The Leonid meteor shower makes its annual return early Monday, and astronomers hope it will bring as many as 80 to 100 "shooting stars" an hour.
That would be six to 10 times the Leonids' normal rate, and continue the buildup toward an eagerly anticipated meteor "storm" at this time next year, or in 1999. That event could bring hundreds or thousands of meteors an hour.
But this year's bright moonlight might dim the show and prove a disappointment to some sky-watchers.
"I don't want to raise their expectations," said Donald K. Yeomans, a senior research scientist at the National Aeronautics and Space Administration's Jet Propulsion Laboratory in Pasadena, Calif. "But it is worth getting up early and seeking a location away from city lights."
The Leonid meteor shower occurs every year about Nov. 17 or 18, when the Earth passes through the dusty orbital trail of Comet Tempel-Tuttle -- like a pedestrian walking through the exhaust of a passing bus.
Tempel-Tuttle visits the inner solar system and crosses Earth's orbital plane every 33 years. This time around, it's due in January 1998, and will round the sun Feb. 28.
Because the Leonid meteor counts often surge just before and after the comet's return, astronomers are watching this year's Leonids, and eagerly awaiting the 1998 and 1999 showers -- both of which will occur on moonless nights.
After passages of the comet, the Leonids filled the sky with stupendous meteor storms in 1833 and 1966. Observers estimated 50,000 to 150,000 meteors an hour in both those events.
Comets are made of dust, ice and frozen gases. They circle the sun in lopsided orbits that carry them from the inner planets to the most distant reaches of the solar system. Tempel-Tuttle wanders between the orbit of Earth -- about 90 million miles from the sun -- and that of Uranus, 1.8 billion miles out.
As comets near the sun, some of their ice and gases vaporize and release captured dust particles. Over the centuries, the dust spreads out along the comet's orbit like an invisible smoke trail.
Each year, when Earth's orbit carries it through that trail of dust, the particles slam into the upper atmosphere like bugs on a windshield -- the Leonids at 43 miles a second. Instead of splattering, the particles burn, creating the flurry of light streaks in the night sky that we call meteor showers.
"The [particles] that cause meteors are about size of grains of sand, or smaller," Yeomans said. Occasionally, Earth encounters chunks the size of golf balls or basketballs that produce spectacular glowing trails and "fireballs."
"We're not likely to see too many of those. The smallest are by far the most numerous," he said.
Meteor showers are fickle because the density of particles within the comet's path is not uniform, Yeomans said. "The Earth could slip through there without running into much of anything. Or, it could run through a clump and be treated to a major storm."
Leonid meteor counts have been rising in advance of Tempel-Tuttle's 1998 return. They jumped from a normal 10 to 15 an hour to 40 in 1995, and 50 to 80 last year.
The Leonid meteors appear to radiate from the constellation Leo. They were first recorded by the Chinese in the 9th century. But Tempel-Tuttle itself has been elusive to naked-eye observers. It was first spotted in 1366, and again in 1699.
"It's only a naked-eye comet when it is very, very close to Earth," Yeomans said. It could be very small or very faint. "It may be a comet in its twilight years in terms of its activity."
He believes 1998 and 1999 may be the last chance for people now living to see a Leonid storm.
In 2029, Tempel-Tuttle's orbital track will be deflected by Jupiter's gravity away from Earth's orbit, foreclosing any significant storm in 2031 or 2065, he said. The next opportunities won't be until 2098, or 2131. After that, the comet's "evolving" orbit won't bring it close to Earth's again for "several millennia."
Where to watch
The best place to be to see the 1997 Leonid shower will be western North America or Hawaii, where the prime viewing hours will arrive just as the Earth makes its hour-long passage through the densest part of the comet trail.
From any vantage point, the best time to look is from 1 a.m. to 6 a.m., as the Earth turns observers head-on into the comet trail, said Dennis O'Leary, director of the Maryland Science Center's Davis Planetarium.
Seek out dark country skies, then relax. "This is a laid-back kind of event," O'Leary said. "Get a comfortable lounge chair and a sleeping bag." Allow your eyes 15 minutes to adjust to the dark.
"Your eyes are the best tools to use for meteors because you don't know where they're going to appear," he said. If Monday morning is a washout, try again Tuesday morning. Or wait until next year.
Pub Date: 11/12/97