Minor-planet moon

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

762 Pulcova and satellite as seen with adaptive optics in 2000

A minor-planet moon is an astronomical object 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 December 2015, there are over 290 minor planets known to have moons.^[2]

Terminology

In addition to the terms satellite and moon, the term "binary" (binary minor planet) is sometimes used for minor planets with moons, and "triple" for minor planets with two moons. If one object is much bigger it can be 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 December 2015, over 290 moons of minor planets have been discovered.^[2] These consist of:

• 60 around near-Earth objects,

• Two with two satellites: (136617) 1994 CC and (153591) 2001 SN263

• 22 orbiting Mars-crossing asteroids,

• One with two satellites: 2577 Litva

• 123 in the asteroid belt,

• Six with two satellites: 45 Eugenia, 87 Sylvia, 93 Minerva, 130 Elektra, 216 Kleopatra, and 3749 Balam

• 4 around Jupiter trojans,
• 2 around centaurs (both in the form of ring systems that are likely contained by shepherd moons), and
• 85 moons of trans-Neptunian objects

• Two with two satellites: (47171) 1999 TC36 and 136108 Haumea
• One with five satellites: 134340 Pluto^[2][6]

Prior to the era of the Hubble telescope and space probes reaching the outer Solar System, attempts to detect satellites around asteroids 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 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

Triple asteroids, or trinary asteroids, are known since 2005, when the asteroid 87 Sylvia was discovered to have two satellites, making it the first known triple system.^[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.^[14]

Additionally, 216 Kleopatra^[15] and 93 Minerva^[16] were discovered to be trinary asteroids in 2008 and 2009 respectively. Since the first few trinary asteroids were discovered, more continue to be discovered at a rate of about one a year. Most recently discovered was a third moon orbiting the Main-belt asteroid 130 Elektra, bringing the number of known trinary asteroids in the asteroid belt up to 6.

List of triple asteroids:

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 50 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.

Two binaries have been found so far among centaurs with semi-major axes smaller than Neptune.^[17] Both are double ring systems around 2060 Chiron and 10199 Chariklo, discovered in 1994-2011 and 2013 respectively.

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 the 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).^[18]

• 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.^[19]

Pluto has five known moons. Its largest moon Charon is more than half the size of Pluto itself, and large enough to orbit a point outside Pluto's surface. In fact, each orbits the common barycenter between them, with Pluto's orbit entirely enclosed by Charon's; thus they form 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).^[20]

Eris has one known moon, Dysnomia. Its radius, based on its brightness, is estimated to be roughly between 150 and 350 km.^[21]

List of minor planets with moons

Near-Earth objects

(357439) 2004 BL86 and its moon

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

There are 58 known near-Earth asteroids with moons (15 Amor, 36 Apollo, and 7 Aten) with a total of 60 moons.

Mars crossers

There are 21 Mars-crossing asteroids with moons with a total of 22 moons.

Asteroid belt

There are 117 asteroids with 123 moons in the asteroid belt.

Jupiter trojans

There are 4 Jupiter trojans with known moons.

Centaurs

There are only 2 centaurs with satellites, both of which are ring systems instead of moons.

Trans-Neptunian objects

There are 79 Trans-Neptunian objects with moons, with a total of 85 moons.

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)

Natural satellites of the Solar System Planetary satellites Terrestrial Martian Jovian Saturnian Uranian Neptunian Other satellites Asteroid moons Plutonian Haumean Eridian Largest satellites (mean radius ≥ 100 km) size Ganymede largest / 2634 km / 0.413 Earths Titan Callisto Io Moon Europa Triton Titania Rhea Oberon Iapetus Charon Umbriel Ariel Dione Tethys [Dysnomia] Enceladus Miranda Proteus Mimas [Hi'iaka] Nereid Hyperion Phoebe smallest / 106 km / 0.017 Earths abc's Ariel Callisto Charon Dione [Dysnomia] Enceladus Europa Ganymede [Hi'iaka] Hyperion Iapetus Io Mimas Miranda Moon Nereid Oberon Phoebe Proteus Rhea Tethys Titan Titania Triton Umbriel Discovery timeline Inner moons Irregular moons List Naming Regular moons Trojan moons NOTE: Italicized moons are not close to being in hydrostatic equilibrium; [bracketed] moons may or may not be close to being in hydrostatic equilibrium.

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