|Discovered by||Charles T. Kowal|
|Discovery date||October 18, 1977|
|MPC designation||2060 Chiron
|Epoch June 18, 2009 (JD 2455000.5)|
|Aphelion||18.891 AU (Q)
|Perihelion||8.5114 AU (q)
|13.708 AU (2,050.7 Gm) (a)|
|50.76 a (18,539 d)|
Average orbital speed
233 ± 14 km
|5.918 h (0.2466 d)|
0.075 ± 0.01
15.6 (Perihelic opposition)
2060 Chiron is a minor planet in the outer Solar System, orbiting between Saturn and Uranus. Discovered in 1977 by Charles T. Kowal (precovery images have been found as far back as 1895), it was the first-identified member of a new class of objects now known as centaurs (minor planets orbiting between the asteroid belt and the Kuiper belt).[a] Furthermore, it is the second-known minor planet to have rings, after 10199 Chariklo, also a centaur.
Although it was initially called an asteroid and classified as a minor planet, it was later found to exhibit behavior typical of a comet. Today it is classified as both, and is accordingly also known by the cometary designation 95P/Chiron.
Discovery and naming
Chiron was discovered on 18 October 1977 by Charles Kowal from images taken two weeks earlier at Palomar Observatory. It was given the temporary designation of 1977 UB. It was found near aphelion and at the time of discovery it was the most distant known minor planet. Chiron was even claimed as the tenth planet by the press. Chiron was later found on several precovery images, going back to 1895, which allowed its orbit to be accurately determined. It had been at perihelion 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.
Chiron's orbit was found to be highly eccentric (0.37), with perihelion just inside the orbit of Saturn and aphelion just outside the perihelion of Uranus (it does not reach the average distance of Uranus, however). According to the program Solex, Chiron's closest approach to Saturn in modern times was in May 720, at just under 30 million kilometres. During this passage Saturn's gravity caused Chiron's semi-major axis to decrease from 14.4 AU to 13.7 AU. It does not come nearly as close to Uranus; Chiron crosses Uranus's orbit where the latter is farther than average from the Sun. It attracted considerable interest because it was the first object discovered in such an orbit, well outside the asteroid belt. Chiron is classified as a centaur, the first of a class of objects orbiting between the outer 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. Centaurs are not in stable orbits and will be removed by gravitational perturbation by the giant planets over a period of millions of years, moving to different orbits or leaving the Solar System altogether. Chiron is probably a refugee from the Kuiper belt and will probably become a short-period comet in about a million years.
The orbit of 2060 Chiron compared with the orbits of Jupiter, Saturn, Uranus and Neptune.
The chaotic unstable motion of Chiron as simulated by Gravity Simulator. It is possible that Chiron will evolve into a 2:1 near resonance with Saturn over the next 10,000+ years.
|2007||117||Spitzer Space Telescope|
|2013||109||Herschel Space Observatory|
|111||Average of measurements|
The assumed size of an object depends on its absolute magnitude (H) and the albedo (the amount of light it reflects). In 1984 Lebofsky estimated Chiron to be about 180 km in diameter. Estimates in the 1990s were closer to 150 km in diameter. Occultation data from 1993 suggests a diameter of about 180 km. The data from the Spitzer Space Telescope in 2007 suggests that Chiron is closer to 233 ± 14 km in diameter. Therefore Chiron may be as large as 10199 Chariklo.
Its rotational period is 5.917813 hours, a value determined by observing its distinct light curve.
In February 1988, at 12 AU from the Sun, Chiron brightened by 75 percent. This is behavior typical of comets but not asteroids. Further observations in April 1989 showed that Chiron had developed a cometary coma, A tail was detected in 1993. Chiron differs from other comets in that water is not a major component of its coma, because it is too far from the Sun for water to sublimate.
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.
Chiron is officially designated as both a comet and a minor planet, an indication of the sometimes fuzzy dividing line between the two classes of object. The term proto-comet has also been used. Being at least 130 km in diameter, it is unusually large for a comet nucleus.
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.
Chiron is a Chiron-type comet with (TJupiter > 3; a > aJupiter). Other Chiron-type comets include: 39P/Oterma, 165P/LINEAR, 166P/NEAT, and 167P/CINEOS. There are also non-centaurs that are classified as comets, such as 4015 Wilson–Harrington, 7968 Elst–Pizarro, and 118401 LINEAR.
Chiron is suspected to have two rings, similar to the better-established rings of Chariklo. 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 are proposed to be 324 (± 10) km in radius and sharply defined. Their changing appearance at different viewing angles may 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.
The preferred pole of Chiron's rings is, in ecliptic coordinates, λ = 144°±10°, β = 24°±10°. The rings' width, separation, and optical depths are nearly identical to those of Chariklo's rings, indicating that the same type of structure is responsible for both. Moreover, both their rings are within their respective Roche limits.
- 944 Hidalgo, discovered in 1920, also fits this definition, but was not identified as belonging to a distinct population.
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- Dual-Status Objects
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- Patrick Moore Guinness book of Astronomy ISBN 0-85112-375-9
- SOLEX 9.1
- 95P/Chiron at Cometography
- A single clone run of centaur 2060 Chiron showing how Chiron may someday become an active comet (Solex 10)
|Periodic comets (by number)|