Nov 20, 2000

Leonids Galore




The art of predicting Leonid meteors officially became a science this weekend as sky watchers around the globe enjoyed three predicted episodes of shooting stars.


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November 21, 2000 -- A bright moon, city lights and scattered clouds weren't enough to keep the 2000 Leonid meteor shower at bay. Sky watchers who ventured outdoors after midnight on Nov. 17th and 18th enjoyed sporadic flurries of bright shooting stars numbering more than 200 per hour over parts of Europe, Africa, and the Americas.

"I could see plenty of Leonids from [brightly-lit] downtown Boston," reported a reader on Saturday morning after a break in cloudcover briefly revealed clear skies. "One meteor was even brighter than the Prudential Building!"

Right: Amateur astronomer George Varros captured this video of a Leonid fireball over Mount Airy, MD at 0837 UTC on Nov. 18th. Jupiter is the brightest object near the fireball, followed by Saturn and Aldebaran. The constellation Orion appears to the left. [More video and images from Leonids 2000]




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This year's Leonid meteor shower consisted of three main episodes lasting several hours each. There was a modest flurry of 50 to 100 meteors per hour on Nov. 17th, followed by two more outbursts of 150 to 450 per hour on the 18th. [details]

For many North Americans the times of greatest activity coincided with local midnight when the constellation Leo was lying low on the eastern horizon. Normally, low-hanging radiants are bad news because they make shooting stars hard to see. In this case, however, sky watchers were treated to a vivid display of Earthgrazing meteors. "Earthgrazers" are shooting stars that emerge from just below the horizon and streak through the upper atmosphere nearly parallel to the ground. They often display colorful halos and long-lasting trails stretching 90 degrees or more across the sky.

"I [started watching] just before 11:00 pm on Friday," recounted Pierre Martin from eastern Ontario. "Even with the Leonid radiant only 3 degrees over the eastern horizon, it was obvious that some fairly high activity was in progress. Several spectacular Earthgrazers appeared! .... A most impressive orange-colored Leonid split the sky in half. It traveled 70 degrees. A multi-colored one at 11:55 pm really blew me away... It went from vivid blue to green to orange before it extinguished and left behind a train that lingered for 3 seconds."



Above: The "zenithal hourly rate" (ZHR) of Leonid meteors vs solar longitude during the interval Nov. 16 (0000 UT) to Nov. 18 (1300 UT), 2000, based on early reports by observers to the International Meteor Organization. Grey arrows indicate times when astronomers predicted Earth would cross dusty debris streams from comet Tempel-Tuttle. [more information]

The bright Moon was not a serious impediment to meteor watching as many feared it would be. The Leonids were bright and they tended to streak far from the shower's moonlit radiant.

At 2:45 am EST on Saturday, the Moon was high in the sky when Marjory Moeller of Atol, NY, peered out her bedroom window. "I was immediately rewarded by a long yellow meteor coming from the east," she said. "Incredible sight! It would have been scary if I hadn't known what it was!" Minutes later, Jeannie Moorhead of Warwick, NY, says "I saw an incredible fireball explode as white as the moon -- it left a very thick trail that remained in the sky for at least 5 minutes."

"I never expected a shower [to be] this good with the Moon up," added Ted Nichols of the Astronomical Society of Harrisburg, PA. "During one 15 minute interval I counted 45 meteors!" Altogether, he saw 275 shooting stars between 10:30 pm on Friday and 3:30 am on Saturday.

But, not everyone was so fortunate.

"Like a lot of people in the southeastern US, all we saw in Louisiana were rain showers -- about 10.5 inches worth at my house," lamented meteor enthusiast Dave Hostetter. "We've been having a drought for a year and a half -- I should have known which weekend would get rain!"


"The experts had predicted a strong Leonid shower... and by golly, that's what happened here," agreed Kim Youmans in Georgia. "I can't remember when it last rained so hard."


Fortunately for such observers, more Leonids are on the way. The triple-peaked character of this year's shower appears to confirm new research that predicts powerful Leonid meteor storms in the future.

Right: Surely the most unusual picture from the 2000 Leonids meteor shower, this graphic overlays two images of the same fireball captured by twin brothers George and David Varros located 15 miles apart in Maryland. The arrows "1" and "2" trace the path of the fireball as seen from each site; the apparent trajectories are different as a result of parallax. The ghostly web-like object between Jupiter and Saturn is a basketball net in David Varros's driveway.

"We're very confident that Leonid storms are coming in 2001 and 2002," says forecaster David Asher of the Armagh Observatory in Northern Ireland. "Peak rates during those years should reach at least 10,000 meteors per hour when Earth passes through debris streams from comet Tempel-Tuttle."

Asher and collaborator Robert McNaught (Australian National University) drew attention last year when they predicted the onset of a Leonid meteor storm over Europe within minutes of the time it actually occurred. They had carefully studied the orbits of myriad debris streams shed by comet Tempel-Tuttle during its periodic 33-year visits to the inner solar system. By noting the time when Earth passed close to one of those dust trails, Asher & McNaught were able to forecast the1999 Leonids with unheard-of precision.

Astronomers have long regarded the Leonids as stubbornly unpredictable. The failure of a major Leonid storm to appear in 1899, after scientists had urged millions to stay up and watch it, was "the worst blow ever suffered by astronomy in the eyes of the public," according to 19th-century astronomer Charles Olivier. For the next hundred years astronomers fared little better with hit-or-miss forecasts based on historical records.


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Left: Earth's path through space is shown in blue. Auburn-colored ellipses denote dust streams laid down by comet Tempel-Tuttle in the indicated years. This year our planet passed close to trails deposited in 1932, 1733 and 1866. Credit: Dr. David Asher (copyright 2000, all rights reserved). [more information]

As mid-November 2000 approached, meteor watchers were anxious to learn if the dust stream models developed by Asher & McNaught would work again. It seemed to be a testable question because -- according to the models -- Earth was heading for the outskirts of three debris streams. Expectations were tempered by the fact that the expected encounters were not very close. Earth would pass half a lunar distance (LD) from one stream and 0.3 LD from two others. Researchers suspected that these might be great distances compared to the average width of a dust filament. If the outer reaches of the debris fields were rarefied, observers might see very little meteor activity or possibly none at all. (Note: one "lunar distance" or LD equals 384 thousand km, the average separation of the Earth and the Moon.)


The doubters became believers by day's end on Nov. 18th as sky watchers reported strong meteor activity during all three encounters. There is still some uncertainty about the exact times of the maxima and their amplitudes, and how those compare to the predictions, but the essentials are clear: Asher & McNaught-style models can predict Leonid meteor showers, and for more than one year in a row. Other researchers are already working to improve the basic predictive models by, e.g., adding the effects of radiation pressure on meteoroids and considering in detail the trajectories of debris particles ejected from the parent comet. Decades of uncertain Leonid meteor forecasts may soon be a distant memory.


Right: Mike Boschat captured this image of a colorful Leonid streaking past the Pleiades on Nov. 18, 2000 from an observing site in Halifax, Nova Scotia, Canada.

Indeed, the future looks bright for Leonid meteors. In mid-November 2001 Earth will pass almost directly through three more Leonid dust streams. Observers in the Americas, east Asia, Australia and the Pacific Ocean will be favored for a good display. Even the Moon is expected to cooperate -- its phase will be nearly New, affording dark skies for observers.

So, if rain or clouds (or simply a faulty alarm clock) spoiled your view of the 2000 Leonids, don't despair. The best may be yet to come!


Web Links


Leonid Predictions for 1999-2006 -- from David Asher and Rob McNaught.

History of the Leonids -- compiled by Gary Kronk


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