HEAVENLY VAGABOND

By MICHAEL D. LEMONICK

SAY THE WORD KOHOUTEK, AND MOST astronomers wince. Back in 1973 there were predictions that the comet of that name might be the brightest of the century. It turned out to be a total dud. Halley's comet was just as heavily overhyped 12 years later; that time around, at least, history's most celebrated comet was dim and unimpressive.

But scientists aren't likely to be embarrassed by C/1996 B2, better known as Comet Hyakutake. By last week the recently discovered visitor from the edge of the solar system was already being spotted without telescopes or binoculars by stargazers from the Azores to Australia, and many of them rushed to the Internet to report their observations. "This thing is starting to look amazing," wrote Marcus Featherston of Panama City, Florida, in a Usenet newsgroup called sci.astro.amateur. "I could see it through my car window!" And that was while Hyakutake was still brightening. When it reaches maximum intensity this week, the comet should be nearly as bright as Vega, one of the most luminous stars in the heavens. Astronomy clubs, colleges and planetariums have organized "star parties" for comet wannasees; World Wide Web users have created a dozen fact-packed Hyakutake sites, including NASA's Night of the Comet home page http://ccf.arc.nasa.gov/comet/)

Even scientists have developed a touch of comet fever. They have commandeered the world's most powerful telescopes, including the high-flying Hubble, to plumb the secrets of one of the most ancient objects orbiting the sun. Says Daniel Green, an astronomer at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts: "It's the brightest one since 1976, and we're dropping everything to study it."

When Japanese amateur astronomer Yuji Hyakutake first caught sight of the comet through a pair of binoculars on Jan. 30 (it was his second comet discovery; the first came just a month earlier) there was no reason to think it would be especially bright. But when professionals calculated the orbit, they realized that Hyakutake would be approaching to within a mere 9.3 million miles of Earth, only 40 times as distant as the moon.

That makes it the closest comet since 1983 and gives it a leg up on visibility (Halley's came no nearer than 39 million miles). Better yet, Hyakutake's trajectory will place it high in the northern sky when it reaches peak brightness this week, so that it will be visible for most of the night over most of the northern hemisphere.

Finally, the shape of Hyakutake's orbit tells astronomers that it has been here before, perhaps 9,000 years ago. That's crucial: the only reason a comet is visible at all is that its tiny core, just a few miles across, is surrounded by a giant cloud of gas and dust that can spread over a million miles. The cloud spews from the comet itself, as the sun's heat turns its dirty, icy surface into dirty water vapor mixed with other gases. If a comet is on its maiden voyage to the inner solar system, though, its surface is encrusted with dust, which prevents much evaporation. The first visit may only loosen the crust, while later trips allow the ice to evaporate freely. That was evidently the problem with Kohoutek.

Hyakutake's status as a return visitor makes it ideal for amateur stargazers but a bit less enticing to scientists. Comets are believed to be leftover material from the formation of the solar system 4.5 billion years ago. Millions are still orbiting lazily in a halo called the Oort cloud, far beyond Pluto, where they are perfectly preserved. It's only occasionally that one changes orbit and plunges into the relative warmth of Earth's neighborhood. And the more often a comet returns to be heated, the more its structure and chemical composition are altered.

Although a truly primordial comet would tell them more about the original chemistry of the solar system, astronomers are still thrilled to see any comet this close. "We can't exactly reach out and touch them," says Green. "We have to wait for them to come to us."

Nonscientists will be thrilled as well--as long as they have realistic expectations. Hyakutake is as bright as a bright star overall, but its light is diluted over an area twice the size of the full moon; it's a glowing blob rather than a fiery point. There won't be much of a tail at first, but one will grow over the next few weeks, making the comet even more prominent through April. And after it fades from view later in the spring, comet lovers have a treat to look forward to next year: a second new comet, called Hale-Bopp. Astronomers are predicting--cautiously--that this one could be even brighter.