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An asteroid moon is an asteroid orbiting an asteroid.


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Asteroid moon is an asteroid that orbits another asteroid as its natural satellite. Many asteroids may possess moons, in some cases quite substantial in size. Discoveries of asteroid moons (and binary objects, in general) are very important because the determination of their orbits provides estimates (or least constraints) on their density and mass allowing an insight into their physical properties, impossible otherwise.

Terminology of asteroid moon.

asteroid moon.
243 Ida and its asteroid moon Dactyl.

In addition to the term satellite and the popular term moon, the term binary (triple, quadruple, multiple system) is used for objects with a companion (respectively 2, 3 or more companions). The term binary is often used independently from the relative sizes of the components. If one object is much bigger it is usually referred to as the primary and its companion as secondary. Asteroids with moons are commonly referred to as binary asteroids. The term double asteroid is sometimes used for systems in which the asteroid and its moon are roughly the same size.

Typically, masses/albedos are not actually known so the determination of the barycentre would be problematic anyway.

Discovery milestones of asteroid moons.

As early as 1978, following a stellar occultation, 532 Herculina had been suggested to have a moon and there were reports of other asteroids having companions (usually referred to as satellites) in the following years. A letter in Sky & Telescope magazine at this time pointed to pairs of large craters (e.g. the Clearwater Lakes in Quebec) also suggesting asteroids having companions. However, the first asteroid moon to be confirmed was Dactyl which orbits 243 Ida. It was discovered by the Galileo probe in 1993. The second was discovered around 45 Eugenia in 1998. The first TNO binary, 1998 WW31 was resolved optically in 2002.

As of February 2004, nearly 37 more asteroid moons had been discovered by Earth-bound telescopes. Asteroid moons have been discovered orbiting Main belt asteroids, Trojan asteroids, near-Earth objects, and Kuiper Belt objects. In 2005, the asteroid 87 Sylvia was discovered to have two moons, making it the first known triple asteroid. Later the same year, the KBO (136108) 2003 EL61 was also discovered to have two moons, making it the second known KBO to have at least two moons after Pluto.

An example of a double asteroid is 90 Antiope, where two roughly equal-sized components orbit the common centre of gravity. 617 Patroclus and its same-sized companion Menoetius is the only known binary system in the Trojan population.

Asteroid moons: are they common or rare?

The data about the populations of binary objects are still patchy. In addition to the inevitable observational bias (dependence on the distance from Earth, size, albedo and separation of the components) the frequency appears to be different among different categories of objects. Among asteroids, an estimated 2% would have satellites. Among trans-Neptunian objects (TNO), an estimated 11% are believed to be binary or multiple objects, but three of the four known large TNO (75%) have at least one satellite.

More than 20 binaries are known in each of the main groupings: Near Earth asteroids, Main belt asteroids, and Trans-Neptunians, not including numerous claims based solely on the light curve variation. No binaries have been found so far among Centaurs, presumably due to the much smaller number and relative faintness of these objects.

Origin of asteroid moons.

The origin of asteroid moons is not currently known with certainty, and a variety of theories exist. A widely accepted theory is that asteroid moons are formed from debris knocked off of the primary asteroid by an impact. Other pairings may be formed when a small object is captured by the gravity of a larger one.

Formation by collision is constrained by the angular momentum of components i.e. by the masses and their separation. Close binaries fit this model (e.g. Pluto/Charon). Distant binaries however, with components of comparable size, are unlikely to have followed this scenario, unless considerable mass has been lost in the event.

The distances of the components for the known binaries vary from a few hundreds of km (243 Ida, 3749 Balam) to more than 3000 km (379 Huenna) for the asteroids. Among TNOs, the known separations vary from 3,000 to 50,000 km.

Notable asteroids with moons.

Name of primaryOrbital typediameter of primary (km)
(or dimensions)
Name of moonDiameter of moon (km)
(or dimensions)
Distance between pair (km)
22 Kalliope main belt (215180150) Linus 38 6 1,0658
45 Eugenia main belt 214.64.2
(305220145)
Petit-Prince 12.70.8 1,18412
87 Sylvia main belt (385265230) Remus (Sylvia II) 72 7065
Romulus (Sylvia I) 184 1,3565
90 Antiope main belt 11016 S/2000 (90) 1 11016 1701
121 Hermione main belt 209.04.7
(265180180)
S/2002 (121) 1 18 794.72.1
243 Ida main belt (59.825.418.6) Dactyl (1.61.41.2) 108
283 Emma main belt 148.14.6 S/2003 (283) 1 12 5963
617 Patroclus Jupiter trojan 121.8 3.2 Menoetius 112.6 3.2 685 40
762 Pulcova main belt 137.13.2 S/2000 (762) 1 20 810
1313 Berna main belt 11 S/2004 (1313) 1 11 35
Trans-Neptunian objects
(47171) 1999 TC36 Plutino 590? S/2001 (47171) 1 250? 7,640460
58534 Logos Cubewano 80 Zoe 66 8,010 80
65489 Ceto SDO 193? Phorcys 146? 1,842 46
(79360) 1997 CS29 Cubewano 305 S/2005 (79360) 1 292 2300
(82075) 2000 YW134 SDO 431 S/2005 (82075) 1 237 1900
Pluto Plutino 230620 Charon (Pluto I) 12073 19,5714
Nix (Pluto II) 44-130 48,675120
Hydra (Pluto III) 44-130 64,78090
Eris SDO 2400100 Dysnomia 300-400 30,000-36,000
1998 WW31 Cubewano 133 15 S/2000 (1998 WW31) 1 110 12 22,300 800
2001 QG298 Plutino 260205185 S/2002 (2001 QG298) 1 265160150 400
2003 EL61
(Santa)
Cubewano 1400 S/2005 (2003 EL61) 1 (Rudolph) 310 49,500 400
S/2005 (2003 EL61) 2 170 39,300


Further reading about the solar system.
The Sun Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Pluto Eris
Planets Dwarf planets Moons: Terran Martian Asteroidal Jovian Saturnian Uranian Neptunian Plutonian Eridian
Small bodies:   Meteorites Asteroids (Asteroid belt) Centaurs TNOs (Kuiper belt/Scattered disc) Comets (Oort cloud)
Solar system related pages. astronomical objects and the solar system's list of solar system objects.



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