteroids into Families at Work". Complex Planetary Systems 310: 130–133. doi:10.1017/S1743921314008035. Bibcode2014IAUS..310..130K.
8. Zappalà, V.; Bendjoya, Ph.; Cellino, A.; Farinella, P.; Froeschle, C. (1997). "Asteroid Dynamical Families". NASA Planetary Data System: EAR-A-5-DDR-FAMILY-V4.1. Retrieved 4 March 2020.  (PDS main page)
9. Roig, F.; Ribeiro, A. O.; Gil-Hutton, R. (June 2008). "Taxonomy of asteroid families among the Jupiter Trojans: comparison between spectroscopic data and the Sloan Digital Sky Survey colors". Astronomy and Astrophysics 483 (3): 911–931. doi:10.1051/0004-6361:20079177. Bibcode2008A&A...483..911R.
10. Milani, Andrea (October 1993). "The Trojan asteroid belt: Proper elements, stability, chaos and families". Celestial Mechanics and Dynamical Astronomy 57 (1–2): 59–94. doi:10.1007/BF00692462. ISSN 0923-2958. Bibcode1993CeMDA..57...59M.

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