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A comet is coming ... again: a newly discovered comet may be the brightest in decades.

Get your telescopes ready! For the next few months, Comet Hale-Bopp--a megalump of rock, dust, and ice from outer space--is expected to create a brilliant sky show as it whizzes by Earth. That's right. Another cool comet is coming our way, just 11 months after Comet Hyakutake, the brightest comet to light up our skies in years, wowed observes last spring.

Having two bright comets show up within a year is rare. Though billions of comets exist, such bright comets usually appear only about once every 20 years, says Daniel Green of the Harvard-Smithsonian Center for Astrophysics. One reason is that most comets remain in orbits (paths around the Sun) that are billions of miles away, beyond Neptune. Those few that do make their way to the inner solar system have extremely elongated orbits. It can take a comet thousands of years to make one orbital trip.

Astronomers have calculated that Hale-Bopp last passed through our cosmic neighborhood 4,200 years ago. That makes this comet's appearance a real once-in-many-lifetimes event!

People didn't always get so excited about comets. In centuries past, many people believed the appearance of a comet meant something terrible was about to happen. They gazed at a strange, new object blazing across the sky and imagined empires falling.

Icy rocks from outer space still have a creepy reputation. Some scientists think a comet (or another kind of space rock) hit Earth 65 millions years ago, and changed the planet's environment enough to wipe out the dinosaurs. If another comet crashes into Earth . . . Yikes! We could be next!

"We don't have to worry about a collision with Hale-Bopp," though, says astronomer Paul Thomas of the University of Wisconsin. Astronomers have calculated that Hale-Bopp will stay millions of miles away.

VISITORS FROM SPACE

But what made Hale-Bopp come whizzing into Earth's neighborhood in the first place? Astronomers think comets occasionally get pulled from their orbital paths by the gravity of stars or some undiscovered planet. Then the gravity of our solar system's planets--especially giant Jupiter-"jiggles" the comets' orbits even more. Some comets are thrown out of the solar system completely, Thomas says. A few crash into the Sun. But some, like Hale-Bopp, start to travel in orbits that bring them closer to the Sun--and Earth.

When a comet gets closer to the Sun, it heats up. "That causes the ice to turn to gas and steam outward from the comet's nucleus, [or rocky center]," says Green. The belched-out gas forms a huge atmosphere--up to millions of kilometers in diameter--around the nucleus. This atmosphere, called a coma, is what we see from Earth as the comet's "head."

Many comets also have a tail made of gas. The tail forms when a steady stream of particles from the Sun--the solar wind--"blows" material away from a comet's coma. Gas tails can be hard to see with the naked eye. But some comets, like Hale-Bopp, also have a dust tail Because this trail of rock and dust reflects lots of sunlight, dust tails are highly visible.

PREDICTIONS

But until a comet arrives, astronomers are never certain how well we'll be able to see it. They base their guesses on three main factors.

First: How close will the comet get to the Sun? Astronomers expect Hale-Bopp to glow more and more brightly as it approaches the Sun. The day it will be closest to the Sun, and brightest, is April 1. Hale-Bopp should be almost as bright for several weeks before and after.

But remember: We'll be seeing Hale-Bopp from Earth, millions of miles from the Sun. Our best views should come when the comet is closest to us. So: When--and how close--will the comet come to Earth? Using computers, Green has calculated that, on March 22, Hale-Bopp will fly by at a distance of 193 million kilometers (120 million miles).

Finally, astronomers must ask: How many trips has the comet made around the Sun? After many trips, says Thomas, comets run out of icy "fuel" and fizzle out. Some remain in orbit as large asteroids. Others break up into smaller orbiting rocks called meteoroids. (These meteoroids may eventually crash into Earth's atmosphere and burn up as meteors.)

Thomas says it's tricky to determine how much "fuel" a comet has left. All comets, he says, probably contain the same basic ingredients: rock and dust--plus frozen water, methane, and ammonia "But each comet's particular mix is different," Thomas explains. The proportion of ingredients hidden in Hale-Bopp is a mystery. Still, Green says Hale-Bopp's size and steady output of gas suggest the comet won't run out of fuel anytime soon.

People who live outside cities should be able to see the comet with the unaided eye. Even in cities--where light pollution makes comets harder to see--Hale-Bopp should be visible through binoculars.

One thing is certain: You're never going to get another chance to glimpse Hale-Bopp. It's not due back in Earth's neighborhood for another 2,400 years. For the best views, step outside one evening this March or April, and look toward the northwest just as it's getting dark.(*) The comet will set within an hour of darkness, so don't be late. You wouldn't want to miss your chance to welcome this visitor to our solar system.

(*) The comet's position in the sky will vary depending on where you live. Check for viewing tips in your local newspapers or on this Web site:

http:/www.skypub.com/comets/ comets.shtml

RELATED ARTICLE: COMET FINDER

On July 22, 1995, a clear night in southern New Mexico, Alan Hale looked through his telescope and discovered a new comet. Thomas Bopp, sky-watching in Arizona, made the same discovery at roughly the same time. Comet Hale-Bopp is named for both men.

Hale had spent years looking at comets, hoping to spot a new one. "When I was 11 years old, I badgered my father into buying me a telescope," he remembers. He used that telescope to observe his first comet. In the years since, he's observed more than 200!

Astronomers usually need at least a Ph.D. They also need to be patient, says Hale, and have "an enjoyment of the universe, and a willingness to learn." For information on careers in astronomy, write:

American Astronomical Society

Education Office

Astronomy Department

RLM 15.308

University of Texas

Austin, TX 78712-1083

E-mail aa@astro.as.utexas.edu

RELATED ARTICLE: Historical Hysteria

Are comets a warning from God? Do they mean a prince will soon die? That's what many people thought before scientists studied the space rocks. But science hasn't cured all comet hysteria. Internet chat groups still discuss the possibility that an alien spacecraft is headed toward Earth, hidden behind Comet Hale-Bopp.
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Title Annotation:includes related viewing chart; Comet Hale-Bopp
Author:Costello, Emily
Publication:Science World
Date:Feb 21, 1997
Words:1114
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