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A minor-planet moon is an
astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
that
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
s a
minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''mino ...
as its
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are important because the determination of their orbits provides estimates on the
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
and
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of the primary, allowing insights into their physical properties that are generally not otherwise accessible. Several of the moons are quite large compared to their primaries:
90 Antiope Antiope ( minor planet designation: 90 Antiope) is a double asteroid in the outer asteroid belt. It was discovered on October 1, 1866, by Robert Luther. In 2000, it was found to consist of two almost-equally-sized bodies orbiting each other. At ...
, Mors–Somnus and Sila–Nunam (95%), Patroclus–Menoetius,
Altjira The Dreaming, also referred to as Dreamtime, is a term devised by early anthropologists to refer to a religio-cultural worldview attributed to Australian Aboriginal beliefs. It was originally used by Francis Gillen, quickly adopted by his c ...
and Lempo–Hiisi (90%, with Lempo–Paha at 50%). The largest known minor-planet moon in ''absolute'' size is Pluto's largest moon Charon, which itself has about half the diameter of Pluto. The first modern era mention of the possibility of an asteroid satellite was in connection with an
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
of the bright star
Gamma Ceti Gamma Ceti (γ Ceti, abbreviated Gamma Cet, γ Cet) is a triple star system in the equatorial constellation of Cetus. It has a combined apparent visual magnitude of 3.47. Based upon parallax measurements, this star is located at a distan ...
by the asteroid
6 Hebe Hebe ( minor planet designation: 6 Hebe) is a large main-belt asteroid, containing around 0.5% of the mass of the belt. However, due to its apparently high bulk density (greater than that of the Moon or even Mars), Hebe does not rank among the ...
in 1977. The observer, amateur astronomer Paul D. Maley, detected an unmistakable 0.5 second disappearance of this naked eye star from a site near
Victoria, Texas Victoria is a small city in South Texas and county seat of Victoria County, Texas. The population was 65,534 as of the 2020 census. The three counties of the Victoria Metropolitan Statistical Area had a population of 111,163 as of the 2000 censu ...
. Many hours later, several observations were reported in Mexico attributed to the occultation by 6 Hebe itself. Although not confirmed, this documents the first formally documented case of a suspected companion of an asteroid. There are also several known ring systems around distant objects ''(see:
rings of Chariklo The minor planet and centaur 10199 Chariklo, with a diameter of about , is the smallest celestial object with confirmed rings and the fifth ringed celestial object discovered in the Solar System, after the gas giants and ice giants. Orbiting Cha ...
and
Chiron In Greek mythology, Chiron ( ; also Cheiron or Kheiron; ) was held to be the superlative centaur amongst his brethren since he was called the "wisest and justest of all the centaurs". Biography Chiron was notable throughout Greek mythology ...
)''.


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. A good example of a true binary is the
90 Antiope Antiope ( minor planet designation: 90 Antiope) is a double asteroid in the outer asteroid belt. It was discovered on October 1, 1866, by Robert Luther. In 2000, it was found to consist of two almost-equally-sized bodies orbiting each other. At ...
system, identified in August 2000. Small satellites are often referred to as moonlets.


