Chiron or Charon?

by Joe Orman

[Originally published in the newsletter of the East Valley Astronomy Club, October 1998]

A recent brochure for the Lowell Observatory referred to "enigmatic objects of the outer solar system including Pluto and Chiron." When someone asked me what Chiron was, I immediately said "It's the moon of Pluto." Wrong! I was getting two similarly-named objects confused. A little research set me straight.

Charon, discovered in 1978, is the moon of the planet Pluto. Relative to the planet it orbits, it is the largest satellite in the solar system. At 1220 km diameter, Charon is fully half the size of Pluto (2300 km diameter)! Charon and Pluto are separated by only 19,500 km. Although Pluto develops an atmosphere near perihelion, the surface of Charon is water ice which remains frozen at that distance from the sun. Charon was named after the mythological boatman who ferried the souls of the dead across the River Styx to Hades. NASA has on its drawing board the first mission to explore the Pluto-Charon system, the Pluto-Kuiper Express. This unmanned spacecraft will not be launched for several years, and will take 7 - 10 more years to travel to Pluto.

Chiron, discovered in 1977, is an unusual object whose highly-elliptical, 50-year orbit around the sun brings it within the orbit of Saturn and as far out as the orbit of Uranus. It exhibits properties of both a comet (it has a coma) and an asteroid (it is as big as a large asteroid -- some 200 km in diameter -- much larger than the biggest known comet), so it has been designated as one of a class of objects known as Centaurs. These half-comet/half-asteroid objects were named after the mythical half-man/half-horse; Chiron is named after the wisest of the Centaurs. Several objects in this class have been discovered; they may be objects that have escaped from the Kuiper belt, the vast disk of cometary bodies beyond the orbit of Neptune. Chiron is designated alternately as Comet 95P/Chiron or minor planet (2060) Chiron.


The Pluto-Kuiper Express, originally planned for a 2004 launch and a 2012 rendezvous, has been postponed.

Revised: June 2, 2001
Copyright © 1999 Joe Orman
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