90482 Orcus is a Kuiper belt object with a large moon, Vanth. It was discovered on 17 February
2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz
of Yale University. Precovery images as early as 8 November 1951 were later identified.
Orcus is a plutino, locked in a 2:3 resonance with Neptune, making two revolutions around the
Sun to every three of Neptune's. 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. Moreover, the aphelion of Orcus's orbit points in nearly the opposite direction
from Pluto's, although the eccentricities and inclinations are similar. Because of these similarities
and contrasts, along with its large moon Vanth that recalls Pluto's large moon Charon, Orcus has
been regarded 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
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.
This image comprises 5 x Luminance (180 seconds each) , 0.5m f/2.9 ASA Astrograph with FLI
ML3200 camera at an altitude of 50 degrees on 23 February 2017, 3 days before opposition at
The field of view is 18’ x 18’.
Hills Observatory: 1 January 2013 to 6 May 2018