[meteorite-list] Lunar Leonid Strikes

[Ron Baalke]

http://science.nasa.gov/headlines/y2006/01dec_lunarleonid.htm

Lunar Leonid Strikes
NASA Science News
December 1, 2006

Dec. 1, 2006: Meteoroids are smashing into the Moon a lot more often
than anyone expected.

That's the tentative conclusion of Bill Cooke, head of NASA's Meteoroid 
Environment Office, after his team observed two Leonids hitting the Moon 
on Nov. 17, 2006. "We've now seen 11 and possibly 12 lunar impacts since 
we started monitoring the Moon one year ago," says Cooke. "That's about 
four times more hits than our computer models predicted."

If correct, this conclusion could influence planning for future moon
missions. But first, the Leonids:

Last month, Earth passed through a "minefield" of debris from Comet
55P/Tempel-Tuttle. This happens every year in mid-November and results
in the annual Leonid meteor shower. From Nov. 17th to Nov. 19th both
Earth and the Moon were peppered with meteoroids.

Meteoroids that hit Earth disintegrate harmlessly (and beautifully) in
the atmosphere. But the Moon has no atmosphere to protect it, so
meteoroids don't stop in the sky. They hit the ground. The vast majority
of these meteoroids are dust-sized, and their impacts are hardly felt.
But bigger debris can gouge a crater in the lunar surface and explode in
a flash of heat and light. Some flashes can be seen from Earth.

During the passage through Tempel-Tuttle's debris field, Cooke's team
trained their telescopes (two 14-inch reflectors located at the Marshall
Space Flight Center) on the dark surface of the Moon. On Nov. 17th,
after less than four hours of watching, they video-recorded two impacts:
a 9th magnitude flash in Oceanus Procellarum (the Ocean of Storms) and a
brighter 8th magnitude flash in the lunar highlands near crater Gauss.

"The flashes we saw were caused by Leonid meteoroids 2 to 3 inches (5 to
8 cm) in diameter," says Cooke. "They hit with energies between 0.3 and
0.6 Giga-Joules." In plain language, that's 150 to 300 pounds of TNT.

How do you get so much energy out of a 3-inch meteoroid? "Leonids travel
fast - about 144,000 mph," he explains. "At that speed, even a 3-inch rock
packs tremendous energy."

For comparison, the ESA's SMART-1 probe crashed into the Moon on Sept.
2nd, delivering 0.6 Giga-Joules of energy to the lunar surface - the same
as the brighter of the two Leonids.

"Leonid impacts are as energetic as the crash of a 700-lb spacecraft!"
says Cooke.

With these latest detections, Cooke's group has tallied a dozen "lunar
meteors" since Nov. 2005. Most were sporadic meteoroids--meaning they
are part of no annual shower like the Leonids, but just random chips of
asteroids and comets floating around in space. Cooke estimates that for
every four hours they observe the Moon, they see one bright flash caused
by the impact of a large meteoroid.

And that's surprising.  "Our best models of the lunar meteoroid 
environment predict a much lower rate - only 25% of what we are actually 
seeing." The problem may be with the computer models: "They're based on 
observations of meteors in the skies of Earth," and those data may not 
translate well to the Moon.

The solution? "We need to spend more time watching the Moon," says
Cooke. "With more data, we can draw stronger conclusions about the
impact rate."

NASA needs that kind of information to decide, e.g., if it's safe for
astronauts to go moonwalking during a meteor shower; to calculate the
necessary thickness of shielding for lunar spacecraft; and to answer the
question, how often will a moonbase be punctured by a Leonid?

Next up: The Geminid meteor shower on December 13th-14th. Once again
Earth and Moon will be peppered with meteoroidsi - this time from the
asteroid 3200 Phaethon. Says Cooke, "we'll be watching."