[meteorite-list] Geminid Meteor Shower Defies Explanation (Asteroid 3200 Phaethon)

[Ron Baalke]

http://science.nasa.gov/science-news/science-at-nasa/2010/06dec_geminids/      

Geminid Meteor Shower Defies Explanation
NASA Science News

Dec. 6, 2010:  The Geminid meteor shower, which peaks this year on
Dec. 13th and 14th, is the most intense meteor shower of the year. It
lasts for days, is rich in fireballs, and can be seen from almost any
point on Earth.

It's also NASA astronomer Bill Cooke's favorite meteor shower - but not
for any of the reasons listed above.

"The Geminids are my favorite," he explains, "because they defy
explanation."

Most meteor showers come from comets, which spew ample meteoroids for a
night of 'shooting stars.' The Geminids are different. The parent is not
a comet but a weird rocky object named 3200 Phaethon that sheds very
little dusty debris - not nearly enough to explain the Geminids.

"Of all the debris streams Earth passes through every year, the
Geminids' is by far the most massive," says Cooke. "When we add up the
amount of dust in the Geminid stream, it outweighs other streams by
factors of 5 to 500."

This makes the Geminids the 900-lb gorilla of meteor showers. Yet 3200
Phaethon is more of a 98-lb weakling.

3200 Phaethon was discovered in 1983 by NASA's IRAS satellite and
promptly classified as an asteroid. What else could it be? It did not
have a tail; its orbit intersected the main asteroid belt; and its
colors strongly resembled that of other asteroids. Indeed, 3200 Phaethon
resembles main belt asteroid Pallas so much, it might be a 5-kilometer
chip off that 544 km block.

"If 3200 Phaethon broke apart from asteroid Pallas, as some researchers
believe, then Geminid meteoroids might be debris from the breakup,"
speculates Cooke. "But that doesn't agree with other things we know."

Researchers have looked carefully at the orbits of Geminid meteoroids
and concluded that they were ejected from 3200 Phaethon when Phaethon
was close to the sun - not  when it was out in the asteroid belt breaking
up with Pallas. The eccentric orbit of 3200 Phaethon brings it well
inside the orbit of Mercury every 1.4 years. The rocky body thus
receives a regular blast of solar heating that might boil jets of dust
into the Geminid stream.

Could this be the answer?

To test the hypothesis, researchers turned to NASA's twin STEREO
spacecraft, which are designed to study solar activity. Coronagraphs
onboard STEREO can detect sungrazing asteroids and comets, and in June
2009 they detected 3200 Phaethon only 15 solar diameters from the sun's
surface.

What happened next surprised UCLA planetary scientists David Jewitt and
Jing Li, who analyzed the data. "3200 Phaethon unexpectedly brightened
by a factor of two," they wrote. "The most likely explanation is that
Phaethon ejected dust, perhaps in response to a break-down of surface
rocks (through thermal fracture and decomposition cracking of hydrated
minerals) in the intense heat of the Sun."

Jewett and Li's "rock comet" hypothesis is compelling, but they point
out a problem: The amount of dust 3200 Phaethon ejected during its 2009
sun-encounter added a mere 0.01% to the mass of the Geminid debris
strea - not nearly enough to keep the stream replenished over time.
Perhaps the rock comet was more active in the past...?

"We just don't know," says Cooke. "Every new thing we learn about the
Geminids seems to deepen the mystery."

This month Earth will pass through the Geminid debris stream, producing
as many as 120 meteors per hour over dark-sky sites. The best time to
look is probably between local midnight and sunrise on Tuesday, Dec.
14th, when the Moon is low and the constellation Gemini is high
overhead, spitting bright Geminids across a sparkling starry sky.

Bundle up, go outside, and savor the mystery.


Author: Dr. Tony Phillips
Credit: Science at NASA