2060 Chiron

2060 Chiron is a small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs—bodies orbiting between the asteroid belt and the Kuiper belt.[lower-alpha 1]

Not to be confused with Pluto's moon Charon.

2060 Chiron
95P/Chiron
Hubble Space Telescope image of Chiron and its coma, taken in 1996
Discovery[1][2][3]
Discovered byCharles Kowal
Discovery sitePalomar Observatory
Discovery date1 November 1977
Designations
  • (2060) Chiron
  • 95P/Chiron[4]
Pronunciation/ˈkaɪrɒn/
Named after
Chiron (Greek mythology)[5]
1977 UB
Adjectives
  • Chironian
  • Chironean
  • (both /kaɪˈroʊniən/)
Symbol⚷ (astrological)
Orbital characteristics[6]
Epoch 2021-Jul-01 (JD 2459396.5)
Uncertainty parameter 0
Observation arc126.29 yr
Earliest precovery date24 April 1895[1]
(Harvard Observatory)
Aphelion18.87 AU (2.8 billion km)
(occurred May 2021)[8]
Perihelion8.533 AU (1.3 billion km)
13.70 AU (2.0 billion km)
Eccentricity0.3772
50.71 yr (18,523 days)
7.75 km/s
180.70°
0° 1m 10.2s / day
Inclination6.9299°
209.27°
  • 3 August 2046[9]
  • 14 February 1996 (previous)
  • 29 August 1945
  • 16 March 1895[10]
339.71°
Jupiter MOID3.1 AU (460 million km)
Saturn MOID0.48 AU (72 million km)[1]
Uranus MOID1.4 AU (210 million km)[1]
TJupiter3.363
Physical characteristics
Mean radius
  • 107.8±4.95 km (Herschel 2013)[11]
  • 116.7±7.3 km (Spitzer)[12]
  • 135.69 km (LCDB, derived)[13]
5.918 h[13][14][15][16][17]
18.93[20]
14.9 (Perihelic opposition)[21]
0.035" (max)[26]

    Although it was initially called an asteroid and classified only as a minor planet with the designation "2060 Chiron", in 1989 it was found to exhibit behavior typical of a comet. Today it is classified as both a minor planet and a comet, and is accordingly also known by the cometary designation 95P/Chiron. Chiron is named after the centaur Chiron in Greek mythology.[1]

    History

    Discovery

    Chiron was discovered on 1 November 1977 by Charles Kowal from images taken on 18 October at Palomar Observatory.[2][3] It was given the temporary designation of 1977 UB.[27] It was found near aphelion[2] and at the time of discovery it was the most distant known minor planet.[lower-alpha 2][27] Chiron was even claimed as the tenth planet by the press.[28] Chiron was later found on several precovery images, going back to 1895,[29] which allowed its orbit to be accurately determined.[2] 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.[2] The April 1895 precovery image was one month after the March 1895 perihelion.[10]

    Naming

    This minor planet was named after Chiron, a half-human, half-horse centaur from Greek mythology. Son of the Titan Cronus and the nymph Philyra, Chiron was the wisest and most just of all centaurs, serving as an instructor of the Greek heroes.[5] The official naming citation was published by the Minor 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 ⚷ was devised for it by Al H. Morrison and is mostly used among astrologers: it resembles a key as well as an OK monogram for Object Kowal.[31][32]

    Orbit
    Orbital diagram of Chiron

    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 around May 720, when it came within 30.5±2.0 million km (0.204 ± 0.013 AU) of Saturn. During this passage Saturn's gravity caused Chiron's semi-major axis to decrease from 14.55±0.12 AU[33] to 13.7 AU.[6] Chiron's orbit does not intersect Uranus'.

