95P (2060) Chiron 

Discoverer

C. Kowal 1977.

Diameter (km)

148 to 208

Mass (kg)

2*10^18 to 2*10^19

Rotation period (hrs)

~5.9

Orbital period (yrs)

50.7

Semimajor axis (AU)

13.70354

Orbital eccentricity

0.3831118

Orbital Inclination (deg)

6.9354

Albedo

0.05

Type

B

On February 14, 1996, the enigmatic object Chiron reached perihelion inside the orbit of Saturn and on April 1 it reached perihelion opposition, its closest approach to Earth. Chiron is unusual because it has a detectable coma, indicating that it is a cometary body, but it is over 50,000 times the characteristic volume of a comet, a size more commensurate with a large asteroid, which it was initially assumed to be. Furthermore, its curious orbit is unstable on time scales of a million years, indicating that it hasn't been in its present orbit long. Chiron was the first of bodies discovered so far with similar orbits and properties. These bodies have been designated Centaurs, after the race of half-man/half-horse beings from Greek mythology, in recognition of their dual comet/asteroid nature. Chiron is named after the wisest of the Centaurs, the tutor of Achilles and Hercules.

Chiron's highly elliptical 50.7-year orbit has a semi-major axis of 13.7 AU, a perihelion of 8.46 AU, and an aphelion of about 19 AU. For reference, Saturn is at 9.54 AU and Uranus is at 19.18 AU. The orbit is inclined 6.93 degrees to the ecliptic plane. Estimates of Chiron's diameter range from 148 to 208 km, and lightcurve studies give a rotational period of 5.9 hrs. A gas and dust coma, which varies with time in both size and brightness, has been observed about Chiron's nucleus. The diameter of the coma has been measured to reach almost 2 million km in diameter on occasion, and the brightness can fluctuate by a factor of four over a period of a few hours. In addition, a gravitationally bound "dust atmosphere" appears to be suspended in the inner 1,200 km of the coma, and this dust displays evidence of structure, indicating the possibility of particle plumes emanating from the nucleus.

After Chiron was first discovered by Charles Kowal on November 1, 1977 (on a photographic plate taken on October 18) and designated 1977 UB, earlier views of the object on photographic plates dating back to the 1895 perihelion were recognized. Images of Chiron near aphelion in 1970 showed that it was bright even at that great distance from the Sun. This along with evidence of a coma as early as 1988 indicated continuing surface activity and coma production at low temperature. This points to super-volatile substances such as methane, carbon monoxide, and molecular nitrogen, which can sublime at these low temperatures from the surface of Chiron, as being the source of the coma.

Two separate arguments indicate that Chiron has not been in its present orbit for more than a few million years. The first is that Chiron's orbit is unstable on time scales of about a million years to perturbations from the large outer planets. The second argument involves the super-volatiles sublimating from Chiron's surface. It is estimated that at Chiron's current orbit these substances would completely vaporize in a few million years, so the fact that Chiron is still active means it has not been in this orbit that long. The fact that Chiron must have come to its present state from another location in the solar system has led investigators to look towards the Kuiper belt.

The argument that Chiron is an escaped member of the Kuiper belt is based on a number of lines of reasoning. Gravitational perturbations from the giant planets should occasionally force Kuiper belt objects into Neptune-crossing orbits from which they can evolve into orbits like the Centaur's. The similarity in size between Chiron and the discovered Kuiper belt objects makes this a likely source. Asteroids are also in this size range, but the observations of a coma on Chiron appear to rule out an asteroidal origin. The evidence that Chiron still retains super-volatiles which would only persist for long times at lower temperatures than it presently experiences indicates a colder source region, beyond Chiron's present orbit. The data gathered by the Chiron Perihelion Campaign should help determine if Chiron is indeed a former inhabitant of the Kuiper belt.

Images of Chiron

Let's go

1P Halley

Kuiper Belt

I'm here if you need me

Last updated: March 15, 2002.

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