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Asteroids with Satellites  by Wm. Robert Johnston last updated 16 November 2025

Contents:

• List of binary asteroids/TNOs
  --with links to separate pages of data and links on specific binary asteroids/TNOs
• Tables of data on binary asteroids and other reports of asteroid/TNO companions (separate pages)
• General discussion
• Images and figures regarding binary asteroids/TNOs (separate pages)
• Links--lists, databases, articles, etc.
  --including link to separate page listing abstracts at ADS

Current Count: 599 asteroids and TNOs with companions: 580 binaries, 17 triple systems, 1 quadruple system (130 Elektra), 1 sextuple system (Pluto); 622 companions total:

• 109 near-Earth asteroids (5 with two satellites),
• 36 Mars crossing asteroids (1 with two satellites),
• 303 main-belt asteroids (8 with two satellites, 1 with three satellites; 1 binary with dual comet designation),
• 8 Jupiter Trojan asteroids, and
• 143 trans-Neptunian objects (3 with two satellites, 1 with five satellites).

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

Orbital elements for asteroids fromJPL Small-Body Database supplemented by the Minor Planet Center. Physical data supplemented from theLowell Asteroid Information database. Links to abstracts generated from NASA'sADS Project at Harvard-CfA/SAO.

Tables, other lists:

Asteroids/TNOs with satellites: summary data table

Asteroids/TNOs with satellites: designation data

Citations for asteroid and TNO satellite designations

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:

• TNOs and centaurs with rings (5 TNOs/centaurs)

• Contact binary asteroids, TNOs, and comets (112 asteroids, 1 asteroid satellite, 4 planetary satellites, 25 TNOs, and 8 comets)

• Doublet craters

• Asteroid pairs

Discussion:

Counts: The above listing includes 599 objects with companions: 580 binaries, 17 triple systems, 1 quadruple system, and the sextuple system of Pluto, for 622 companions total. These systems include the following:

• 109 near-Earth asteroids (5 with two satellites),
• 36 Mars crossing asteroids (1 with two satellites),
• 303 main-belt asteroids (8 with two satellites, 1 with three satellites; 1 binary with dual comet designation),
• 8 Jupiter Trojan asteroids, and
• 143 trans-Neptunian objects (3 with two satellites, 1 with five satellites).

For about two-thirds of the binaries listed above, the detections have been announced by the Minor Planet Center. Only a minority (~10%) 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.

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

Manyadditional reports of asteroid companions are listed here: 915 possible binaries reported (plus 25 reports since refuted). These 915 possible binaries include 120 NEAs, 21 Mars crossers, 638 main belt asteroids, 78 Jupiter Trojans, 1 unusual asteroid, 55 TNOs/Centaurs, and 2 comets. In addition 19 objects with confirmed satellites have suggested third components.

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)

Detections by space-based imaging are mostly detections from the Hubble Space Telescope along with four detections from space probes (Galileo, Lucy, and New Horizons).

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. 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 onthis page.

Near-Earth objects: The near-Earth multiple asteroids include 9 Atens, 66 Apollos, 33 Amors, and 1 Atira. These include five triple systems: Amor asteroids (3122) Florence, (153591) 2001 SN263, (348400) 2005 JF21, Apollo asteroid (136617) 1994 CC, and Aten asteroid (363027) 1998 ST27. 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 33 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 140 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. 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-sizedby Texas standards) are now known to have moons.

Objects with rings: In 2014 discovery of two narrow rings around the centaur (10199) Chariklo was announced; the nature of these rings would be consistent with one or more embedded moonlets and/or shepherd satellites. (136108) Haumea in 2017 found to have a ring system in addition to its two satellites. In 2023 a ring was announced around (50000) Quaoar. Ring material around another centaur, (2060) Chiron, was reported in 2015 though the structure of this material is unclear.

Images and figures regarding binary asteroids/TNOs:

• Scale representations of Earth-approaching binary asteroids
• Scale representations of main-belt and outer Solar System binary asteroids
• Graph of ratio of component sizes vs. separation (click on small graph below for full size version)
  
• Graph of ratio of component sizes vs. primary size (click on small graph below for full size version)
  

Online lists/databases of binary asteroids/TNOs:

