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.
|
Orbits of Orcus (blue) and Pluto (red). |
Orcus is a typical
plutino. Although at one point
1 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.
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.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.
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
.
*
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).
*
Abstract*
Preprint on arXiv.
