Asteroid family

 ( Families ) 

An asteroid family is a population of 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.

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General properties 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 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.

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

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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 family, Pallas family, Hygiea family, and Massalia family 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 asteroid pair. Further evidence for a large number of past families (now dispersed) comes from analysis of chemical ratios in . 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).

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Identification of members, interlopers and background asteroids When the 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.

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Description Strictly speaking, families and their membership are identified by analysing the proper orbital elements rather than the current osculating orbit 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 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.

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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.

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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).

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Clusters, clumps, clans and tribes The term cluster is also used to describe a small asteroid family, such as the Karin Cluster.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. 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.

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List

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

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 . It is the most prominent family in the Kirkwood gap asteroid belt and the 6th-largest family with approximately 1.4% of all main belt asteroids.

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)

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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 , which consisted of almost 400,000 numbered bodies at the time (see LOMP for a current listing of numbered minor planets). The data has been made available at the "Small Bodies Data Ferret". 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.

James Bond familyThis is a joke by Nesvorný et al. In their Table 2 the reference is to the 1995 film, GoldenEye.
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Other families or dynamical groups

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

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 62 members.

Alinda group described by projectpluto.com

Small family of 22 asteroids identified by Zappalà (1995). Most members have been assigned to the encompassing complex of the Flora family by Nesvorný (2014).

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 31 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 59 members.

Large MBA-family (AstDys) according to Milani and Knežević (2014). Total of 6,169 members. Lowest-numbered members: , , , , and . Not a listed family by Zappalà (1995). Considered a HCM-artifact by Nesvorný (2014) due to a resonant alignment (z1 g + s − g6 − s6 = 0).

Small family of 23 asteroids identified by Zappalà (1995). Most members have been assigned to the Flora family by Nesvorný (2014).

Single member. Unsourced. Member of the Vesta family according to AstDyS-2 and Nesvorný (2014).

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 13 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 58 members.

Micro-family with 10 members as per Zappalà (1995). Adj. Bowerian. Alternative name Endymion (Endymionian) family after 342 Endymion. All members: , , , , , , , , and . This family corresponds in large parts with the König family by Nesvorný (2014).

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 19 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 17 members. Subset of the large Alauda family as per Nesvorný (2014). All members: , , , , , , , , , , , , , , , and .

Cybele group according to Asteroids, Meteorites, and Comets – by Linda T. Elkins-Tanton and projectpluto.com. Corresponding wiki-category lists a total of 32 members. Not a listed family in HCM by Zappalà (1995), Nesvorný (2014) and AstDyS-2 ( Src), where these bodies are predominantly assigned to the background population.

Micro-family with 5 members as per Zappalà (1995). All members: , , , and . All belong to the background population according to Nesvorný (2014).

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 19 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 133 members.

Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.

Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.

Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.

C-type asteroid family with 12 identified members as per Zappalà (1995). All members: , , , , , , , , , , and . Predominantly background population with 3 bodies belonging to the stony Maria family per Nesvorný (2014). Not a listed family at AstDyS-2 ( Src)

Griqua group (not a collisional family) described by projectpluto.com. A marginally unstable group of asteroids observed in the 2 :1 resonance with Jupiter.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 116 members.

This is a TNO-family. As of 2017, and current categorization, the family consists of 10 members (including parent body).

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 50 members.

Nesvorný moved family (formerly FIN 503) to candidate status, and to background. Also background according to Milani and Knežević ( AstDyS-2).

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 17 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 7 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 26 members. Nesvorný moved family to candidate status.

Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.

Category with 2 members. 507 Laodica and 635 Vundtia are core members of the Eos family according to AstDyS-2 ( 507; 635) and background asteroid per Nesvorný (; ), respectively.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 1534 members.

3 listed members. 125 Liberatrix is a background asteroid according to AstDyS-2, and a member of the Nemesis family according to . Background asteroid: 301 Bavaria (both AstDyS-2 and ). 9923 Ronaldthiel is a core member of the Agnia family at AstDyS-2.

Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 16 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 481 members. Largest asteroids are members of the Erigone family according to Nesvorný (; ).

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 169 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 78 members.

Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.

Jupiter trojan family according Milani (1993). Part of the Menelaus clan according to Roig and Gil-Hutton (2008).

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 344 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 186 members.

Previously known as the "1982 QG" family. Second member: ; both are background asteroids according to AstDyS-2 and .

Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 379 members.

Claimed subgroup of the Flora family. Background asteroid according to both AstDyS-2 and .

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 24 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 57 members. Nesvorný moved family to candidate status.

Jupiter trojan family according to Roig and Gil-Hutton (2008). Part of the Menelaus clan.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 56 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 13 members.

MBA-family (AstDys) according to Milani and Knežević (2014). Total of 23 members.

Legend: These are families listed as "robustly" identified in Bendjoya and Zappala (2002). TNOs are not considered asteroids, but are included here for completeness. Candidate families (Nesvorný): 929 Algunde, 1296 Andrée, 1646 Rosseland, 1942 Jablunka, 2007 McCuskey, 2409 Chapman, 4689 Donn, 6246 Komurotoru. , 539 Pamina, , 3567 Alvema, 260 Huberta, 928 Hildrun, 2621 Goto, 1113 Katja, 8737 Takehiro, 46 Hestia, 5 Astraea, 1044 Teutonia, 3110 Wagman, 4945 Ikenozenni, , 8905 Bankakuko, , .

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See also

• Proper orbital elements
• Asteroid groups and families

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Notes

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Further reading

• Bendjoya, Philippe; and Zappalà, Vincenzo; "Asteroid Family Identification", in Asteroids III, pp. 613–618, University of Arizona Press (2002),
• V. Zappalà et al. "Physical and Dynamical Properties of Asteroid Families", in Asteroids III, pp. 619–631, University of Arizona Press (2002),
• A. Cellino et al. "Spectroscopic Properties of Asteroid Families", in Asteroids III, pp. 633–643, University of Arizona Press (2002),
• Hirayama, Kiyotsugu; "Groups of asteroids probably of common origin", Astronomical Journal, Vol. 31, No. 743, pp. 185-188 (October 1918).
• Nesvorný, David; Bottke Jr., William F.; Dones, Luke; and Levison, Harold F.; "The recent breakup of an asteroid in the main-belt region", Nature, Vol. 417, pp. 720-722 (June 2002).
• Zappalà, Vincenzo; Cellino, Alberto; Farinella, Paolo; and Knežević, Zoran; "Asteroid families I - Identification by hierarchical clustering and reliability assessment", Astronomical Journal, Vol. 100, p. 2030 (December 1990).
• Zappalà, Vincenzo; Cellino, Alberto; Farinella, Paolo; and Milani, Andrea; "Asteroid families II - Extension to unnumbered multiopposition asteroids", Astronomical Journal, Vol. 107, pp. 772-801 (February 1994)
• M. S. Kelley & M. J. Gaffey 9 Metis and 113 Amalthea: A Genetic Asteroid Pair, Icarus Vol. 144, p. 27 (2000).

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External links

• Planetary Data System - Asteroid Families dataset, as per the Zappalà 1995 analysis.
• Latest calculations of proper elements for numbered minor planets at astDys.
• Asteroid (and Comet) Groups by Petr Scheirich (with excellent plots).
• Asteroid Families Portal

Categories: Families

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