# (90482) Orcus

Asteroid
(90482) Orcus
Orcus with moon Vanth. Photo from the Hubble Space Telescope
Properties of the orbit ( animation )
Epoch:  April 27, 2019 ( JD 2,458,600.5)
Orbit type Plutino ,
"Distant Object"
Major semi-axis 39.266  AU
eccentricity 0.224
Perihelion - aphelion 30.465 AU - 48.068 AU
Inclination of the orbit plane 20.6 °
Length of the ascending node 268.8 °
Argument of the periapsis 72.2 °
Time of passage of the perihelion February 11, 2142
Sidereal period 246 a 0.7 M
Mean orbital velocity 4.714 km / s
Physical Properties
Medium diameter ${\ displaystyle 917 _ {- 25} ^ {+ 25} \ mathrm {km}}$
Dimensions 6:41 ± 0:19  ·  10 20 kg
Albedo ${\ displaystyle 0 {,} 231 _ {- 0.011} ^ {+ 0.018} \ mathrm {}}$
Medium density ${\ displaystyle 1 {,} 53 _ {- 0.13} ^ {+ 0.15} \ mathrm {}}$ g / cm³
Rotation period 9.5393 ± 0.0001 h (0.397 d )
9.7 ± 0.3 h (0.404 d )
Absolute brightness 2.31 ± 0.03 mag
Spectral class C
B-V = 0.700 ± 0.020
VR = 0.370 ± 0.020
VI = 0.740 ± 0.040
BR = 1.070 ± 0.020
history
Explorer Michael E. Brown
David L. Rabinowitz
Date of discovery February 17, 2004
Another name 2004 DW
«Anti-Pluto»
Source: Unless otherwise stated, the data comes from JPL Small-Body Database Browser . The affiliation to an asteroid family is automatically determined from the AstDyS-2 database . Please also note the note on asteroid items.

(90482) Orcus (former designation 2004 DW ) is a large trans-Neptunian object in the Kuiper belt , which is classified as Plutino in terms of orbital dynamics . Because of its size, the asteroid is a dwarf planet candidate . It has a well-known moon called Vanth , which, according to current knowledge, is about half the diameter of the mother asteroid. Therefore this system can also be understood as a double asteroid system.

## Discovery and naming

Orcus was discovered on February 17, 2004 by a team of astronomers consisting of Mike Brown , Chad Trujillo and David Lincoln Rabinowitz of the California Institute of Technology (CalTech) at the 1.2 m Oschin Schmidt telescope at the Palomar Observatory ( California ) discovered. The discovery was announced on February 19, 2004, the planetoid was given the preliminary designation 2004 DW and later the minor planet number 90482 . Orcus was the fifth discovery of a large TNO and likely dwarf planet by Mike Brown's team of astronomers. Brown's team successively discovered Quaoar and 2002 MS 4 (2002), Sedna (2003) and Haumea (2003, controversial); Orcus was followed by Salacia (2004) and the dwarf planets Eris and Makemake (2005) and Gonggong (2007).

Orcus is sometimes referred to as "anti-Pluto" due to the similar orbital elements and some other parallels to the Pluto - Charon system, especially due to the fact that both celestial bodies are on the opposite side of the sun. Mike Brown's discoverers suggested naming him after Orkus , the Roman god of the underworld. This proposal was officially accepted by the IAU on November 26, 2004 . The name corresponds to the IAU conventions that objects of similar sizes and orbital elements like Pluto should be named after underworld gods. The Etruscan god Orcus is associated with Pluton and represents his evil side, who punished the oath breakers. He was depicted in drawings in Etruscan tombs as a hairy, bearded giant. The name was also a private reference to the island of Orcas , where Brown's wife Diana had lived as a child and who both visit regularly.

Like all other Trans-Neptunian objects except Pluto, Orcus has no official or commonly used astronomical symbol . Orcus symbols circulating on the Internet such as B. are designs from private individuals. An official symbol assignment is not to be expected, since astronomical symbols only play a subordinate role in modern astronomy.

After his discovery, Orcus could be identified in photos going back up to November 8, 1951, which were also taken at the Palomar Observatory as part of the Digitized Sky Survey project, and thus extended his observation period by 53 years, making his orbit more precise to calculate. Since then, the asteroid has been observed through various telescopes such as the Hubble , Herschel and Spitzer Space Telescopes, as well as Earth-based telescopes. In April 2017, there were a total of 542 observations over a period of 66 years. The last observation so far was made in February 2019 at the ATLAS telescope ( Hawaii ). (As of March 4, 2019)

## properties

 Orcus' orbit (red above, blue below) compared to Pluto, Neptune and others.

