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A few hundred miles away, in Stanfield, Arizona, Thomas Bopp was going through a similar exercise at almost precisely the same time. Bopp is a supervisor at a construction-materials company and, like Hale, a longtime amateur astronomer. He too saw the intruder and sent his own E-mail to the bureau. Thanks to their nearly simultaneous discoveries, Hale and Bopp share the honor of giving the comet their names.

Within a few days, Green and Marsden had calculated that Hale-Bopp was incredibly far away and must therefore be unusually bright. At this rate, they determined, it should be absolutely brilliant when it finally arrived in March 1997. Should was the operative word, however. Comets are not especially well-behaved creatures. All too often they show great promise early in their career but turn out--like the infamous Kohoutek in the early 1970s--to be celestial duds.

The uncertainty has to do with the way comets are put together. They're basically chunks of ice--chiefly H2O with a fair amount of carbon dioxide and other frozen gases mixed in, plus a lot of sooty dust. Billions upon billions of comets orbit lazily out beyond Neptune--most of which we'll never even see. When one happens to fall in toward the sun, though, the ice begins to vaporize, surrounding the solid core with a hazy cloud of dusty gas.

This cloud, which can grow to thousands of miles across, is the comet's head, the light-reflecting shroud that turns an otherwise insignificant iceberg into a brilliant object. Just how brilliant depends on many factors. The solid comet's size is one, and Hale-Bopp, an estimated 20 miles across, is bigger than most. (Halley's was less than half as large.) Its history is another. Out in deep space, a comet can get encrusted with a layer of gummy dust. This layer can seal in most of the ice and prevent it from vaporizing. Some gas may spurt out through cracks in the crust, giving a comet a premature air of greatness that amounts to not much at all.

Or maybe the comet has been around the block too many times. The first visit loosens a comet's crust, making later go-rounds more impressive. If the comet comes through too often, however, a new crust can form out of dust falling back onto the surface. This too can lead to false optimism. "With Comet Halley, which has been back many times," says University of Texas astronomer Anita Cochran, "only about 15% to 20% of the surface is active." Admits Hale: "It's been kind of nerve-racking to sit through all those months wondering if the comet would fizzle."

It didn't. Hale-Bopp has steadily grown in brightness, giving amateur astronomers an increasingly satisfying show. Professional astronomers too have been watching Hale-Bopp, and not always with detachment. "We're delirious," says Tobias Owen of the University of Hawaii. "It's been 20 years since a really bright comet came by, and now, within just a year, we've had Hyakutake and Hale-Bopp."

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