The Chiron Perihelion Campaign

On February 14, 1996, the enigmatic object Chiron reached perihelion inside the orbit of Saturn and on April 1 will reach 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 four 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.) It is believed that the Centaurs may be objects which have escaped from the Kuiper belt, a disk of objects orbiting beyond Neptune. Chiron is showing increased activity as it approaches perihelion. In an effort to coordinate observations and gain an unprecedented view of a possible Kuiper belt body, the Chiron Perihelion Campaign was organized. This page contains information on Chiron and the campaign to observe its perihelion passage as well as links to images of Chiron from various sources.

[Chiron images, Denis Bergeron]

Images of Chiron taken by Denis Bergeron, 2 and 3 April 1995.
More information available in the caption

 Images of Chiron


Chiron's highly elliptical (e=0.383) 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.

The Kuiper Belt Connection

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 Kuiper belt is a hypothetical disk-shaped reservoir of objects of sizes ranging from tiny particles to Pluto or larger sized bodies. It was theorized to exist as early as 1949 as a source of comets additional to the more distant Oort comet cloud. It may also be the origin of Pluto and Neptune's moon Triton. Since 1992, almost 20 objects have been discovered in the Kuiper belt with diameters of 100 to 300 km, leading to estimates that 20,000 to 40,000 such bodies exist in this region. A recent Hubble study has discovered about 30 objects 6 to 10 km in diameter which may be part of the 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.

The Perihelion Campaign

The Chiron Perihelion Campaign was established to coordinate observations of Chiron over the next couple of years. The campaign is a loose association of observatories around the world, with the PDS Small Bodies Node at the University of Maryland acting as the repository for the data. The next opportunity to observe Chiron at perihelion will not be until the year 2047, so scientists want to learn as much as possible during this occurence. If Chiron is in fact from the Kuiper belt, this represents a unique chance to view one of these objects at close range. Chiron will be four times closer and 250 times brighter than a typical Kuiper belt object, and will be undergoing over ten times more heating by the Sun. This will cause large scale sublimation of surface material and possibly dramatic outbursts of dust and gases. Observations of these gases may help distinguish the character of these objects. Carbon monoxide is a typical volatile associated with comets, while Pluto and Triton show more evidence of molecular nitrogen. Much may also be learned during this time about surface processes on Chiron, as well as the structure of the inner atmosphere, the development and make-up of the coma, and the size and composition of the nucleus.

Observatories involved in this campaign include many major Earth based observatories and proposals have been made for the Hubble Space Telescope. These images of Chiron will be posted here as they are made available, and results from the observations will be reported. Over the next couple of years we will certainly come to better understand the mysterious object known variously as Comet 95P/Chiron or minor planet (2060) Chiron.

 Images of Chiron

 Chiron Fact Sheet
 Asteroids and Comets Page

 Chiron Home Page - Vrije Universiteit Brussel

 Plot of Chiron's Orbit - Minor Planet Center
 Occultations by Chiron - 1990-1995 - Harvard University
 Comet Observations Page - JPL
 Kuiper Belt Objects - David Jewitt
 The Kuiper Belt - from "The Nine Planets"
 Kuiper Belt and Centaur Objects - Gary Kronk
 List of Centaur Objects - Minor Planet Center
 List of Trans-Neptunian Objects (TNO's) - Brian Marsden, Minor Planet Center

 Hubble Observations of Chiron planned for 16 March 1996 - call for concurrent observations.
 Request for Chiron Observation Information - from the HST/FOS Chiron observation team (21 August 1996)

 Planetary Home Page

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Dr. David R. Williams,
NSSDC, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771

NASA Official: Ed Grayzeck,
Last Updated: 11 December 2003, DRW