| Discovery | |
|---|---|
| Discovered by | Charles T. Kowal |
| Discovery date | October 18, 1977 |
| Designations | |
| Alternate name | 1977 UB |
| Minor planet category | Centaur,[1] Comet |
| Epoch June 18, 2009 (JD 2455000.5) | |
| Aphelion | 18.891 AU (Q) 2,826 Gm |
| Perihelion | 8.5114 AU (q) 1,273 Gm |
| Semi-major axis | 13.708 AU (2,050.7 Gm) (a) |
| Eccentricity | 0.37911 |
| Orbital period | 18,539 days (50.76 a) |
| Average orbital speed | 7.75 km/s |
| Mean anomaly | 94.716° (M) |
| Inclination | 6.9311° |
| Longitude of ascending node | 209.31° |
| Argument of perihelion | 339.98° |
| Physical characteristics | |
| Dimensions | 233 ± 14 km[3] 132–152 km[4] |
| Mass | ≈1 × 1019 kg (assumed)[5] |
| Mean density | unknown |
| Equatorial surface gravity | 0.037–0.040 m/s² |
| Escape velocity | 0.070–0.075 km/s |
| Rotation period | 0.2466 d (5.918 h)[2] |
| Albedo | 0.075 ± 0.01[3] 0.11 ± 0.02 [4] |
| Temperature | ~75 K |
| Spectral type | B,Cb[2] |
| Apparent magnitude | ~18.7[6] 15.6 (Perihelic opposition) |
| Absolute magnitude (H) | 6.5[2] |
| Angular diameter | 0.035" (max)[7] |
| | |
2060 Chiron (pronounced /ˈkaɪrən/ KYE-rən, or as in Greek: Χείρων) is a planetoid in the outer Solar System. Discovered in 1977 by Charles T. Kowal (precovery images have been found as far back as 1895),[8] it was the first known member of a new class of objects now known as centaurs, with an orbit between those of Saturn and Uranus.
Although it was initially classified as an asteroid, it was later found to exhibit behaviour typical of a comet. Today it is classified as both, and accordingly it is also known by the cometary designation 95P/Chiron.
Chiron is named after the centaur Chiron in Greek mythology. It should not be confused with the dwarf planet partner of Pluto named Charon, discovered in 1978.
Discovery and naming
Chiron was discovered on November 1, 1977 by Charles Kowal from images taken two weeks earlier at Palomar Observatory.[9] It was given the temporary designation of 1977UB.[10] It was found near aphelion[9] and at the time of discovery it was the most distant known minor planet.[10] Chiron was later found on several precovery images, going back to 1895, which allowed its orbit to be accurately determined.[9] 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.[9] 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 1940's.[9]
It was named 2060 Chiron in 1979.[10] Chiron was one of the centaurs, and it was suggested that the names of other centaurs be reserved for other objects of the same type.[9]
Physical characteristics
The visible and near-infrared spectrum of Chiron is neutral,[10] and is similar to that of C-type asteroids and the nucleus of Halley's comet.[11]
| Year | Radius (km) | Notes |
|---|---|---|
| 1984 | 90 | Lebofsky |
| 1991 | <186 | IRAS |
| 1994 | 74 | Campins |
| 1996 | 90 | asteroid occultation |
| 2007 | 166[3] | Spitzer Space Telescope |
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.[4] Estimates in the 1990s were closer to 150 km in diameter.[2][4] Asteroid occultation data from 1993 suggests a diameter of about 180 km.[4] The data from the Spitzer Space Telescope in 2007 suggests that Chiron is closer to 233 ± 14 km in diameter.[3] Therefore Chiron may be as large as 10199 Chariklo.[3]
Its rotational period is 5.917813 hours, a value determined by observing its distinct light curve.[10]
Cometary behaviour
In February 1988, at 12 AU from the Sun, Chiron brightened by 75 percent.[12] This is behaviour typical of comets but not asteroids. Further observations in April 1989 showed that Chiron had developed a cometary coma,[12] and a tail was detected in 1993.[10] 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.[11]
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 likely means it has not been in this orbit that long.[8]
Chiron is officially designated as both a comet and an asteroid, 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 asteroids 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, centaur 52872 Okyrhoe significantly brightened.[13]
There are other non-centaur asteroids that are also classified as main-belt comets: 4015 Wilson-Harrington, 7968 Elst-Pizarro, and 118401 LINEAR.
Orbit
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 Gm. During this passage Saturn's gravity caused Chiron's semi-major axis to decrease from 14.4AU[14] to 13.7AU.[2] It does not come nearly as close to Uranus; Chiron crosses Uranus' 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 SU object since its perihelion lies within Saturn's zone of control and its aphelion lies within Uranus' zone of control.[15] Centaurs are not in stable orbits and over millions of years will eventually be removed by gravitational perturbation by the giant planets, moving to different orbits or leaving the solar system altogether.[16] Chiron is probably a refugee from the Kuiper belt and will likely become a short period comet in about a million years.[15]
Chiron came to perihelion (closest point to the Sun) in 1996.[2]
 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. |
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