90482 Orcus

90482 Orcus (originally known by the provisional designation 2004 DW) 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.

Orbit

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 are above the ecliptic. Orcus is approaching the aphelion (2019) while Pluto has passed the perihelion (1989) and is descending toward the ecliptic.

¹Orcus’s 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.

Size and magnitude

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 , Pluto, , and 90377 Sedna.

Physical characteristics

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.

Name

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.

Possible satellite

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.

External links and sources

MPEC 2004-D09 announcing the discovery but attributing it to Raymond J. Bambery, Steven H. Pravdo, Michael D. Hicks, Kenneth J. Lawrence, Daniel MacDonald, Eleanor F. Helin and Robert Thicksten / NEAT
MPEC 2004-D13 correcting MPEC 2004-D09
Chad Trujillo's page on 2004 DW
First BBC article
First New Scientist article
Updated orbital elements from Lowell Observatory
AstDys orbital elements
Orbit simulation from JPL (Java).

References

Trans-Neptunian objects