90482 Orcus

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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.

[edit] Orbit

This diagram shows the orbits of Orcus (blue), Pluto (red) and Neptune (grey). The three objects are shown in the current positions (as of April 2006). The dates of their perihelia (q) and aphelia (Q) are also marked.

Orcus is a typical plutino (an object in 2:3 orbital resonance with Neptune). Orcus's orbit is shaped similarly to Pluto's (both have perihelia above the ecliptic), but is differently oriented. Although at one point its orbit approaches that of Neptune, the resonance between the two bodies means that Orcus itself is always a great distance away from Neptune (there is always an angular separation of over 60 degrees between them).

[edit] Physical characteristics

[edit] Size and magnitude

Orcus compared to Eris, Pluto, (136472) 2005 FY₉, (136108) 2003 EL₆₁, Sedna, Varuna, Quaoar, and Earth.

The absolute magnitude of Orcus is 2.3 (comparable with the 2.6 of another KBO, 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 after Pluto/Charon and the sixth-largest known trans-Neptunian object, after Eris, Pluto, (136472) 2005 FY₉, (136108) 2003 EL₆₁ and 90377 Sedna.

[edit] Colours and spectra

Observations in infrared by the European Southern Observatory give results consistent with mixtures of water ice and carbonaceous compounds.^[2] Further, the infrared spectra taken with the Gemini telescope confirmed a modest water ice signature, compatible with a cover of 15–30%, but no more than 50% of the surface. This means there is less ice than on Charon, but a similar amount to that on 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.^[3]

KBOs display a diversity of colours and spectra even among objects with similar orbits. Orcus presents a neutral colour in comparison with the redness of an object like Ixion.

[edit] Satellite

The discovery of a satellite of Orcus was reported in IAUC 8812 on 22 February 2007 [1]. The orbit of this satellite has yet to be determined.

The satellite was found at 0.25 arcsec from Orcus with magnitude difference of 2.7^[4]. Assuming an albedo similar to that of the primary the magnitude suggests a quite substantial diameter (~220 km) compared with Orcus.

[edit] Name

Under the guidelines of the International Astronomical Union's naming conventions, objects with a similar size and orbit to that of Pluto are named after underworld deities. Accordingly, the discoverers suggested naming the object after Orcus, a god of the dead in Roman mythology. The name was approved and published on November 22, 2004.

[edit] 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
• Orbital simulation from JPL (Java)

[edit] References

1. ^ Tegler, Stephen C. (2006-01-26). Kuiper Belt Object Magnitudes and Surface Colors. Retrieved on 2006-11-05.
2. ^ C. de Bergh, A. Delsanti, G. P. Tozzi, E. Dotto, A. Doressoundiram and M. A. Barucci (2005). "The Surface of the Transneptunian Object 9048 Orcus". Astronomy & Astrophysics 437: 1115-1120. DOI:10.1051/0004-6361:20042533. 
3. ^ Chadwick A. Trujillo, Michael E. Brown, David L. Rabinowitz, Thomas R. Geballe (2005). "Near Infrared Surface Properties of the Two Intrinsically Brightest Minor Planets (90377) Sedna and (90482) Orcus". The Astrophysical Journal 627: 1057–1065. DOI:10.1086/430337.  Preprint on arXiv.
4. ^ Distant EKO The Kuiper Belt Electronic newsletter, March 2007