Although Europeans and North Americans will have the best chance to spot Leonids activity this year, meteor watchers everywhere should be on the alert. Brief flurries of shooting stars could materialize over any part of the globe between local midnight and dawn on November 17th and 18th.

"We're very, very confident of the storms coming in 2001 and 2002," says David Asher of the Armagh Observatory. "Peak rates during those years should reach at least 10,000 meteors per hour as Earth passes through debris trails from comet Tempel-Tuttle." Asher expects Leonids activity to reach 100 per hour on Nov. 18, 2000, similar to the recent Perseids display. Other forecasters anticipate higher rates -- around 700 per hour.

"The last quarter Moon will be in the constellation Leo on Nov. 18th, practically on top of the Leonids radiant," says Mitzi Adams, an astronomer at the NASA Marshall Space Flight Center. "Moonlight will make fainter meteors hard to spot, but if there's a strong outburst stargazers could see plenty of Leonids in spite of the bright Moon."

Leonid meteors are caused by tiny meteoroids that burn up in our planet's atmosphere as Earth passes close to the dust-strewn orbit of periodic comet Tempel-Tuttle. Every 33 years when the comet swings through the inner solar system it lays down a new trail of dusty debris.

For Tempel-Tuttle, no two circuits through the solar system are exactly alike. Jupiter's powerful gravity slightly alters the comet's orbit each time around. As a result, each new debris trail is in a different spot. The comet's many dust streams form a sort of cosmic minefield that Earth navigates every year in mid-November.

Most years the Leonids are a minor event -- boasting no more than 10 to 15 shooting stars per hour. That's what happens when Earth passes through the rarefied zones between Tempel-Tuttle's debris trails. On the rare occasions when our planet plows directly into a dense filament of dust, meteor rates can soar to 100,000 per hour or more.

James Young, who watched such a storm in 1966 from JPL's Table Mountain Observatory, recalls:

"This very noteworthy [1966] meteor shower was nearly missed altogether... The shower was expected to occur over the European continent. There were 2-5 meteors every second as we scrambled to set up the only two cameras we had. The shower peaked around 4 a.m. with some 50 meteors falling each second. We all felt like we needed to put on 'hard hats'! To further understand the sheer intensity of this event, we blinked our eyes open for the same time we normally blink them closed, and saw the entire sky full of streaks ... everywhere!"

The Leonids, which surprised Young in 1966, are notoriously difficult to predict. They tend to happen at 33 year intervals when the parent comet is in the neighborhood -- but not always. For instance, after major Leonid meteor storms in 1833 and 1866, astronomers confidently forecast another historic display on Nov. 18, 1899. Millions around the world were watching and waiting as Earth glided by Tempel-Tuttle's orbit. And nothing happened.

"[It was] the worst blow ever suffered by astronomy in the eyes of the public," wrote 19th century astronomer Charles Olivier after the much-touted storm failed to materialize.

The Leonids are like that -- dazzling at their best, but capricious and occasionally heart-breaking to forecasters.

Times may be changing, though, as astronomers are finally learning the whereabouts of Tempel-Tuttle's many dust trails. Like the comet itself, Tempel-Tuttle's debris streams can be nudged into different orbits by gravitational encounters with Jupiter, Saturn, and Uranus. By carefully tracking the orbit and shape of each filament, several groups of astronomers now believe they can predict when Earth will hit one.

"[Gravitational perturbations by planets] are the mechanism that determines whether the relevant sections of dust trails are shifted inwards or outwards or exactly on to the Earth's orbit," explains David Asher.

"Ejection velocities from the comet nucleus and/or radiation pressure change the orbital period of [cometary debris] particles, which causes them to stretch into trails -- those with longer periods fall behind in the stream and vice versa. But planetary perturbations are the most important factor in moving the debris streams towards or away from our planet's orbit."

If comet Tempel-Tuttle were to disappear entirely, he added, the existing dust streams could continue to exist and to trigger Leonid meteor storms for hundreds of years before they dispersed.

