The Truth about the 2002 Leonid Meteor Storm
All night long Leonid meteors had streaked across the sky -- thousands of them. Even as the horizon brightened at dawn, I could still see fireballs flashing in the distance. There was only one thought on my mind when the night sky finally vanished:
"I can't wait until next year."
Right: A 2001 Leonid meteor streaking through the dawn sky, photographed by Science@NASA reader Brent Price of Antioch, Illinois. [more]
The Leonid meteor shower happens every year in mid-November. That's when our planet has a close encounter with Comet Tempel-Tuttle's orbit -- a region of space littered with streams of comet dust. Usually we pass through the rarefied gaps between streams and sky watchers see no more than 10 or 15 Leonids per hour. But sometimes (like last year) Earth plows through a debris stream more or less head-on and a full-fledged meteor storm erupts.
That's the good news. The bad news is that the Moon will be full when the storm begins on Nov. 19th. Glaring moonlight will completely overwhelm many faint shooting stars. Indeed, I often hear that the Moon is going to "ruin the show."
I doubt it.
Moonlight will indeed reduce the number of visible meteors by some factor between 2 and 5, but that's not enough to wipe out a storm of bright Leonids. If predictions are correct, peak rates this year would reach 3,000 to 10,000 meteors per hour under ideally dark skies. Even if the Moon's glare obliterates all but a thousand of those, it's still a marvelous show -- one of the best in decades.
Above: Bill Cooke of the NASA/MSFC Space Environments Group prepared this summary of Leonid prospects for 2002. The colored curves denote predictions by three teams who successfully forecast the 2001 storm. Curves that peak above the yellow zone exceed the maximum rate observed in 2001. "ZHR" is the zenithal hourly rate and "UT" is Universal Time.
Furthermore, there are steps you can take to minimize lunar glare. First, don't stare at the Moon. Face away from it; look toward the darkest part of the sky. If possible, choose an observing site where you can stand in the shadow of a building or some other Moon-baffle.
Second, travel if necessary to a place where the air is dry and clear. Even when you face away from the Moon the air glows because of moonlight scattered from air molecules and aerosols (e.g., water droplets, dust and pollution). The glow will be less in places where the air is dry and pollution-free. Mountaintops are excellent because they rise above the humid lower atmosphere and most aerosols.
Another place to escape Moon-glow is on board an airliner. A commercial jet flying at 33,000 feet (10 km), for example, is above 75% of Earth's light-scattering atmosphere. Meteors disintegrate much higher than that, about 80 km above Earth's surface, so watching the storm from an airplane is not dangerous. However, there are other problems: Airplane windows are small and blurry, and you'll have to convince someone to turn off the cabin lights.
For most sky watchers, the ground is a better choice. But when and where?
This year on Nov. 19th, Earth will pass through two of Comet Tempel-Tuttle's debris streams. The first encounter should cause a flurry of meteors over Europe and Africa around 0430 UT. The second encounter favors North Americans who are likely to see an outburst around 5:30 a.m. EST or 10:30 UT. (Note: UT is Universal Time, also known as Greenwich Mean Time.)
Below: Auburn-colored ellipses denote dust clouds shed by Comet Tempel-Tuttle in the indicated years. Earth's path through space in 2002 is blue. This model was formulated by D. Asher and R. McNaught.
Many observers plan to visit the American southwest to see the Leonids. The Moon will be higher there when the storm erupts at 2:30 to 3:30 a.m. local time. Nevertheless, there are many high-altitude sites in that part of North America with dry, clear air -- and thus relatively little Moon-glow.
No matter where you plan to be -- on a mountaintop, in an airplane or at home in the city -- don't miss the 2002 Leonids. Earth won't plow head-on into another Leonid debris stream for decades. The Moon will be a nuisance this year, but not enough to ruin the show if the Leonids are as bright and numerous as they were in 2001.
When the Sun rises on Nov. 19, 2002, I hope to be outside again, stiff-necked and spellbound ... just like last year.
Listening to Leonids -- (Science@NASA) On Nov. 18, 2001, millions of sky watchers saw a dazzling storm of Leonid meteors. Some observers heard them too!
Explosions on the Moon -- (Science@NASA) During the 2001 Leonid meteor storm, astronomers observed a curious flash on the Moon -- a telltale sign of meteoroids hitting the lunar surface and exploding.
Jaw-dropping Leonids -- (Science@NASA) On Nov. 18, 2001, sky watchers somewhere will see a dazzling storm of Leonid meteors.
Leonids 2001 Gallery -- (SpaceWeather.com) Sky watchers who saw it will never forget it: the 2001 Leonid meteor storm.
What some experienced meteor watchers say:
"I remember the Geminids a few years ago when there was a nearly-full Moon. A couple of hours before dawn a patch of (low-lying) clouds covered the Moon. The sky went lovely and dark, like turning a light off. The remainder of the watch was almost like a moonless night." Gary Wilson, Nottingham, England.
"Crucial for moonlight observations (of meteors) is a moisture-free sky. Good conditions are possible even at sea level. For example, in the Netherlands arctic air circulation can bring clear skies and dry air. I have occasionally observed in the past under full Moon conditions with limiting magnitudes approaching +6.0. In general, when the sky is clear and dry, magnitude +5.5 should be possible. If the upcoming Leonids are anything like the two 2001 peaks, then plenty of bright Leonids should still be visible." Marco Langbroek, Dutch Meteor Society
"My experience (observing meteors during a full Moon) suggests a reduction of limiting magnitudes by 0.7-1.0 magnitudes. Assuming a population index of about 2.2, we will see 60% down to 45% of the meteors seen without the Moon. Observations into dawn have shown that amazingly many meteors are still visible even though the stellar limiting magnitude has dropped a great deal. I think that gives us hopes to see a nice display (of Leonids in 2002) despite the Moon." Rainer Arlt, Visual Commission - International Meteor Organization
"With a full Moon high in the sky I typically experience a limiting magnitude (LM) around 4.0. About two hours before moonset it improves to 5.0, then during the final hour it gets toward 6.0. [I have observed] Geminids with a nearly-full Moon several times. The limiting magnitude was 4.0 and I consistently saw 18 Geminids/hour. My dark-sky rate is in the 80's per hour, so the correction factor from LM=4.0 to LM=7.0 is thus 4 to 5. During the Perseid meteor shower in 1984 [when the Moon was bright] I saw 9/hour for 3 straight hours. A normal Perseid shower back then was around 40/hour; again the correction factor is between 4 and 5." Norman W. McLeod III, Staff Advisor, American Meteor Society
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