5145 Pholus (/ˈfoʊləs/; from Greek: Φόλος) provisional designation 1992 AD, is an eccentric centaur in the outer Solar System, approximately 100 to 200 kilometers in diameter, that crosses the orbit of both Saturn and Neptune. It was discovered on 9 January 1992, by American astronomer David L. Rabinowitz (unaccredited) of UA's Spacewatch Project at Kitt Peak National Observatory, and named after the mythological creature Pholus.
The minor planet orbits the Sun at a distance of 8.7–32.0 AU once every 91 years and 12 months (33,601 days). Its orbit has an eccentricity of 0.57 and an inclination of 25° with respect to the ecliptic. The first precovery was taken at the Australian Siding Spring Observatory in 1977, extending the centaur's observation arc by 15 years prior to its discovery. Pholus has not come within one astronomical unit of a planet since 764 BC, and will not until 5290. It is believed that Pholus originated in the Kuiper belt.
Pholus was the second centaur to be discovered and was quickly found to be quite red in color, for which it has been occasionally nicknamed "Big Red". The color has been speculated to be due to organic compounds on its surface. It is classified as a Z class object on the Tholen taxonomic scheme.
The surface composition of Pholus has been estimated from its reflectance spectrum using two spatially segregated components: dark amorphous carbon and an intimate mixture of water ice, methanol ice, olivine grains, and complex organic compounds (tholins). The carbon black component was used to match the low albedo of the object. Unlike Chiron, Pholus has shown no signs of cometary activity. The diameter of Pholus is estimated to be 7002185000000000000♠185±16 km.
The minor planet was named by the Minor Planet Names Committee for the creature Pholus, a centaur from Greek mythology, like his brother Chiron, after which 2060 Chiron was named, in order to follow the tradition of naming this class of outer planet-crossing objects after centaurs. Pholus died by a self-inflicted wound from a poisoned arrow used by Heracles (see 5143 Heracles), who buried Pholus on the mountain Pholoe. Naming citation was published on 14 July 1992 (M.P.C. 20523).