 Hubble Space Telescope image of 2060 Chiron, taken in 2015. | |
| Discovery[1][2][3] | |
|---|---|
| Discovered by | Charles Kowal |
| Discovery site | Palomar Observatory |
| Discovery date | 1 November 1977 |
| Designations | |
| |
| Pronunciation | /ˈkaɪrɒn/ |
Named after | Chiron(Greek mythology)[5] |
| 1977 UB | |
| Adjectives |
|
| Symbol | .svg) (astrological) |
| Orbital characteristics[6] | |
| Epoch 2021-Jul-01 (JD 2459396.5) | |
| Uncertainty parameter 0 | |
| Observation arc | 126.29 yr |
| Earliestprecovery date | 24 April 1895[1] (Harvard Observatory) |
| Aphelion | 18.87 AU (2.8 billion km) (occurred May 2021)[8] |
| Perihelion | 8.533 AU (1.3 billion km) |
| 13.70 AU (2.0 billion km) | |
| Eccentricity | 0.3772 |
| 50.71 yr (18,523 days) | |
Averageorbital speed | 7.75 km/s |
| 180.70° | |
| 0° 1m 10.2s / day | |
| Inclination | 6.9299° |
| 209.27° | |
| 339.71° | |
| Jupiter MOID | 3.1 AU (460 million km) |
| Saturn MOID | 0.48 AU (72 million km)[1] |
| Uranus MOID | 1.4 AU (210 million km)[1] |
| TJupiter | 3.363 |
| Physical characteristics | |
| Dimensions | 126±22 km x109±19 km x68±12 km[11] |
| 5.918h[13][14][15][16][17] | |
| 18.93[20] 14.9(Perihelic opposition)[21] | |
| 0.035" (max)[26] | |
2060 Chiron is a ringedsmall Solar System body in theouter Solar System,orbiting theSun betweenSaturn andUranus. Discovered in 1977 byCharles Kowal, it was the first-identified member of a new class of objects now known ascentaurs—bodies orbiting between theasteroid belt and theKuiper belt.[a] Chiron is named after thecentaurChiron inGreek mythology.
Although it was initially called anasteroid and classified only as aminor planet with the designation "2060 Chiron", in 1989 it was found to exhibit behavior typical of acomet. Today it is classified as both a minor planet and a comet, and is accordingly also known by the cometary designation95P/Chiron. More recently, a series ofoccultation events through the 2010s and early 2020s revealed that Chiron hostsrings, making it one of four minor planets and the only known comet to host a ring system.
Chiron was discovered on 1 November 1977 byCharles Kowal from images taken on 18 October atPalomar Observatory.[2][3] It was given the temporary designation of1977 UB.[27] It was found nearaphelion[2] and at the time of discovery it was the most distant known minor planet.[b][27] Chiron was even claimed as the tenth planet by the press.[28] Chiron was later found on severalprecovery images, going back to 1895,[29] which allowed its orbit to be accurately determined.[2] It had been atperihelion in 1945 but was not discovered then because there were few searches being made at that time, and these were not sensitive to slow-moving objects. The Lowell Observatory's survey for distant planets would not have gone down faint enough in the 1930s and did not cover the right region of the sky in the 1940s.[2] The April 1895 precovery image was one month after the March 1895 perihelion.[10]
Thisminor planet was named afterChiron, a half-human, half-horsecentaur fromGreek mythology. Son of the TitanCronus and the nymphPhilyra, Chiron was the wisest and most just of all centaurs, serving as an instructor of the Greek heroes.[5] The officialnaming citation was published by theMinor Planet Center on 1 April 1978 (M.P.C. 4359).[5][30] It was suggested that the names of other centaurs be reserved for objects of the same type.[2]
Chiron, along with most major and minor planetary bodies, is not generally given a symbol in astronomy. A symbol.svg)
was devised for it by Al H. Morrison and is mostly used among astrologers: it resembles a key as well as an OK monogram forObjectKowal.[31][32]
Chiron's orbit was found to be highlyeccentric (0.37), withperihelion just inside the orbit ofSaturn andaphelion just outside the perihelion ofUranus (it does not reach the average distance of Uranus, however). According to the programSolex, Chiron's closest approach to Saturn in modern times was around May 720, when it came within30.5±2.0 million km (0.204 ± 0.013 AU) of the planet. During this passage Saturn's gravity caused Chiron'ssemi-major axis to decrease from14.55±0.12 AU[33] to 13.7 AU.[6] Chiron's orbit does not intersect Uranus' orbit.
