Science World
Science World for grades 6–10 brings science to life with fascinating feature articles and hands-on activities that reinforce science concepts and help students build test-taking and critical-thinking skills.

A Comet is Coming... Again

A newly discovered comet may be the brightest in decades.

By Emily Costello | null , null

Get your telescopes ready! For the next few month, 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 observers last spring.

Having two bright comets show up within a year is rare. Though billions of comets exist, such bright comets usually appear only 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, may 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 space still have a creepy reputation. Some scientists think a comet (or another kind of space rock) hit Earth 65 million 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 cause 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 dust tail. Because this tail of rock and dust reflects lots of sunlight, dust tails are highly visible.

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 may 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 — Comet 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.

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 discover 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 by 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."

More Facts About Comets

  • Comet Hale-Bopp is circling the Sun in an orbit that is nearly perpendicular (at right angles) to the Earth's orbit.

  • Hale-Bopp is the brightest comet discovered in about 400 years. That's why it will look bright from Earth even though it's far way.

  • Hale-Bopp zips by at speeds up to 44 kilometers (27 miles) per second.

  • At the farthest point in its orbit, Hale-Bopp travels 10 times farther from the Sun than Neptune.

  • About 10 new comets are discovered every year. Most aren't visible without telescopes.

  • The solar wind, a stream of charged particles emitted by the Sun, always "blows" a comet's tail away from the Sun.

  • Most comets range in size from 1 to 10 kilometers in diameter. Some my be 100 kilometers across.

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