Asteroids with Satellites 

by Wm. Robert Johnston
last updated 26 March 2014


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

(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

Combined chronological listing of reported asteroid/TNO companions

Other data sets related to binary asteroid origins and fates:


Counts: The above listing includes 243 objects with companions: 232 binaries, 10 triple systems, and the sextuple system of Pluto, for 257 companions total. It also includes (10199) Chariklo and its ring system. These systems include the following:

For nearly all of the binaries listed above, the detections have been announced by the Minor Planet Center. While some have been assigned provisional designations and some have not, this distinction does not always 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) 33 (33) 7 (7) 48 (50)
Mars crossers 0 (0) 0 (0) 5 (6)14 (14) 19 (20)
main belt asteroids 5 (7) 9 (10) 41 (43)38 (38) 93 (98)
Jupiter Trojans 1 (1) 0 (0) 1 (1) 2 (2) 4 (4)
trans-Neptunian objects12 (17)24 (24) 20 (20)23 (24) 79 (85)
total 18 (25)41 (44)100 (103)84 (85)243 (257)

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

Many additional reports of asteroid companions are listed here: 111 possible binaries reported (plus 16 reports since refuted). These 111 possible binaries include 16 NEAs, 6 Mars crossers, 81 main belt asteroids, 1 Jupiter Trojan, and 7 TNOs/Centaurs/other outer solar system objects.

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)33 (35) 15 (15) 48 (50)
Mars crossers 0 (1) 0 (0) 0 (0) 19 (19) 19 (20)
main belt asteroids 17 (21) 4 (4) 0 (0) 72 (73) 93 (98)
Jupiter Trojans 2 (2) 0 (0) 0 (0) 2 (2) 4 (4)
trans-Neptunian objects14 (15)64 (69) 0 (0) 1 (1) 79 (85)
total 33 (39)68 (73)33 (35)109 (110)243 (257)

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. 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.

Near-Earth objects: The near-Earth binary asteroids include 7 Atens, 28 Apollos, and 11 Amors, plus the triple Amor asteroid (153591) 2001 SN263 and the triple Apollo asteroid (136617) 1994 CC. In addition, 19 other Mars crossers are known binaries. These near-Earth asteroids are all close binaries (not all of the Mars crossers are however). It appears likely that these binaries are "rubble piles" which have been spun up by the YORP effect. (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.

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. While some may prove to be fissioned rubble piles, it is expected that most will be collision fragments having 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, 17 are Hungaria asteroids.

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.

Trans-Neptunian objects: With 76 binary TNOs, 2 triple TNO systems, and the sextuple system of Pluto, among a total of 1,657 known TNOs and Centaurs, such binaries are relatively common. The first companion discovered was that of 1998 WW31, found in December 2000. Most (45 of 79) 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) 1999 TC36, 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 sheperd satellites. Note that Pluto, (136108) Haumea, and (136199) Eris are recognized as dwarf planets; Pluto formerly counted as a planet (and still does for some of us).

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.

© 2001-2013, 2014 by Wm. Robert Johnston.
Last modified 26 March 2014.
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