Chiron attracted considerable interest because it was the first object discovered in such an orbit, well outside theasteroid belt. Chiron is classified as a centaur, the first of a class of objects orbiting between theouter planets. Chiron is a Saturn–Uranus object because its perihelion lies in Saturn's zone of control and its aphelion lies in that of Uranus.[34] Centaurs are not in stable orbits and will be removed by gravitationalperturbation by the giant planets over a period of millions of years, moving to different orbits or leaving the Solar System altogether.[35] Chiron likely comes from theKuiper belt and will probably become ashort-period comet in about a million years.[34] Chiron came toperihelion (closest point to the Sun) in 1996 and aphelion in May 2021.[8]
The visible and near-infrared spectrum of Chiron is neutral,[27] and is similar to that ofC-type asteroids and the nucleus ofHalley's Comet.[15] The near-infrared spectrum of Chiron shows absence of water ice.[36]
Four rotationallight curves ofChiron were taken fromphotometric observations between 1989 and 1997. Lightcurve analysis gave a concurring, well-definedrotational period of 5.918 hours with a small brightness variation of 0.05 to 0.09magnitude, which indicates that the body has a rather spheroidal shape (U=3/3/3).[13][14][15][16][17]
| Year | Diameter | Notes | Refs |
|---|---|---|---|
| 1984 | 180 km | Lebofsky (1984) | [37] |
| 1991 | 186 km | IRAS | [37] |
| 1994 | 188 km | Campins (radius94±6 km) | [27] |
| 1996 | 180 km | occultation | [37] |
| 1998 | 166 km | Dunham occultation list (Dunham 1998) | [6] |
| 2007 | 233 km | Spitzer Space Telescope | [12] |
| 2013 | 218 km | Herschel Space Observatory (PACS and SPIRE) | [18] |
| 2017 | 271 km | LCDB | [13] |
| 2023 | 196 km | Sickafoose | [11] |
The assumed size of an object depends on itsabsolute magnitude (H) and thealbedo (the amount of light it reflects). In 1984 Lebofsky estimated Chiron to be about 180 km in diameter.[37] Estimates in the 1990s were closer to 150 km in diameter.[6][37]Occultation data from 1993 suggests a diameter of about 180 km.[37] Combined data from theSpitzer Space Telescope in 2007 and the Herschel Space Observatory in 2011 suggests that Chiron is218±20 km in diameter.[18] Therefore, Chiron may be as large as10199 Chariklo.[12] The diameter of Chiron is difficult to estimate in part because the true absolute magnitude of its nucleus is uncertain due to its highly variable cometary activity.[18]
In February 1988, at 12 AU from the Sun, Chiron brightened by 75 percent.[38] This is behavior typical of comets but not asteroids. Further observations in April 1989 showed that Chiron had developed a cometarycoma,[39] A tail was detected in 1993.[27] Chiron differs from other comets in thatwater is not a major component of its coma, because it is too far from theSun for water to sublimate.[40] In 1995carbon monoxide was detected in Chiron in very small amounts, and the derived CO production rate was calculated to be sufficient to account for the observed coma.[41]Cyanide was also detected in the spectrum of Chiron in 1991.[42] At the time of its discovery, Chiron was close to aphelion, whereas the observations showing a coma were done closer to perihelion, perhaps explaining why no cometary behavior had been seen earlier. The fact that Chiron is still active probably means it has not been in its current orbit very long.[29]
Chiron is officially designated as both a comet—95P/Chiron—and a minor planet,[4][18] an indication of the sometimes fuzzy dividing line between the two classes of object. The term proto-comet has also been used. Being about 220 km in diameter, it is unusually large for acomet nucleus. Chiron was the first member of a new family ofChiron-type comets (CTC) with (TJupiter > 3;a > aJupiter).[6] Other CTCs include:39P/Oterma,165P/LINEAR,166P/NEAT, and167P/CINEOS. There are also non-centaur asteroids that are simultaneously classified as comets, such as4015 Wilson–Harrington,7968 Elst–Pizarro, and118401 LINEAR.[4]
Since the discovery of Chiron, other centaurs have been discovered, and nearly all are currently classified as minor planets, but are being observed for possible cometary behavior.60558 Echeclus has displayed a cometary coma and now also has the cometary designation 174P/Echeclus. After passing perihelion in early 2008,52872 Okyrhoe significantly brightened.[43]
Chiron hasrings, similar to the better-establishedrings of10199 Chariklo.[44][45][46][c] Based on unexpected occultation events observed in stellar-occultation data obtained on 7 November 1993, 9 March 1994, and 29 November 2011, which were initially interpreted as resulting from jets associated with Chiron's comet-like activity, Chiron's rings were proposed to be324±10 km in radius and sharply defined. The rings' changing appearance at different viewing angles can largely explain the long-term variation in Chiron's brightness and hence estimates of Chiron's albedo and size. Moreover, it can, by assuming that the water ice is in Chiron's rings, explain the changing intensity of the infrared water-ice absorption bands in Chiron's spectrum, including their disappearance in 2001 (when the rings were edge-on). Also, the geometric albedo of Chiron's rings as determined by spectroscopy is consistent with that used to explain Chiron's long-term brightness variations.[44]
Further evidence of the rings was provided by two independent observations of occultations on 28 November 2018 and 15 December 2022, which suggests that their structure is constantly evolving.[47] In the 2018 event Chiron's rings were observed to have less material than in 2011, but seemed to be developing a partial third ring; by the 2022 event there was more material than either of the previous observations, and the third ring had become fully developed.[11] J.L. Ortiz speculated that the extra material in the 2022 event could be from an outburst observed in 2021, which left more material in orbit and thus bolstered the generation of the third ring–this is also expected to be cyclical, maintaining the rings.[47]
The preferred pole of Chiron's rings is, inecliptic coordinates,λ =151°±8°,β =18°±11°. The rings' width, separation, and optical depths were observed to be nearly identical to those of Chariklo's rings until the 2018 observation, indicating that the same type of structure had been responsible for both. Moreover, both their rings are within their respectiveRoche limits, though Chiron's newly developed third ring may be outside of it depending on its density.[44][47]
TheChiron Orbiter Mission was a mission proposed for NASA'sNew Frontiers program orFlagship program. It was published in May 2010 and proposed an orbiter mission to Chiron. Its launch date could have varied from as early as 2023 to as late as 2025, depending on budget and propulsion type.[48]
There was another mission proposed, part of theDiscovery Program known asCentaurus; if approved, it would have launched between 2026 and 2029 and made a flyby of 2060 Chiron and one other Centaur sometime in the 2030s.
| Numbered comets | ||
|---|---|---|
| Previous 94P/Russell | 95P/Chiron | Next 96P/Machholz |
| Planets | |
|---|---|
| Minor planets | |
| Moons |
|
| Related | |
| International | |
|---|---|
| National | |
| Other | |
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