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Discovery[1]
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| Discovered by | M. Brown, C. Trujillo, D. Rabinowitz |
| Discovery date | February 17, 2004 |
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Designations
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| MPC designation | 90482 Orcus |
| Pronunciation | /ˈɔrkəs/ OR-kəs, Latin: Orcus |
| Alternate name | 2004 DW |
| Minor planet category |
Plutino[2][3] Plutoid candidate |
| 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) |
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Physical characteristics
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| Dimensions | 946.3 +74.1−72.3 km (diameter)[4] |
| Mass | 6.32 ± 0.05 × 1020 kg (system)[5] |
| Mean density | ~1.6 g/cm³ (comparable to that of Charon)[6] |
| Equatorial surface gravity | ~0.2 m/s² |
| Escape velocity | ~0.44 km/s |
| Rotation period | 10.08 ± 0.01 hr[6] |
| Albedo | 0.28 ± 0.04[5] 19.75 +3.40−2.76 %[4] |
| Temperature | ~44 K[6] |
| Spectral type | (neutral)[6] B-V=0.68; V-R=0.37[7] |
| Apparent magnitude | 19.1 (opposition)[8][9] |
| Absolute magnitude (H) | 2.3[1]
(2.27 ± 0.05)[5] Moon: 4.88 ± 0.05[5] |
90482 Orcus is a large Kuiper Belt object (KBO) with a large companion and is likely a dwarf planet. The discovery images of this object were acquired on February 17, 2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified.[1]
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Orcus is a large plutino (an object in 2:3 orbital resonance with Neptune).[2] 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). Over a 14,000 year period Orcus stays more than 18 AU from Neptune.[10] Because their mutual resonance with Neptune constrains Orcus and Pluto to remain on opposite sides of the Sun in their otherwise very similar motions, Orcus is sometimes described as the "anti-Pluto".[11]
Orcus is currently 47.8 AU from the Sun[8] and will come to aphelion (farthest distance from the Sun) in 2019.[9] Simulations by the Deep Ecliptic Survey (DES) show that over the next 10 million years Orcus can acquire a perihelion distance (qmin) as small as 27.8 AU.[2]
The absolute magnitude of Orcus is 2.3[1] (comparable with the 2.6 of cubewano 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.[4] Orcus appears to have an albedo of about 20%[4] to 30%[5] which may be typical of trans-Neptunian objects approaching the 1000 km diameter range.[12]
Since Orcus is known to be a binary system, the mass of the system has been estimated to be 6.32 ± 0.05 × 1020 kg, or about 3.8% the mass of largest known dwarf planet Eris.[5]
Infrared observations in 2004 by the European Southern Observatory give results consistent with mixtures of water ice and carbonaceous compounds, such as tholins.[13] 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.[14] 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.[14] Reflectance spectrum shows the deepest water ice absorption of any Kuiper belt object (KBO) that is not associated with the Haumea collisional family.[5] Orcus straddles the edge for objects massive enough to retain volatiles such as methane on the surface.[5] The large icy satellites of Uranus have an infared spectra quite similar to the infrared spectrum of Orcus.[5]
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 plutinos like Ixion or Huya.
Spectroscopic observations with the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in 2007 show the presence of crystalline water ice, and possibly ammonia ice on the surface. This may indicate a renewal mechanism on the surface and geological activity.[6] Ammonia has not been detected on any icy satellite of the outer planets other than Miranda.[6] The 1.65 μm band is large and deep (12%), as on Charon, Quaoar, Haumea (2003 EL61), and icy satellites of giant planets.[6] Water ice irradiation to the conditions of a trans-Neptunian object (TNO) at 40 AU indicates that crystalline water ice on the surface of TNOs should be completely amorphized by irradiation in about 10 million years.[6] Cryovolcanism, which is considered the most probable mechanism to explain geological activity on some satellites of the outer planets, may indeed be possible for TNOs larger than about 1000 km.[6]
Using observations with the Hubble Space Telescope from November 13, 2005, Mike Brown and T.A. Suer detected a satellite.[15] The discovery of a satellite of Orcus was reported in IAUC 8812 on 22 February, 2007.[16] The satellite has been given the designation S/2005 (90482) 1. It orbits Orcus in a nearly face-on circular orbit with an eccentricity less than 0.0036, and an orbital period of 9.53 days.[5] It orbits only 8980 ± 20 km from Orcus and is too close to Orcus for ground-based spectroscopy to determine the surface composition of the satellite.[5] Mike Brown also suspects that like the Pluto-Charon system, Orcus and its moon are likely tidally locked.[11]
In 2005, the satellite was found at 0.25 arcsec from Orcus with magnitude difference of 2.7 ± 1.[15] Assuming an albedo similar to that of the primary, the magnitude suggested a quite substantial diameter; perhaps ~250 km or about 1/4th to 1/3rd the diameter of Orcus.[16] Estimates in 2009 by Mike Brown show Vanth is apparent magnitude 21.97 ± 0.05 which is 2.54 ± 0.01 magnitudes fainter than Orcus.[5] This suggests a diameter 3.2 times smaller assuming similar albedos.[5] Assuming equal densities and albedos, measurements imply sizes for Orcus and Vanth of 900 and 280 km, respectively, and a mass ratio of 33.[5] Though if Vanth has an albedo half of that of Orcus, the diameters could instead be 820 km and 640 km with an equal-density mass ratio of only 2.1.[5]
The satellite does not resemble other known collisional satellites and may be a captured KBO.[11] If the satellite was created from an impact event, Mike Brown's team estimates a synchronization time scale of 100–400 Myr for the system.[5]
On March 23, 2009, Brown asked readers of his weekly column to suggest possible names for the satellite, with the best one to be submitted to the International Astronomical Union (IAU) on April 5.[11] The name Vanth, after the Etruscan goddess who guided the souls of the dead to the underworld, was eventually chosen from among a large pool of submissions.[17] This submission will be assessed by the IAU's Committee for Small Body Nomenclature, which will vote on whether to approve it, in accordance with the normal object naming procedures.[18]
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 Etruscan and Roman mythology. The name was also a private reference to the homonymous Orcas Island, where Brown's wife Diane had lived as a child and which they visit frequently.[17] Orcus was approved and published on November 22, 2004.
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   | First BBC article - BBC NEWS | Science/Nature | New world found far beyond Pluto |
| | First New Scientist article - The World's No.1 Science and Technology News Service - New Scientist - New Scientist |
| | MPEC 2004-D13 - MPEC 2004-D13 : 2004 DW |
| | MPEC 2004-D09 - MPEC 2004-D09 : 2004 DW |
| | Preprint on arXiv. - [astro-ph/0504280] Near Infrared Surface Properties of the Two Intrinsically Brightest Minor Planets (90377) Sedna and (90482) Orcus |
| | Kuiper Belt Object Magnitudes and Surface Colors - Magnitudes & Colors |
| | The Surface of the Transneptunian Object 9048 Orcus - The surface of the transneptunian object 90482 Orcus |
| | Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope - [astro-ph/0702538v1] Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope |
| | Distant EKO - Distant EKOs #51 |
| | Ephemeris - HORIZONS Web-Interface |
| | Orbital simulation - JPL Small-Body Database Browser |
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