Asteroid family

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Plot of proper inclination vs. semi-major axis for numbered asteroids. Asteroid families are visible as distinct clumps. Prominent Kirkwood gaps divide the core region. (A, B+C, D, E+F+G)

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 and interlopers

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 background distribution of generic asteroids. These concentrations are the asteroid families. 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 object that once fragmented upon a collisional impact.


Asteroid orbital elements: standard Keplerian 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 times of at least tens of millions of years, and perhaps longer.

The Japanese 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.

Present day computer-assisted searches have identified several tens of 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 the Wavelet Analysis Method (WAM) 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). 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.


Most prominent families

Mean of the "core" members found in HCM and WAM analyses by Zappala et al. (1995), rounded to 2 significant digits. That analysis considered 12,487 asteroids, but currently almost half a million are known (see § All families for a 2015 census). Hence, the number of currently catalogued asteroids that are members of a given family is likely to be greater than the value in this column by a similar factor of roughly 40.

Family Name Named After orbital elements Size Alternate Names
a (AU) e i (°) approx. % of asteroids Members in Zappalà
HCM analysis
Eos (Eoan) 221 Eos 2.99 to 3.03 0.01 to 0.13 8 to 12 480
Eunomia(n) 15 Eunomia 2.53 to 2.72 0.08 to 0.22 11.1 to 15.8 5% 370
Flora (Florian) 8 Flora 2.15 to 2.35 0.03 to 0.23 1.5 to 8.0 4–5% 590 Ariadne(an) family after 43 Ariadne
Hygiea(n) 10 Hygiea 3.06 to 3.24 0.09 to 0.19 3.5 to 6.8 1% 105
Koronis (Koronian) 158 Koronis 2.83 to 2.91 0 to 0.11 0 to 3.5 310
Maria(n) 170 Maria 2.5 to 2.706 12 to 17 80
Nysa (Nysian) 44 Nysa 2.41 to 2.5 0.12 to 0.21 1.5 to 4.3 380 Hertha (Herthian) family after 135 Hertha
Themis (Themistian) 24 Themis 3.08 to 3.24 0.09 to 0.22 0 to 3 530
Vesta (Vestian) 4 Vesta 2.26 to 2.48 0.03 to 0.16 5.0 to 8.3 6% 240

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 (FIN).