### Orbit

Orcus orbits the Sun in 246.06 years in an elliptical orbit between 30.46  AU and 48.07 AU from its center. The orbital eccentricity is 0.224; the orbit is inclined 20.59 ° to the ecliptic . Currently, the planetoid is 48.07 AU from the sun. So it is almost at its aphelion , the point of its orbit furthest from the sun. The next time it passes through perihelion in 2142, the last perihelion should have been in 1896. Simulations by the Deep Ecliptic Survey showed that Orcus can achieve a minimum perihelion distance of 27.8 AU over the next 10 million years.

The 2: 3 orbit resonance to Neptune keeps Orcus almost on the opposite side of the sun . Orcus is at its aphelion when Pluto is at perihelion, and vice versa; the perihelia of both planetoids are above the ecliptic. Although Orcus approaches Neptune's orbit at one point, it never comes close to the planet; the angular distance between the two bodies is always more than 60 °. Orcus always remains more than 18 AU away from Neptune over a period of 14,000 years.

Both Marc Buie ( DES ) and the Minor Planet Center classify the planetoid as Plutino ; the latter also generally lists it as a "distant object" .

### size

Until the discovery of the dwarf planet Eris was published in July 2005, Orcus - with a diameter of 1,600 to 1,800 km at the time - was possibly the largest newly discovered celestial body in the solar system since the discovery of Pluto. Investigations in 2013 with the Herschel space telescope (instruments SPIRE and PACS) combined with the revised data from the Spitzer space telescope (instrument MIPS), however, came to the conclusion that the diameter of Orcus is 917 ± 25 km, that of Vanth 276 ± 17 km , based on a retroreflective power of 23.1% and an absolute brightness of 2.31  m. Recent studies based on a star occultation showed a diameter of 442.5 ± 10.2 km for Vanth.

Based on a diameter of 917 km, the total area is about 2,642,000 km². The apparent magnitude of Orcus is 19.09  m ; the mean surface temperature is estimated at 44 K (−229 ° C) based on the distance from the sun  .

Most likely Orcus belongs to the class of dwarf planets . Both Mike Brown and Gonzalo Tancredi come to the conclusion that Orcus is almost certainly a dwarf planet, since due to its estimated size and mass it is probably in hydrostatic equilibrium , i.e. almost spherical ( Maclaurin ellipsoid ) . Gonzalo Tancredi proposes that the IAU officially recognize him as such. Orcus is believed to be only slightly smaller than the dwarf planet Ceres , which is 975 km in diameter.

Provisions of the diameter for Orcus
year Dimensions km source
2008 1540.0 Tancredi
2007 946.3 + 074.1- 072.3 (System) Stansberry et al. a.
2009 940 ± 70.0 (system)
900.0
Brown et al. a.
2013 850.0 ± 90.0 (system) Lim u. a.
2010 946.0 Tancredi
2011 1086.0 (system)
1040 ± 240.0
Grundy et al. a.
2011 850.0 ± 90.0 (system)
807.0 ± 100.0
Carry u. a.
2013 958.4 ± 22.9 (system)
917.0 ± 25.0
Fornasier et al. a.
2013 936.0 Mommert et al. a.
2014 <782.0 (system)
<749.0
Thirouin et al. a.
2017 965.0 ± 40.0 (system)
885.0 +55.0−80.0
Brown et al. a.
2017 960.0 + 045.0- 042.0 (System) Lellouch u. a.
2018 1027.0 (system)
910.0 +50.0−40.0
Brown et al. a.
2018 983.0 Brown
2019 1525.90 LightCurve DataBase
The most precise determination is marked in bold .

### rotation

There is still some uncertainty about the rotation period. Different light curve observations gave different results. Some showed slight variations with orbital times of 7 to 21 hours, while others showed no changes at all. The value of 9.7 hours, which was determined in 2010 by a team led by Ortiz, seems to be the most likely. The poles of rotation may coincide with the orbital poles of Vanth, which means that one pole of Orcus is currently pointing towards the Earth; this would explain the near-absence of the brightness changes. Brown assumes that Vanth has a bound rotation , so Orcus always shows the same side.

Based on periods of rotation of 9.54 and 9.7, the result is that the planetoid performs 226111.9 and 222365.9 rotations (“days”), respectively, in an Orcus year .