Discovery milestones

Prior to the era of the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
and space probes reaching the
outer Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
, attempts to detect satellites around asteroids were limited to optical observations from Earth. For example, in 1978,
stellar occultation Stellar means anything related to one or more stars (''stella''). The term may also refer to: Arts, entertainment, and media * ''Stellar'' (magazine), an Irish lifestyle and fashion magazine * Stellar Loussier, a character from ''Mobile Suit Gu ...
observations were claimed as evidence of a satellite for the asteroid 532 Herculina. 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. There were other similar reports of asteroids having companions (usually referred to as satellites) in the following years. A letter by astronomer Thomas Hamilton in the ''
Sky & Telescope ''Sky & Telescope'' (''S&T'') is a monthly American magazine covering all aspects of amateur astronomy, including the following: *current events in astronomy and space exploration; *events in the amateur astronomy community; *reviews of astronomic ...
'' magazine at this time pointed to apparently simultaneous impact craters on Earth (for example, the
Clearwater Lakes The Lac Wiyâshâkimî (the official name, in French, formerly Lac à l'Eau Claire, a calque of the lake's name, Wiyâšâkamî, in Northern East Cree, changed form of ''wâšâkamî'' or ''wâšekamî'' in more southerly Cree dialects), also ...
in Quebec), suggesting that these craters were caused by pairs of gravitationally bound objects. Also in 1978, Pluto's largest moon Charon was discovered; however, at the time Pluto was still considered to be one of the major planets. In 1993, the first asteroid moon was confirmed when the '' Galileo'' probe discovered the small
Dactyl Dactyl may refer to: * Dactyl (mythology), a legendary being * Dactyl (poetry), a metrical unit of verse * Dactyl Foundation, an arts organization * Finger, a part of the hand * Dactylus, part of a decapod crustacean * "-dactyl", a suffix used ...
orbiting 243 Ida in the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, c ...
. The second was discovered around
45 Eugenia Eugenia ( minor planet designation: 45 Eugenia) is a large asteroid of the asteroid belt. It is famed as one of the first asteroids to be found to have a moon orbiting it. It was also the second triple asteroid to be discovered, after 87 Sylvia ...
in 1998. In 2001, 617 Patroclus and its same-sized companion Menoetius became the first known binary asteroids in the Jupiter trojans. The first
trans-Neptunian A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...
binary after Pluto–Charon, , was optically resolved in 2002.


Multiple systems

In 2005, the asteroid 87 Sylvia was discovered to have two satellites, making it the first known triple system (also called trinary minor planets). This was followed by the discovery of a second moon orbiting
45 Eugenia Eugenia ( minor planet designation: 45 Eugenia) is a large asteroid of the asteroid belt. It is famed as one of the first asteroids to be found to have a moon orbiting it. It was also the second triple asteroid to be discovered, after 87 Sylvia ...
. Also in 2005, the dwarf planet was discovered to have two moons, making it the second trans-Neptunian object after
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
known to have more than one moon. Additionally, 216 Kleopatra and
93 Minerva Minerva (minor planet designation: 93 Minerva) is a large trinary main-belt asteroid. It is a C-type asteroid, meaning that it has a dark surface and possibly a primitive carbonaceous composition. It was discovered by J. C. Watson on 24 August ...
were discovered to be trinary asteroids in 2008 and 2009 respectively. Since the first few triple minor planets were discovered, more continue to be discovered at a rate of about one a year. Most recently discovered were two moons orbiting large near-earth asteroid 3122 Florence, bringing the number of known trinary systems in the Solar System up to 16 (including the Pluto system). The following table lists all satellites of triple systems (or higher multiplicities) chronologically by their discovery date, starting with Charon, discovered in 1978. The highest known multiplicities are for Pluto (a sextuple system) and
130 Elektra Elektra ( minor planet designation: 130 Elektra) is a large outer main-belt asteroid and quadruple system with three minor-planet moons. It was discovered on 17 February 1873, by astronomer Christian Peters at Litchfield Observatory, New York, ...
(a quadruple system).


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 Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...
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 object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically ...
s (TNOs), an estimated 11% are thought to be binary or multiple objects, and the majority of the large TNOs have at least one satellite, including all four IAU-listed dwarf planets. More than 50 binaries are known in each of the main groupings: near-Earth asteroids, belt asteroids, and
trans-Neptunian objects A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typicall ...
, not including numerous claims based solely on light-curve variation. Two binaries have been found so far among
centaur A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as being ...
s with semi-major axes smaller than Neptune. Both are double ring systems around 2060 Chiron and
10199 Chariklo 10199 Chariklo is the largest confirmed centaur (small body of the outer Solar System). It orbits the Sun between Saturn and Uranus, grazing the orbit of Uranus. On 26 March 2014, astronomers announced the discovery of two rings (nicknamed ...
, 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 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 In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
of the components, i.e. by the masses and their separation. Close binaries fit this model (e.g.
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
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.


