The Night Visitor

KOHOUTEK FELL SO SHORT OF its hype in 1973 that when one astronomer threw a comet party he served flat, booze-free drinks: fake punch, he said, for a fake comet. Halley raised such expectations that the hordes of amateur observers dusting off their 'scopes in the winterar of '86 expected something whizzing across the sky, not the static smudge they actually saw (or thought they saw). But comet groupies are as optimistic as serial blind-daters, no matter how often their expectations crash and burn. The next one, they are sure, will be a heavenly vision.

The next one is here. Using eight-inch telescopes atop suburban school roofs and the most powerful astronomical instrument ever crafted by the hand of man, sky buffs worldwide are squinting their eyes and craning their necks this week for a peek at Comet Hyakutake. Even usually restrained astronomers can't resist superlatives. "I've never seen anything like this in the sky," says Steven Ostro of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "This may turn out to be the comet of the century."

And this is the week to see it. Hyakutake makes its closest approach to Earth at 2 a.m. Monday, and for the next few nights should be as bright as the stars of the Big Dipper and as large as a full moon. It will be visible to the naked eye all night at any cloudless spot in the Northern Hemisphere (diagram), even where bright lights of the big city act like a contrast-dimming knob.

Although astronomers got little advance warning about Hyakutake--Japanese photoengraver Yuji Hyakutake spied it on Jan. 30--they have scrambled to turn any telescope they can toward the surprise visitor. Hyakutake could become the best-studied comet ever. And, fittingly for an object discovered by an amateur, the pros will have lots of company as cometeers assemble to glimpse the luminous fuzzball with a bright point at its center. In Lincoln, Neb., the astronomy club promises to make its telescope available "until everyone is tired of looking," says club official John Bruce. In Boston, hundreds are expected at a "viewing party," and in L.A. telescopes and binoculars will be available in Griffith Park until the comet disappears.

For centuries, even scholars considered these cosmic streakers divine portents (usually of the maleficent variety) and therefore Not Their Job. But with the comet of 1577, astronomers began studying these apparitions. They know now that comets are frozen dirtballs -- or dirty snowballs, take your pick--created at the same time as the planets and the Sun, and biding their time in a cosmic bullpen called the Oort Cloud. Extending 300 times farther from the Sun than Pluto, the Oort Cloud contains an estimated 100 billion comets, each waiting for the gravitational nudge that will send it hurtling toward the Sun.

Hyakutake came from the Oort Cloud, too--originally. But its brightness indicates that it has passed this way before. A comet "cooked" by a previous trip emits the dust and gases that create its luminous coma, or atmosphere, later, explains Brian Marsden of the Smithsonian Astrophysical Observatory. Hyakutake last came through 18,000 years ago, calculates JPL astronomer Donald Yeomans, whose 1991 book "Comet"* presents the lore and science of these celestial streakers. Hyakutake's orbit angles through the planets' and will bring it within 9.5 million miles of Earth. That, in cosmic terms, is a close call. Just a nudge when Hyakutake was near the far point of its orbit, 130 trillion miles away, would have put the 93,000-mile-per-hour missile on a collision course with Earth.

Whatever amateur sky gazers hope for, astronomers expect even more. NASA's Goldstone telescope in the California desert will bounce radar beams off the comet to determine the size and shape of its nucleus, and whether it is spinning. Rotation, explains the JPL's Ostro, would hint that the comet is studded with "jets" that blow off ice and dust and make it spin as madly as a pinwheel. The radar echoes should also reveal whether the nucleus is one solid chunk. The way Comet Shoemaker-Levy shattered when it smashed into Jupiter in July 1994 revived a discredited idea that the nucleus is instead a pile of rubble. Even the Hubble Space Telescope will study Hyakutake, glimpsing details, like craters, as small as four miles across. NEAR, a spacecraft making a three-year trip to the asteroid Eros, will shoot 50 images.

Back on Earth, telescopes will be determining what the comet is made of. The 140-foot National Radio Astronomy Observatory (NRAO) in Green Bank, W.Va., will look for signs of formaldehyde, ammonia and methyl alcohol. The Berkeley-Illinois-Maryland Array in northern California will seek amino acids (which form proteins) and acetic acid (vinegar). The Very Large Array in New Mexico will take a closer look at whatever these other telescopes find. Why this interest in the recipe for a comet? "The molecules froze out eons ago, when the gases in the nebula that formed the Sun and planets started condensing," explains Alwyn Wootten of the NRAO. So a comet "tells us what the nebula was like," says University of Chicago astronomer Patrick Palmer. Knowing that is the first step in reconstructing the solar system's history. Changes in its composition might reveal, for instance, that its molecules were once cooked by the radiation of a supernova.

Another reason for wondering what comets are made of is that they probably gave Earth an atmosphere, and thus life. Geophysicists believe that "Earth's atmosphere must have vanished very early on," says Wootten. "We think that its current atmosphere--nitrogen, oxygen--was brought in by comets." And not only its atmosphere. "Maybe half the carbon compounds on the early Earth, and all the water in the oceans, came from space," says University of Illinois astronomer Lewis Snyder. "Deliveries from comets may be how chemistry [leading to life] got started on Earth." Perhaps the most eagerly sought molecule is ammonia. According to theories of how life began, ammonia in the primordial atmosphere created the conditions by which nonliving molecules combined into the amino acids (for proteins) and nucleic acids (for DNA) required for life.

Hyakutake will beat its retreat through Perseus, remaining visible through April. It will glow brighter as it nears the Sun for its closest encounter, May 1. Predawn watchers in the Southern Hemisphere should be able to pick it out after mid-May as it starts its return trip toward the outer reaches of the solar system. And if Hyakutake disappoints, comet lovers can pin their hopes on Comet Hale-Bopp. It's coming in March 1997. It might even be the comet of the century. After all, one of them has to be.

This week, the newly discovered comet C/1996 B2, also called Comet Hyakutake, will be visible in the skies of the Northern Hemisphere. Passing 9.5 million miles from Earth, it is the brightest comet since 1976.

Venus: The very bright "Evening Star' in the west, low in the horizon in March, then slowly rising

Polaris, the North Star: The star at the end of the Little Dipper's handle

Vantage point: The darker the better. Rural areas away from city lights and smog are best.

Time of night: The comet will be visible after twilight, but from March 27 to April 5 the moon may overwhelm it.

Gear: The naked eye will work, but binoculars with their wide field also help. Use telescopes on lowest magnification.