Chiron: asteroid or comet?

Chiron is a solar system object having the characteristics of both a comet and an asteroid. Discovered in October 1977, Chiron is peculiar because it has a coma - a cloud of water, carbon dioxide and other gases sublimed from its nucleus indicating it is a cometary body—yet is also 75 to 100 miles in diameter, more than 50,000 times the size of a normal comet. This size is characteristic of a large asteroid, which it was first thought to be. Its unusually elliptical orbit from just inside Saturn’s orbit to approximately that of Uranus’ is also unstable over millions of years, indicating that, in astronomical terms, it hasn’t been there very long. This is supported by the fact that Chiron’s coma is still active, yet the super-volatiles (its coma) sublimating from its surface would have completely vaporized in a few million years at its current orbit’s position.

Dozens of bodies have since been discovered with similar orbits and properties. In recognition of their dual comet/asteroid nature they have been designated Centaurs, the mythological Greek race that was half man, half horse. They are hypothesized to be escaped Kuiper belt objects because gravitational perturbations from Jupiter and Saturn would occasionally force Kuiper belt objects into Neptune-crossing orbits that could evolve into orbits like the Centaur’s. The similarity in size between Chiron and other Kuiper belt objects also makes it a likely source. Although asteroids are in this size range too, Chiron’s coma rules out an asteroidal origin.

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Virgo and Venus in "You and the Universe"