    Chiron 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.[34] 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.[35] Chiron is probably from the Kuiper belt and will probably become a short-period comet in about a million years.[34] Chiron came to perihelion (closest point to the Sun) in 1996 and aphelion in May 2021.[8]

    Physical characteristics

    Spectral type

    The visible and near-infrared spectrum of Chiron is neutral,[27] and is similar to that of C-type asteroids and the nucleus of Halley's Comet.[15] The near-infrared spectrum of Chiron shows absence of water ice.[36]

    Rotation period

    Four rotational light curves of Chiron were taken from photometric observations between 1989 and 1997. Lightcurve analysis gave a concurring, well-defined rotational period of 5.918 hours with a small brightness variation of 0.05 to 0.09 magnitude, which indicates that the body has a rather spheroidal shape (U=3/3/3).[13][14][15][16][17]

    Diameter
    Summary – size estimates for Chiron:
    YearDiameterNotesRefs
    1984180 kmLebofsky (1984)[37]
    1991186 kmIRAS[37]
    1994188 kmCampins (radius 94±6 km)[27]
    1996180 kmoccultation[37]
    1998166 kmDunham occultation list
    (Dunham 1998)    		[6]
    2007233 kmSpitzer Space Telescope[12]
    2013218 kmHerschel Space Observatory
    (PACS and SPIRE)    		[11]
    2017271 kmLCDB[13]
    2017206 kmBrown[18]

    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.[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 the Spitzer Space Telescope in 2007 and the Herschel Space Observatory in 2011 suggests that Chiron is 218±20 km in diameter.[11] Therefore, Chiron may be as large as 10199 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.[11]

    Cometary behavior

    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 cometary coma,[39] A tail was detected in 1993.[27] 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.[40] In 1995 carbon 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][11] 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 a comet nucleus. Chiron was the first member of a new family of Chiron-type comets (CTC) with (TJupiter > 3; a > aJupiter).[6] Other CTCs include: 39P/Oterma, 165P/LINEAR, 166P/NEAT, and 167P/CINEOS. There are also non-centaur asteroids that are simultaneously classified as comets, such as 4015 Wilson–Harrington, 7968 Elst–Pizarro, and 118401 LINEAR.[4] Michael Brown lists it as possibly a dwarf planet with a measured diameter of 200 km (120 mi),[18] which may be near the lower limit for an icy object to have been a dwarf planet at some point in its history.

    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]

    Rings
    Depiction of Chiron with rings

    Chiron possibly has rings, similar to the better-established rings of 10199 Chariklo.[44][45][46][lower-alpha 3] 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 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]

    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.[44]

    Further evidence of the rings was provided by observations of an occultation on 15 December 2022.[47]

    Exploration

    The Chiron Orbiter Mission is a mission proposed for NASA's New Frontiers program or Flagship program. It was published in May 2010 and proposes an orbiter mission to Chiron. Its launch date could vary from as early as 2023 to as late as 2025, depending on budget and propulsion type.[48]

    There is another mission proposed, part of the Discovery Program known as Centaurus; if approved it would launch between 2026 and 2029 and make a flyby of 2060 Chiron and one other Centaur sometime in the 2030s.

    Wikimedia Commons has media related to 2060 Chiron.
    • Hubble Space Telescope image of the centaur 2060 Chiron, taken on 14 September 2015
      Hubble Space Telescope image of the centaur 2060 Chiron, taken on 14 September 2015
    • Chaotic, unstable motion of Chiron with Saturn (stationary, white dot at 10 o'clock) and Jupiter (blue)
      Chaotic, unstable motion of Chiron with Saturn (stationary, white dot at 10 o'clock) and Jupiter (blue)
    • Animated orbital diagram with Chiron (violet) not showing perturbation
      Animated orbital diagram with Chiron (violet) not showing perturbation

    See also

    Notes
    1. 944 Hidalgo, discovered in 1920, also fits this definition, but was not identified as belonging to a distinct population.
    2. Pluto, now considered to be a dwarf planet and hence a minor planet, was known at the time, but was considered a planet.
    3. A stellar occultation in 2017 of another minor planet, Haumea (a trans-Neptunian object), indicated the presence of a ring.

    References
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    Further reading
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