Populations and classes

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). * 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 The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
asteroids'', the satellites are usually much smaller than the primary (a notable exception being
90 Antiope Antiope ( minor planet designation: 90 Antiope) is a double asteroid in the outer asteroid belt. It was discovered on October 1, 1866, by Robert Luther. In 2000, it was found to consist of two almost-equally-sized bodies orbiting each other. At ...
), and orbit around 10 primary radii away. Many of the binary systems here are members of
asteroid families An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic o ...
, 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 object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically ...
s'', 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. **
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
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 In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...
between them, with Pluto's orbit entirely enclosed by Charon's; thus they form a
binary system A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that th ...
informally referred to as a double dwarf planet. Pluto's four other moons,
Nix Nix or NIX may refer to: Places * Nix, Alabama, an unincorporated community, United States * Nix, Texas, a ghost town in southwestern Lampasas County, Texas, United States * Nix (moon), a moon of Pluto People * Nix (surname), listing people with ...
, Hydra, Kerberos, and Styx, are far smaller and orbit the Pluto–Charon system. **
Haumea , discoverer = , discovered = , earliest_precovery_date = March 22, 1955 , mpc_name = (136108) Haumea , pronounced = , adjectives = Haumean , note = yes , alt_names = , named_after = Haumea , mp_category = , orbit_ref = , epoc ...
has two moons with radii estimated around 155 km (
Hiʻiaka In Hawaiian religion, Hiʻiaka is a daughter of Haumea and Kāne. Attributes and history Hiiaka is the patron goddess of hula dancers, chant, sorcery, and medicine. Owls are her messengers and are sacred to her. Conceived in Tahiti, Hiiaka was c ...
) and 85 km ( Namaka). ** has one known moon,
S/2015 (136472) 1 S/2015 (136472) 1, nicknamed MK2 by the discovery team, is the only known moon of the trans-Neptunian dwarf planet Makemake. It is estimated to be in diameter (for an assumed albedo of 4%) and has a semi-major axis at least from Makemake. Its ...
, estimated to be some in diameter. ** 47171 Lempo is a unique trans-Neptunian triple system: Lempo and its moon of roughly equal mass, Hiisi, form a close-proximity binary, separated by roughly 867 km. A second moon, Paha, orbits the Lempo–Hiisi binary at about 7411 km. ** has one known moon, Dysnomia. Its radius, based on its brightness, is estimated to be roughly between 150 and 350 km.


List

, there are 457 minor planets (systems) with 477 known companions. The following table is a listing of the total number of these systems by orbital class:


Near-Earth objects

This is a list of near-Earth asteroids with companions. Candidate binaries with an unconfirmed status are displayed on a dark background. For an overview, see
summary may refer to: * Abstract (summary), shortening a passage or a write-up without changing its meaning but by using different words and sentences * Epitome, a summary or miniature form * Abridgement, the act of reducing a written work into a sho ...
and
introduction Introduction, The Introduction, Intro, or The Intro may refer to: General use * Introduction (music), an opening section of a piece of music * Introduction (writing), a beginning section to a book, article or essay which states its purpose and g ...
.


Mars crossers

This is a list of
Mars-crossing asteroid A Mars-crossing asteroid (MCA, also Mars-crosser, MC) is an asteroid whose orbit crosses that of Mars. Some Mars-crossers numbered below 100000 are listed here. They include the two numbered Mars trojans 5261 Eureka and . Many databases, for in ...
s with companions. Candidate binaries with an unconfirmed status are displayed on a dark background. For an overview, see
summary may refer to: * Abstract (summary), shortening a passage or a write-up without changing its meaning but by using different words and sentences * Epitome, a summary or miniature form * Abridgement, the act of reducing a written work into a sho ...
and
introduction Introduction, The Introduction, Intro, or The Intro may refer to: General use * Introduction (music), an opening section of a piece of music * Introduction (writing), a beginning section to a book, article or essay which states its purpose and g ...
.


