2060 Chiron

Not to be confused with Pluto's moon Charon.
Chiron

Chiron in Celestia with rings. Surface details and shape are imaginary, as it may not be in hydrostatic equilibrium.
Discovery
Discovered by Charles T. Kowal
Discovery date October 18, 1977
Designations
MPC designation 2060 Chiron
95P/Chiron
Pronunciation /ˈkaɪərən/
Named after
 Chiron
1977 UB
centaur [1][2]
comet
Adjectives Chironean, Chironian
Orbital characteristics [3]
Epoch June 18, 2009 (JD 2455000.5)
Aphelion 18.891 AU (Q)
2,826 Gm
Perihelion 8.5114 AU (q)
1,273 Gm
13.708 AU (2,050.7 Gm) (a)
Eccentricity 0.37911
50.76 a (18,539 d)
7.75 km/s
94.716° (M)
Inclination 6.9311°
209.31°
339.98°
Physical characteristics
Dimensions 218±20 km [4]
5.918 h (0.2466 d)[3]
0.16±0.03[4]
Temperature ~ 75 K
B,Cb[3]
~ 18.7 [5]
15.6 (Perihelic opposition)
5.92±0.20[4]
0.035" (max)[6]

    2060 Chiron, also known as 95P/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),[7] 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] Besides the four giant planets, Chiron and 10199 Chariklo, also a centaur, are the only bodies in the Solar System known to have rings.[8]

    Although it was initially called an asteroid and classified only as a minor planet with the designation "2060 Chiron", it was later 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.

    Michael Brown lists it as possibly a dwarf planet with a measured diameter of 206 km[9] (128 miles) which is near the lower limit for an icy dwarf planet (around 200 km, or 124 miles).

    History

    Discovery

    Chiron was discovered on 18 October 1977 by Charles Kowal from images taken two weeks earlier at Palomar Observatory.[10] It was given the temporary designation of 1977 UB.[11] It was found near aphelion[10] and at the time of discovery it was the most distant known minor planet.[lower-alpha 2][11] Chiron was even claimed as the tenth planet by the press.[12] Chiron was later found on several precovery images, going back to 1895, which allowed its orbit to be accurately determined.[10] 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.[10] 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.[10]

    Naming

    It was named Chiron in 1979[11] after Chiron, one of the centaurs; it was suggested that the names of other centaurs be reserved for objects of the same type.[10]

    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 around May 720, when it came within 30.5±2.0 million km of Saturn. During this passage Saturn's gravity caused Chiron's semi-major axis to decrease from 14.55±0.12 AU[13] to 13.7 AU.[3] It does not come nearly as close to Uranus; Chiron crosses Uranus's orbit where the latter is farther than average from the Sun.

    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.[14] 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.[15] Chiron is probably a refugee from the Kuiper belt and will probably become a short-period comet in about a million years.[14]

    Chiron came to perihelion (closest point to the Sun) in 1996.[3]

    Orbit

    The orbit of 2060 Chiron compared with the orbits of Jupiter, Saturn, Uranus and Neptune.
    Chaotic motion

    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.

    Physical characteristics

    Size estimates for Chiron:[16]
    YearRadius (km)Notes
    198490Lebofsky
    1991<186IRAS
    199474Campins
    199690occultation
    2007117[17]Spitzer Space Telescope
    2013109[4]Herschel Space Observatory
    111Average of measurements

    The visible and near-infrared spectrum of Chiron is neutral,[11] and is similar to that of C-type asteroids and the nucleus of Halley's Comet.[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.[16] Estimates in the 1990s were closer to 150 km in diameter.[3][16] Occultation data from 1993 suggests a diameter of about 180 km.[16] 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.[4] Therefore, Chiron may be as large as 10199 Chariklo.[17]

    Its rotational period is 5.917813 hours, a value determined by observing its distinct light curve.[11]

    Cometary behavior

    In February 1988, at 12 AU from the Sun, Chiron brightened by 75 percent.[19] This is behavior typical of comets but not asteroids. Further observations in April 1989 showed that Chiron had developed a cometary coma,[20] A tail was detected in 1993.[11] 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.[21]

    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.[7]

    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.[22]

    Chiron is a Chiron-type comet with (TJupiter > 3; a > aJupiter).[3] 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.[23]

    Rings

    Chiron is suspected to have two rings,[24][25] 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 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.[25]

    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.[25]

    Notes

    1. ↑ 944 Hidalgo, discovered in 1920, also fits this definition, but was not identified as belonging to a distinct population.
    2. ↑ Pluto, now known to be a dwarf planet and hence a minor planet, was known at the time, but was considered a planet.

