Orcus
Orcus information Average distance from Sun: 5896946000 km (39.4181 AU) Perihelion (closest): 4567091000 km Perihelion (farthest): 7226801000 km Equatorial radius: 420 - 940 km (Diameter: 840 km) Mass: 620 - 700000000 kg Density: 2 g/cm³ Equatorial surface gravity: 0.2348 - 0.5254 m/s² Escape velocity: 444.1 - 993.9 m/s Siderial orbit period (length of year): 90396.4 earth days Mean orbit velocity: 4680 m/s Orbital eccentricity: 0.22552 Orbital inclination to ecliptic: 20.55233 degrees Equatorial inclination to orbit: 79.561 degrees Surface temperature (min/max): 45 K		Orcus, Sun
Orcus Orcus was originally known by the provisional designation 2004 DW. It is a Kuiper Belt object (KBO) that was discovered by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. The discovery images of this object were acquired on February 17, 2004. Precovery images as early as November 8, 1951 were later identified. Orcus is a typical plutino. Although at one point1 it approaches Neptune's orbit, the orbital resonance keeps an angular separation of more than 60 deg between the two bodies. Orcus and Pluto follow similar but differently oriented orbits as shown on the diagram (Orcus in blue, Pluto in red, Neptune in grey, positions as of April 2006). Both perihelia (cf. apsis) are above the ecliptic. Orcus is approaching the aphelion (2019) while Pluto has passed the perihelion (1989) and is descending toward the ecliptic. Orcus' perihelion is ~30.7AU to be compared with Neptune's axis of 30.07 but the perihelion is not the closest point of the two orbits as it is situated well above the ecliptic. The absolute magnitude of the object is 2.3 to compare with 2.6 for 50000 Quaoar. Using an assumed albedo of 0.09, Trujillo estimates its diameter to be approximately 1600 km, which makes Orcus the largest known plutino and the sixth-largest discovered KBO, after 2003 UB313, Pluto, 2005 FY9, 2003 EL61 and 90377 Sedna. Observations in infrared by ESO give results consistent with mixtures of water ice and carbonaceous compounds. Further, the infrared spectra taken with Gemini telescope confirmed a modest water ice signature, compatible with a cover of 15–30%, but no more than 50% of the surface. It is less than found on Charon, but similar to Triton. Limitations were also placed on the amount of methane ice (less than 30%) leaving open the possibility for discovery of other components in the future. The diversity of colours and spectra even for objects following currently similar orbits is puzzling. Orcus presents a neutral colour as opposed to the second largest known plutino, Ixion (<820km) which is red, had no water ice discovered so far and has albedo higher than Orcus. Since it shares a similar size and orbit to that of Pluto, it too must be named after a deity of the underworld. In accordance with International Astronomical Union astronomical naming conventions, the discoverers' suggested name of Orcus was approved and published November 22, 2004. Orcus is both another name for the Greek deity Hades and a separate god of the dead in Roman mythology. A recent proposal for Hubble Space Telescope observations submitted by Michael Brown mentions, in the abstract, 'the Pluto-Charon analog system of Orcus and its satellite', which indicates that Orcus has a large satellite.
More Discovered by M. Brown, C. Trujillo, D. Rabinowitz in February 17, 2004
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