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Watching Comet Halley come to life.

For most of a comet's liftime, it is just an inert ball of ice, dust and perhaps larger chunks of rock. Only when it nears the sun does the growing heat trigger the formation of the familiar, fuzzy "coma" and tail that make it stand out from other objects in the sky. Comet Halley is no exception, and when astronomers awaiting its 1910 "apparition" first saw it coming, about eight months earlier in 1909, its coma was already developing.

Now Halley is on its way again, due to make its closest passes to the sun next Feb. 9, but thanks in part to today's greatly improved instrumentation, it was "recovered" this time with more than three years to go, in October of 1982 (SN: 10/30/82, p.277). An international fleet of spacecraft are on their way to study the comet when it gets near, but a unique advantage of the long period of ground-based observations has been the chance to study the coma as it begins to form -- as the comet's icy nucleus first "comes to life."

It is a difficult change to identify -- the time at which the ice actually starts to sublimate, or vaporize, and free the dust that forms a comet's visible coma. But Susan Wyckoff, Mark Wagner and colleagues from Arizona State University in Tempe and the Institute of Astrophysics in Paris have now eported in the July 12 NATURE what they call, "to our knowledge ... the first observation of the onset of sublimation in any comet."

Changes in the apparent brightness of a comet's nucleus need not be due only to the formation of the coma, whose growth gives the nucleus a larger reflective surface. Another factor can be the rotation of the nucleus, exposing variously brighter and darker material to the sun.

A more exotic contributor, suggest Asoka Mendis, Harry Houpis and other researchers from the University of California at San Diego, could be the raising of dust from the surfacd of the nucleus by electrostatic charges due to the solar wind. One member of the UCSD group, for example, has found a correlation between some of Halley's brightenings and the position of coronal holes on the sun -- possibly acting as openings to release high-speed solar-wind streams.

Whatever is freeing the dust -- be it mere sublimation of the ice that traps it, electrostatic "levitation," or sudden bursts produced by the exposure of ices whose sublimation pressure is lower than that of water ice -- the expansion of the area covered by dust provides a conspicuous signature.

The observations reported by Wyckoff's group were made last Nov. 26, Feb. 17, MArch 24 and April 2, using the Multiple Mirror Telescope near Tucson, and combined with other researchers' observations. The result (to which more data are being added) indicates that the dust coma began to grow sometimes between early 1984 and 1985.

Other sensors, however, offer less clear clues to the subtle timing of the coma's birth. The photo shown (left), made last Dec. 30-31 with the 3.6-meter Canada-France-Hawaii Telescope on Mauna Kea, shows little if any indication of a developing coma, even when computer-divided into discrete brightness contours (right). These images, however, produced by Bruce Goldberg of Jet Propulsion Laboratory in Pasadena, Calif., and colleagues, will be part of a series documenting the comet's approach through standardized filters being made available to Halley-watchers at many different observatories.
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Author:Eberhart, Jonathan
Publication:Science News
Date:Jul 20, 1985
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