The Geminid Meteor Shower

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The 2006 Geminid Meteor Shower

Dec. 12 , 2006: he best meteor shower of the year peaks this week on Dec. 13th and 14th.

see caption"It's the Geminid meteor shower," says Bill Cooke of NASA's Meteoroid Environment Office in Huntsville, Alabama. "Start watching on Wednesday evening, Dec. 13th, around 9 p.m. local time," he advises. "The display will start small but grow in intensity as the night wears on. By Thursday morning, Dec. 14th, people in dark, rural areas could see one or two meteors every minute."


Above: Geminid meteors photographed in Dec. 2004 by
Jason A.C. Brock of Roundtimber, Texas. [More]

The source of the Geminids is a mysterious object named 3200 Phaethon. "No one can decide what it is," says Cooke.
The mystery, properly told, begins in the 19th century: Before the mid-1800s there were no Geminids, or at least not enough to attract attention. The first Geminids appeared suddenly in 1862, surprising onlookers who saw dozens of meteors shoot out of the constellation Gemini. (That's how the shower gets its name, the Geminids.)

Astronomers immediately began looking for a comet. Meteor showers result from debris that boils off a comet when it passes close to the Sun. When Earth passes through the debris, we see a meteor shower.

For more than a hundred years astronomers searched in vain for the parent comet. Finally, in 1983, NASA's Infra-Red Astronomy Satellite (IRAS) spotted something. It was several kilometers wide and moved in about the same orbit as the Geminid meteoroids. Scientists named it 3200 Phaethon.

Just one problem: Meteor showers are supposed to come from comets, but 3200 Phaethon seems to be an asteroid. It is rocky (not icy, like a comet) and has no obvious tail. Officially, 3200 Phaethon is catalogued as a "PHA"—a potentially hazardous asteroid whose path misses Earth's orbit by only 2 million miles.

If 3200 Phaethon is truly an asteroid, with no tail, how did it produce the Geminids? "Maybe it bumped up against another asteroid," offers Cooke. "A collision could have created a cloud of dust and rock that follows Phaethon around in its orbit."

This jibes with studies of Geminid fireballs. Some astronomers have studied the brightest Geminid meteors and concluded that the underlying debris must be rocky. Density estimates range from 1 to 3 g/cm³. That's much denser than flakes of comet dust (0.3 g/cm³), but close to the density of rock (3 g/cm3).

So, are the Geminids an "asteroid shower"?

Cooke isn't convinced. 3200 Phaethon might be a comet after all--"an extinct comet," he says. The object's orbit carries it even closer to the Sun than Mercury. Extreme solar heat could've boiled away all of Phaethon's ice long ago, leaving behind this rocky skeleton "that merely looks like an asteroid."

In short, no one knows. It's a mystery to savor under the stars—the shooting stars—this Thursday morning.

A note about time: All times in this story are local to the reader. So "Thursday morning" means Thursday morning wherever you happen to live. --the Editor

Author: Dr. Tony Phillips | Editor: Dr. Tony Phillips | Credit: Science@NASA

More to the story...

Geminid sky map -- from Spaceweather.com

3200 Phaethon -- 3D orbit

History of the Geminids

Geminid Photo Galleries: 2004, 2002, 2001

The Vision for Space Exploration

Source: Science@NASA

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I saw a couple nices ones this morning on the way to School...

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Asteroid Shower

Dec. 03, 2007: Mark your calendar: The best meteor shower of 2007 peaks on Friday, December 14th.

see caption"It's the Geminid meteor shower," says NASA astronomer Bill Cooke of the Marshall Space Flight Center. "Start watching on Thursday evening, Dec. 13th, around 10 pm local time," he advises. "At first you might not see very many meteors—but be patient. The show really heats up after midnight and by dawn on Friday, Dec. 14th, there could be dozens of bright meteors per hour streaking across the sky."

Above: A Geminid meteor in 2006 photographed by Christopher Colley of Lombard, Illinois. [Larger image]

The Geminids are not ordinary meteors. While most meteor showers come from comets, Geminids come from an asteroid—a near-Earth object named 3200 Phaethon.

"It's very strange," says Cooke. How does an asteroid make a meteor shower?

Comets do it by evaporating. When a comet flies close to the sun, intense heat vaporizes the comet’s "dirty ice" resulting in high-speed jets of comet dust that spew into interplanetary space. When a speck of this comet dust hits Earth's atmosphere traveling ~100,000 mph, it disintegrates in a bright flash of light—a meteor!

Asteroids, on the other hand, don't normally spew dust into space—and therein lies the mystery. Where did Phaethon's meteoroids come from?

One possibility is a collision. Maybe it bumped against another asteroid. A collision could have created a cloud of dust and rock that follows Phaethon around in its orbit. Such collisions, however, are not very likely.

Cooke favors another possibility: "I think 3200 Phaethon used to be a comet."

Exhibit #1 in favor of this idea is Phaethon's orbit: it is highly elliptical, like the orbit of a typical comet, and brings Phaethon extremely close to the sun, twice as close as Mercury itself. Every 1.4 years, Phaethon swoops through the inner solar system where repeated blasts of solar heat could easily reduce a flamboyant comet to the rocky skeleton we see today.

If this scenario is correct, Phaethon-the-comet may have produced many rich streams of dust that spent hundreds or thousands of years drifting toward Earth until the first Geminid meteors appeared during the US Civil War. Since then, Geminids have been a regular shower peaking every year in mid-December.

3200 Phaethon is now catalogued as a "PHA"—a potentially hazardous asteroid whose path misses Earth's orbit by only 2 million miles. It measures 5 km wide, about half the size of the asteroid or comet that wiped out the dinosaurs 65 million years ago, and can be seen through backyard telescopes—in fact, now is a good time to look:

"3200 Phaethon is flying past Earth just a few days before this year’s Geminid meteor shower," notes Cooke. On Dec. 10th, Phaethon will be about 11 million miles away shining like a 14th magnitude star in the constellation Virgo: ephemeris. That's too dim for the naked eye, he says, but a good target for amateur telescopes equipped with CCD cameras.

Cooke doesn't expect the flyby to boost the Geminids—"11 million miles is too distant to affect meteor rates"—but the Geminids don't really need boosting. "It's always a great shower," he says. "Don't miss it."

Author: Dr. Tony Phillips | Editor: Dr. Tony Phillips | Credit: Science@NASA

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Web Links

NASA Meteoroid Environment Office -- home page

NASA's Future: The Vision for Space Exploration

Source: Science@NASA