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The Kuiper Belt and the Oort Cloud | Quaoar | Eris | Sedna

Chiron is a curious object. It's the size of an asteroid, but is composed more like a comet, and is called Chiron after one of the centaurs, the half-horse, half-human creatures of Greek mythology. In fact, Chiron is over 50,000 times the regular size for a comet, but has a coma that can be detected, showing that it is more comet than asteroid.

Discovery

Charles Kowal discovered Chiron in 1977, and subsequent research showed that it had been photographed as far back as 1895. Chiron's coma was discovered in 1988. Chiron's radius is thought to be between 148 and 208km, and it rotates every 5.9 hours. Since Chiron, other objects have been found which fit into its 'Centaur' class.

Orbit

Chiron is on an orbit around the Sun that over a million years is unstable. This means that somehow Chiron got into this orbit within the last million years, relatively recently on a solar system timescale.

Chiron takes 50.7 years to orbit the Sun, on a very elliptical orbit. At closest approach it is 8.46AU from the Sun, within the orbit of Saturn, but at its furthest it is 19AU, almost as far out as Uranus. Its orbit is inclined to the ecliptic too (in other words it's inclined to the plane in which all the other planets orbit the Sun).

Where Did Chiron Come From?

It is thought that Chiron may have originally been a Kuiper Belt object. It can't have been where it is for very long, for two reasons. The first of these is the instability of its orbit. Over about a million years the gravitational fields of the other planets will cause big perturbations in it. The other reason comes from studying Chiron's composition. Chiron has a coma and a tail, like a comet, so it must be carrying some volatile elements such as methane and molecular nitrogen to produce this. Studies have shown that if Chiron had been in an orbit at that distance from the Sun for long, all of these elements would have disappeared and Chiron would appear more like an asteroid in a strange place.

Theoretically, larger Kuiper belt objects could be sent closer in to the Sun by getting close to the gravitational influence of the planet Neptune. If Chiron had been orbiting farther out for most of its life, its volatile elements would have remained inert in the extreme cold of the outer solar system.