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Right: Lorenzo Lovato captured this stunning picture of a -10 magnitude Leonid fireball on November 17, 1998 from Monteromano, Italy. He used Fuji 800 film with a 16 mm f/2.8 lens for an exposure time of 15 minutes. Copyright 1998, Lorenzo Lovato, all rights reserved. One day, space weather forecasts like this could be commonplace.
As our society comes to rely on satellites, cell phones, and
other space-age gadgets, forecasting solar storms and meteor
showers can be just as important as knowing the chances of rain
tomorrow. Three weeks from now we may be treated to a very visible
reminder of space weather when the Leonid meteor shower strikes
on November 18, 1999.
Most experts would agree that predicting the Leonids can be tricky. To understand why it's helpful to know the difference between a "meteor shower" and a "meteor storm." Simply put, meteor showers are small and meteor storms are big. Meteor showers produce a few to a few hundred shooting stars per hour. Meteor storms produce a few thousand to a few hundred thousand meteors per hour. A meteor storm, like a total solar eclipse, ranks as one of Nature's rarest and most beautiful wonders. A Leonid meteor shower happens every year around November 17 when Earth passes close to the orbit of comet Tempel-Tuttle. Usually not much happens. The Earth plows through a diffuse cloud of old comet dust that shares Tempel-Tuttle's orbit, and the debris burns up harmlessly in Earth's atmosphere. A typical Leonid meteor shower consists of a meager 10 to 20 shooting stars per hour.
Every 33 years something special happens. Comet Tempel-Tuttle swings through the inner solar system and brings a dense cloud of debris with it. For 3 or 4 years after its passage the Leonids can be very active. In 1966 for instance, over 100,000 meteors per hour were seen from parts of North America. Curiously, there isn't a full-fledged storm every time Tempel-Tuttle passes by. Sometimes there's simply a stronger-than-average shower. Sometimes nothing happens at all! Will there be a storm in 1999? (Probably, yes.)Tempel-Tuttle visited the inner solar system most recently in late 1997 and early 1998. The subsequent Leonids display, in Nov. 1998, was marvelous as observers all over the world were treated to a dazzling display of fireballs (shooting stars with magnitudes brighter than -3). Nevertheless, the 1998 Leonids were a shower, not a storm. The maximum rate of meteors last year was about 250 per hour. Scientists have learned that if Earth crosses the orbit of Tempel-Tuttle too soon after the comets passage, then there is no storm, just a strong shower. Apparently that's what happened in 1998. In recent history no Leonid storm has ever occurred less than 300 days after Tempel-Tuttle passed by Earth's orbit. In 1998, Earth followed the comet to the orbit-crossing point by only 257 days [ref].
Above: 1998 Leonids activity based on visual records from 217 observers who saw more than 47,000 Leonids in 858 observing hours. The vertical axis is the "zenithal hourly rate" of visual meteors, or the hourly rate of meteors an observer would witness under ideal conditions with the meteors appearing directly overhead. The horizontal axis is the solar longitude of Earth, and may also be regarded as time increasing from left to right. The "Fireball peak" corresponds to the impressive fireball display of Nov. 17, 1998. The smaller "Storm peak" occurred approximately 12 hours later just as Earth was crossing the orbital plane of Tempel-Tuttle. Credit: The International Meteor Organization. If the 1999 Leonids are anything like the 1966 storm, stargazers are definitely in for a treat. The 1966 event was, predictably, somewhat unexpected. The comet had passed by Earth's orbit in 1965, so astronomers were aware that something might happen. But, judging by the paucity of the 1899 and 1932 showers, it was widely thought that the orbit of the debris stream had been deflected so much by gravitational encounters with other planets (mainly Jupiter) that a close encounter with Earth's orbit was no longer possible. The best predictions suggested a strong shower over Western Europe with 100 or so meteors per hour. Instead, there was an stunning display of shooting stars over western North America. This recollection by James Young at JPL's Table Mountain Observatory in California gives a sense of what the storm was like:
The 1966 return of the Leonids was one of the greatest displays
in history, with a maximum rate of 2400 meteors per minute or
144,000 per hour.
Rao is also a meteorologist for News 12 Westchester, which seems a suitable occupation for predicting meteor showers. In 1999, the Earth will pass nearly three times as far from the comet's orbital path as it did in 1966 and more than six times farther than it did during the great storm of 1833. If the peak of the Leonids arrives exactly when the Earth passes through the comet's orbital plane, Donald Yeomans of JPL gives 01:48 UT on November 18, 1999 as the most likely time for the 1999 maximum [ref]. That would make Europe and West Africa the best places to watch the show. However, Leonid meteor showers frequently arrive much earlier or later than predicted, so any place on the globe could be favored. If the peak of the Leonids occurs over Europe or the Atlantic Ocean, then observers in the USA could be in for an unusual treat. The Leonid radiant would just be rising over North America at the time. In the eastern US sky watchers would see a large number of earth-grazing meteors skimming horizontally through the upper atmosphere. "Earth grazers" are typically long and dramatic, streaking far across the sky. To look or not to look, that is the question
Most experts agree that 1999 is the most likely year for a Leonids meteor storm during the current 33 year cycle. However, if 1999 turns out to be a disappointment, don't despair! There are other studies that suggest 2000, 2001 or even 2002 could be better years. The Leonids are simply hard to forecast. If 1999 is the year, when should you look? Most experts predict
that the Leonids peak will
occur between 0100 and 0400 Universal Time on November 18th.
However, it is important to remember that
such predictions are always uncertain. The 1998 Leonid fireball display
occurred nearly 16 hours before the predicted maximum!
No matter where on Earth you live, the morning of November 18 will
probably be
the best time to look for Leonids in 1999. This is true even
if morning where you live occurs much earlier or later than
0100-0400 UT.
In the coming weeks Science@NASA will post more stories about the Leonids with observing tips for meteor watching with the naked eye, video cameras and other types of recording devices. One thing seems sure, no matter where you live: The Leonids are coming and, on Nov 18, 1999 the place to be is outside, looking up! |
October 27, 1999: Imagine tuning
in to the local TV weather report and hearing this from the weatherman:
| Web Links |
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Leonids Live! -site of the live webcast of the 1998 Leonids Awaiting the Storm - by Joe Rao in Sky&Telescope Bulletin 13 of the International Leonid Watch: The 1998 Leonid Meteor Shower - by Rainer Arlt of the International Meteor Organization The November Leonids: Will They Roar? - Leonids predictions from Donald Yeomans of JPL Asher, DJ, Bailey, ME and Emel'yanenko, VV "Resonant meteoroids from Comet Tempel-Tuttle in 1333: the cause of the unexpected Leonid outburst in 1998," Mon. Not. R. Astron. Soc. 304, L53-L56 (1999) Related Stories: |
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