90482 Orcus

90482 Orcus

Discovery
Discovered by:	M. Brown, C. Trujillo, D. Rabinowitz
Discovery date:	February 17, 2004
Orbital characteristics
Epoch November 10, 1951 (JD 2433960.5)
Aphelion	7226.801 Gm (48.31 AU)
Perihelion:	4567.091 Gm (30.53 AU)
Semi-major axis:	5896.946 Gm (39.419 AU)
Eccentricity:	0.22552
Orbital period:	90396.4 d (247.492 a)
Avg. orbital speed:	4.68 km/s
Mean anomaly:	79.561°
Inclination:	20.55233°
Longitude of ascending node:	268.586°
Argument of perihelion:	73.8325°
Satellites:	1 (92-432 km)
Physical characteristics
Dimensions:	946.3+74.1-72.3 km[1]
Mass:	~7.5×1020 kg
Mean density:	~1.5 g/cm³ (assumed)
Equatorial surface gravity:	~0.2 m/s²
Escape velocity:	~0.44 km/s
Rotation period:	? d
Albedo:	19.75+3.40-2.76 %
Temperature:	~45 K
Spectral type:	B-V=0.68; V-R=0.37 [2]
Apparent magnitude:	19.11
Absolute magnitude:	2.3

90482 Orcus (IPA: /ˈɔrkəs/ or'-kəs, Latin: 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

This diagram shows the orbits of Orcus (blue), Pluto (red) and Neptune (grey). Orcus and Pluto are shown in the April 2006 positions. 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 247 year 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).

Physical characteristics

Size and magnitude

<imagemap> Image:EightTNOs.png|thumb|250px|left|Orcus compared to Eris, Pluto, (136472) 2005 FY₉, (136108) 2003 EL₆₁, Sedna, Varuna, Quaoar, and Earth.

1. Earth

rect 646 1714 2142 1994 The Earth

1. Eris and Dysnomia

circle 226 412 16 Dysnomia circle 350 626 197 (136199) Eris

1. Pluto and Charon

circle 1252 684 86 Charon circle 1038 632 188 (134340) Pluto

1. 2005 FY9

circle 1786 614 142 (136472) 2005 FY9

1. 2003 EL61

circle 2438 616 155 (136108) 2003 EL61

1. Sedna

circle 342 1305 137 (90377) Sedna

1. Orcus

circle 1088 1305 114 (90482) Orcus

1. Quaoar

circle 1784 1305 97 (50000) Quaoar

1. Varuna

circle 2420 1305 58 (20000) Varuna desc none

1. - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
2. Notes:
3. Details on the new coding for clickable images is here:
4. While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
5. Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.

</imagemap> The absolute magnitude of Orcus is 2.3 (comparable with the 2.6 of another KBO, 50000 Quaoar). In the first quarter of 2007 a paper was published, showing the Spitzer space telescope had detected Orcus in the far infrared, during its first three years in operation, constraining the size to 946.3^+74.1_-72.3 km.^[1]. Orcus appears to have a high albedo of ~ 20 %.

Colours and spectra

Observations in infrared by the European Southern Observatory give results consistent with mixtures of water ice and carbonaceous compounds.^[3] 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.^[4] 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.

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^[5]. Assuming an albedo similar to that of the primary the magnitude suggests a quite substantial diameter (~220 km) compared with Orcus.

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.

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) / Ephemeris

References

1. ^ ^{a b} Stansberry, J.; Grundy, W.; Brown, M.; et.al. (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope".
2. ^ Tegler, Stephen C. (2006-01-26). Kuiper Belt Object Magnitudes and Surface Colors. Retrieved on 2006-11-05.
3. ^ 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.
4. ^ 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.
5. ^ Distant EKO The Kuiper Belt Electronic newsletter, March 2007