[meteorite-list] The 2006 Geminid Meteor Shower

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 12 Dec 2006 17:00:34 -0800 (PST)
Message-ID: <200612130100.RAA15376_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2006/12dec_geminids.htm

The 2006 Geminid Meteor Shower
NASA Science News
December 12, 2006

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

"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."

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.
Received on Tue 12 Dec 2006 08:00:34 PM PST