In 1999, Asher and colleague Robert McNaught (Australian National University) predicted a strong Leonid outburst on Thursday, Nov. 18th. In years past, astronomers had struggled to decide whether a storm might occur within a day or so of Earth's closest approach to the comet's orbit. Asher and McNaught's audacious prediction was for 0208 UT plus or minus 10 minutes. No one had ever dared predict the Leonids with such precision.

The 1999 Leonid meteor storm broke over Western Europe, right on schedule, at 0205 UT. It was an unprecedented success in Leonids forecasting.

According to the Asher-McNaught model of Leonid dust streams, the 1999 outburst of 1500 meteors per hour occurred when our planet passed through a trail deposited by Tempel-Tuttle in 1899 -- the very year that astronomy suffered its "greatest blow" at the hands of the Leonids!

The same model predicts that Earth is now heading for encounters with two dust streams on November 18th, 2000: one from the year 1733 and another from 1866. Unfortunately for meteor lovers, we won't pass as close to these streams as we did to the one in 1999. Astronomers aren't sure how wide the dusty ribbons really are -- gliding through the outskirts might not produce much of a display.

This year's likely encounters with dust streams are tabulated below. The higher estimates for Leonid meteor rates, in the range 200 to 700 per hour, come from astronomers Esko Lyytinen and Tom van Flandern, who are analyzing the streams in much the same fashion as Asher and McNaught have done.

"They've done some interesting work on the effect of radiation forces in dispersing meteoroids," says Asher of Lyytinen and van Flandern, "and their predictions could well turn out to be correct. If pushed, I would go with our lower estimates of 100 per hour."

Asher also noted that sky watchers should be alert for a possible outburst on Nov. 17th, when Earth skirts by the 1932 dust trail. "There should be some activity although the level may not be very high."

Predictions for Leonids 2000

Date (2000),   Maximum visual  Time (UT) meteor rate  Best seen from:

N. America,    Nov. 17, 07:50  1932              215  C.Am & NW S.Am

W. Africa, W.   Nov 18, 03:44  1733          100-700  Euro & NE S.Am

N. America,     Nov 18, 07:51  1866       1   00-700  C.Am & NW S.Am

Leonid Observing Tips

With the science of Leonid forecasting still in its infancy, meteor enthusiasts might not wish to place too much confidence in the exact times of this year's predicted outbursts -- they could be wrong. Earth will be passing through comet Tempel-Tuttle's "minefield" of debris for two full days spanning Nov. 17th to 18th. Flurries of shooting stars could erupt at any time during that interval.

No matter where you live, the best time to look for Leonids will be between local midnight and sunrise on Nov. 17th and again on Nov. 18th. Observers in the northern hemisphere are favored because the constellation Leo reaches a maximum altitude of 60 degrees at mid-northern latitudes, but only 20 degrees in, say, Australia.

In mid-November, Leo rises above the eastern horizon just after midnight local time. (Local time means the time it is where you live.) Finding Leo will be easy because the waning quarter Moon will lie inside the boundaries of the constellation -- you can't miss it! Once you've located Leo, look away toward a darker area of the sky. Although meteors will stream from the general direction of Leo, they can appear anywhere overhead. In fact, they are often easiest to spot about 90 degrees from the shower's radiant point.

If you begin observing before Leo rises -- that is, before local midnight -- and a Leonid meteor outburst occurs, then you may spot a rare variety of shooting star called "Earth-skimmers." These are disintegrating meteoroids that fly over the horizon nearly parallel to the atmosphere. They produce long colorful tails.

Contrary to some reports, 2000 is not a make-or-break year for the Asher-McNaught model. If there are no outbursts this November, it may simply be that Earth passed too far from the dusty filaments that lie ahead. The real test will come in November 2001 when our planet is due to pass through the hearts of at least two dust streams.

What will happen? The only way to know in either year is to be outside on November 18th, under clear skies and looking up!

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