Meteor Shower Defies Explanation
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
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.
Credit: An artist's concept
of an impact event on Pallas. Credit: B. E. Schmidt and S. C. Radcliffe
"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
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?
The path of 3200 Phaethon through
STEREO's HI-1A coronagraph camera. False-color green and blue streamers
come from the sun.
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
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 stream—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.
Tony Phillips | Credit: Science@NASA