Asteroids with Satellites 

by Wm. Robert Johnston
last updated 28 November 2023


Current Count: 508 asteroids and TNOs with companions: 492 binaries, 14 triple systems, 1 quadruple system (130 Elektra), 1 sextuple system (Pluto); 528 companions total:

List of binary asteroids/TNOs--by class or by designation, with orbital class of object in parenthesis
  (Follow links to pages with data and links on specific objects, including links to abstracts at ADS):

Link to list by date of announcement of detection

by dynamical class:

near Earth objects:

Mars crossers:

main belt members:

Jupiter Trojans:

trans-Neptunian objects:

by designation:

Data sources for individual object pages
Explanation of quantities

(Orbital elements for asteroids from JPL Small-Body Database supplemented by the Minor Planet Center. Links to abstracts generated from NASA's ADS Project at Harvard-CfA/SAO.)

Tables, other lists:

Asteroids/TNOs with satellites: summary data table

Asteroids/TNOs with satellites: designation data

Other reports of asteroid/TNO companions

Other reports of asteroid/TNO companions: additional data

Combined chronological listing of reported asteroid/TNO companions

Other data sets related to binary asteroid origins and fates:


Counts: The above listing includes 508 objects with companions: 492 binaries, 14 triple systems, 1 quadruple system, and the sextuple system of Pluto, for 528 companions total. It also includes (10199) Chariklo and its ring system. These systems include the following:

For about two-thirds of the binaries listed above, the detections have been announced by the Minor Planet Center. Only a minority of these have been assigned provisional designations, but in any case this distinction does not reliably separate candidates of higher confidence from those of lower confidence. I have attempted to draw some distinctions regarding level of confidence as summarized in the table below, followed by a link to a page with a detailed listing. Please note, however, that these classifications are subjective in some cases--corrections are welcome.

Asteroids/TNOs with companions by type and level of confirmation
number of systems, with total number of companions in parenthesis

near-Earth asteroids 0 (0) 8 (10) 58 (59) 31 (31) 97 (100)
Mars crossers 0 (0) 0 (0) 6 (7) 26 (26) 32 (33)
main belt asteroids 6 (9) 8 (9) 57 (60)177 (180)248 (258)
Jupiter Trojans 3 (3) 0 (0) 2 (2) 2 (2) 7 (7)
trans-Neptunian objects18 (24)34 (34) 19 (19) 53 (53)124 (130)
total 27 (36)50 (53)142 (147)289 (292)508 (528)

Detailed listing by type and level of confirmation, with links to individual pages.

Many additional reports of asteroid companions are listed here: 451 possible binaries reported (plus 21 reports since refuted). These 451 possible binaries include 103 NEAs, 15 Mars crossers, 243 main belt asteroids, 71 Jupiter Trojans, 18 TNOs/Centaurs/other outer solar system objects, and 1 comet. In addition there are possible third components for (1830) Pogson, (2006) Polonskaya, (2343) Siding Spring, (4435) Holt, (8306) Shoko, (10123) Fideoja, (16635) 1993 QO, (17700) Oleksiygolubov, (276049) 2002 CE26, (300163) 2006 VW139, and (348400) 2005 JF21, plus a possible ring system for (2060) Chiron.

Discovery methods: The following table gives counts by type and by method of detection. (Number of systems given, with total number of companions in parenthesis)

near-Earth asteroids 0 (0) 0 (0)54 (57) 43 (43)0 (0) 97 (100)
Mars crossers 0 (1) 0 (0) 0 (0) 32 (32)0 (0) 32 (33)
main belt asteroids 18 (25) 6 (6) 0 (0)221 (224)3 (3)248 (258)
Jupiter Trojans 2 (2) 2 (2) 0 (0) 2 (2)1 (1) 7 (7)
trans-Neptunian objects28 (29) 94 (99) 0 (0) 1 (1)1 (1)124 (130)
total 48 (57)102 (107)54 (57)299 (302)5 (5)508 (528)

Detections by space-based imaging are detections with the Hubble Space Telescope along with a single detection from a space probe (the Galileo probe's detection of Ida's satellite).

History: The first reports of observations suggesting asteroid satellites were obtained during stellar occultations, such as visual observations of (6) Hebe in 1977 and photometric lightcurve observations of (532) Herculina in 1978. These and similar reports over following years were eventually discounted for lack of confirmation. The first confirmed asteroid satellite discovery was made by Galileo during its flyby of (243) Ida in 1993. Several others have been discovered using direct imagery by the Hubble Space Telescope and ground-based adaptive optics telescopes. Lightcurve measurements have been analyzed to indicate the signature of eclipsing binaries for several near-Earth asteroids. Some of these have been confirmed by radar observations. Radar observations have also independently revealed some companions. The first minor planet with multiple satellites, (87) Sylvia, was identified in 2005. The first main belt asteroid showing cometary activity found to be binary, 288P/(300163) 2006 VW139, was confirmed in 2017. Regarding outer solar system objects, Pluto's first satellite was discovered in 1978, long before discovery of other trans-Neptunian objects. Other binary/multiple TNOs have been discovered since 2000.

