90482 Orcus

90482 Orcus

Discovery
Discovered by	M. Brown, C. Trujillo, D. Rabinowitz
Discovery date	February 17, 2004
Designations
Alternate name	2004 DW
Minor planet category	Plutino[1]
Orbital characteristics[2]
Epoch November 30, 2008 (JD 2 454 800)
Aphelion	7 188.17 Gm (48.05 AU)
Perihelion	4 535.80 Gm (30.32 AU)
Semi-major axis	5 862.44 Gm (39.188 AU)
Eccentricity	0.226 18
Orbital period	89 606 d (245.33 yr)
Average orbital speed	4.68 km/s
Mean anomaly	164.68°
Inclination	20.593°
Longitude of ascending node	268.722°
Argument of perihelion	72.474°
Satellites	1 (92-432 km)
Physical characteristics
Dimensions	946.3 +74.1−72.3 km (diameter)[3]
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	13.19h[4]
Albedo	19.75 +3.40−2.76 %
Temperature	~45 K
Spectral type	B-V=0.68; V-R=0.37 [5]
Apparent magnitude	19.1 (opposition)[6]
Absolute magnitude (H)	2.3[2]

90482 Orcus (pronounced /ˈɔrkəs/ OR-kəs, Latin: Orcus, originally known by the provisional designation 2004 DW) is a Kuiper Belt object (KBO) and a likely dwarf planet 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 large plutino (an object in 2:3 orbital resonance with Neptune).^[1] 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

Orcus compared to Eris, Pluto, Makemake, Haumea, Sedna, Varuna, Quaoar, and Earth.

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 diameter to 946.3^+74.1_−72.3 km.^[3] 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.^[7] 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.^[8]

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.^[9] 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.^[10] Assuming an albedo similar to that of the primary, the magnitude suggests a quite substantial diameter (~220 km) compared with Orcus. The high ice content of Orcus makes Brown think its satellite is not a collisional fragment, but a small captured KBO.

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