Minor-planet moon

243 Ida and its moon Dactyl as imaged by the Galileo spacecraft in 1993

762 Pulcova and satellite as seen with adaptive optics in 2000

A minor-planet moon is an astronomical body that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size. Discoveries of minor-planet moons (and binary objects, in general) are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights of their physical properties that is generally not otherwise possible.^[1] As of November 2011, there are over 200 minor planets known to have moons.^[2]

Terminology

In addition to the terms satellite and moon, the term binary is sometimes used for minor planets with moons (or triple for minor planets with two moons). If one object is much bigger it is usually referred to as the primary and its companion as secondary. The term double asteroid is sometimes used for systems in which the asteroid and its moon are roughly the same size, while binary tends to be used independently from the relative sizes of the components. When binary minor planets are similar in size, the Minor Planet Center (MPC) refers to them as "binary companions" instead of referring to the smaller body as a satellite.^[3] A good example of a true binary is the 90 Antiope system, identified in August 2000.^[4] Small satellites are often referred to as moonlets.^[1][5]

Discovery milestones

As of June 2013, over 230 moons of minor planets have been discovered.^[2] These consist of:

• 84 in the asteroid belt (5 with two satellites),
• 4 around the (Jupiter) Trojan asteroids,
• 46 around near-Earth objects (2 with two satellites each),
• 18 circulating objects that cross the orbit of Mars, and
• 79 known moons of trans-Neptunian objects, including 2 TNOs with two moons each and one (Pluto) with 5.^[2][6]

Prior to the era of the Hubble telescope and space probes reaching the outer Solar System, attempts to detect satellites around asteriods were limited to optical observations from Earth. For example, in 1978, stellar occultation observations were claimed as evidence of a satellite for the asteroid 532 Herculina.^[7] However, later more-detailed imaging by the Hubble Telescope did not reveal a satellite, and the current consensus is that Herculina does not have a significant satellite.^[8] There were other similar reports of asteroids having companions (usually referred to as satellites) in the following years. Also, a letter in Sky & Telescope magazine^{[citation needed]} at this time pointed to apparently simultaneous impact craters on Earth (for example, the Clearwater Lakes in Quebec), suggesting that these craters were caused by pairs of gravitationally-bound objects.

In 1993, the first asteroid moon was confirmed when the Galileo probe discovered the small Dactyl orbiting 243 Ida in the asteroid belt. The second was discovered around 45 Eugenia in 1998.^[9] In 2001, 617 Patroclus and its same-sized companion Menoetius became the first known binary asteroids in the Jupiter Trojans.^[10] The first trans-Neptunian binary, 1998 WW₃₁, was optically resolved in 2002.^[11]

Triple systems

In 2005, the asteroid 87 Sylvia was discovered to have two satellites, making it the first known triple asteroid.^[12] This was followed by the discovery of a second moon orbiting 45 Eugenia.^[13] Also in 2005, the Kuiper belt object (KBO) Haumea was discovered to have two moons, making it the second KBO after Pluto known to have more than one moon. Other known triple systems include: 3749 Balam (March 2008),^[14] 216 Kleopatra (September 2008),^[15] and 93 Minerva (August 2009).^[16]

Commonality

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 light-curve variation.

No binaries have been found so far among centaurs with semi-major axis smaller than Neptune.^[17] However, using an extended definition of Centaurs, as the objects on unstable orbits with the perihelion inside the orbit of Neptune, the first binary centaur, 42355 Typhon (previously known as 2002 CR₄₆), was identified in 2006.^[18]

Origin

The origin of minor-planet moons is not currently known with certainty, and a variety of theories exist. A widely accepted theory is that minor-planet moons are formed from debris knocked off of the primary 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.^[17]

Populations

What is "typical" for a binary 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).^[19]

• 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, although collisions are thought to also be a factor in the creation of these satellites.
• 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.

Dwarf planets

Among the dwarf planets, it is 90 percent certain that Ceres has no moons larger than 1 km in size, assuming that they would have the same albedo as Ceres itself.^[20]

Pluto has five known moons. Its largest moon Charon is more than half as large as Pluto itself, and large enough to orbit a point outside Pluto's surface. In effect, each orbits the other, forming a binary system informally referred to as a double dwarf planet. Pluto's four other moons, Nix, Hydra, Kerberos, and Styx, are far smaller and orbit the Pluto–Charon system.

Haumea has two moons with radii estimated around 155 km (Hiʻiaka) and 85 km (Namaka).

Makemake has no known moons. A satellite having 1% Makemake's brightness would have been detected if it had been located at an angular distance from Makemake farther than 0.4 arcseconds (0.0001 degrees; 2 microradians).^[21]

Eris has one known moon, Dysnomia. Its radius, based on its brightness, is estimated to be 50 to 125 km.^[22]

List of minor planets with moons

Near-Earth objects

Radar images of asteroid (66391) 1999 KW₄ and its moon. The 'streaks' on the image are the moon's trail as it moved while the images were created.

Near-Earth encounter of asteroid (136617) 1994 CC in June 2009, showing two moons

Mars crossers

Main-belt asteroids

Jupiter trojans

Trans-Neptunian objects

See also

• Yarkovsky–O'Keefe–Radzievskii–Paddack effect (YORP effect)

References

• Asteroids with Satellites web page, maintained up to date by W. Robert Johnston; and references therein. (last accessed 13-03-2007)
• The VOBAD database a web page built and designed by F. Marchis and his collaborators (UC-Berkeley/SETI Institute) which contains the parameters of 169 multiple asteroid systems (last update May 9, 2009)

External links

• Orbits of Binary Asteroids with Adaptive Optics (Franck Marchis)
• Satellites and Companions of Minor Planets (CBAT)
• Asteroids with Satellites (Robert Johnston)

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