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Earthgrazers and Fireballs: The Strange Side of The Leonid Meteor Shower

By Robert Roy Britt
Senior Science Writer, SPACE.com
The Leonid meteor shower is a strange show. Its meteors are among the fastest known. It is notoriously difficult to predict. And it is a total night owl, refusing to show its best stuff until well after midnight. But while the 2001 Leonids will likely be remembered for the sheer volume of shooting stars, there are some strange characters to look for as the shower's source ekes above the eastern horizon late Saturday night and early Sunday morning. A handful of meteors will first zoom across the horizon for long stretches of time. Earthgrazers, they're called. And if you're real lucky, you might spot some fireballs -- larger meteors that explode upon impact with Earth's atmosphere, generating spectacular blazes of light (not to mention fear of alien spacecraft and calls to local law enforcement offices).

Earthgrazers

Leonid meteors will take their time arriving Saturday night. Wherever you are on Earth, you're viewing location has to rotate into the stream of space dust that causes the Leonids. The shooting stars will appear to emanate from a point in the sky known as the radiant, which for the Leonids happens to be in the constellation Leo (hence the name). No knowledge of this is needed to find an earthgrazer. Just go out and look to the East. The timing depends on where you live. Figure mid-evening for high northern latitudes, such as Canada; late evening hours for mid-northern latitudes, as in most of the United States; and after midnight for equatorial regions and the Southern Hemisphere.

What might you see? "When the radiant lies near the horizon the Leonid meteors cannot penetrate far into the Earth's atmosphere," explains Robert Lunsford of the American Meteor Society. "At this time they are only able to skim the upper atmosphere." These earthgrazers, as scientists call them, often last several seconds and can span a great distance of the sky, Lunsford said.

To see an earthgrazer, you'll need an unobstructed view of the eastern horizon. Later, as Earth continues rotating, the Leonid radiant moves higher into the sky, along with its host constellation and all the stars. Meteors will strike the atmosphere at a more direct angle, Lunsford explains, creating shorter paths. But the paths will still span much of the sky, so you don't need to face East. In fact, the best views will be everywhere but directly East. Just go out, look up.

Fireballs

Most Leonid meteors are created by sand-sized grains of dust that vaporize about 60 miles up due to the friction caused by Earth's atmosphere. But Tempel-Tuttle, the comet that has left all this Leonid raw material in space, also deposits a few larger chunks of itself each time it swings around the Sun (which it does every 33 years). A comet fragment the size of a marble can generate a glorious fireball of light as it burns up. Instead of slicing through the atmosphere like a small bit of dust, such a pebble sometimes goes splat upon meeting up with a certain density of air.

"The Leonids can have fireballs, but they're not especially noted for them," said Bill Cooke, a meteor researcher at NASA (news - web sites)'s Marshall Space Flight Center. Cooke said the number of fireballs each year depends in part on which streams of cometary debris Earth plows through.

In 1998, observers noted several fireballs when the planet moved through a stream that comet Tempel-Tuttle had deposited in the 14th Century. The Sun's radiation had blown much of that ancient dust into a widely dispersed region of space, so the 1998 Leonids did not produce a great number of shooting stars. But the larger material -- fireball material -- was still relatively concentrated. In fact, Cooke said, scientists are learning that gravity acts on these larger fragments, causing them to be huddled more closely together over time. They call the process "gravitational focusing."

So what are the chances for fireballs this year? People in Asia will see shooting stars caused by material that has been waiting to be swallowed up by Earth since 1633, so there should be some fireballs there, Cooke said. The North American peak will be caused by material left by Tempel-Tuttle in the 1700s, however, and should provide fewer fireballs, but probably still some.

Cooke is quick to point out that the Leonids can surprise, however. There could be fewer meteors overall. Or there could be more fireballs. Meteor forecasting is a young profession. And, for now at least, meteor showers are still somewhat strange -- even to the scientists.