Age of the planets

Whistling and moaning, a 50-mile-an-hour (80-kilometer-an-hour) wind whipped amidst the telescope domes atop Kitt top. Just a couple of feet below, rotating gray in the dusk, skidded a stream of clouds that had been rising and dropping all day. And high above, comet Hale-Bopp dangled hovering like a feathery fishing lure, its follow arching off a bit, as if blown to the side by the penalizing wind.

One by one, stars winked on in a blackening atmosphere. In each of the telescope domes, teams of astronomers pleaded that the breeze would drop underneath 40 miles per hour (64 kilometers an hour), the issue at which they'd be adept to open the sliding doors and get back to work.

The sky turned indigo. Then very dark. Viewed from the summit, 6,873 feet (2,095 meters) above Arizona's Sonoran wasteland, Hale-Bopp's brilliant dirt tail, along with a dimmer, all but clear blue one, appeared to grow by qualifications. amidst the brightest comets ever glimpsed, Hale-Bopp had been visible for months from midtown Manhattan, of all locations. But here, on a moonless evening in the hills in the wasteland, the extent of Hale-Bopp's tail became visible—a wispy, delicate veil.

Along with eclipses, comets have been the most feared and adored atmosphere scenes of all. But while astronomers have been adept to forecast eclipses for thousands of years, only in the 1700s was a comet's return rightly forecast, by Edmond Halley.

Some comets sway round the sun every few years. Others, like Hale-Bopp, may take thousands of years. Most can be seen only with a telescope. But every one time in a while—a few times a years, perhaps—an outstanding one is visible to the naked eye. And in the past two years the world has seen not one but two of them.

Hyakutake in 1996 had one of the longest follows on record, extending more than halfway across the atmosphere; Hale-Bopp in 1997 had one of the most bright heads, almost as bright as the celebrity Sirius. Add the Jupiter smash into 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 state 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 retained in the Oort cloud, a gigantic, diffuse sphere of cometary nuclei in the far reaches of the solar scheme. Close to the sun, yet still after Neptune, around what may well be their brethren, in a large computer disk called the Kuiper belt.

Comets are leftovers, scraps of material that didn't make it to planethood in the events creating our solar scheme. one time, numerous astronomers believe, the solar scheme was full of comet nuclei, chunks of ice and dirt left over from the formation of the sun. Most clumped together to form satellites, departing a relation handful—averaging perhaps a couple of miles broad, with temperatures as low as minus 400 qualifications Fahrenheit (minus 240 qualifications Celsius)—as time capsules of the early solar scheme.

They orbit in a perpetual deep freeze until some subtle gravitational nudge upsets the dainty balance. Then the large drop starts. Imperceptibly at first, a snowball wanders in the direction of the sun and gradually accelerates. As solar emission warms up the comet, the ice within sublimates, escaping as gas from vents at the exterior. occasionally jets of sublimating ice whirl off the rotating comet nucleus like a fireworks pinwheel. Dust tricked in the ice breaks free. Pushed back by the pressure of the sun's emission, the dust streams out behind the comet in what seems as a fiery tail.