By Bill Burton
Special to The Washington Post
Wednesday, November 11, 1998; Page H03

In the early morning hours of Nov. 17, 1966, in Flagstaff, Ariz., it stormed. But there wasn't a cloud in the sky.

That was the night of the famous Leonid meteor shower, a torrent that seemed to originate from the direction of the constellation Leo. Geologist Howard Pohn, then stationed in Flagstaff, describes what he and his colleagues saw:

"We started by keeping track of the number of Leonid meteors we were seeing, but after a while they were appearing too quickly to count. The meteor rate kept on rising until, an hour or so later, the peak of the storm arrived.

"It was like standing under an enormous umbrella of fire, with the spokes of the umbrella traced by hundreds of brilliant meteors, raining downward from a single point in the sky. The meteors that most impressed me were the ones at the peak of the umbrella that remained perfectly motionless and just brightened and faded away. They were the meteors that were heading straight at you."

Although he may have felt like ducking, Pohn was in no danger. The objects that create those fiery streaks are particles of space grit rarely larger than a grain of sand. They burn in the atmosphere at least 50 miles high.

No Deep Impacts or Armageddons here -- just a harmless but spectacular and prolific celestial show. One observer farther south in Tucson estimated that, at its peak, the brief 1966 storm produced about 40 meteors a second for 20 minutes.

Now anticipation is rising again. It has long been known that the annual Leonid meteor shower peaks about every 33 years, and 1999 should produce another.

Uncertainty about distribution of particles that produce these meteors, however, means that 1998 could very well be the year, and that early next Tuesday morning could be the time.

Of course, there could be no storm in either year. The 1833 shower was so vivid, peaking at an estimated 150,000 "falling stars" an hour, that it provoked fear about the end of the world and spurred religious movements. The 1866 Leonids also were intense. But 1899 was a disappointment, and 1933 was an outright flop.

The aerospace industry, however, is taking no chances with the approximately 500 communications satellites orbiting above the protective blanket of Earth's atmosphere. These satellites are always at risk from impacts by meteoroids, the potential meteor-producing particles that travel outer space.

During a Leonid storm, millions of meteoroids encounter Earth at velocities of about 71 kilometers (44 miles) a second, or about 155,000 mph. At such speeds, meteoroids the size of sand grains have a kinetic energy equivalent to bullets, and satellites could find themselves in a shooting gallery. The Hubble Space Telescope will be turned so that its back faces the incoming Leonids.

Concern about the welfare of these orbiting objects is prompting the Air Force to send a team of Leonid observers to eastern Asia, thought to be the most likely place to see a major storm this year.

What causes meteor showers, and why do they occur at the same time every year?

Meteoroids that produce most showers originate from comets, balls of ice and dust that begin to warm when their orbits carry them near the sun. As they become warmer, they evaporate, shedding "tails" of dust, gas and meteoroid particles. The comet's orbital path thus becomes a highway of sorts for millions of these moving bits of debris, even far from the comet itself.

Most comets span the sun in highly elliptical orbits, in contrast to the nearly circular orbit of Earth and most other planets, and some of those cometary orbits intersect Earth's path.

The clouds of meteoroids in those comet tracks produce meteor showers when Earth arrives at the intersection. Since these intersections are fixed in space and Earth completes an orbit once a year, meteor showers occur on about the same calendar date every year. When meteoroid streams are much wider than Earth's diameter, the shower can last more than a day.

The parent bodies for the three best-known meteor showers, each named for the constellation from which it appears to radiate, are the Leonids this month, the Geminids (from the direction of Gemini) about Dec. 13 and the familiar Perseids (from the direction of Perseus) on Aug. 12.

The Leonids are caused by Comet Tempel-Tuttle and the Perseids by Comet Swift-Tuttle, both named for their 19th-century discoverers. The Geminids are produced by debris surrounding the asteroid 3200 Phaethon, which was the 3,200th asteroid discovered. It has a much smaller orbit than the two comets and rounds the sun much more frequently. More than a dozen minor meteor showers occur elsewhere along Earth's orbit.

The 33-year peak of the Leonids occurs because Comet Tempel-Tuttle, which orbits the sun in a direction opposite to Earth's, crosses our path every 33 years, accompanied by a stream of meteoroids ahead of and behind it. The comet passed through our neighborhood again early this year.

If Earth plows into a particularly dense part of the stream following the comet, a meteor storm will result from the nearly head-on collision. Or we may miss dense portions of the stream entirely, as happened in 1933.

By precisely plotting the intersection of Earth's orbit with the Leonid meteoroid stream, which changes slightly from year to year, and comparing it to the intersection points of good and bad Leonid years, meteor experts predict that the peak this year or next could be quite good but probably not as spectacular as in 1966.

Because of Earth's position in its daily rotation as it enters the stream, the most likely place to see a storm will be on the other side of the world from us. On the East Coast, the best viewing opportunity would occur between 11:30 p.m. next Monday and dawn the next day. Second best is the same time period the next evening.

For several years, the Leonids have been intensifying, producing many bright meteors, including spectacular green fireballs with persistent, glowing trails. These bright meteors are visible even from the National Mall if the sky is clear.

The moon will not be up, allowing more meteor sightings. It also will be absent from the night sky next month during the Geminid shower and for the Perseids next summer, a shower well known for abundant faint meteors.

Where's the best place to watch a meteor shower? The single most important rule is that the darker the sky, the more you will see. In a typical shower, faint meteors are much more common than bright ones.

For anyone in the Washington metropolitan area's light-polluted night skies, a trip to a rural site is necessary to see a lot of meteors. In dark, rural skies, faint meteors are visible, the bright ones are much more impressive and fireballs can cast shadows.

A second important factor is openness of view -- the more sky visible, the better. Meteor observation is done by scanning the whole sky with both eyes, without binoculars or other optical aids. Most meteors are detected first out of the corner of the eye, so it is better to get away from trees, buildings or any other obstructions that might block direct or peripheral vision.

A treeless mountaintop is ideal, but any large, level field will do. Overlooks along Skyline Drive in Shenandoah National Park are excellent locations.

First, check the weather forecast to see if clear skies are likely Monday night. If the forecast is "mostly cloudy" or worse, you might want to stay home and watch the sky for holes in the clouds, which could allow glimpses of brighter meteors, even from your backyard.

If the weather looks promising, prepare for staying comfortable outdoors on a cold night. Since you will spend all of your time looking up, a reclining lawn chair or insulated sleeping pad is essential.

The best orientation is probably with feet pointed east toward the rising radiant. Most radiants, including that of the Leonids, don't move very high in the sky until after midnight. You may see few, if any, meteors before then.

To try meteor-counting, count the number you see each hour. Only those that appear to be coming from the direction of Leo should be recorded as Leonid meteors. You also may see as many as 10 meteors traveling in random directions in an hour. These are non-Leonid, or "sporadic," meteors.

You can make a contribution to astronomy by producing an accurate count of meteors you see, along with other observations, and sending them to the International Meteor Organization (IMO). After a shower, the IMO compiles meteor counts by observers worldwide to produce a graph showing when the peak of the shower occurred. The more data gathered via observer reports, the more precise this graph will be.

To make a meteor count useful to the IMO, you must record, along with number of meteors, such things as brightness of the faintest star visible to the unaided eye and percentage of sky covered by clouds at any one time.

Full details on how to make these observations and submit your data can be found in the web pages of Sky and Telescope magazine at http://www.skypub.com/meteors/metwatch.html. Also, you may want to examine the IMO web site at www.imo.net.

Bill Burton is a geologist with the U.S. Geological Survey.