Main-belt asteroids

This is a list of
main-belt asteroid The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
s with companions. Candidate binaries with an unconfirmed status are displayed on a dark background. For an overview, see
summary may refer to: * Abstract (summary), shortening a passage or a write-up without changing its meaning but by using different words and sentences * Epitome, a summary or miniature form * Abridgement, the act of reducing a written work into a sho ...
and
introduction Introduction, The Introduction, Intro, or The Intro may refer to: General use * Introduction (music), an opening section of a piece of music * Introduction (writing), a beginning section to a book, article or essay which states its purpose and g ...
. The following binaries are
double asteroid A binary asteroid is a system of two asteroids orbiting their common barycenter. The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993. Since then numerous binary asteroids and several triple ast ...
s, with similarly sized components, and a
barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...
outside of the larger object. #
90 Antiope Antiope ( minor planet designation: 90 Antiope) is a double asteroid in the outer asteroid belt. It was discovered on October 1, 1866, by Robert Luther. In 2000, it was found to consist of two almost-equally-sized bodies orbiting each other. At ...
— S/2000 (90) 1 # 854 Frostia — S/2004 (854) 1 # 1313 Berna — S/2004 (1313) 1 # 2478 Tokai — S/2007 (2478) 1 # 3169 Ostro — S/2005 (3169) 1 # 3749 Balam — S/2002 (3749) 1 #
3905 Doppler 3905 Doppler, provisional designation , is a stony asteroid and Binary asteroid, binary system from the middle region of the asteroid belt, approximately 8 kilometers in diameter. The asteroid was discovered on 28 August 1984, by Czech astronomer ...
— S/2013 (3905) 1 # 4674 Pauling — S/2004 (4674) 1 # 4951 Iwamoto — S/2007 (4951) 1 # 5674 Wolff — S/2015 (5674) 1 # 8474 Rettig — S/2015 (8474) 1 # 17246 Christophedumas — S/2004 (17246) 1 # — S/2011 (300163) 1 In addition, these bodies might be
double asteroid A binary asteroid is a system of two asteroids orbiting their common barycenter. The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993. Since then numerous binary asteroids and several triple ast ...
s, but due to errors in their size and orbit, it is uncertain. #
809 Lundia 809 Lundia is a small, binary, V-type asteroid orbiting within the Flora family in the main belt. It is named after Lund Observatory, Sweden. Characteristics Lundia orbits within the Flora family. However, its V-type spectrum indicates that it ...
— S/2005 (809) 1 # 1089 Tama — S/2003 (1089) 1 # 1509 Esclangona — S/2003 (1509) 1 # 4492 Debussy — S/2004 (4492) 1 # 11264 Claudiomaccone — S/2003 (11264) 1 # 22899 Alconrad — S/2003 (22899) 1


Jupiter trojans

This is a list of Jupiter trojans with companions. Candidate binaries with an unconfirmed status are displayed on a dark background. For an overview, see
summary may refer to: * Abstract (summary), shortening a passage or a write-up without changing its meaning but by using different words and sentences * Epitome, a summary or miniature form * Abridgement, the act of reducing a written work into a sho ...
and
introduction Introduction, The Introduction, Intro, or The Intro may refer to: General use * Introduction (music), an opening section of a piece of music * Introduction (writing), a beginning section to a book, article or essay which states its purpose and g ...
.


Trans-Neptunian objects

This is a list of
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically ...
s with companions. Candidate binaries with an unconfirmed status are displayed on a dark background. This list gives the companion's orbital period (Ps) in days rather than hours. For an overview, see
summary may refer to: * Abstract (summary), shortening a passage or a write-up without changing its meaning but by using different words and sentences * Epitome, a summary or miniature form * Abridgement, the act of reducing a written work into a sho ...
and
introduction Introduction, The Introduction, Intro, or The Intro may refer to: General use * Introduction (music), an opening section of a piece of music * Introduction (writing), a beginning section to a book, article or essay which states its purpose and g ...
.


See also

*
Lists of astronomical objects This is a list of lists, grouped by type of astronomical object. Solar System * List of Solar System objects * List of gravitationally rounded objects of the Solar System * List of Solar System objects most distant from the Sun * List of So ...
* * *
Satellite system (astronomy) A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object (incl. sub-brown dwarfs and rogue planets) or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), alt ...


References


External links

* , Lecture at SETI talk
Orbits of Binary Asteroids with Adaptive Optics
(Franck Marchis)

(CBAT)

Johnston's Archive *

' 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) {{Portal bar, Stars, Spaceflight, Outer space, Science Moons