Asteroids with Satellites 

 by Wm. Robert Johnston
last updated 27 May 2006

Contents:

List of confirmed and suspected 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):

by dynamical class:

near Earth objects:

Mars crossers:

main belt members:

Jupiter Trojans:

trans-Neptunian objects:

by designation:

Link to list by date of announcement of detection

Data sources for individual asteroid with satellites 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.)

Asteroids/TNOs with satellites: summary data table

Other reports of asteroid/TNO companions

Combined chronological listing of reported asteroid/TNO companions

Doublet craters in the inner solar system

Discussion:

Counts: The above listing includes 93 minor planets with companions: 91 binaries and 2 triple systems, for 96 companions total. 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. Below is an attempt to draw some distinctions in this regard, but please note: this listing is still under revision; I do not claim to be an authoritative judge in this area, and corrections are welcome.

The above list is divided here by type, with links to individual pages.

History: The first observations purporting to reveal asteroid satellites were lightcurve measurements during stellar occultations by asteroids, such as those of (6) Hebe in 1977 and (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. Later in 2005, 2003 EL61 was identified as the first trans-Neptunian object with multiple satellites.

Near-Earth objects: The near-Earth binary asteroids include 6 Atens, 13 Apollos, and 7 Amors. In addition, 5 other Mars crossers are known binaries. These near-Earth asteroids are all close binaries. It appears likely that these binaries are "rubble piles" which have either been spun up by collisions sufficiently to fission into two bodies or have been tidally split by a close approach to a planet. Many other near-Earth asteroids have high rotation rates--close to the limit where fission will occur--and a few 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 implies that binary near-Earth asteroids may be 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.

Trojan asteroids: The one known double Trojan asteroid, (617) Patroclus, has components which are very similar in size. Another Trojan asteroid, (624) Hektor, shows light curve variations which may indicate that it is a contact binary.

Trans-Neptunian objects: With 23 binary TNOs and 1 triple TNO system among a total of 1,031 known TNOs, such binaries seem to be relatively common. The first companion discovered was that of 1998 WW31, found in December 2000. Most (14 of 23) known TNO binaries are "Cubewanos" (or possible Cubewanos), orbiting in relatively low eccentricity orbits beyond Pluto. Additionally, many of the discovered binaries are more widely separated than the Pluto-Charon system. An exception is 2001 QG298, currently described as a contact binary. One binary TNO is a "Plutino", orbiting near the 3:2 orbital resonance with Neptune, and one is a scattered disk object, ranging up to 101 AU from the Sun. 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 the count above does not include the Pluto-Charon system, but does include 2003 UB313 and its satellite discovered in 2005--2003 UB313 might be considered a planet as well, being larger than Pluto.

Images and figures regarding binary asteroids/TNOs:

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).

© 2001-2005, 2006 by Wm. Robert Johnston.
Last modified 27 May 2006.
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