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Dec.
03, 2007: Mark your calendar: The best meteor shower
of 2007 peaks on Friday, December 14th.
 "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."
Right:
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."
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Author: Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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