Circle Nov. 17 on your calendar, for early that morning a moderate to possibly very strong showing of annual Leonid meteor shower is likely.
The very strong display will favor those living across most of central and eastern Asia. In this region, meteor rates might briefly rise to a few hundred per hour (the time frame for the most intense activity is anticipated sometime around 21:40 GMT).
A far more modest, but still potentially enjoyable display of a few dozen Leonid meteors per hour is expected to favor North America. In the United States and Canada, eastern observers will be particularly well-positioned for maximum activity, expected sometime between 3:30 and 5:30 a.m. EST, when the radiant of the Leonid shower will be well up in the dark southeastern sky.
A meteor shower's radiant is the perspective point from which all the meteors would appear to originate if their paths were traced backward far enough. The higher the radiant is, the more meteors flash into view all over the sky (though meteors can appear anywhere in the sky, so focusing on the radiant is not necessary).
The Leonid radiant is within the so-called "Sickle" of Leo; a backwards question-mark pattern of stars that outlines the head and mane of the constellation Leo, the Lion. Hence the meteors are known as "Leonids."
Not in the East? Don't fret. Observers all across North America may experience a good Leonid show with "shooting stars" streaking across the sky every few minutes.
Also a big plus in 2009 is the lack of any interference from the moon. New moon is on Nov. 16, so skies will be dark for catching the fainter meteor streaks. And the first light of dawn will not break until shortly after 5 a.m. local time.
Europe and Africa appear largely out of luck. This year's first round of expected enhanced activity will happen chiefly during their daylight hours. The second, stronger outburst will occur during early evening, but that's hours before Leo comes above the horizon. Europeans might try watching before sunrise on the morning of Nov. 17, but are not likely to see more than 10 or 15 Leonids per hour.
It may not sound sexy, but simply put, the reason for this year's anticipated good Leonid showing is due to cosmic garbage.
The Leonids are known to be made up of cosmic litter from a small – 2.2 mi (3.6 km) – dusty comet discovered by two astronomers in the late 19th century and christened Tempel-Tuttle. The Leonid meteors are thought to be the dusty legacy of Comet Tempel-Tuttle because the dust is moving around the sun in virtually the same orbit as the comet.
As the Earth encounters the debris left behind by the comet's previous passes through our orbit, these tiny fragments of the comet – typically no bigger than a sand grain or the occasional pea – impact our atmosphere at speeds of up to 45 miles per second (71 kps), causing them to blaze briefly but brightly in the night sky.
The Leonids are not a one-night stand. The dust from Tempel-Tuttle spangles the sky for a few nights every year in mid-November. This year, the peak is expected during the predawn hours of Nov. 17, but early-morning hours on the dates surrounding Nov. 17 could provide a decent show, too.
Tempel-Tuttle last passed near the sun and Earth more than a decade ago (in 1998) and for several years thereafter the Leonids put on some spectacular displays, producing many hundreds – even thousands of meteors per hour. But with the departed comet now cruising through the outer part of the solar system, we typically would not expect to see more than 8 or 10 Leonids during an hour's watch.
Yet if several meteor scientists are correct, this year will be atypical. The researchers have produced various models of the Leonid stream and all of them are indicating that the Earth will intersect a few "rivers of rubble" left behind by Comet Tempel-Tuttle.
In particular, French astronomer, Jeremie Vaubaillon, Russian astronomer, Mikhail Maslov and Americans Bill Cooke and Danielle Moser of NASA's Meteoroid Environment Office are all in agreement that material that was ejected from the comet's nucleus during the years 1466 and 1533 will likely produce a very strong meteor display over much of Asia.
"The year 2009 will not see a Leonid storm, but an outburst for sure," Vaubaillon said, adding that "there are still some uncertainties."
Last year, the material that was shed by the comet back in 1466 produced about 100 Leonids per hour.
This year, Earth will cross through the 1466 stream again, but this time much closer – 42,000 mi. (68,000 km) – to the center on Nov. 17 at around 21:40 GMT. This time favors central and eastern Asia (and it comes during daylight in North America). In addition, at about this same time, the Earth will also be passing through dust ejected by the comet in 1533. The consensus forecast among the astronomers for this year suggests anywhere from 130 to perhaps 300 Leonids per hour in Asia, but this spread has been calibrated chiefly using last year's Earth interaction solely with the 1466 stream.
"But nothing is known (about) the 1533 stream," Vaubaillon said.
North America forecast
About 12 to 14 hours before the main event, Earth is forecast to sideswipe a stream of dust that was loosed from the nucleus of Comet Tempel-Tuttle in the year 1567, passing to within 188,000 mi. (302,000 km.) of the stream's center. This interaction could provide smodest activity for North Americans.
The best guesstimate is for perhaps 25 to 30 Leonids per hour, which would most likely target a time frame sometime between roughly 3:30 and 5:30 a.m. ET (12:30 to 2:30 a.m. PT).
For North American observers, the emphasis might be on quality, not quantity; for while the numbers might not be exceptionally high as compared to Asia, a few of these meteors, though visible for a just a fraction of a second, might leave bright trails of ionized atoms in their wake that hang in the sky for many seconds – or possibly even minutes – as these tiny dust particles streak through our atmosphere at altitudes of 80 to 100 miles (130 to 160 km).
And seeing even just one such outstandingly bright meteor like that can make a cold early morning vigil worthwhile.
If you want to get started early, you can practice for the big event.
The first Leonid forerunners might be sighted as early as Nov. 10, although overall activity will be rather low and spotty – perhaps only a few per hour at most. Around Nov. 16, in the predawn, Leonid activity will noticeably increase to perhaps four to eight per hour.
Observers on the lookout for early Leonids might also notice a number of rather slow moving meteors appearing to emanate from around the constellation of Taurus, the Bull (high in the southwest sky after midnight). These are the Taurid meteors and are most active between Nov. 5 and 12 when they can produce as many as five or 10 per hour.
Editor's Note: SPACE.com will provide a Leonids 2009 Viewer's Guide on Friday, Nov. 13.
- Leonid Image Galleries 1, 2, 3, 4
- Top 10 Leonid Meteor Shower Facts
- Leonids 2009 on Twitter: Forecasts and Your Reports
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.