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5145 Pholus (/ˈfləs/ FOE-ləs; from Φόλος) provisional designation 1992 AD, is an eccentric centaur in the outer Solar System, approximately 180 kilometers (110 miles) in diameter, that crosses the orbit of both Saturn and Neptune. It was discovered on 9 January 1992, by American astronomer David Rabinowitz (unaccredited) of UA's Spacewatch survey at the Kitt Peak National Observatory in Arizona, United States. The very reddish object has an elongated shape and a rotation period of 9.98 hours.[17] It was named after the creature Pholus from Greek mythology.[2]

5145 Pholus
Discovery [2]
Discovered bySpacewatch
(D. Rabinowitz uncredited)[1]
Discovery siteKitt Peak National Obs.
Discovery date9 January 1992
MPC designation(5145) Pholus
Pronunciation/ˈfləs/ · FOE-ləs
Named after
(Greek mythology)[3]
1992 AD
centaur[4][5][6] · distant[2]
Orbital characteristics[4]
Epoch 27 April 2019 (JD 2458600.5)
Uncertainty parameter 1
Observation arc31.74 yr (11,593 d)
Aphelion32.007 AU
Perihelion8.7785 AU
20.393 AU
92.09 yr (33,637 d)
0° 0m 38.52s / day
Jupiter MOID3.469 AU
Physical characteristics
Dimensions310 km × 160 km × 150 km[7]
Mean diameter
99±15 km[8]
107 km[5]
185±16 km[9]
190 km[4][7]
9.980 h[7]
Tholen = Z [4]
RR (very red)[10]
B−V = 1.19[7]
V−R = 0.78[7]
7.198±0.056 (R)[13]

Orbit and classificationEdit

Orbital diagram of 5145 Pholus

Pholus was the second centaur to be discovered.[18] Centaurs are objects in between the asteroid and trans-Neptunian populations of the Solar System – that is, beyond Jupiter's and within Neptune's orbit – which behave with characteristics of both asteroids and comets.

It orbits the Sun at a distance of 8.8–32.0 AU once every 92 years and 1 month (33,637 days; semi-major axis of 20.39 AU). Its orbit has an eccentricity of 0.57 and an inclination of 25° with respect to the ecliptic.[4] It is a Saturn-, Uranus- and Neptune-crosser, crossing the orbits of these giant planets at a mean-distance of 9.6, 11.9 and 30.1 AU from the Sun, respectively. Pholus has not come within one astronomical unit of a planet since 764 BC, and will not until 5290.[19] It is believed that it originated in the Kuiper belt.

Discovery and namingEdit

Pholus was discovered by David Rabinowitz (not officially credited), working with the Spacewatch program, at Kitt Peak National Observatory on 9 January 1992.[2] Rabinowitz'es discovery was confirmed by Eugene and Carolyn Shoemaker who identified the object on images they previously took on 1 January 1992. The discovery was announced by James Scotti on 23 January 1992 in a IAU Circular (IAUC 5434) of the Central Bureau for Astronomical Telegrams.[1] A first precovery was taken at the Australian Siding Spring Observatory in 1977, extending the centaur's observation arc by 15 years prior to its discovery.[2] It was the second discovery of a centaur after 2060 Chiron discovered by Charles Kowal in 1977. In 1993, while with the Spacewatch program, David Rabinowitz went on to discover another centaur, 7066 Nessus.

This minor planet was named by the Minor Planet Names Committee for Pholus, a centaur from Greek mythology. As with 2060 Chiron, named after his brother Chiron, the tradition is to name this class of outer planet-crossing objects after the half-human, half-horse mythological creatures. In the mythological account, Pholus died by a self-inflicted wound from a poisoned arrow used by Heracles (see 5143 Heracles), who buried Pholus on the mountain Pholoe.[3] The official naming citation was published by the Minor Planet Center on 14 July 1992 (M.P.C. 20523).[20]

Physical characteristicsEdit

Spectral type and colorEdit

After its discovery, Pholus was quickly found to be very red in color, for which it has been occasionally nicknamed "Big Red". The color has been speculated to be due to organic compounds on its surface.[18] It is classified as a Z class object on the Tholen taxonomic scheme.[4]

The object has been classified by astronomers as RR and RR-U type, respectively.[10][11] Polarimetric observations with ESO's Very Large Telescope in 2007 and 2008, revealed noticeable negative polarization at certain phase angles, distinctly different from that of trans-Neptunian objects. Pholus appears to have a rather homogeneous surface with small amount of water frost on its darker parts.[11]

The surface composition of Pholus has been estimated from its reflectance spectrum using two spatially segregated components:[21] dark amorphous carbon and an intimate mixture of water ice, methanol ice, olivine grains, and complex organic compounds (tholins). The carbon black component was used to match the low albedo of the object. Unlike Chiron, Pholus has shown no signs of cometary activity.

