|
|
  |
|
That is, except for the fireballs. I was sky watching with a friend, both of us experienced astronomers. Nevertheless, we stared upwards like novices, slack-jawed, as if we had never seen the sky before. We were witnessing the annual Leonid meteor shower. But these were no ordinary Leonids. They were bright, vivid, shadow-casting fireballs. Every five minutes or so we saw one as bright as Venus, and a fair number would have outshined a Full Moon. Some of the most startling left behind glowing trails of debris that lingered in the sky, twisting and turning as they were sheared by high-altitude winds. Above: A wide angle view of Leonid fireballs on November 17, 1998. Photo credit: Juraj Toth. [more] It was unforgettable.
But the Leonids of 2001 will be. Cooke and other experts agree that when the Leonids return later this month sky watchers in some parts of the world will see a display even better than the one in 1998. Indeed, says Cooke, "what's coming on Nov. 18th could be the biggest event since 1966 [when North Americans enjoyed a Leonid storm numbering 100,000 shooting stars per hour]." Observers in North America, Hawaii, Australia and Asian countries along the Pacific Rim will be favored for the best views of the 2001 Leonids. Meteor rates in those places could climb as high as 8000 per hour -- not quite as intense as the 1966 storm, but more than enough to make a sky watcher's jaw drop.
Table notes: (1) UT is Universal Time, also known as Greenwich Mean Time or GMT. UT values above refer to Nov. 18th. (2) Because of the international date line, observers in Australian and Asian countries will see their Leonids before dawn on Nov. 19th local time. (3) ZHR is the Zenithal Hourly Rate -- that is, the number of meteors a observer with ideally dark skies would see if the constellation Leo were directly overhead. The range in predicted ZHRs reflects differences among the models of various forecasters. [more information] Leonid meteor storms happen when Earth passes through clouds of dusty debris shed by comet 55P/Tempel-Tuttle when it comes close to the Sun every 33 years. This year our planet is heading for close encounters with four such clouds. They bubbled off Tempel-Tuttle in 1699, 1766, 1799 and 1866.
Right: Auburn-colored ellipses denote dust clouds shed by comet Tempel-Tuttle in the indicated years. Earth's path through space is blue. This model was formulated by D. Asher and R. McNaught; models advocated by other Leonid forecasters differ somewhat. "These clouds are long and narrow like a comet's tail," says Cooke. "The younger ones are only 10 or so Earth-diameters wide." Our chances of hitting something so narrow and filamentary are slim. Indeed, most years in November we miss them altogether. Earth glides between the clouds where there is only a sprinkling of meteoroids. At such times Leonid rates remain low: only 10 or 15 meteors per hour. "In 1998 we passed through material shed by the comet in 1333," says Cooke. "That filament was old and somewhat spread out," so rates never climbed to storm levels. It was nevertheless spectacular because "the smallest bits of dust inside that cloud had been blown away long ago by solar radiation pressure. Only the largest meteoroids remained -- hence the fireballs." "In 2001 we're running into relatively young clouds, richer in small meteoroids," added Cooke. "Observers from '98 who remember mostly fireballs will be dazzled this year instead by a greater number of ordinary meteors."  Above: Still frames from a digital movie showing Earth as it glides through a filamentary dust cloud. The encounter triggers a meteor shower from the constellation Leo. Click to view the full 565 kb Quicktime animation created by Digital Radiance, Inc. Although certain parts of the world are favored for intense activity this year, Cooke encourages people everywhere to watch the sky on Nov. 18th. "The Leonids might surprise us," he says. Predicted outbursts might fizzle, and activity could surge at unexpected times. Veteran meteor watchers are wary of Leonid predictions because the science of forecasting Leonid meteor storms is still young. The basic techniques were pioneered only three years ago by astronomers David Asher (Armagh Observatory) and Rob McNaught (Australian National University). They correctly predicted a brief meteor storm over the Middle East and Europe in 1999. Then, in 2000, they and others used similar methods to forecast the times of three more Leonid flurries. It's a promising track record, but by no means well-established. Below: The Leonid radiant, denoted by a red dot, will appear high in the southern sky at 5 o'clock in the morning (local time) as viewed from mid-Northern latitudes. Click to view a southern hemisphere sky map.  If you're determined to spot some Leonids this year, here is the best strategy: Dress warmly and travel (if necessary) to a dark-sky site away from urban light pollution. Be prepared to watch the sky between midnight and sunrise on Sunday morning, Nov. 18th. Meteor rates will probably be low near midnight -- although that is a good time to see beautiful Earthgrazing Leonids -- then climb to 10 or 20 per hour by dawn. If you're lucky you might witness a storm-level outburst and count thousands of shooting stars. With the Leonids there are no guarantees. No matter, the coming shower will surely send some sky watchers home with life-long memories. "I'll never forget the night of Nov 18th, 2001," they might recall years from now -- just as I remember the Leonids of 1998. Others, perhaps, will gain little more than a quiet night under the stars. One thing is certain: if you stay indoors you won't see anything! Visit SpaceWeather.com for more observing tips and for real-time updates and images during the coming meteor shower. |
Listen to this story
via 
"Each
encounter with a dust cloud will produce an outburst of Leonids
over some part of our planet," explains Cooke. "For
example, the best place to view the 1799 meteoroids is Hawaii.
That's where I'll be!" The 1766 cloud will produce a flurry
of Leonids over North America, while the 1699 and 1866 clouds
will rain meteors over Australia and east Asia.
Headlines