### surface

The surface of Orcus is relatively light with a reflectivity of 23%, has a gray color and an abundance of water ice . The ice is predominantly in crystalline form, which indicates previous cryovolcanic activity. Other components such as methane and ammonia ice could also appear on the surface.

First spectroscopic investigations in 2004 showed that the visible spectrum of Orcus is flat (color-neutral) and structureless, while investigations in the near infrared showed moderately strong water absorption bands at 1.5 and 2.0 μm. Orcus seemed to differ from other TNOs like Ixion with red visible and often structureless spectra. Further research by the European Southern Observatory and the Gemini Observatory produced results that are consistent with a mixture of water ice and carbonaceous components such as tholines . Water and methane ice can make up more than 50% and 30% of the surface; this means that the ratio of ice on the surface is lower than on Charon , but similar to that on Triton .

Further investigations from 2008 to 2010 in the infrared revealed additional spectral structures. Among them is a deep water absorption band at 1.65 μm, which is evidence of the crystalline water ice on the surface, and a new absorption band at 2.22 μm. The reason for this has not been conclusively clarified. It can be created by ammonia or (ionized) ammonium that is dissolved in the water ice or by the methane / ethane ice . Radiation transmission models showed that a mixture of water ice, tholines (as a darkening component), ethane ice and ammonium fits the spectrum best, while a combination of water ice, tholines, methane ice and ammonium hydrate produces a slightly deeper result. On the other hand, a mixture of ammonium hydrate only, tholines, and water ice did not make a suitable match. So (as of 2010) the only reliably identified components are crystalline water ice and possibly dark tholines. A more reliable identification of ammonia, methane and other hydrocarbons requires better infrared spectra.

Orcus is on the limit for TNOs large enough to hold volatiles like methane on the surface. Its spectrum shows the deepest water ice absorption band of all Kuiper belt objects that are not associated with the Haumea collision family. The largest ice moons of Uranus have very similar infrared spectra to Orcus. Among the other Trans-Neptunian objects, Pluto's companion, Charon, appears to be most similar. Charon has a higher albedo, but a very similar visible and near-infrared spectrum, a similar mean density and both bodies have water-rich surfaces. The Plutino 2003 AZ 84 also has similar spectral properties. Quaoar is comparable in size, but its water ice absorption structures are higher in the spectrum and the surface has a strong red color in visible light, which is an indication of the presence of ultra-red matter. The members of the Haumea family have much higher albedos and much deeper water ice absorption bands than Orcus.

### Cryovolcanism

The presence of crystalline water ice, and perhaps ammonia ice, is an indication that surface altering processes have taken place in Orcus' past. Ammonia has so far not been found on any TNO or ice moon on the outer planets with the exception of Miranda . The 1.65 μm absorption band on Orcus is wide and deep (12%), as on Charon, Quaoar, Haumea and the icy moons of the outer planets. On the other hand, the crystalline water ice on the surface of the Trans-Neptune would have to be completely transformed into amorphous water ice by the solar radiation within about 10 million years without any regenerative processes . Some calculations show that cryovolcanism - as one of the regenerative processes - is quite possible for TNO around 1000 km. Orcus may have experienced such a period at least once in his past that turned the amorphous ice into crystalline. The preferred form of volcanism may have been an explosive aqueous cryoulcanism, which results from the explosive breakdown of methane from melting water and ammonia.

Models of internal heating from radioactive decay show that Orcus may be able to maintain an ocean of liquid water below its surface.

## moon

In February 2007, a team led by Mike Brown announced the discovery of the moon Vanth, which was discovered on recordings from 2005. By analyzing the orbit, the mass of the Orcus-Vanth system could be determined to be 6.41 ± 0.19  ·  10 20 kg. This corresponds to about 3.8% of the mass of the dwarf planet Eris . How the mass is distributed between Orcus and Vanth depends on their size ratio. If the diameter of Vanth corresponds to about a third of the Orcus diameter, its mass would be only 3% of the system mass, at half the Orcus diameter - which corresponds approximately to the current value - the mass of Vanth could be up to 1/12 of the system mass or 8 % of the Orcus mass (1.10  ·  10 15  kg). The mean density of Orcus is about 1.5 g / cm³, the same density is assumed for Vanth.