Taxonomy: The sample of known binary/multiple asteroids and TNOs is large enough to show distinct classes of systems. This is discussed on this page.

Near-Earth objects: The near-Earth multiple asteroids include 8 Atens, 58 Apollos, 30 Amors, and 1 Atira. These include three triple systems: Amor asteroids (3122) Florence and (153591) 2001 SN263 and Apollo asteroid (136617) 1994 CC. The near-Earth systems are all close systems. It appears likely that most of these binaries are "rubble piles" which have been spun up by the YORP effect to the point where mass shedding leads to formation of a satellite. (The Yarkovsky-O'Keefe-Radzievskii-Paddack effect, or YORP effect, results from momentum and torque imparted by thermal radiation emitted from the asteroid.) Many other near-Earth asteroids have high rotation rates--close to the limit where fission will occur--and at least 29 other such objects are double-lobed, like contact binaries. Additionally, the inner planets and the Moon show a number of double craters, apparently formed by the near-simultaneous impact of two objects. Together, the evidence indicates that binary near-Earth asteroids are common.

Mars crossing asteroids: There are 30 non-NEA Mars crossing asteroids and a Mars trojan that are known binaries, plus one Mars crossing triple system. Whereas all near-Earth multiple asteroids are close binaries, there are a few widely separated binaries among the Mars crossing asteroids.

Main belt objects: The companions of main-belt asteroids discovered to date show a wider variety both in terms of separations between components and in terms of relative sizes. Some may be collision fragments that mutually captured each other. The only system yet examined close up is (243) Ida and Dactyl; Galileo imagery shows a very irregular shape for Ida, implying that it is not a simple rubble pile. In 2005, (87) Sylvia was announced to have a second satellite, making it the first known triple component asteroid system. Such systems were predicted as an outcome of collisional disruption. Since collisions play such a large role in the history of main-belt asteroids, fragments as satellites have long been expected by astronomers. Of main belt binaries, 20 are Hungaria asteroids. Probably many of the smaller binaries are YORP effect spun-up rubble piles. A number of small binaries are associated with separated objects in very similar orbits.

Trojan asteroids: The first confirmed double Trojan asteroid, (617) Patroclus, has components which are very similar in size. The second identified binary, (624) Hektor, has a primary which is a contact binary accompanied by a smaller secondary. The unconfirmed binary (17365) Thymbraeus is recently suggested to be rather a contact binary.

Trans-Neptunian objects (including Centaurs): With 121 binary TNOs, 2 triple TNO systems, and the sextuple system of Pluto, among a total of 4,377 known TNOs/Centaurs, such binaries are relatively common. The first companion discovered was that of 1998 WW31, found in December 2000. Most (80) of the known TNO systems are "Cubewanos", orbiting in relatively low eccentricity orbits beyond Pluto. Two are Centaurs: (42355) Typhon and (65489) Ceto. Many of the discovered binaries are more widely separated than the Pluto-Charon system. Exceptions include (139775) 2001 QG298 and the central components of (47171) Lempo, which are near-contact binaries. It has been suggested that the extreme variations in the lightcurve of some TNOs could be explained by the presence of a close companion. In 2014 discovery of two narrow rings around (10199) Chariklo was announced; the nature of these rings would be consistent with one or more embedded moonlets and/or shepherd satellites. Haumea was later found to have a ring system in addition to its two satellites. Note that Pluto, (136108) Haumea, (136199) Eris, and (136472) Makemake are recognized as dwarf planets; Pluto formerly counted as a planet (and still does for some of us). The Pluto and Haumea systems are believed to be products of collisional disruption of their parent body. Along with (225088) Gonggong, all the dwarf planet-sized objects (or alternately, all TNOs that are planet-sized by Texas standards) are now known to have moons.

Images and figures regarding binary asteroids/TNOs:

Online lists/databases of binary asteroids/TNOs:

Overview articles on binary asteroids/TNOs: (this is not an exhaustive list by any means)

General links on binary asteroids/TNOs:

General articles on binary asteroids/TNOs (non-technical, mostly older):

General links on asteroids/TNOs:

List of abstracts on binary asteroids/TNOs (from ADS.)

Banner image: Ida and Dactyl (right) in an enhanced color image from Galileo taken 28 August 1993 (credit: NASA).

Comments? Questions? Corrections? Contact me.

© 2001-2022, 2023 by Wm. Robert Johnston.
Last modified 28 November 2023.
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