It's time for a celestial shower


Meteors: This weekend's Leonid meteor display could be the most spectacular in years.

November 16, 2001|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Astronomers say it could be the meteor shower of a lifetime, and they're more sure of it this time than ever before.

Where the skies stay clear, they say, North Americans willing to roll out of bed before dawn Sunday and drive to a dark place in the country should see a starry sky streaked by hundreds -perhaps thousands - of meteors per hour.

Even if you've been disappointed by past meteor predictions, crawl out of bed this time, says Kelly Beatty, executive editor of Sky & Telescope magazine: "You're going to see more meteors in those few hours than you've probably seen in your whole life up to that point."

It's the annual Leonid meteor shower, an event that routinely produces 10 to 15 meteors an hour as Earth passes through the dusty orbital path of the comet 55P/Tempel-Tuttle.

But three times a century, in the years around the comet's return, meteor rates can soar into the thousands, and the shower becomes a "storm."

The Leonids have been storming each November since the comet's last visit in 1998, especially in the Middle East and Europe. This year, forecasters say, North Americans with the darkest skies might finally be rewarded with up to 4,200 meteors an hour.

In the past, such forecasts have been notoriously unreliable. But thanks to recent advances, "we seem to have it mastered now," says Steve Maran, spokesman for the American Astronomical Society.

In the past two years forecasters "hit the Leonids bang-on, predicting the right time of night and the right spot on the globe."

East Coast residents should be watching between 4 a.m. and 6 a.m. The farther they get from urban lights, the more meteors they will see. The weather forecast in Maryland calls for clear skies, with no moon to wash out the view.

The annual Leonid shower occurs when specks of dust and sand thrown off by Tempel-Tuttle smack into Earth's atmosphere at 155,000 mph, or 43 miles per second. Friction burns them up, creating streaks of light ranging from faint to fireball.

The great Leonid storm of 1833 terrified observers around the world. It was witnessed by a young Abraham Lincoln and a Maryland slave named Frederick Douglass, both of whom remembered the sight for the rest of their lives.

The 1998 Leonid shower startled observers with an unexpected barrage of fireballs. NASA astronomer Tony Phillips said some were bright enough to cast shadows.

"Some of the most startling left behind glowing trails of debris that lingered in the sky, twisting and turning as they were sheared by high-altitude winds," he said.

There is no risk to anyone on the ground. But satellite owners will be shutting down vulnerable systems, rotating solar panels edge-on into the meteor stream, and putting their most experienced controllers on duty to reduce the risk of collisions and short-circuits, says Bill Ailor, of the Aerospace Corp., which advises the operators of both military and commercial satellites.

A meteor struck a European communications satellite during the Perseid meteor shower one August several years ago, Ailor said. It sent a jolt of electricity to the satellite's thrusters. By the time operators realized it, the satellite had burned up all its maneuvering fuel, putting it out of commission.

Even in a heavy Leonid shower, says Ailor, "the chances are still pretty slight we will see any problems, but it pays to be aware."

Until about 20 years ago, scientists didn't know why annual meteor showers were so unpredictably variable. Then Donald Yeomans, of the NASA Jet Propulsion Laboratory, figured out that there were many more-concentrated ribbons of dust embedded within the comet's broader dust trail.

"Each time the comet went past the sun, every 33 years, it would dump off a ribbon of this debris, which stayed pretty concentrated in time," Beatty says.

Yeomans and others began mapping these dust lanes. They calculated how they would be shoved around by the effects of solar radiation, and the gravitational pull of the sun and the planets - especially giant Jupiter.

After the comet's return in 1998, they put their calculations to the test. They found there were fewer meteors than they had forecast, and the shower peaked several hours earlier than expected.

But the results helped them refine their models.

In 1999, astronomers David Asher, of the Armagh Observatory in Northern Ireland, and Robert McNaught, of the Siding Spring Observatory in Australia, predicted almost exactly when Earth would move through one of the comet's ribbons of dust.

"The ribbons are so narrow" - 20,000 miles across - "that they [forecasters] can pin down when the Earth is going to plow into it to within a half-hour," says Beatty.

They can also predict how heavy the meteor shower will be and what parts of the globe will have the best view.

Astronomers have even figured out when each trail was laid down by the comet. This year, they say, Earth will pass through skeins of dust cast off by Tempel-Tuttle in 1699, 1766 and 1866.

But the forecasters don't always agree.

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