Minor-planet moon


A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. As of January 2020, there are [|384 minor planets] known or suspected to have moons. Discoveries of minor-planet moons 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.
The first modern era mention of the possibility of an asteroid satellite was in connection with an occultation of the bright star Gamma Ceti by the asteroid 6 Hebe 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. 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.

Terminology

In addition to the terms satellite and moon, the term "binary" 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 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 system, identified in August 2000. Small satellites are often referred to as moonlets.

Discovery milestones

Prior to the era of the Hubble Space 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. 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 in the following years. Also, a letter in Sky & Telescope magazine at this time pointed to apparently simultaneous impact craters on Earth, 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. In 2001, 617 Patroclus and its same-sized companion Menoetius became the first known binary asteroids in the Jupiter trojans. The first trans-Neptunian binary after Pluto–Charon,, was optically resolved in 2002.

Triple systems

In 2005, the asteroid 87 Sylvia was discovered to have two satellites, making it the first known triple system. This was followed by the discovery of a second moon orbiting 45 Eugenia. Also in 2005, the dwarf planet was discovered to have two moons, making it the second trans-Neptunian object after Pluto known to have more than one moon. Additionally, 216 Kleopatra and 93 Minerva 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.
The following table lists all satellites of triple systems chronologically by their discovery date, starting with Charon, discovered in 1978.

Commonality

The data about the populations of binary objects are still patchy. In addition to the inevitable observational bias the frequency appears to be different among different categories of objects. Among asteroids, an estimated 2% would have satellites. Among trans-Neptunian objects, 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, 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.
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. 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 to more than 3000 km 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.
, there are 384 minor planets with 403 known companions. These consist of:

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] and [|introduction].

Mars crossers

This is a list of Mars-crossing asteroids with companions. Candidate binaries with an unconfirmed status are displayed on a dark background. For an overview, see summary and introduction.

Main-belt asteroids

This is a list of main-belt asteroids with companions. Candidate binaries with an unconfirmed status are displayed on a dark background. For an overview, see summary and introduction.
The following binaries are double asteroids, with similarly-sized components, and a barycenter outside of the larger object.
  1. 90 Antiope — S/2000 1
  2. 854 Frostia — S/2004 1
  3. 1313 Berna — S/2004 1
  4. 2478 Tokai — S/2007 1
  5. 3169 Ostro — S/2005 1
  6. 3749 Balam — S/2002 1
  7. 3905 Doppler — S/2013 1
  8. 4674 Pauling — S/2004 1
  9. 4951 Iwamoto — S/2007 1
  10. 5674 Wolff — S/2015 1
  11. 8474 Rettig — S/2015 1
  12. 17246 Christophedumas — S/2004 1
  13. — S/2011 1
In addition, these bodies might be double asteroids, but due to errors in their size and orbit, it is uncertain.
  1. 809 Lundia — S/2005 1
  2. 1089 Tama — S/2003 1
  3. 1509 Esclangona — S/2003 1
  4. 4492 Debussy — S/2004 1
  5. 11264 Claudiomaccone — S/2003 1
  6. 22899 Alconrad — S/2003 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 and introduction.

Centaurs

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

Trans-Neptunian objects

This is a list of trans-Neptunian objects with companions. Candidate binaries with an unconfirmed status are displayed on a dark background. This list gives the companion's orbital period in days rather than hours. For an overview, see summary and introduction.