A natural satellite, or moon, is, in the most common usage, an astronomical body that orbits a planet or minor planet (or sometimes another small Solar System body). In the Solar System, there are six planetary satellite systems containing 205 known natural satellites. Four IAU-listed dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris. As of September 2018, there are 334 other minor planets known to have moons.
The Earth–Moon system is unique among planetary systems in that the ratio of the diameter of the Moon to the diameter of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km (2,158 miles) across, the Moon is 0.273 times the diameter of Earth. This is five times greater than the next largest moon-to-planet diameter ratio (with Neptune’s largest moon at 0.055, Saturn’s at 0.044, Jupiter’s at 0.038 and Uranus’ as 0.031). For the category of planetoids, among the five that are known in the Solar System, Charon has the largest ratio, being half (0.52) the diameter of Pluto.
Definition of a moon
There is no established lower limit on what is considered a “moon”. Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometre across, has been considered a moon, though objects a tenth that size within Saturn’s rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets. The upper limit is also vague. Two orbiting bodies are sometimes described as a double planet rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto–Charon system to be a double (dwarf) planet. The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary because it depends on distance as well as relative mass.
A temporary satellite is an object which has been captured by the gravitational field of a planet and thus became the planet’s natural satellite, but, unlike irregular moons of the larger outer planets of the Solar System, will eventually either leave its orbit around the planet or collide with the planet. The only observed examples are 2006 RH120, a temporary satellite of Earth for nine months in 2006 and 2007, and 2020 CD3, which was discovered in 2020. Some defunct space probes or rockets have also been observed on temporary satellite orbits.
As of February 2020, two objects have been observed at the time when they were temporary satellites: 2006 RH120 and 2020 CD3. According to orbital calculations, on its solar orbit, 2006 RH120 passes Earth at low speed every 20 to 21 years, at which point it can become a temporary satellite again. As of March 2018, there is one confirmed example of a temporarily captured asteroid that didn’t complete a full orbit, 1991 VG. This asteroid was observed for a month after its discovery in November 1991, then again in April 1992, after which it wasn’t seen until May 2017. After the recovery, orbital calculations confirmed that 1991 VG was a temporary satellite of Earth in February 1992. In astrophysics, a temporary satellite is anybody that enters the Hill sphere of a planet at a sufficiently low velocity such that it becomes gravitationally bound to the planet for some period of time.
Most regular moons (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. The only known exception is Saturn’s natural satellite Hyperion, which rotates chaotically because of the gravitational influence of Titan. In contrast, the outer natural satellites of the giant planets (irregular satellites) are too far away to have become locked. For example, Jupiter’s Himalia, Saturn’s Phoebe, and Neptune’s Nereid have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days.
Satellites of satellites
No “moons of moons” or subsatellites (natural satellites that orbit a natural satellite of a planet) are currently known. In most cases, the tidal effects of the planet would make such a system unstable. However, calculations performed after the 2008 detection of a possible ring system around Saturn’s moon Rhea indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow, a phenomenon normally associated with shepherd moons. However, targeted images taken by the Cassini spacecraft failed to detect rings around Rhea. It has also been proposed that Saturn’s moon Iapetus had a satellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge.
Two natural satellites are known to have small companions at both their L4 and L5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are called trojan moons, as their orbits are analogous to the trojan asteroids of Jupiter. The trojan moons are Telesto and Calypso, which are the leading and following companions, respectively, of the Saturnian moon Tethys; and Helene and Polydeuces, the leading and following companions of the Saturnian moon Dione.
The discovery of 243 Ida’s natural satellite Dactyl in the early 1990s confirmed that some asteroids have natural satellites; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two comparably sized components. A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. As of October 2020, there are 416 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 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.Template_SolarMoonSummary-1