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The Age of Comets

The Age of Comets

Photo: Comet streaking through sky

No telescope was needed to see comet Hyakutake above the Palomar Observatory in San Diego County, California, as it came within 9.3 million miles (15 million kilometers) of Earth in 1996.

Photograph by Don Bartletti

Written by William Newcott

Republished from the pages of National Geographic magazine

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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