Leonid Meteor Shower Peaks Tonight

Brian Handwerk
for National Geographic News

November 18, 2004

Tonight marks the return of the Leonid meteor shower. The heavenly show is expected to peak at 1:40 a.m. ET on Friday for sky-watchers in North America. The spectacle looks to be the second-to-last chance to see the shower in this century.

The Leonids, which recur each November, had a spectacular run between 1999 and 2002. The meteor showers of those years approached the intensity of "meteor storms," a threshold generally marked when viewers can see a thousand meteors an hour.

"Those terrific years have passed, and we're now back to a more normal level," said Kelly Beatty, executive editor of Sky and Telescope magazine and editor of Night Sky magazine. "Still, at a good dark site, you might see 15 [meteors] per hour."

A second peak, also on the 19th, will treat Asian observers to some 60 meteors per hour.

Astronomers say the show should be a good one. They note, however, that predicting meteor shower intensity is not always an exact science.

"There's always a chance for a surprise," said Bill Cooke, a meteor-shower expert with the Space Environment Group at NASA's Marshall Space Flight Center in Huntsville, Alabama.

"The art of meteor forecasting has been around for only a few years," Cooke added. "We do very well on the timing. We can nail that to within a few minutes. But we're not always as good on the intensity."

Lingering "Trains"

While the frequency of the 2004 Leonid shower may not match that of previous years, the show will still pack quite a punch, observers say.

The meteors will zip into our atmosphere at 44 miles a second (71 kilometers a second). Most will vaporize between 56 to 62 miles (90 to 100 kilometers) above the Earth's surface.

"They are the fastest meteors in a known shower," said Beatty, the Sky and Telescope editor. "They tend to be very bright, and they leave behind a kind of glowing trail for a few seconds that a lot of other meteors do not."

The glowing "trains," sometimes compared to the visible exhaust plumes of rockets, hang in the sky for a few seconds before fading.

These visible meteor streaks are not caused by the meteor particles burning up, but rather by energy in our atmosphere. As meteors streak toward Earth, molecules in the atmosphere absorb tremendous energy. So much so, that they are heated to high temperatures. As they cool, the molecules give off light.

"People see meteors in the sky, and they are there and gone in a fraction [of a second]," Beatty said. "But these persistent trains are really great and a little spooky. When the bright flash has gone, there's a kind of ghostly ribbon hanging behind in the sky."

Why the Leonids leave such trains, and why many other meteors do not, is uncertain—but it's likely that speed plays a part.

"If they were any faster, we wouldn't see them at all," Cooke said. "They wouldn't be held by the sun's gravity. They'd slingshot right out of the solar system."

This limitation means that no annual meteors can be faster than the Leonids, though random meteors encountered by Earth could travel more rapidly.

The Leonids "also tend to be on the fluffy side," Cooke added. "They tend to fragment, and that means that they leave nice streaks."

Like many recurring meteor showers, the Leonids take their name from the area of the sky from which they appear to originate. In this case, the spot is the constellation Leo.

In North America, Leo doesn't rise over the Northeast horizon until about 11:00 p.m. local time.

"It might be worthwhile to go outside around 11 or 12 [ET] at night," Beatty suggested for North American viewers. "What could happen is that, as the constellation is rising, meteors could be skimming the atmosphere over your head. You can get some really dramatic long streak and trains [at that time].

Comet Debris

Annual meteor showers occur when Earth intersects debris left along the orbit of the shower's parent comet. Each time a comet orbits the sun, the star's heat strips away a layer of dust and ice.

"For a select few [comets], the Earth goes through their orbits at the same time every year," Beatty explained. "The analogy I like to use is a garbage truck full of sand. As it barrels down the road, the sand billows out the back end. And that's what Earth plows through."

The tiny particles of ice and rock, some as small as a grain of sand, put on quite a show as shooting stars or meteor showers. They are among the smallest celestial particles visible to the eye.

The Leonids' parent, comet Tempel-Tuttle, travels on a 33-year oval orbit that extends as far as the planet Uranus.

The meteors visible in North America next Friday are remnant's of the comet's flyby in 1333. The later, larger display for Asian sky-watchers features circa-1733 debris.

The annual show will soon close down for the rest of the century, as the Earth's orbit seems unlikely to intersect with concentrated paths of Tempel-Tuttle debris.

"They'll make one final brief burst of activity in 2006, but for the rest of the century they'll be at a pretty low level I think," Cooke said. "But I could be surprised."

Meteor lovers need not despair. Mid-December sees the return of the Gemenids, perhaps the best of all the annual meteor showers.

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