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.