First view of Chiron's farthest fringes.

First View of Chiron's Fartheast Fringes

In 1977, when astronomer Charles Kowal discovered the object now called Chiron circling the sun between the orbits of Saturn and Uranus, he could not tell whether it was a comet or an asteroid. The first evidence for Chiron as a comet did not appear until late 1987, as the rocky object's orbit carried it closer to the sun. Measurements showed it brightening more rapidly than would a mere asteroid -- which is just what the icy nucleus of a comet ought to do as the sun's heat vaporizes the ice and releases it as a diffuse but growing cloud, or coma, surrounding the comet's head.

Now, Jane X. Luu of MIT and David Jewitt of the University of Hawaii at Manoa have taken what they say is "believed to be the deepest (most revealing) image of this object [Chiron] ever recorded."

They made the image on Jan. 29, using the University of Hawaii's 2.24-meter telescope on Mauna Kea. The researchers combined 40 separate exposures, each 500 seconds long and taken one after another, that essentially add up to a single time exposure lasting more than 5.5 hours. The cumulative exposures allowed a computer to integrate enough light to reveal even the faint outer fringes of the coma. The resulting composite shows the coma, as seen from Earth, extending about 80,000 kilometers from Chiron's center to each side, while single exposures show only the inner 36,000 km to each side, Luu and Jewitt say.

Jewitt points out, however, that the coma is actually "a highly elongated structure almost parallel to the line of sight from Earth, preventing us from seeing its full length." Its true length is 2 million km, he told SCIENCE NEWS.

Chiron's brightness increased steadily through 1988. The comet reached its maximum brightness -- about twice that of an asteroid -- in December 1988, and has faded ever since. Astronomers made their first direct observations of the coma itself about a year ago, having only measured changes in Chiron's overall brightness before that time.

One reason the brightness increase has reversed, Jewett says, could be that Chiron's nucleus contained pockets of material that turned to gas and burst forth when heated by the sun, and the amount of released material diminished over time. Candidates for these now-exhausted materials include carbon monoxide, carbon dioxide and nitrogen, he suggests. Another possible coma-forming candidate might be "amorphous ice" in the nucleus, which could give off heat as it crystallized, but Jewitt suspects that any water released would freeze rather than evaporate to form a coma.