| Discovery[1] | |
|---|---|
| Discovered by |
|
| Discovery site | Kitt Peak National Obs. |
| Discovery date | 9 January 1992 |
| Designations | |
| (5145) Pholus | |
| Pronunciation | /ˈfoʊləs/[3] |
Named after | ΦόλοςPholos (Greek mythology)[4] |
| 1992 AD | |
| Symbol | .svg) or.svg) (astrological) |
| Orbital characteristics[5] | |
| Epoch 1 July 2021 (JD 2459396.5) | |
| Uncertainty parameter 1[5] · 0[1] | |
| Observation arc | 40.74yr (14,881 d) |
| Earliestprecovery date | 22 July 1977 |
| Aphelion | 31.943AU |
| Perihelion | 8.7531 AU |
| 20.348 AU | |
| Eccentricity | 0.5698 |
| 91.79 yr (33,526 d) | |
| 117.26° | |
| 0° 0m 38.52s / day | |
| Inclination | 24.617° |
| 119.44° | |
| 354.77° | |
| Jupiter MOID | 3.495 AU |
| Saturn MOID | 0.34961[1] |
| TJupiter | 3.21 |
| Physical characteristics | |
| Dimensions | 310 km × 160 km × 150 km[8] |
| 9.980 h[8] | |
| 21.62[13] 16.3(Perihelic opposition) | |
5145 Pholus/ˈfoʊləs/ is an eccentriccentaur in theouter Solar System, approximately 180 kilometers (110 miles) in diameter, that crosses the orbit of bothSaturn andNeptune. It was discovered on 9 January 1992 by American astronomerDavid Rabinowitz (uncredited) ofUA'sSpacewatch survey at theKitt Peak National Observatory in Arizona, United States. The veryreddish object has an elongated shape and arotation period of 9.98 hours.[18] It was named after the centaurPholus from Greek mythology.[1]
Pholus was the secondcentaur to be discovered.[19] Centaurs are objects in between theasteroid andtrans-Neptunian populations of the Solar System – that is, beyond Jupiter's and within Neptune's orbit – which behave with characteristics of both asteroids andcomets.
It orbits the Sun at a distance of 8.8–31.9 AU once every 91 years and 9 months (33,526 days;semi-major axis of 20.35 AU). Its orbit has aneccentricity of 0.57 and aninclination of 25° with respect to theecliptic.[5] It is aSaturn-,Uranus- andNeptune-crosser, crossing the orbits of thesegiant planets at a mean distance of 9.6, 11.9, and 30.1 AU from the Sun, respectively. Pholus has not come within oneastronomical unit of a planet since 764 BC, and will not until 5290.[20] It is believed that it originated in theKuiper belt.
Pholus was discovered byDavid Rabinowitz (not officially credited), working with theSpacewatch program, atKitt Peak National Observatory on 9 January 1992.[1] Rabinowitz's discovery was confirmed byEugene andCarolyn Shoemaker who identified the object on images they previously took on 1 January 1992. The discovery was announced byJames Scotti on 23 January 1992 in anIAU Circular (IAUC 5434) of theCentral Bureau for Astronomical Telegrams.[2] A firstprecovery was taken at the AustralianSiding Spring Observatory in 1977, extending the centaur'sobservation arc by 15 years prior to its discovery.[1] It was the second discovery of a centaur after2060 Chiron discovered byCharles Kowal in 1977. In 1993, while with the Spacewatch program, David Rabinowitz went on to discover another centaur,7066 Nessus.
Thisminor planet was named by theMinor Planet Names Committee forPholus, acentaur fromGreek mythology. As with 2060 Chiron, named after his brotherChiron, 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 from a self-inflicted wound from a poisoned arrow used byHeracles(see5143 Heracles), who buried Pholus on the mountain Pholoe.[4] The officialnaming citation was published by theMinor Planet Center on 14 July 1992 (M.P.C. 20523).[21]
A symbol derived from that for2060 Chiron,U+2BDB ⯛PHOLUS (
), was devised in the late 1990s by German astrologer Robert von Heeren. It replaces Chiron's K with a P for Pholus.[22] A common variant,.svg)
, uses a Greekφ.[23]
After its discovery, Pholus was quickly found to be very red in color. The color has been speculated to be due to organic compounds on its surface.[19] It is classified as a Z-class object on theTholen taxonomic scheme.[5]
The object has been classified by astronomers asRR andRR-U type, respectively.[11][12] Polarimetric observations with ESO'sVery 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 a small amount of water frost on its darker regions.[12]
The surface composition of Pholus has been estimated from its reflectance spectrum using two spatially segregated components:[24] darkamorphous carbon and an intimate mixture ofwater ice,methanol ice,olivine grains, and complex organic compounds (tholins). Thecarbon black component was used to match the lowalbedo of the object. UnlikeChiron, Pholus has shown no signs ofcometary activity.
Diameter calculations range from 99 to 190 kilometers with a correspondingalbedo between 0.155 and 0.04.[6][8][9][18][10]
According to theHerschel Space Observatory with itsPACS instrument, Pholus measures 99 kilometers in diameter and its surface has analbedo of 0.155,[9] while a study from 1996 derived a diameter of 185 km.[10] During 2003–2004, observations with the 1.8-meterVatican Advanced Technology Telescope (VATT) onMount 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.[8] Johnston's archive lists a diameter of 107 km with an albedo of 0.126,[6] andCollaborative Asteroid Lightcurve Link assumes a standard albedo of acarbonaceous body of 0.057 and derives a diameter of 165 km based on anabsolute magnitude of 7.64.[18]
In March 2005, a rotationallightcurve of Pholus was obtained fromphotometric observations by Tegler using the VATT at Mount Graham. Lightcurve analysis gave arotation period of 9.980 hours with a brightness amplitude of 0.60magnitude (U=3-).[8] 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).[25][26][27]
