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MSNBC / Sky & Telescope

What causes meteor showers?
How comets cause those shooting stars
By Joe Rao
Skywatching Columnist
updated 10:46 a.m. PT, Wed., Dec. 12, 2007

What could be the best meteor display of the year is reaching its peak on Thursday night and Friday.

Here is what astronomers David Levy and Stephen Edberg have written of the annual Geminid meteor shower: "If you have not seen a mighty Geminid fireball arcing gracefully across an expanse of sky, then you have not seen a meteor."

The Geminids get their name from the constellation of Gemini the Twins, because the meteors appear to emanate from a spot in the sky near the bright star Castor in Gemini

Also in Gemini this month is the planet Mars, nearing a close approach to Earth later this month and shining brilliantly with yellow-orange hue. Mars is certain to attract the attention of prospective Geminid watchers this week.

The Geminid meteors are usually the most satisfying of all the annual showers, even surpassing the famous Perseids of August.

Studies of past find the "Gems" have a reputation for being rich both in slow, bright, graceful meteors and fireballs as well as faint meteors, with relatively fewer objects of medium brightness.

They are of medium speed, encountering Earth at 22 miles per second (35 kilometers per second). They are bright and white, but unlike the Perseids, they leave few visible trails or streaks. They are four times denser than most other meteors, and have been observed to form jagged or divided paths.

Geminids also stand apart from the other meteor showers in that they seem to have been spawned not by a comet, but by 3200 Phaethon, an Earth-crossing asteroid. Then again, the Geminids may be comet debris after all, for some astronomers consider Phaethon to really be the dead nucleus of a burned-out comet that somehow got trapped into an unusually tight orbit.

On Monday, Phaethon passed about 11 million miles (18 million kilometers) from Earth, its closest approach since its discovery in 1983.

The prospects for this year
The Geminids perform excellently in any year, but British meteor astronomer, Alastair McBeath, has categorized 2007 as a "great year."

Last year's display was hindered somewhat by the moon, two days past last quarter phase. But this week, the moon is just past its new phase. On the peak night, the moon will be a fat crescent, in the south-southwest at dusk and setting soon after 8 p.m. local time. That means that the sky will be dark and moonless for the balance of the night, making for perfect viewing conditions for the shower.

According to McBeath, the Geminids are predicted to reach peak activity at 16:45 GMT (11:45 a.m. ET) Friday. That means those places from central Asia eastward across the Pacific Ocean to Alaska are in the best position to catch the very crest of the shower, when the rates conceivably could exceed 120 per hour.

"But," he adds, "maximum rates persist at only marginally reduced levels for some six to 10 hours around the biggest ones, so other places (such as North America) should enjoy some fine Geminid activity as well."

Indeed, under normal conditions on the night of maximum activity, with ideal dark-sky conditions, at least 60 to 120 Geminid meteors can be expected to burst across the sky every hour on the average. (Light pollution greatly cuts the numbers.)

Earth moves quickly through this meteor stream producing a somewhat broad, lopsided activity profile. Rates increase steadily for two or three days before maximum, reaching roughly above a quarter of its peak strength, then drop off more sharply afterward. Late Geminids, however, tend to be especially bright. Renegade forerunners and late stragglers might be seen for a week or more before and after maximum.

What to do
Generally speaking, depending on your location, Gemini begins to come up above the east-northeast horizon right around the time evening twilight is coming to an end. So you might catch sight of a few early Geminids as soon as the sky gets dark.

There is a fair chance of perhaps catching sight of some "Earth-grazing" meteors. Earth grazers are long, bright shooting stars that streak overhead from a point near to even just below the horizon. Such meteors are so distinctive because they follow long paths nearly parallel to our atmosphere.

The Geminids begin to appear noticeably more numerous in the hours after 10 p.m. local time, because the shower's radiant is already fairly high in the eastern sky by then. The best views, however, come around 2 a.m. local time, when their radiant point will be passing very nearly overhead.

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