Asteroid Shower (Geminids Meteor Shower)
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Asteroid Shower
NASA Science News
December 3, 2007
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
meteorsâbut 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."
The Geminids are not ordinary meteors. While most meteor showers come
from comets, Geminids come from an asteroid - 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
<http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=Phaethon&orb=1>: 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
<http://www.cfa.harvard.edu/iau/Ephemerides/Unusual/03200.html>.
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."