    References

    1. ↑ Marc W. Buie (2007-08-18). "Orbit Fit and Astrometric record for 2060". SwRI (Space Science Department). Retrieved 2008-10-13.
    2. ↑ "List Of Centaurs and Scattered-Disk Objects". Minor Planet Center. Retrieved 2014-11-13.
    3. 1 2 3 4 5 6 7 "JPL Small-Body Database Browser: 2060 Chiron (1977 UB)" (2008-11-28 last obs). Retrieved 2009-03-15.
    4. 1 2 3 4 5 Fornasier, S.; Lellouch, E.; Müller P., T.; et al. (2013). "TNOs are Cool: A survey of the trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of 9 bright targets at 70–500 µm.". Astronomy & Astrophysics 555: A92. arXiv:1305.0449v2. Bibcode:2013A&A...555A..15F. doi:10.1051/0004-6361/201321329.
    5. ↑ "AstDys (2060) Chiron Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2009-03-15.
    6. ↑ Meech, Karen (19 February 1994). "The Structure of the Inner Coma of Comet Chiron: Imaging The Exopause". Institute for Astronomy, University of Hawaii. Retrieved 2007-10-19.
    7. 1 2 Grayzeck, Ed (2003-12-11). "The Chiron Perihelion Campaign". NASA Goddard Space Flight Center. Archived from the original on 11 October 2007. Retrieved 2007-10-18.
    8. ↑ "A second minor planet may possess Saturn-like rings". Space Daily. 17 March 2015.
    9. ↑ Michael E. Brown. "How many dwarf planets are there in the outer solar system? (updates daily)". California Institute of Technology. Retrieved 2012-04-28.
    10. 1 2 3 4 5 6 Kowal, C. T.; Liller, W.; Marsden, B.G. (1979). "The Discovery and Orbit of (2060) Chiron". Dynamics of the Solar System; Proceedings of the Symposium, Tokyo, Japan, May 23–26, 1978. Tokyo: Reidel Publishing Co. pp. 245–250. Bibcode:1979IAUS...81..245K.
    11. 1 2 3 4 5 6 Campins, H.; Telesco, C. M.; Osip, D. J.; Rieke, G. H.; Rieke, M. J.; Schulz, B. (December 1994). "The Color Temperature of (2060) Chiron: A Warm and Small Nucleus". The Astronomical Journal 108 (6): 2318–2322. Bibcode:1994AJ....108.2318C. doi:10.1086/117244.
    12. ↑ Collander-Brown, S.; Maran, M.; Williams, I.P. (2000). "The effect on the Edgeworth-Kuiper Belt of a large distant tenth planet". Monthly Notices of the Royal Astronomical Society 318 (1): 101–108. Bibcode:2000MNRAS.318..101C. doi:10.1046/j.1365-8711.2000.t01-1-03640.x.
    13. ↑ "Chiron's Osculating Elements 700AD generated with Solex 11, and data of close approach in 720". Retrieved 2015-07-12. (Solex 10 results)
    14. 1 2 Horner, J.; Evans, N.W.; Bailey, M. E. (2004). "Simulations of the Population of Centaurs II: Individual Objects". arXiv:astro-ph/0408576.
    15. ↑ Jewitt, David C.; A. Delsanti (2006). "The Solar System Beyond The Planets". Solar System Update: Topical and Timely Reviews in Solar System Sciences. Springer-Praxis Ed. ISBN 3-540-26056-0. (Preprint version (pdf))
    16. 1 2 3 4 Groussin (2004). "Properties of the nuclei of Centaurs Chiron and Chariklo". Astronomy and Astrophysics 413: 1163–1175. Bibcode:2004A&A...413.1163G. doi:10.1051/0004-6361:20031564.
    17. 1 2 John Stansberry; Will Grundy; Mike Brown; Dale Cruikshank; John Spencer; David Trilling; Jean-Luc Margot (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". arXiv:astro-ph/0702538 [astro-ph].
    18. ↑ Luu, J.X.; Jewitt, D.C. (September 1990). "Cometary activity in 2060 Chiron". The Astronomical Journal 100: 913–932. Bibcode:1990AJ....100.913L. doi:10.1086/115571.
    19. ↑ Hartmann, William K.; Tholen, D.J.; Meech, K.J.; Cruikshank, D.P. (January 1990). "2060 Chiron - Colorimetry and cometary behavior". Icarus 83: 1–15. Bibcode:1990Icar...83....1H. doi:10.1016/0019-1035(90)90002-Q.
    20. ↑ Meech, Karen J.; Belton, M.J.S (April 1989). "(2060) Chiron". International Astronomical Union Circular 4770. Bibcode:1989IAUC.4770....1M.
    21. ↑ Meech, Karen J.; Belton, M.J.S (October 1990). "The Atmosphere of 2060 Chiron". The Astronomical Journal 100: 1323–1338. Bibcode:1990AJ....100..1323M. doi:10.1086/115600.
    22. ↑ Trigo-Rodríguez; Melendo; García-Hernández; Davidsson; Sánchez; Rodriguez (2008). "A continuous follow-up of Centaurs, and dormant comets: looking for cometary activity." (PDF). European Planetary Science Congress. Retrieved 2008-10-12.
    23. ↑ Dual-Status Objects
    24. ↑ Lakdawalla, E. (2015-01-27). "A second ringed centaur? Centaurs with rings could be common". Planetary Society. Retrieved 2015-01-31.
    25. 1 2 3 Ortiz, J.L.; Duffard, R.; Pinilla-Alonso, N.; Alvarez-Candal, A.; Santos-Sanz, P.; Morales, N.; Fernández-Valenzuela, E.; Licandro, J.; et al. "Possible ring material around centaur (2060) Chiron". arXiv:1501.05911.

    Further reading

    External links

    Periodic comets (by number)
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