C H I R O N

With the shortest orbital period of 50.78 years and having the mean solar distance of 13.89 AU, Chiron changes position more rapidly than the other more distant TBO’s. An highly eccentric orbit (e) of 0.382, finds that distances from the Sun between periastron to apastron may vary anywhere from 8.46 AU to 18.91 AU — a difference of almost 75%! The slight inclination of 6.94° means that Chiron follows the ecliptic similar to all the main planets. Physically, Chiron is only 163 kilometres across and is unusually dark. Detailed photometry has revealed the short rotation period of 5.92 hours.

This is certainly an unusual planetary body, showing characteristics similar to both comets and asteroids. Almost since discovery, Chiron was unexpectedly found with a very thin atmosphere of cyanogen (HCN) and other gases — similar to what has been found in comets. During opposition in 1988 and 1989, and likely for several years around perihelion, Charon suddenly brighten just over one magnitude. Remarkably it then displayed characteristics of cometary comas, making some to speculated that Chiron was just a very large distant comet or proto-comet. Hence, Chiron shares its name as asteroid 2060 and also comet 95P/Chiron!

Other examples of Centaur-type asteroids include 5335 Damocles, whose orbit ranges from just ouside of Mars to just beyond Uranus. Another is 5145 Pholus, that orbits between Saturn to just beyond Neptune. Since the discovery by Kowal, many more Centaurs have been found, but all these planet-crossing orbits are unstable and suffer very much from the gravitational perturbations from the main outer planets. Like Charon, most Centaurs are more like comets, the distant Kuiper belt objects, the other outer dwarf planets or trans-Neptunian bodies.

Presently, yellowish Chiron is very faint at about 18.5 magnitude, varing by 0.3 magnitudes between opposition and conjuction. In 2006-08, it remains close to the border of Carpicornus and Aquarius, and will cross into Pisces by 2014. Perihelion last occurred 14th February 1996, making next aphelion as early May 2020 (2020.48). At the Pisces opposition in 2020 the maximum magnitude peaks at 19.2, dropping to the minimum brightness of 19.6 magnitude. Like most of these small outer solar system bodies, they are almost totally invisible to amateur telescopes, but this seems to be the exception. For only several years around 1996, Chiron reached its oppositions in Virgo rose to the maximum of 15.6v magnitude. It was easily visible in dark skies using either in 35cm. to 40cm. telescopes as a faint star, whose movement could be detected over one or two nights. After 2002, Chiron became more difficult to see, when the maximum brightness will drop below 17th magnitude. Amateurs will likely have their chances of again of seeing the minor planet between 2043 and 2048 AD, whose opposition is again brightest in 2046 AD.

In ancient Greek mythology the principal leader of the mythical Centaurs — creatures who are half human and half horse. The Centaurs were established in the early history of the world, whose creation was made by the union of Cronus (Saturn) and the nymph Philyra. (Although to detailed for our discussion here, the detailed mythology of Cronus can be readed in Saturn : Part 2 and about the nymphs Neptune : Part 2.) Chiron, was unlike his often drunken and violent fellow Centaurs, as he was considered to be highly cultured and wise. He was often associated as a great healer and teacher, and being the son of Cronus, he was an immortal. However, he was eventually accidentally killed by a dropped poison arrow, and because of his charity and the healing of the sick, he was respectfully placed among the stars by the ancient Greeks as the constellation of Sagittarius. In more recent times, others associated him with the mainly southern constellation of Centaurus.