COMET OF THE DECADE, PART II

By MICHAEL D. LEMONICK

Until now, you had to be pretty much of an astronomy nut to see Comet Hale-Bopp. Not that the comet is especially hard to spot. For weeks it has been putting on a show to rival last year's Comet Hyakutake. People have seen Hale-Bopp, without a telescope or even binoculars, from such unpromising, light-polluted vantage points as midtown Manhattan and downtown Chicago. Amateur astronomers have been taking telescopic photos of the comet for well over a year; the Jet Propulsion Laboratory's Hale-Bopp home page on the World Wide Web NewProducts.jpl.nasa.gov/comet has posted more than a thousand already, and more are streaming in all the time. The problem, though, is that Hale-Bopp is still what is called an early-morning object, up before the sun and invisible before most folks are out of bed. A spectacular comet is one thing; getting up at 5 a.m. is another.

But that's about to change dramatically. By the end of next week, Hale-Bopp's path across the sky will take the comet right into prime time. By April 1 or so, when it makes its closest approach to Earth, the comet will be high in the evening skies over the northern hemisphere, brighter than ever and showing a short but prominent tail. And there it will sit, not for a measly week like Hyakutake, but for more than a month. "I predict that this could be the most viewed comet in all of human history," says Daniel Green, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "This will be one of the brightest objects in the sky. It'll be hard for the average person not to see it."

All this may come as news to most of us, but astronomers, amateur and professional alike, have been buzzing about Hale-Bopp ever since its discovery nearly two years ago. At that point the comet was more than half a billion miles from the sun, well beyond the orbit of Jupiter, and invisible without a telescope. But not necessarily a huge telescope: like most comets, this one was found by a pair of amateurs as familiar with their favorite regions of the sky as most people are with their own neighborhoods.

On July 23, 1995, Alan Hale, who has a Ph.D. in astronomy and makes his living running a research and educational company, was scanning the skies above his home in Cloudcroft, New Mexico. He was waiting for an already discovered comet to rise over his house when he trained his telescope on M70, a well-known cluster of stars in the constellation Sagittarius. "As soon as I looked," he says, "I saw a fuzzy object nearby. It was strange, because I'd looked at M70 a couple of weeks earlier and the object hadn't been there."

Hale checked his sky atlas, then logged on to the computer at the quaintly named Central Bureau for Astronomical Telegrams, located at the Harvard-Smithsonian Observatory. Maybe this would turn out to be a known object. It didn't. "Now I felt I had a pretty live suspect," he says. He fired off an E-mail message to Daniel Green and Brian Marsden, who run the Bureau for the International Astronomical Union, reporting a possible new comet. A few hours later he looked again, and the object had moved. It was a comet for sure.

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."

In the intervening two decades, astronomical hardware has improved markedly, giving scientists the chance to study comets in unprecedented detail. They want to know precisely what these bodies are made of, since comets are believed to be the only objects that have remained unaltered since the solar system was born, 4.5 billion years ago. The planets and the asteroids have been heated and cooled, smashed apart and re-formed, but the comets, lingering on the solar system's periphery, have stayed relatively pristine.

So goes the theory, at least, and early studies of Hale-Bopp's gases bear this out. "We've found a type of hydrogen cyanide that's otherwise seen only in interstellar space," says Owen. "We saw it in Hyakutake too, but we thought it could be a fluke." Astronomers have found other gases they suspected would be there, including ammonia, methane, alcohol, formaldehyde and other organic compounds. Says Michael Mumma, an astronomer at NASA's Goddard Space Flight Center: "It's been suggested that both the building blocks of life and the water in our oceans fell to Earth on comets. Our observations of Hale-Bopp may help settle that question."

Astronomers also believe a comet impact is probably what did in the dinosaurs 65 million years ago. That object was perhaps 10 miles across. At double the size, Hale-Bopp packs a lot more potential energy. Luckily for civilization, Hale-Bopp will miss Earth by 120 million miles.

That should reassure worriers, but it's too bad for sky watchers. Hale-Bopp will be brighter than Hyakutake was, but it's also 15 times as far away. "If Hale-Bopp came as close as Hyakutake did," says Harvard's Green, wistfully, "it would be incredible. You'd even be able to see it easily in the daytime." Along with the rest of us, he's going to have to settle for what will merely be the best celestial show in decades.