FIN Family Lbl # of Members Loc. Taxonomy Parent LoMP
001 Hilda family HIL 409 rim C 153 Hilda; adj. Hildian; a-e-i: (3.7 to 4.2; > 0.07; < 20°) list
002 Schubart family SHU 352 rim C 1911 Schubart (Hildian) list
003 Hungaria family H 2965 rim E 434 Hungaria list
004 Hektor family HEK 12 rim 624 Hektor (Jupiter trojan) list
005 Eurybates family ERY 218 rim CP 3548 Eurybates (Jupiter trojan) list
006 RJ family 006 7 rim (9799) 1996 RJ (Jupiter trojan) list
007 James Bond family 007 1 rim ASP 9007 James Bond list
008 Arkesilaos family ARK 37 rim 20961 Arkesilaos list
009 Ennomos family ENM 30 rim 4709 Ennomos list
010 UV209 family 010 13 rim (247341) 2001 UV209 list
401 Vesta family V 15252 inner V 4 Vesta list
402 Flora family FLO 13786 inner S 8 Flora list
403 Baptistina family BAP 2500 inner X 298 Baptistina list
404 Massalia family MAS 6424 inner S 20 Massalia, adj. Massalian, a-e-i: (2.37 to 2.45; 0.12 to 0.21; 0.4 to 2.4) list
405 Nysa family (Polana) NYS 19073 inner SFC 44 Nysa/142 Polana list/(b)
406 Erigone family ERI 1776 inner CX 163 Erigone, adj. Erigonian list
407 Clarissa family CLA 179 inner X 302 Clarissa list
408 Sulamitis family SUL 303 inner C 752 Sulamitis list
409 Lucienne family LCI 142 inner S 1892 Lucienne list
410 Euterpe family EUT 474 inner S 27 Euterpe list
411 Datura family DAT 6 inner S 1270 Datura list
412 Lucascavin family LCA 3 inner S 21509 Lucascavin list
413 Klio family KLI 330 inner C 84 Klio list
414 Chimaera family CIM 108 inner CX 623 Chimaera list
415 Chaldaea family CHL 132 inner C 313 Chaldaea list
416 Svea family SVE 48 inner CX 329 Svea list
417 GB11 family 417 9 inner (108138) 2001 GB11 list
701 Phocaea family PHO 1989 inner S 25 Phocaea list
501 Juno family JUN 1684 middle S 3 Juno list
502 Eunomia family EUN 5670 middle S 15 Eunomia list
504 Nemesis family NEM 1302 middle C 128 Nemesis, adj. Nemesian; Concordia(n) family after 58 Concordia) list
505 Adeona family ADE 2236 middle C 145 Adeona list
506 Maria family MAR 2940 middle S 170 Maria list
507 Padua family PAD 1087 middle X 363 Padua list
508 Aeolia family AEO 296 middle X 396 Aeolia list
509 Chloris family CLO 424 middle C 410 Chloris, adj. Chloridian list
510 Misa family MIS 702 middle C 569 Misa, adj. Misian list
511 Brangane family BRG 195 middle S 606 Brangane list
512 Dora family DOR 1259 middle C 668 Dora, adj. Dorian list
513 Merxia family MRX 1215 middle S 808 Merxia, adj. Merxian list
514 Agnia family AGN 2125 middle S 847 Agnia list
515 Astrid family AST 489 middle C 1128 Astrid, adj. Astridian list
516 Gefion family GEF 2547 middle S 1272 Gefion, adj. Gefionian; a-e-i: (2.74 to 2.82; 0.08 to 0.18; 7.4 to 10.5); alt: Ceres (Cererian) family after 1 Ceres and Minerva (Minervian) family after 93 Minerva list
517 König family KON 354 middle CX 3815 König list
518 Rafita family RAF 1295 middle S 1644 Rafita, adj. Rafitian list
519 Hoffmeister family HOF 1819 middle CF 1726 Hoffmeister list
520 Iannini family IAN 150 middle S 4652 Iannini list
521 Kazuya family KAZ 44 middle S 7353 Kazuya list
522 Ino family INO 463 middle S 173 Ino list
523 Emilkowalski family EMI 4 middle S 14627 Emilkowalski list
524 YC2 family 524 3 middle S (16598) 1992 YC2 list
525 Schulhof family SHF 6 middle S 2384 Schulhof list
526 BY6 family 526 58 middle C (53546) 2000 BY6 list
527 Lorre family LOR 2 middle C 5438 Lorre list
528 Leonidas family LEO 135 middle CX 2782 Leonidas list
529 Vibilia family VIB 180 middle C 144 Vibilia list
530 Phaeo family PAE 146 middle X 322 Phaeo list
531 Mitidika family MIT 653 middle C 2262 Mitidika list
532 Henan family HEN 1872 middle L 2085 Henan list
533 Hanna family HNA 280 middle CX 1668 Hanna list
534 Karma family KRM 124 middle CX 3811 Karma list
535 Witt family WIT 1618 middle S 2732 Witt list
536 Xizang family XIZ 275 middle 2344 Xizang list
537 Watsonia family WAT 99 middle L 729 Watsonia list
538 Jones family (asteroids) JNS 22 middle T 3152 Jones list
539 Aeria family AER 272 middle X 369 Aeria list
540 Julia family (asteroids) JUL 33 middle S 89 Julia list
541 Postrema family POS 108 middle CX 1484 Postrema list
