If the skies clear and the cosmos cooperates, Marylanders who sacrifice some sleep early Tuesday could experience the most spectacular display of celestial fireworks in their lifetime.

The annual Leonid meteor shower, which last November unleashed hundreds of "shooting stars" an hour in the skies over Maryland, is expected to peak again early Tuesday at "storm" intensities - as many as 5,000 an hour.

Even though the glare of the full moon could obscure many of the fainter streaks of light, scientists say, sky-watchers who get up to watch the show still should see more than 3,000 meteors an hour - up to 55 a minute.

There may be no second chances. Meteor forecasters say it will be 2099 or 2131 before the Leonids are likely to hit storm levels again.

"This will be the last meteor storm of our lifetime," said Jim O'Leary, director of the Maryland Science Center's Davis Planetarium.

The Leonids return every year when the Earth passes through the orbital track of a comet called 55/P Tempel-Tuttle. The show begins as the planet plows through dust and pebbles shed by the comet, like a car hitting a swarm of bugs at 155,000 mph.

In most years, the shower produces 10 to 15 meteors an hour. But those rates can surge to hundreds, even thousands, an hour in the years after the comet swings around the sun on its 33-year orbit and crosses Earth's path. No other annual meteor shower has equaled the Leonids' most intense displays.

Tempel-Tuttle's last pass through this region of the solar system was in 1998, and the Leonids have been active ever since.

As particles the size of sand grains strike the atmosphere, they heat the air molecules around them and create fleeting streaks of light. Larger debris can produce bright fireballs so intense they cast a shadow, and leave lingering trails twisting in the high-altitude winds.

The Leonids get their name from the constellation Leo, the point in space from which all the shower's meteors appear to radiate.

The world's top meteor forecasters are all predicting two short, sharp peaks in this year's Leonid display.

The first is expected soon after 11 p.m. tomorrow as Earth passes through the dust trail left behind by Tempel-Tuttle during its 1767 turn around the sun.

'Grazing fireballs'

Observers in the United States will miss most of those meteors, though, because Leo is still just below the eastern horizon at that hour, according to Peter Jenniskens, a meteor scientist at NASA's Ames Research Center in California.

But a few of the larger ones will likely streak over the horizon and skip across the outer atmosphere, producing what Jenniskens called "a nice display of grazing fireballs" for the East Coast.

After a lull of several hours, the second Leonid peak should begin after 4 a.m., as Earth passes through another debris train shed by Tempel-Tuttle in 1866. The frequency of meteors will intensify sharply after 5 a.m. and keep rising into the dawn, which occurs in Baltimore at 5:20 a.m.

"My forecast is about 5,000 meteors per hour at the peak," Jenniskens said - three to five times last year's rate. But the most intense display may last less than an hour. "It's very critical that people are out there at the right time," he said.

The weekend rains should end, and skies should clear in time for the Leonids. The weather service forecasts clear skies for tomorrow, with partly cloudy conditions Tuesday.

The Davis Planetarium's Jim O'Leary said the moon will be low in the west by 5 a.m. Meteor watchers can minimize its glare by positioning themselves so there's a house or a hill between themselves and the moon.

While meteor forecasts are always chancy, this year's Leonid predictions should be the most reliable ever because of major advances in the last four years.

Modern meteor science was born after a spectacular Leonid storm in 1833, a cascade of meteors that astonished and terrified people throughout North America.

Researchers in 1866 discovered the link between Tempel-Tuttle's 33-year return cycle and showy Leonid displays noted in historical documents as early as the 10th century. But highly publicized Leonid forecasts flopped in 1899 and 1933. Scientists were baffled, and unprepared in 1966 when the Leonids stormed over the western United States.

A breakthrough

As it turned out, Jenniskens said, "the problem was simpler than we thought."

When returning comets near the sun, the water ice and other frozen gases evaporate from the surface, releasing trapped dust and stony particles. The result is a trailing ribbon of dust and debris that continues to orbit the sun. Attempts to map Tempel-Tuttle's dusty bands proved frustrating until a Russian team discovered that they follow a different, and slightly longer path around the sun than the comet itself. They're also shoved around by the pressure of solar radiation and the gravitational influence of the planet Jupiter.

Each time the comet returns, it leaves a discrete dust trail behind, like parallel jet contrails along a busy air corridor. "When one of these trails happens to be in Earth's orbit, that's when we see a meteor storm," said Jenniskens. Otherwise, Leonid rates are low.

Scientists are mapping the dust trails with growing precision. Calculations for the 1998 Leonids were off by 16 hours. Forecast teams refined their assumptions and ran the numbers again for the 1999 Leonids. Those predictions were off by 5 minutes.

90 years to regret it

The same forecasters now agree that this year's predicted storm will be the last for many years.

Celestial mechanics, especially the tug of Jupiter's gravity, are dragging the heaviest known ribbons of Tempel-Tuttle's dust away from the Earth's orbit. The comet's 2033 and 2066 returns are not expected to produce big meteor displays.

If this year's Leonids fizzle, "all you've lost is a couple hours sleep," said Geoff Chester, spokesman for the U.S. Naval Observatory. But sleep through a Leonid spectacular, and "you'll have 90 years to kick yourself in the pants for missing it."

Tips for viewing

Find a viewing spot with low horizons as far from urban lighting as possible. Wear warm clothing, bring a blanket or lawn chair and something warm to drink. Meteors may appear anywhere in the sky.

The bright moon, and work-night timing have discouraged some public observations of the Leonids. But a few events are planned:

The Howard Astronomical League will meet from 11 p.m. tomorrow until dawn Tuesday, Alpha Ridge Community Park, off Old Frederick Road (Route 99), 0.8 mile west of Marriottsville Road.

Baltimore's "Street Corner Astronomer," Herman M. Heyn, will speak and offer telescope views at 7:30 p.m. tomorrow at the Oregon Ridge Park Nature Center in Hunt Valley. Attendees may join him later at a nearby farm for the first Leonid peak.