Stargazers await meteor showers

San Francisco Chronicle

Brilliant showers of meteors called the Leonids should streak across the late night sky this weekend, and stargazers are expecting one of the most spectacular displays our solar system can provide.

It may even become a full-fledged meteor storm, with hundreds if not thousands of “shooting stars” appearing after midnight early Sunday, astronomers say.

Beginning late Saturday and climaxing about 2 a.m. Pacific time on Sunday, the earth in its orbit will swing through clouds of tiny fragments ejected from the tail of a comet that flashes between the sun and Earth roughly every 33 years and leaves the wake of its debris behind to flare and burn up in Earth’s high atmosphere.

Before those fragments do vanish, some scientists believe, they may even have been carrying the very same organic chemicals that seeded the earth billions of years ago with the starting materials for life.

“It’s now or never,” said Robert Naeye of the Astronomical Society of the Pacific. “Astronomers don’t think we’ll see another storm like this one until the year 2099. We will probably never see a better meteor shower in our lifetimes.”

And if the sky is clear over the Bay Area from late Saturday night until just before dawn on Sunday, the Leonid showers should be clearly visible even above city lights, while they create a more fantastic show for anyone who ventures into darker country well beyond the urban glow.

Amateur astronomers, and professionals too, will be on hand at seven Bay Area locations Saturday night to explain the phenomenon to the public here, while scientists across the United States and Asia will be studying the Leonids from aircraft, through telescopes, spectroscopes, and special wide-angle photographic instruments.

The Leonid meteors are so called because they appear to originate from the constellation Leo the Lion, although in fact they are infinitely closer to Earth – flashing less than 100 miles above us. Like many lesser showers from other comets that occurr regularly every year, the Leonids are usually sparse during ther annual November visitations.

But this year’s display could be the most spectacular since 1966 because Earth will be passing through three distinct trails of debris left from three separate flights of the comet into the solar system in 1699, 1767 and 1866, according to Peter Jenniksens, a meteor expert at the SETI Institute and NASA’s Ames Research Center in Mountain View.

The comet, called Tempel-Tuttle, was discovered independently by astronomers William Tempel in December 1865 and Horace Tuttle a month later in January 1866. Other astronomers later calculated its orbit closely, and showed that it was the same comet whose flashing meteor trail had filled the skies and terrorized chroniclers as long ago as the year 1366.

There should be a repeat show next year, Jenniksen says, but the moon over the Bay Area will be full then, and the sky will be too bright for the meteors to be clearly visible.

“This is the year to see the show,” Jenniskens said in an interview, “and the best way is to make a whole weekend of it. Get out into the country – up to the mountains if you can – and be patient. After dark on Saturday night you won’t see many Leonids at first, but by 1 a.m. on Sunday you may see 1,000 an hour and by 2 a.m. the count should reach more than 2,000 – a classic meteor storm – and then the numbers will fade away rapidly.

“You should make an all-nighter of it.”

Although Jenniskens has predicted a maximum count of slightly more than 2,000 an hour for this year’s Leonids over the Bay Area as the earth passes through the wake of the 1767 Tempel-Tuttle passage, other scientists, based on other models of Leonid behavior, have forecast counts as low as 400 an hour.

For folks who can’t leave home, urban light pollution will dim the show, although many Leonids should still be strikingly visible. “So be friendly to your neighbors,” Jenniskens said, “and keep your porch lights off if you can – you’ll all see better.”

As Earth passes through the three trails of debris from Tempel-Tuttle’s tail the meteors – ranging in size from sand grains to marbles – will hit the upper edges of the earth’s atmosphere at 71 kilometers per second, according to Jenniskens. That’s nearly 160,000 miles an hour.

120 miles high, they will still be invisible, but at roughly half that altitude each meteor will encounter enough of Earth’s thin upper atmosphere to heat into incandescence and turn into a tiny cloud of ionized gas as hot as 7,300 degrees Fahrenheit before it vanishes, according to Jenniskens. Some of the larger meteors may even explode as blazing fireballs in the sky, as they did during the Leonid shower of 1998, Jenniskens said.

Over the weekend Jenniskens and colleagues from several research centers will be flying from Edwards Air Force Base aboard a modified Air Force NKC135 tanker aircraft with 20 observing windows built in for the scientists.

Originally NASA invited meteor researchers from five nations to fly with Jenniksens, but because the plane has been on standby since the terror attack of September 11, the foreign scientists will not be aboard and their instruments will be manned by American researchers.

One major purpose of the flight is an effort to analyze the ionized organic gases that emerge in the dying instants of each meteor. If those gases survive the intense heat, Jenniskens believes, they may be altered chemically, but still remain in the atmosphere – a strong indication that similar meteors could well have carried the chemicals essential for life to the surface of the early earth. Excellent Leonid details are on the Astronomical Society of the Pacific’s Web site at www.astrosociety.org and the NASA site at leonids.arc.nasa.gov.