An asteroid moon
is an asteroid
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 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 moons of Trans-Neptunian objects will also be covered in this article.
In addition to the terms satellite
, the term binary
is sometimes used for asteroids with moons (or triple
for asteroids 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.
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, it was not until until 1993 that the first asteroid moon was confirmed when the Galileo probe
orbiting 243 Ida
. The second was discovered around 45 Eugenia
in 1998. The first Trans-Neptunian
binary, was optically resolved in 2002.
As of September 2008, 104 asteroid moons had been discovered, 60 in the main belt, 2 orbiting Trojan asteroids, 42 near-Earth objects and Mars-crossers. There were at that time also 58 Trans-Neptunian moons. In 2005, the asteroid 87 Sylvia was discovered to have two moons, making it the first known triple asteroid. This was followed by the discovery of a second moon orbiting 45 Eugenia. Also in 2005, the KBO was discovered to have two moons, making it the second KBO after Pluto known to have more than one moon.
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.
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
, presumably due to the much smaller number and relative faintness of these objects.
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.
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).
- 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.
Notable asteroids and TNOs with moons