Giant Rocks and Snowballs

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A new comet is speeding in from the dark. If it comes as a surprise to you, well, join the crowd. The comet has probably changed little since it clumped together some 4.5 billion years ago. Ten miles wide, it is an uneven mass of the same ice, rock and dust that swirled around our early sun during the solar system's formation. Our planet grew from the same stuff. But the comet's antique grains are chemically unaltered, frozen together with water ice and frozen gases.

The comet's surface may appear reddish black, resulting from the organic molecules that form when cosmic rays bombard the ices. If you could smell it, you might recall a tar pit. If this newcomer settles into a relatively close elliptical orbit around the sun, as have the comets Halley and Tempel, it could become a regular visitor to our neighborhood. If it doesn't, we'll never see it again.

The comet's name?

"Nineteen-ninety-nine S-four," says Brian Marsden, who catalogs newly discovered "minor planets" for the International Astronomical Union. "Sorry," he says with a tidy, British chuckle, "that's not very romantic."

Equally unromantic was 1999 S4's discovery: It was mistaken for an asteroid by an agency that surveys space for "near-Earth objects," mainly asteroids with orbits that cross that of Earth's. The hunt turned up droves of the asteroids' icy cousins — 17 comets in 1999 alone.

Among them, 1999 S4 stands out. Firstly, astronomers noticed it. Of the 100 new and returning comets that might dive around the sun in an average year, astronomers only spot about half.

Secondly, you, too, might notice it. Of the 50 that astronomers spot each year, only the rare comet brightens enough to dazzle the naked eye. Already, as it swiftly passes Jupiter's chilly orbit, 1999 S4 is getting fuzzy. The faint warmth of the sun is awakening its most volatile gases. Ammonia and methane are thawing, expanding and blowing out from beneath its crust. The blowing gas lifts off little particles of dust, too. Both gas and dust hover, reflecting sunlight.

But Marsden counts no "coma" — the shining head of a sun-bound comet — until it truly blooms.

"We've had so many embarrassing comets that are supposed to turn bright and don't," he says. "If this comet is coming from the Oort cloud for the first time, that's the worst."

The Oort cloud is a spherical shell of comets, trillions of them, orbiting 10,000 times farther from the sun than the Earth does. No one has ever laid eyes on the Oort cloud. But theoretically, the Oort comets were born much nearer to the sun, in the frigid vicinity of the giant planets Jupiter, Saturn, Uranus and Neptune. Then, as the solar system settled into being the place we know, more than 4 billion years ago, one or more of those bullies gravity-whipped the comets deep into space.

If 1999 S4 is an Oort comet, then it may have spent at least 2 million years traveling back toward its birthplace. And if it is a first-timer, then, for reasons unknown, it may not blaze as it nears the sun.

But perhaps 1999 S4's home was much closer. Until recently, another theoretical comet hideout called the Kuiper Belt was thought to stretch into the darkness just beyond Neptune, with icy Pluto marking the belt's inner boundary. Astronomers Jane Luu and David Jewitt verified its existence after searching five years for small, dark-colored bodies in that dim region. In 1992, they spotted the first Kuiper Belt object. Now, more than 200 are known.

The comets we admire with the naked eye tend to be irregular chunks 6 or 10 miles wide. Luu calculates the Kuiper belt is hiding some 70,000 comets about 60 miles wide, including additional Pluto– bodies about 1,500 miles wide. Wherever 1999 S4 resided, its serene life has taken a sharp turn. If it was in the Oort cloud, perhaps a passing star perturbed its delicate gravitational relationship with the sun. If 1999 S4 had inhabited the Kuiper Belt, then a nudge from Neptune might have upset its balance.

Now, after a long flight, 1999 S4 is closing in on the sun. As it traverses the band of asteroids between Jupiter and Mars, and approaches the orbit of Mars, its water ice will begin to thaw. As the gas explodes off the cold nucleus, it will loosen more dust.

By May, a person with binoculars might catch a faint blur in the northeast, before sunrise. On July 26, 2000, comet 1999 S4 will be at its closest approach to the sun. If the solar wind blows its halo of gas and dust into a tail 100 million miles long, we will see quite a sight.

Alas, a shining comet is a dying comet. Glamorous Halley was hemorrhaging at a rate of 24 metric tons of dust each second when it passed the Earth in 1986. If 1999 S4 slides past us with less fanfare and less damage, well, perhaps the brightest comets are those clever enough to trade glory for longevity.