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Science & Space

Leonid meteor shower to peak Wednesday

By Joe Rao
SPACE.comexternal link

Italian astrophotographer Lorenzo Lovato captured this Leonid fireball in 1998.
Italian astrophotographer Lorenzo Lovato captured this Leonid fireball in 1998.

( -- This week brings us the return of the famous Leonid meteor shower, a display that has brought great anticipation and excitement to sky watchers around the world.

From a viewing standpoint, this will be a favorable year to look for these meteors. The moon will be only a thin crescent and will set in the west long before the constellation Leo (from where the meteors get their name) begins to rise.

This is in stark contrast to 2002 when a nearly full moon lit up the sky like a spotlight and wiped out all but the brightest meteors. Last year, conditions were much better. The moon slimmed to a narrow crescent in the predawn morning sky and offered little interference. This year, the moon will be completely absent and provide perfect viewing conditions.

The Leonid meteors are debris shed into space by the Tempel-Tuttle comet, which swings through the inner solar system at intervals of 33 years. With each visit the comet leaves behind a trail of dust in its wake.

Lots of the comets' old dusty trails litter the mid-November part of Earth's orbit and the planet glides through these debris zones every year. Occasionally we'll pass directly through an unusually concentrated dust trail, or filament, which can spark a meteor storm resulting in thousands of meteors per hour. That's what happened in 1999, 2001 and 2002.

Since the Tempel-Tuttle comet passed the sun in 1998, the Leonids put on their best show in the years immediately following its passage.

But now, the comet -- and its dense trails of dust -- have receded far beyond Earth's orbit and back into the outer regions of the solar system. So odds are that there is little, if any chance of any unusual meteor activity.

Peak activity

This year's peak activity should occur on the morning of Wednesday, November 17, according to meteor experts Margaret Campbell-Brown and Peter Brown, in the 2004 Observer's Handbook of the Royal Astronomical Society of Canada.

They cite 9:00 GMT, which corresponds to 4 a.m. EST and 1 a.m. PST, as the moment when the Earth will be passing closest to the orbit of the long-departed comet and is most likely to encounter residual comet material. This time is advantageous for North Americans, especially those in the eastern United States and eastern Canada.

But while Leonid rates are unpredictable, it is unlikely that more than a dozen meteors per hour will be seen this year during peak activity, at least for viewer's with dark skies away from cities.

Meteor researchers such as Jeremie Vaubaillon of France, David Asher of Ireland and Esko Lyytinen of Finland have examined Leonid prospects for this year and also suggest watching for some meteor activity two days later on November 19.

Sometime around 6:40 GMT (1:40 a.m. EST) on November 19th, the Earth might interact with material that was shed by comet Tempel-Tuttle back in the year 1333. At best, only about 10 Leonids per hour may be seen. Lyytinen even suggests that because of gravitational perturbations affecting these particles after making 20 trips around the sun, that this sparse activity might actually take place several hours earlier, which would favor observers in Europe.

Later that same day, the Earth could possibly sweep through a fresher and somewhat more concentrated trail of material dating back to 1733. Perhaps a brief bevy of 30 to 60 Leonids per hour may seen, but the time frame when these meteors are predicted to be most numerous (approximately 19:00 to 22:00 GMT) favors observers in Asia and Australia in the early morning hours of November 20.

How to watch

The meteors will appear to emanate from the so-called "Sickle" of Leo, but prospective viewers should not concentrate on that area of the sky around Leo, but rather keep their eyes moving around to different parts of the sky.

Leo does not begin coming fully into view until after midnight which is the best time to concentrate on looking for the Leonid meteors.

The hours after midnight are generally best for watching for "shooting stars" anyway. Before midnight we are riding on the back side of the Earth in its orbit around the sun. After midnight, we are on the front or advancing side and the only meteoroids escaping collision are those ahead of the Earth and moving in the same direction at velocities exceeding 18.5 miles per second.

Therefore, on the average, morning meteors appear brighter and faster than those we see in the evening.

Because the Leonids are moving along in their orbit around the sun in a direction opposite to that of Earth, they slam into our atmosphere nearly head-on, resulting in the fastest meteor velocities possible: 45 miles per second (72 kilometers per second). Such speeds tend to produce bright meteors, which leave long-lasting streaks or trains in their wake.

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