The Orcus system at a glance:

Components Physical parameters Path parameters discovery
Surname Throughput
diameter
(km)
Relative
size
%
Mass
(kg)
Major
semi-axis
(km)
Orbital time
(d)
eccentricity
Inclination
to Orcus'
equator
Date of discovery
Date of publication
(90482) Orcus
917.0 100.00 6.41 · 10 20 - - - - February 17, 2004
February 19, 2004
Vanth
(Orcus I)
442.5 48.26 1.10 · 10 15 8999 9.53916 0.0009 90.2 ° November 13, 2005
February 22, 2007

Commons : (90482) Orcus  - collection of images, videos and audio files

## Individual evidence

1. ^ A b Marc W. Buie : Orbit Fit and Astrometric record for 90482 . SwRI (Space Science Department). Retrieved March 4, 2019.
2. a b MPC : MPEC 2010-S44: Distant Minor Planets (2010 OCT.11.0 TT) . IAU . September 25, 2010. Accessed March 4, 2019.
3. E. Lellouch et al. a .: “TNOs are Cool”: A survey of the trans-Neptunian region. IX. Thermal properties of Kuiper belt objects and Centaurs from combined Herschel and Spitzer observations (PDF) . In: Astronomy and Astrophysics . 557, No. A60, June 10, 2013, p. 19. bibcode : 2013A & A ... 557A..60L . doi : 10.1051 / 0004-6361 / 201322047 .
4. a b c (90482) Orcus at IAU Minor Planet Center (English) Retrieved March 4, 2019.
5. v ≈ π * a / period (1 + sqrt (1-e²))
6. S. Fornasier u. a .: “TNOs are Cool”: A survey of the trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of nine bright targets at 70-500 µm . In: Astronomy and Astrophysics . 555, No. A15, June 19, 2013, p. 22. arxiv : 1305.0449v2 . bibcode : 2013A & A ... 555A..15F . doi : 10.1051 / 0004-6361 / 201321329 .
7. a b c B. Carry u. a .: Integral-field spectroscopy of (90482) Orcus-Vanth . In: Astronomy & Astrophysics . 534, No. A115, October 18, 2011. arxiv : 1108.5963 . doi : 10.1051 / 0004-6361 / 201117486 .
8. a b c W. Grundy u. a .: Five New and Three Improved Mutual Orbits of Transneptunian Binaries (PDF) . In: Icarus . 213, No. 2, March 14, 2011, pp. 678-692. arxiv : 1103.2751 . bibcode : 2011Icar..213..678G . doi : 10.1016 / j.icarus.2011.03.012 .
9. a b J. L. Ortiz et al. a .: A mid-term astrometric and photometric study of trans-Neptunian object (90482) Orcus (PDF) . In: Astronomy and Astrophysics . 525, No. A31, October 29, 2010, p. 12. arxiv : 1010.6187 . bibcode : 2011A & A ... 525A..31O . doi : 10.1051 / 0004-6361 / 201015309 .
10. a b LCDB Data for (90482) Orcus . MinorPlanetInfo. 2019. Retrieved March 4, 2019.
11. a b c S. Tegler u. a .: Two Color Populations of Kuiper Belt and Centaur Objects and the Smaller Orbital Inclinations of Red Centaur Objects (PDF) . In: The Astronomical Journal . 152, No. 6, December 2016, p. 210, 13. bibcode : 2016AJ .... 152..210T . doi : 10.3847 / 0004-6256 / 152/6/210 .
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13. MPC : MPEC 2004-D09: 2004 DW . IAU . February 19, 2004. Retrieved March 4, 2019.
14. MPC : MPEC 2004-D13: 2004 DW . IAU . February 20, 2004. Retrieved March 4, 2019.
15. MPC : MPEC 2004-D15: 2004 DW . IAU . February 20, 2004. Retrieved March 4, 2019.
16. MPC : MPC / MPO / MPS Archive . IAU . Retrieved on March 4, 2019, reference there: MPC 53177
17. (90482) Orcus in the Small-Body Database of the Jet Propulsion Laboratory (English). Retrieved March 4, 2019.
18. AstDyS-2: (90482) Orcus . Universita di Pisa. Retrieved March 4, 2019.
19. a b Mike Brown : How many dwarf planets are there in the outer solar system? . CalTech . November 12, 2018. Retrieved March 4, 2019.
20. a b Gonzalo Tancredi: Physical and dynamical characteristics of icy “dwarf planets” (plutoids) (PDF) . In: International Astronomical Union (Ed.): Icy Bodies of the Solar System: Proceedings IAU Symposium No. 263, 2009 . 2010. doi : 10.1017 / S1743921310001717 . Retrieved March 4, 2019.
