This weekend brings us the return of the famous Leonid Meteor Shower, a meteor display that in recent years has brought great anticipation and excitement to skywatchers around the world.
While the Leonids have been spectacular in years past, this year a modest display is expected.
Solely from the standpoint of viewing circumstances, this will be a favorable year to look for these meteors, since the Moon will be at first quarter phase and will have set in the West long before the constellation Leo (from which the meteors get their name) has climbed high in the sky.
What they are
The Leonid meteors are debris shed into space by the Tempel-Tuttle comet, which swings through the inner solar system at intervals of 33.25 years.
With each visit the comet leaves behind a trail of dust in its wake.
Lots of the comet's old dusty trails litter the mid-November part of Earth's orbit and the Earth glides through this debris zone 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, because Tempel-Tuttle had gone through the inner solar system in 1998.
But now, the comet — and its dense trails of dust — have all receded far beyond Earth's orbit and back into the outer regions of the solar system. So this year there there is little if any chance of heightened activity.
In the 2007 Observer's Handbook of the Royal Astronomical Society of Canada, meteor experts Margaret Campbell-Brown and Peter Brown indicate that this year's peak activity should occur on the night of Nov. 17-18.
They cite 4:00 GMT on Nov. 18, which corresponds to 11 p.m. EST and 8 p.m. PST on the evening of Nov. 17.
This is the moment when the Earth will be passing closest to the orbit of the long-departed comet, and when our planet seemingly is most likely to encounter some residual comet material.
This time is highly favorable for those in Europe and Africa. But for North American observers, Leo will still be below the horizon; they will have to wait until later in the night to catch a view of the Leonids.
While Leonid rates are unpredictable, it is unlikely that more than about 15 meteors per hour will be seen this year.
Other meteor researchers concur that Leonid activity will be modest at best.
In the 2007 Astronomical Calendar, British meteor astronomer Alastair McBeath also cites the night of Nov. 17-18 as the best night for Leonid viewing: "This year may see a return to more typical meteor numbers, perhaps 15+?"
McBeath gives 3:00 GMT on Nov. 18 for this year's Leonid peak and states that observing conditions "should be impressive for covering this likely maximum, especially from Europe, North Africa and the Near East."
Possible brief outburst over Asia?
Other meteor researchers, however, such as NASA's Peter Jenniskens, Jeremie Vaubaillon of France, Esko Lyytinen of Finland, David Asher of Ireland and Mikhail Maslov of Russia have examined Leonid prospects for this year and also suggest watching for some meteor activity also on Nov. 18, but much later in the day.
For instance: sometime between 22:36 and 23:03 GMT, the Earth might interact with material that was shed by comet Tempel-Tuttle back in the year 1932.
But even in this case, the intensity of the shower will fall far short of the memorable Leonid displays that occurred as we transitioned from the 20th to the 21st century.
"Unfortunately it isn't possible to have a 'once in a lifetime' chance every year," Asher notes.
It is possible the dusty material from 1932 will create a brief bevy of 30 to 60 Leonids per hour. But the time frame when these meteors are predicted to be most numerous only favors observers in central and eastern Asia (where it will be the early morning hours of November 19).
And unfortunately, for Japan and Australia, the sun will have already risen!
How to watch
The meteors will appear to emanate from out of the so-called "Sickle" of Leo. 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.
Because Leo does not start coming fully into view until the after midnight hours, that would be the best time to concentrate on looking for the Leonid meteors.
The hours after midnight are generally best for watching for "shooting stars" anyway, because before midnight we are riding on the back side of the Earth in its orbit around the sun, whereas after midnight we are on the front or advancing side.
After midnight the only meteoroids escaping collision are those ahead of the Earth and moving in the same direction with velocities exceeding 18.5 miles (29.8 kilometers) per second. All others we will either overtake or meet head-on.
But before midnight, when we are on the backside, the only meteoroids we encounter are those with velocities high enough to overtake the Earth.
Therefore, on the average, morning meteors appear brighter and faster than those we see in the evening.
And 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 (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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