Asteroid moon

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An asteroid moon is an asteroid that orbits another asteroid as its natural satellite. It is thought that 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.

1 Terminology
2 Discovery milestones
3 Common or rare?
4 Origin
5 Notable asteroids with moons
6 See also
7 References
8 External links

[edit] Terminology

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.

[edit] Discovery milestones

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.<ref name="Chiang2006" > E. Chiang, Y. Lithwick, M. Buie, W. Grundy, M. Holman A Brief History of Trans-Neptunian Space. to appear in Protostars and Planets V (August 2006) Final preprint on arXiv </ref>

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.

[edit] 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 <ref name="Noll2005">Noll, Keith S. "Solar System binaries", Asteroids, Comets, Meteors, Proceedings of the 229th Symposium of the IAU, Rio de Janeiro, 2005, Cambridge University Press, 2006., pp.301-318 Preprint</ref>, presumably due to the much smaller number and relative faintness of these objects.

[edit] Origin

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.<ref name="Noll2005"/>

[edit] Notable asteroids with moons

Main article: List of asteroid moons

Name of primary	Orbital type	Diameter of primary (km) (or dimensions)	Name of moon	Diameter of moon (km) (or dimensions)	Distance between pair (km)
22 Kalliope	main belt	(215×180×150)	Linus	38 ± 6	1,065±8
45 Eugenia	main belt	214.6±4.2 (305×220×145)	Petit-Prince	12.7±0.8	1,184±12
87 Sylvia	main belt	(385×265×230)	Remus (Sylvia II)	7±2	706±5
87 Sylvia	main belt	(385×265×230)	Romulus (Sylvia I)	18±4	1,356±5
90 Antiope	main belt	110±16	S/2000 (90) 1	110±16	170±1
121 Hermione	main belt	209.0±4.7 (265×180×180)	S/2002 (121) 1	18	794.7±2.1
243 Ida	main belt	(59.8×25.4×18.6)	Dactyl	(1.6×1.4×1.2)	108
283 Emma	main belt	148.1±4.6	S/2003 (283) 1	12	596±3
617 Patroclus	Jupiter trojan	121.8 ± 3.2	Menoetius	112.6 ± 3.2	685 ± 40
762 Pulcova	main belt	137.1±3.2	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?	7,640±460
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	2306±20	Charon (Pluto I)	1207±3	19,571±4
			Nix (Pluto II)	44-130	48,675±120
			Hydra (Pluto III)	44-130	64,780±90
Eris	SDO	2400±100	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	260×205×185	S/2002 (2001 QG₂₉₈) 1	265×160×150	400
2003 EL₆₁ (Santa)	cubewano	1400	S/2005 (2003 EL₆₁) 1 (Rudolph)	310	49,500 ± 400
2003 EL₆₁ (Santa)	cubewano	1400	S/2005 (2003 EL₆₁) 2	170	39,300

[edit] See also

Yarkovsky-O'Keefe-Radzievskii-Paddack effect (YORP effect)

[edit] References

[edit] External links

v • d • e</span> Small Solar System bodies
Vulcanoids \| Near-Earth asteroids \| Main belt \| Jupiter Trojans \| Centaurs \| Damocloids \| Comets \| Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud)
For other objects and regions, see: asteroid groups and families, binary asteroids, asteroid moons and the Solar system For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.

Solar System Natural Satellites v • d • e</div>
Planetary satellites:	Terran • Martian • Jovian • Saturnian • Uranian • Neptunian
Other satellite systems:	Plutonian • Eridian • Asteroid satellites

Largest satellites:	Ganymede • Titan • Callisto • Io • Earth's Moon • Europa • Triton Titania • Rhea • Oberon • Iapetus • Charon • Umbriel • Ariel • Dione • Tethys
See also: inner satellites • trojans • irregulars • list by diameter • discovery timeline • naming

The Solar System v • d • e</div>
Image:Solar System XXVII.png
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: Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.