Asteroid moon

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Image:243 ida.jpg
243 Ida and its moon Dactyl.

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 at least constraints) on their density and mass allowing an insight into their physical properties, impossible otherwise.

Contents

[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, 4 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.[1]

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,[2] 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 kilometres (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.[2]

[edit] Populations

What is "typical" for a binary asteroid system tends to depend on its location in the Solar System (presumably because of different modes of origin and lifetimes of such systems in different populations of minor planets).[3]

  • Among Near-Earth Asteroids, satellites tend to orbit at distances of the order of 3-7 primary radii, and have diameters two to several times smaller than the primary. Since these binaries are all inner-planet crossers, it is thought that tidal stresses that occurred when the parent object passed close to a planet may be responsible for the formation of many of them.
  • Among main belt asteroids, the satellites are usually much smaller than the primary (a notable exception being 90 Antiope), and orbit around 10 primary radii away. Many of the binary systems here are members of asteroid families, and a good proportion of satellites are expected to be fragments of a parent body whose disruption after an asteroid collision produced both the primary and satellite.
  • Among Trans-Neptunian Objects, it is common for the two orbiting components to be of comparable size, and for the semi-major axis of their orbits to be much larger − about 100 to 1000 primary radii. A significant proportion of these binaries are expected to be primordial.

[edit] Notable asteroids with moons

Main article: List of asteroid moons
Name of primary Orbital type Diameter
of primary (km)
(dimensions) 		Name of moon Diameter
of moon (km)
(dimensions) 		Distance between
pair (km)
22 Kalliope main belt (215×180×150) Linus 38 ± 6 1,065 ± 8
45 Eugenia 214.6 ± 4.2
(305×220×145) 		Petit-Prince 12.7 ± 0.8 1,184 ± 12
87 Sylvia (385×265×230) Remus (Sylvia II) 7 ± 2 706 ± 5
Romulus (Sylvia I) 18 ± 4 1,356 ± 5
90 Antiope 110 ± 16 S/2000 (90) 1 110 ± 16 170 ± 1
121 Hermione (254×125) S/2002 (121) 1 12 ± 4 768 ± 11
243 Ida (59.8×25.4×18.6) Dactyl (1.6 × 1.4 × 1.2) 108
283 Emma 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 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[4] 146? 1,842 ± 46
66652 Borasisi TNO 166? Pabu[5] 137? 4,660 ± 170
(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
(136108) 2003 EL61
(Santa) 		cubewano 1400 S/2005 (2003 EL61) 1 (Rudolph) 310 49,500 ± 400
S/2005 (2003 EL61) 2 (Blitzen) 170 39,300
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

[edit] See also

[edit] References

  1. ^ Chiang, E.; Lithwick, Y.; Buie, M.; Grundy, W.; Holman, M.; A Brief History of Trans-Neptunian Space, to appear in Protostars and Planets V (August 2006) Final preprint on arXiv
  2. ^ a b 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
  3. ^ "T. Michałowski et al. (2004). "Eclipsing binary asteroid 90 Antiope". Astronomy & Astrophysics 423: 1159.
  4. ^ Discovered April 11, 2006 by K. Noll, H. Levison, W. Grundy and D. Stephens using the Hubble Space Telescope
  5. ^ Discovered 8 September 1999 by Chadwick A. Trujillo, Jane X. Luu, and David C. Jewitt from Mauna Kea, HI, USA

[edit] External links


v • d • e
Small Solar System bodies
Vulcanoids · Near-Earth asteroids · Main belt · Jupiter Trojans · Centaurs · Damocloids · Neptune Trojans · Comets · Trans-Neptunians (Kuiper belt • Scattered disc objects • Oort cloud)
For other objects and regions, see Asteroid groups and families, Binary asteroids, Asteroid moons, meteoroids and the Solar System.
For a complete listing, see List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.
v • d • e
Natural satellites of the Solar System
Planetary satellitesTerrestrial · Martian · Jovian · Saturnian · Uranian · NeptunianImage:Rhea true color.jpg
Other satellite systemsPlutonian · Eridian · Asteroid satellites
Largest satellitesGanymede · Titan · Callisto · Io · Moon · Europa · Triton
Titania · Rhea · Oberon · Iapetus · Charon · Umbriel · Ariel · Dione · Tethys  · Enceladus · Miranda · Proteus · Mimas
Inner satellites • Trojans • Irregulars • List • List by diameter • Timeline of discovery • Naming
 v • d • e The Solar System
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  1. The Sun

circle 0 0 90 35 The Sun

  1. Mercury

circle 112 18 6 Mercury

  1. Venus

circle 153 18 8 Venus

  1. Earth and the Moon

circle 203 8 4 The Moon circle 194 18 8 Earth

  1. Mars and satellites

circle 239 13 3 Phobos and Deimos circle 233 18 8 Mars

  1. Ceres and the asteroid belt
  2. - by placing the rectangle code for the asteroid belt AFTER Ceres, Ceres is "on top" (and can co-exist)

circle 271 18 8 Ceres rect 256 0 288 35 The asteroid belt

  1. Jupiter and satellites

circle 316 18 15 Jupiter circle 329 5 6 Moons of Jupiter

  1. Saturn and satellites

circle 372 18 10 Saturn circle 381 7 6 Moons of Saturn

  1. Uranus and satellites

circle 418 18 9 Uranus circle 427 10 6 Moons of Uranus

  1. Neptune and satellites

circle 471 10 3 Moons of Neptune circle 462 18 12 Neptune

  1. Pluto, satellites, and the Kuiper belt
  2. - by placing the rectangle code for the Kuiper belt AFTER Pluto, Pluto is "on top" (and can co-exist)

circle 508 13 3 Moons of Pluto circle 504 18 8 Pluto rect 492 0 527 35 The Kuiper Belt

  1. Eris, Dysnomia, and the Scattered disc
  2. - by placing the rectangle code for the Scattered disc AFTER Eris, Eris is "on top" (and can co-exist)

circle 544 14 3 Dysnomia circle 540 18 8 Eris rect 528 0 567 35 The Scattered Disc rect 568 0 597 35 The Oort Cloud

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  1. - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
  1. Notes:
  2. Details on the new coding for clickable images is here: [1]
  3. The smaller planets have a bit of an overlap just to ensure they're locatable, especially in the belts.
  4. While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
  5. - I've placed moons on "top" so that their smaller circles won't disappear "under" their respective planets or dwarf planets.
  6. The "poly" code would be more appropriate for the moons of Jupiter, Saturn, and Uranus. However, there appears to be a bug with that aspect of the code.
  7. - I've compensated by using oversized circles for those moon groups, and tucking them UNDER their planets for now.
  8. The Sun is a rectangle as that approximates the edge closely enough for the purposes of this template.
  9. I've guessed as to the boundaries for the KB, SD, and OC - if they need adjustment, load the image into Paint and use the pencil tool to find the appropriate coordinates.

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Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
Small bodies:   Meteoroids · Asteroids/Asteroid moons (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the Solar System Portal
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