Diameter and albedoEdit

Diameter calculations range from 99 to 190 kilometers with a corresponding albedo between 0.155 and 0.04.[5][7][8][17][9]

According to the Herschel Space Observatory with its PACS instrument, Nessus measures 99 kilometers in diameter and its surface has an albedo of 0.155,[8] while a study from 1996 derived a diameter of 185 km.[9] During 2003–2004, observations with the 1.8-meter Vatican Advanced Technology Telescope (VATT) on Mount Graham Observatory, Arizona, determined an elongated shape, 310 km × 160 km × 150 km, with a mean-diameter of 190 kilometers, based on a low albedo of 0.04.[7] Johnston's archive lists a diameter of 107 km with an albedo of 0.126,[5] and Collaborative Asteroid Lightcurve Link assumes a standard albedo of a carbonaceous body of 0.057 and derives a diameter of 165 km based on an absolute magnitude of 7.64.[17]

Rotation periodEdit

In March 2005, a rotational lightcurve of Pholus was obtained from photometric observations by Tegler using the VATT at Mount Graham. Lightcurve analysis gave a rotation period of 9.980 hours with a brightness amplitude of 0.60 magnitude (U=3-).[7] Alternative period determinations were also conducted by Hoffmann, Franham and Buie with concurring results of 9.977, 9.982 and 9.983 hours, respectively (U=3/3/3).[22][23][24]