801 Pallas family PAL 128 middle B 2 Pallas list
802 Gallia family GAL 182 middle S 148 Gallia list
803 Hansa family HNS 1094 middle S 480 Hansa adj. Hansian; a-e-i: (~2.66; ~0.06; ~22.0°)[5] list
804 Gersuind family GER 415 middle S 686 Gersuind list
805 Barcelona family BAR 306 middle S 945 Barcelona list
806 Tina family TIN 96 middle X 1222 Tina list
807 Brucato family BRU 342 middle CX 4203 Brucato list
601 Hygiea family HYG 4854 outer CB 10 Hygiea list
602 Themis family THM 4782 outer C 24 Themis list
603 Sylvia family SYL 255 outer X 87 Sylvia list
604 Meliboea family MEL 444 outer C 137 Meliboea, adj. Meliboean list
605 Koronis family KOR 5949 outer S 158 Koronis list
606 Eos family EOS 9789 outer K 221 Eos list
607 Emma family EMA 76 outer C 283 Emma list
608 Brasilia family BRA 579 outer X 293 Brasilia, adj. Brasilian list
609 Veritas family VER 1294 outer CPD 490 Veritas, adj. Veritasian; alt: Undina (Undinian) family after 92 Undina list
610 Karin family KAR 541 outer S 832 Karin list
611 Naëma family NAE 301 outer C 845 Naëma, adj. Naëmian list
612 Tirela family TIR 1395 outer S 1400 Tirela (Klumpkea) list
613 Lixiaohua family LIX 756 outer CX 3556 Lixiaohua list
614 Telramund family TEL 468 outer S 9506 Telramund list
615 FY12 family 615 104 outer CX (18405) 1993 FY12 list
616 Charis family CHA 808 outer C 627 Charis list
617 Theobalda family THB 376 outer CX 778 Theobalda, adj. Theobaldian; a-e-i: (3.16 to 3.19; 0.24 to 0.27; 14 to 15) list
618 Terentia family TRE 79 outer C 1189 Terentia list
619 Lau family LAU 56 outer S 10811 Lau list
620 Beagle family BGL 148 outer C 656 Beagle list
621 Koronis family (II) K-2 246 outer S 158 Koronis "second family" list
622 Terpsichore family TRP 138 outer C 81 Terpsichore list
623 Fringilla family FIR 134 outer X 709 Fringilla list
624 Durisen family DUR 27 outer X 5567 Durisen list
625 Yakovlev family YAK 67 outer C 5614 Yakovlev list
626 San Marcello family SAN 144 outer X 7481 San Marcello list
627 YB3 family 627 38 outer CX (15454) 1998 YB3 list
628 CG1 family 628 248 outer S (15477) 1999 CG1 list
629 XT17 family 629 58 outer S (36256) 1999 XT17 list
630 Aegle family AEG 99 outer CX 96 Aegle list
631 Ursula family URS 1466 outer CX 375 Ursula list
632 Elfriede family ELF 63 outer C 618 Elfriede list
633 Itha family ITH 54 outer S 918 Itha list
634 Inarradas family INA 38 outer CX 3438 Inarradas list
635 Anfimov family ANF 58 outer S 7468 Anfimov list
636 Marconia family MRC 34 outer CX 1332 Marconia list
637 UJ87 family 637 64 outer CX (106302) 2000 UJ87 list
638 Croatia family CRO 93 outer X 589 Croatia list
639 Imhilde family IMH 43 outer CX 926 Imhilde list
640 Gibbs family GBS 8 outer 331P/Gibbs "P/2012 F5 (Gibbs)"
641 Juliana family JLI 76 outer CX 816 Juliana list
901 Euphrosyne family EUP 2035 outer C 31 Euphrosyne list
902 Alauda family ALA 1294 outer B 702 Alauda list
903 Ulla family ULA 26 outer X 909 Ulla list
904 Luthera family LUT 163 outer X 1303 Luthera list
905 Armenia family ARM 40 outer C 780 Armenia list

Other families

Other divergent and non main-belt families[C]

Family Name Named After orbital elements Size Alternate Names
a (AU) e i (°) approx. % of asteroids Members in Zappalà
HCM analysis
Bower(ian) 1639 Bower 13 Endymion (Endymionian) family after 342 Endymion
Lydia(n) 110 Lydia 38
TNO families:[D]
Haumea(n) 136108 Haumea ~43 ~0.19 ~28
  • C These are families listed as "robustly" identified in Bendjoya and Zappala (2002).
  • D TNOs are not considered asteroids, but are included here for completeness.

See also Category:Asteroid groups and families, which names some less prominent and uncertain groupings.

See also


  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" (PDF). Asteroids IV: 297–321. arXiv:1502.01628Freely accessible. doi:10.2458/azu_uapress_9780816532131-ch016. Retrieved 23 June 2017. 
  4. ^ "Small Bodies Data Ferret". Nesvorny HCM Asteroid Families V3.0. Retrieved 22 July 2017. 
  5. ^ The Hansa Family: A New High-Inclination Asteroid Family

Further reading

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