CHIRON

In 1977 Charles T. Kowal found on telescope photographs he had taken, an object inside the orbit of Uranus. The object later named Chiron, was a new minor planet and the first of a new class of object termed Centaurs, asteroids with elliptical orbits that cross the paths of the giant planets in the region mainly between Saturn and Neptune. By April 2002, a total of 41 Centaurs had been found, but Chiron is definitely the most unusual.

In 1988 when its elliptical orbit brought it closer to the Sun, Chiron suddenly doubled in brightness - developing a coma, a gaseous halo like that of a comet. It was this discovery which started the debate whether Chiron was a comet or an asteroid. Its elliptical path and out-gassing are consistent with the usual definition of a comet, but its size - with a diameter of about 200km- makes it much larger than any previously known comet and large enough for its gravity to retain the gases released from the surface when it warms up in a thin dusty atmosphere extending about 1200 km from Chiron itself. When Chiron moves away form the Sun the gases are frozen and fall back to the surface.

Chiron, when discovered, was given the provisional number 1977 UB, and not long after given the minor Planet name and number (2060) Chiron. In 1995 Chiron was also given a comet designation: 95P/Chiron. Like several other bodies (7968) Elst-Pizzaro or 133P/Elst-Pizzaro and (4015) Wilson-Harrington or 107P/Wilson-Harrington, Chiron has joint status and can be regarded as either or both an asteroid or a comet.

The Centaurs Pholus, Nessus and others do not display the comet-like properties of Chiron and have straightforward minor planet designations. However, astronomers are now certain that the Centaurs originated in the Kuiper Belt, a collection icy (and comet-like) asteroids, orbiting beyond Neptune and scattered inwards by gravitational interactions.