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Chiron's brightening hints it's a comet.

Chiron's brightening hints it's a comet

When astronomer Charles Kowal in 1977 discovered the object now called Chiron between the orbits of Saturn and Uranus, it was known enigmatically as "Object Kowal." Astronomers could not tell whether it was an asteroid or a comet, since it orbited farther from the sun than most known asteroids but for years failed to show a comet's fuzzy "coma." Now Chiron may have revealed its true identity.

Astronomers had speculated that the object might never warm enough to release the frozen surface material that would give it a coma. But as early as November 1987, as it approaced the sun, Chiron seemed to brighten more rapidly than would be expected of a bare, rocky object. Now Karen J. Meech of the University of Hawaii Institute for Astronomy in Honolulu and Michael J.S. Belton of Kitt Peak National Observatory in Tucson, Ariz., report a coma around Chiron in images made with the Kitt Peak 4-meter telescope when the object was about 11.8 astronomical units from the sun (1 astronomical unit equals about 92.9 million miles). Chiron is expected to get no closer than about 8.5 astronomical units, but the brightening is apparently due to "sunlight reflected from an extended dust atmosphere," they say, "indicating that it is a very large comet."

The researchers note that Chiron appears about 112 miles in diameter, 10 to 20 times the size of Comet Halley. Ices such as carbon dioxide, which evaporate at much lower temperatures, may have been freed in what could be Chiron's first trip this close to the sun.
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Author:Eberhart, J.
Publication:Science News
Date:Apr 22, 1989
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