21. ^ Gonzalo Tancredi, Sofía Favre: DPPH List . In: Dwarf Planets and Plutoid Headquarters, from Which are the dwarfs in the solar system? . August. Retrieved March 4, 2019.
22. J. Stansberry et al. a .: Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope (PDF) . In: University of Arizona Press . 592, No. 161-179, February 20, 2007. arxiv : astro-ph / 0702538 . bibcode : 2008ssbn.book..161S .
23. M. Brown et al. a .: The size, density, and formation of the Orcus-Vanth system in the Kuiper belt (PDF) . In: The Astronomical Journal . 139, No. 6, October 26, 2009, pp. 2700-2705. arxiv : 0910.4784 . bibcode : 2010AJ .... 139.2700B . doi : 10.1088 / 0004-6256 / 139/6/2700 .
24. T. Lim et al. a .: “TNOs are Cool”: A survey of the trans-Neptunian region. III. Thermophysical properties of 90482 Orcus and 136472 Makemake (PDF) . In: Astronomy and Astrophysics . 518, No.L148, April 2010, p. 5. bibcode : 2010A & A ... 518L.148L . doi : 10.1051 / 0004-6361 / 201014701 .
25. M. Mommert et al. a .: Remnant planetesimals and their collisional fragments: Physical characterization from thermal-infrared observations . September 23, 2013. Accessed March 4, 2019.
26. A. Thirouin et al. a .: Rotational properties of the binary and non-binary populations in the Trans-Neptunian belt . In: Astronomy and Astrophysics . 569, No. A3, July 5, 2014, p. 20. arxiv : 1407.1214 . bibcode : 2014A & A ... 569A ... 3T . doi : 10.1051 / 0004-6361 / 201423567 .
27. ^ M. Brown : The Density of Mid-sized Kuiper Belt Objects from ALMA Thermal Observations . In: The Astronomical Journal . 154/1, July 7, 2017, pp. 19, 7 pp. arxiv : 1702.07414 . bibcode : 2017AJ .... 154 ... 19B . doi : 10.3847 / 1538-3881 / aa6346 .
28. E. Lellouch et al. a .: The thermal emission of Centaurs and Trans-Neptunian objects at millimeter wavelengths from ALMA observations . In: Astronomy and Astrophysics . 608, No. A45, September 20, 2017, p. 21. arxiv : 1709.06747 . bibcode : 2017A & A ... 608A..45L . doi : 10.1051 / 0004-6361 / 201731676 .
29. M. Brown et al. a .: Medium-sized satellites of large Kuiper belt objects . In: The Astronomical Journal . 156, No. 4, January 22, 2018, p. 164, 6. arxiv : 1801.07221 . bibcode : 2018AJ .... 156..164B . doi : 10.3847 / 1538-3881 / aad9f2 .
30. a b c A. Delsanti u. a .: Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO? A portrait of Plutino (90482) Orcus (PDF) . In: Astronomy and Astrophysics . 520, No. A40, June 25, 2010, p. 15. arxiv : 1006.4962 . bibcode : 2010A & A ... 520A..40D . doi : 10.1051 / 0004-6361 / 201014296 .
31. S. Fornasier et al. a .: Water ice on the surface of the large TNO 2004 DW . In: Astronomy and Astrophysics . 422, July 2004, pp. L43-L46. bibcode : 2004A & A ... 422L..43F . doi : 10.1051 / 0004-6361: 20048004 .
32. C. de Bergh et al. a .: The surface of the transneptunian object 90482 Orcus . In: Astronomy and Astrophysics . 347, No. 2, July 3, 2005, pp. 1115-1120. bibcode : 2005A & A ... 437.1115D . doi : 10.1051 / 0004-6361: 20042533 .
33. C. Trujillo et al. a .: Near-Infrared Surface Properties of the Two Intrinsically Brightest Minor Planets: (90377) Sedna and (90482) Orcus . In: The Astrophysical Journal . 627, No. 2, April 12, 2005, pp. 1057-1065. arxiv : astro-ph / 0504280 . bibcode : 2005ApJ ... 627.1057T . doi : 10.1086 / 430337 .
34. a b M. Barucci et al. a .: Surface composition and temperature of the TNO Orcus . In: Astronomy and Astrophysics . 549, No. 1, February 2008, pp. L13-L16. bibcode : 2008A & A ... 479L..13B . doi : 10.1051 / 0004-6361: 20079079 .
35. H. Hussmann et al. a .: Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects (PDF) . In: Icarus . 185, No. 1, November 2006, pp. 258-273. arxiv : 0910.4784 . bibcode : 2006Icar..185..258H . doi : 10.1016 / j.icarus.2006.06.005 .
36. Asteroids with Satellites - (90482) Orcus and Vanth