90482 Orcus is a trans-Neptunian object in the Kuiper belt with a large moon. It was discovered on February 17, 2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified. It is believed to be a dwarf planet by some astronomers, though the IAU has not formally recognized it as one.
Orcus is a plutino, locked in a 2:3 resonance with Neptune, making two revolutions around the Sun, while Neptune makes three. This is much like Pluto, except that it is constrained to always be in the opposite phase of its orbit from Pluto: Orcus is at aphelion when Pluto is at perihelion and vice versa. Because of this, along with its large moon Vanth that recalls Pluto's large moon Charon, Orcus has been seen as the anti-Pluto. This was a major consideration in selecting its name, as the deity Orcus was the Etruscan equivalent of the Roman Pluto, and later became an alternate name for Pluto.
The surface of Orcus is relatively bright with albedo reaching 30%, grey in color and water rich. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane or ammonia may also be present. The existence of a satellite allowed astronomers to determine the mass of the system, which is approximately equal to that of the Saturnian moon Tethys. The ratio of masses of Orcus and Vanth is uncertain, possibly anywhere from 1:33 to 1:12. The diameter of Orcus is estimated to be 761 or 807 km and the diameter of Vanth 378 or 267 km respectively, depending on their relative albedos.
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... The discovery of a satellite of Orcus was reported in IAUC 8812 on 22 February 2007 ... was given the designation S/2005 (90482) 1 before later being named Vanth ... It orbits Orcus in a nearly face-on circular orbit with an eccentricity of about 0.007, and an orbital period of 9.54 days ...