  1. ^ a b "IAUC 5434: 1992 AD". Central Bureau for Astronomical Telegrams. 23 January 1992. Retrieved 26 November 2018.
  2. ^ a b c d e f "5145 Pholus (1992 AD)". Minor Planet Center. Retrieved 26 November 2018.
  3. ^ a b Schmadel, Lutz D. (2007). "(5145) Pholus". Dictionary of Minor Planet Names – (5145) Pholus. Springer Berlin Heidelberg. p. 443. doi:10.1007/978-3-540-29925-7_4997. ISBN 978-3-540-00238-3.
  4. ^ a b c d e f g h "JPL Small-Body Database Browser: 5145 Pholus (1992 AD)" (2009-04-18 last obs.). Jet Propulsion Laboratory. Retrieved 26 November 2018.
  5. ^ a b c d Johnston, Wm. Robert (7 October 2018). "List of Known Trans-Neptunian Objects". Johnston's Archive. Retrieved 26 November 2018.
  6. ^ "Asteroid 5145 Pholus". Small Bodies Data Ferret. Retrieved 26 November 2018.
  7. ^ a b c d e f g h i Tegler, S. C.; Romanishin, W.; Consolmagno, G. J.; Rall, J.; Worhatch, R.; Nelson, M.; et al. (June 2005). "The period of rotation, shape, density, and homogeneous surface color of the Centaur 5145 Pholus". Icarus. 175 (2): 390–396. Bibcode:2005Icar..175..390T. doi:10.1016/j.icarus.2004.12.011. Retrieved 16 November 2016.
  8. ^ a b c d Duffard, R.; Pinilla-Alonso, N.; Santos-Sanz, P.; Vilenius, E.; Ortiz, J. L.; Mueller, T.; et al. (April 2014). ""TNOs are Cool": A survey of the trans-Neptunian region. XI. A Herschel-PACS view of 16 Centaurs". Astronomy and Astrophysics. 564: 17. arXiv:1309.0946. Bibcode:2014A&A...564A..92D. doi:10.1051/0004-6361/201322377.
  9. ^ a b c Davies, J. K.; Tholen, D. J.; Ballantyne, D. R. (December 1995). "Infrared Observations of Distant Asteroids". Completing the Inventory of the Solar System. 107: 97–105. Bibcode:1996ciss.conf...97D. Retrieved 26 November 2018.
  10. ^ a b Perna, D.; Barucci, M. A.; Fornasier, S.; DeMeo, F. E.; Alvarez-Candal, A.; Merlin, F.; et al. (February 2010). "Colors and taxonomy of Centaurs and trans-Neptunian objects". Astronomy and Astrophysics. 510: A53. arXiv:0912.2621. Bibcode:2010A&A...510A..53P. doi:10.1051/0004-6361/200913654. Retrieved 16 November 2016.
  11. ^ a b c Belskaya, Irina N.; Barucci, Maria A.; Fulchignoni, Marcello; Dovgopol, Anatolij N. (April 2015). "Updated taxonomy of trans-neptunian objects and centaurs: Influence of albedo". Icarus. 250: 482–491. Bibcode:2015Icar..250..482B. doi:10.1016/j.icarus.2014.12.004. Retrieved 16 November 2016.
  12. ^ "(7066) Nessus – Ephemeris at epoch 58448 MJD". AstDyS-2, Asteroids – Dynamic Site; Department of Mathematics, University of Pisa, Italy. Archived from the original on 16 November 2016. Retrieved 26 November 2018.
  13. ^ Peixinho, N.; Delsanti, A.; Guilbert-Lepoutre, A.; Gafeira, R.; Lacerda, P. (October 2012). "The bimodal colors of Centaurs and small Kuiper belt objects". Astronomy and Astrophysics. 546: 12. arXiv:1206.3153. Bibcode:2012A&A...546A..86P. doi:10.1051/0004-6361/201219057. Retrieved 16 November 2016.
  14. ^ Romanishin, W.; Tegler, S. C. (March 1999). "Rotation rates of Kuiper-belt objects from their light curves". Nature. 398 (6723): 129–132.(NatureHomepage). Bibcode:1999Natur.398..129R. doi:10.1038/18168. Retrieved 16 November 2016.
  15. ^ Romanishin, W.; Tegler, S. C. (December 2005). "Accurate absolute magnitudes for Kuiper belt objects and Centaurs". Icarus. 179 (2): 523–526. Bibcode:2005Icar..179..523R. doi:10.1016/j.icarus.2005.06.016. Retrieved 16 November 2016.
  16. ^ Davies, John K.; McBride, Neil; Ellison, Sara L.; Green, Simon F.; Ballantyne, David R. (August 1998). "Visible and Infrared Photometry of Six Centaurs". Icarus. 134 (2): 213–227. Bibcode:1998Icar..134..213D. doi:10.1006/icar.1998.5931. Retrieved 16 November 2016.
  17. ^ a b c "LCDB Data for (5145) Pholus". Asteroid Lightcurve Database (LCDB). Retrieved 16 November 2016.
  18. ^ a b Wilson, P. D.; Sagan, C.; Thompson, W. R. (February 1994). "The organic surface of 5145 Pholus: Constraints set by scattering theory". Icarus. 107 (2): 288–303. Bibcode:1994Icar..107..288W. doi:10.1006/icar.1994.1024. Retrieved 16 November 2016.
  19. ^ "Fifty clones of Centaur 5145 Pholus all passing within ~100Gm of Neptune on 5290-07-07". Retrieved 23 April 2009. (Solex 10)
  20. ^ "MPC/MPO/MPS Archive". Minor Planet Center. Retrieved 26 November 2018.
  21. ^ Cruikshank DP; et al. (1998). "The Composition of Centaur 5145 Pholus". Icarus. 135 (2): 389–407. Bibcode:1998Icar..135..389C. doi:10.1006/icar.1998.5997.
  22. ^ Hoffmann, M.; Fink, U.; Grundy, W. M.; Hicks, M. (December 1991). "Photometric and spectroscopic observations of 5145 1992 AD". Observations and Physical Properties of Small Solar System Bodies. 30: 203. Bibcode:1992LIACo..30..203H. Retrieved 16 November 2016.
  23. ^ Farnham, Tony L. (August 2001). "The Rotation Axis of the Centaur 5145 Pholus". Icarus. 152 (2): 238–245. Bibcode:2001Icar..152..238F. doi:10.1006/icar.2001.6656. Retrieved 16 November 2016.
  24. ^ Buie, Marc W.; Bus, Schelte J. (December 1992). "Physical observations of (5145) Pholus". Icarus. 100 (2): 288–294. Bibcode:1992Icar..100..288B. doi:10.1016/0019-1035(92)90101-C. Retrieved 16 November 2016.

External linksEdit