Whistling and moaning, a 50-mile-an-hour
(80-kilometer-an-hour) wind whipped among the telescope
domes atop Kitt
Peak. Just a few feet below, turning gray in the dusk, slid a river of
clouds that had been rising and dropping all day. And high above, comet
Hale-Bopp hung suspended like a feathery fishing lure, its tail curving
off a bit, as if blown to the side by the punishing wind.
One by
one, stars winked on in a darkening sky. In each of the telescope domes,
teams of astronomers prayed that the wind would drop below 40 miles per
hour (64 kilometers an hour), the point at which they'd be able to open
the sliding doors and get back to work.
The sky turned indigo.
Then black. Viewed from the summit, 6,873 feet (2,095 meters) above
Arizona's Sonoran Desert, Hale-Bopp's bright dust tail, along with a
dimmer, all but transparent blue one, seemed to grow by degrees. Among
the brightest comets ever seen, Hale-Bopp had been visible for months
from midtown Manhattan, of all places. But here, on a moonless night in
the mountains in the desert, the length of Hale-Bopp's tail became
visible—a wispy, delicate veil.
Along with eclipses, comets have
been the most feared and admired sky spectacles of all. But while
astronomers have been able to predict eclipses for thousands of years,
only in the 1700s was a comet's return correctly predicted, by Edmond
Halley.
Some comets swing around the sun every few years. Others,
like Hale-Bopp, may take thousands of years. Most can be seen only with a
telescope. But every once in a while—a few times a century, perhaps—an
impressive one is visible to the naked eye. And in the past two years
the world has witnessed not one but two of them.
Hyakutake in 1996
had one of the longest tails on record, stretching more than halfway
across the sky; Hale-Bopp in 1997 had one of the most brilliant heads,
nearly as bright as the star Sirius. Add the Jupiter crash of comet
Shoemaker-Levy in 1994, Halley's most recent visit in 1986, vivid comet
West in 1976, and the scientifically signifiant—if visually
disappointing—Kohoutek in 1973-74, and you could say that we are indeed
living in the age of comets.
Hovering in the most fragile of
gravitational balances, a fleet of dirty, lumpy snowballs numbering in
the trillions is barely held in orbit by the pull of the sun. They are
stored in the Oort cloud, a huge, diffuse sphere of cometary nuclei in
the far reaches of the solar system. Close to the sun, yet still beyond
Neptune, circle what may well be their brethren, in a great disk called
the Kuiper belt.
Comets are leftovers, scraps of material that
didn't make it to planethood in the events creating our solar system.
Once, many astronomers believe, the solar system was full of comet
nuclei, chunks of ice and dust left over from the formation of the sun.
Most clumped together to form planets, leaving a relative
handful—averaging perhaps a few miles wide, with temperatures as low as
minus 400 degrees Fahrenheit (minus 240 degrees Celsius)—as time
capsules of the early solar system.
They orbit in a perpetual deep
freeze until some subtle gravitational nudge upsets the delicate
balance. Then the great fall begins. Imperceptibly at first, a snowball
drifts toward the sun and steadily accelerates. As solar radiation heats
the comet, the ice within sublimates, escaping as gas from vents at the
surface. Sometimes jets of sublimating ice whirl off the rotating comet
nucleus like a fireworks pinwheel. Dust trapped in the ice breaks free.
Pushed back by the pressure of the sun's radiation, the dust streams
out behind the comet in what appears as a fiery tail.
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