• Small-Body Satellites by JPL (June 2025).
• Asteroid Lightcurve Database by B. Warner et al. (October 2023).
• Orbit Status of Known Binary TNOs by W. M. Grundy et al. (May 2022).
• Binary and Ternary Near-Earth Asteroids Detected by Radar by L. Benner (September 2021).
• Asteroid radar detections by UCLA (June 2021).
• Binary Minor Planets Compilation V3.0 (PDS4) by W. R. Johnston, NASA PDS database (March 2019).
• Binary asteroid parameters by P. Pravec and A. W. Harris (January 2019).
• Ephemerides of the satellites of asteroids (2019).
• List of minor planet companions by IAU (September 2010).
• TNOs and Centaurs with Satellites by J. Parker (2009).
• Summary of the properties of binary Near-Earth asteroids by P. Pravec and P. Scheirich, NASA PDS database (June 2005).
• Binary Asteroid Parameters by Institute of Applied Astronomy of RAS (offline).
• Virtual Observatory for Binary Asteroid Database by F. Marchis et al. (offline).
• List of double Minor Planets by D. Denissenko (offline).

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

• K. S. Noll et al., 2023,Trojan asteroid satellites, rings, and activity,Space Science Reviews, 219(7):59.
• W. M. Grundy et al., 2019,Mutual orbit orientations of transneptunian binaries,Icarus, 334:62-78.
• P. Pravec et al., 2016,Binary asteroid population. 3. Secondary rotations and elongations,Icarus, 267:267-295.
• J.-L. Margot et al., 2015,Asteroid systems: Binaries, triples, and pairs, inAsteroids IV, ed. by P. Michel et al., Univ. of Arizona Press (Tucson, AZ).
• K. J. Walsh and S. A. Jacobson, 2015,Formation and evolution of binary asteroids, inAsteroids IV, ed. by P. Michel et al., Univ. of Arizona Press (Tucson, AZ).
• P. Pravec et al., 2012,Binary asteroid population. 2. Anisotropic distribution of orbit poles of small, inner main-belt binaries,Icarus, 218:125-143.
• P. A. Taylor and J.-L. Margot, 2011,Binary asteroid systems: Tidal end states and estimates of material properties,Icarus, 212:661-676.
• K. Noll et al., 2008,Binaries in the Kuiper Belt, inThe Solar System Beyond Neptune, ed. by M. A. Barucci et al., Univ. of Arizona Press (Tucson, AZ), 345-363.
• P. Descamps and F. Marchis, 2008,Angular momentum of binary asteroids: Implications for their possible origin,Icarus, 193:74-84.
• P. Pravec and A. W. Harris, 2007,Binary asteroid population: 1. Angular momentum content,Icarus, 190:250-259.
• D. C. Richardson and K. J. Walsh, 2006,Binary minor planets,Annu. Rev. Earth Planet. Sci., 34:47-81.
• K. S. Noll, 2006,Solar system binaries, inAsteroids, Comets, Meteors, Proceedings of the 229th Symposium of the International Astronomical Union, ed. by L. Daniela et al., Cambridge Univ. Press (Cambridge, UK), 301-318.
• P. Pravec et al., 2006,Photometric survey of binary near-Earth asteroids,Icarus, 181:63-93.
• W. J. Merline et al., 2002,Asteroids do have satellites, inAsteroids III, ed. by W. F. Bottke Jr. et al., Univ. of Arizona Press (Tucson, AZ).

General links on binary asteroids/TNOs:

• International Astronomical Union: Astronomical Headlines.
  • Recent IAUCs (some require subscription)
  • Recent CBETs (some require subscription)
• Minor Planet Center.
  • Unusual Minor Planets.
• Ondrejov NEO Photometric Program.
  • Photometric Survey for Asynchronous Binary Asteroids.
  • Ondrejov NEO Photometric Program - News.
• JPL Asteroid Radar Research.
  • Asteroid Radar History by L. A. M. Benner.
• Minor Planet Bulletin.
• Distant EKOs: The Kuiper Belt Electronic Newsletter by J. Parker.
• Courbes de rotation d'asteroides et de cometes, CdR (in French).
• Dr. William J. Merline (press releases) at SWRI.
• Satellites of Asteroids by P. Maley.
• Observed Minor Planet Occultation Events by D. Dunham (2005).

General links on asteroids/TNOs:

• Lowell Asteroid Information database
• JPL SSD Small-Body Database Browser.
• Small Bodies Node of the NASA Planetary Data System.
• NEODyS-2--Near Earth Objects--Dynamic Site.
• AstoDyS-2--Asteroids--Dynamic Site.
• Near-Earth Objects--ESA.

List of abstracts on binary asteroids/TNOs (fromADS.)

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

Comments? Questions? Corrections?Contact me.

© 2001-2024, 2025 by Wm. Robert Johnston.
Last modified 16 November 2025.
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