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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."
Right:
Each red dot denotes a meteoroid impact observed since Nov.
2005 by members of the NASA Meteoroid Environment Office.
[More]
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.
Click to view the movie: gif,
wmv
or mpeg.
Above:
An 8th-magnitude Leonid flash near crater Gauss. The movies
play in 7x slow motion; otherwise the explosion would be nearly
invisible to the human eye. [More]
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.
Right:
The lunar meteoroid impact observatory at the Marshall Space
Flight Center. Inset is one of two 14-inch telescopes used
to observe the Moon. [Larger
image]
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; and to calculate the necessary thickness of shielding
for lunar spacecraft and habitats.
Next
up: The Geminid meteor shower on December 13th-14th. Once
again Earth and Moon will be peppered with meteoroids—this
time from the asteroid 3200 Phaethon. Says Cooke, "we'll
be watching."
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Author: Dr. Tony
Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
More
Information |
Lunar
Impact Monitoring -- home page of the NASA Meteoroid
Environment Office
2006
Lunar Leonid Impact Details |
Flash
#1
Time:
Nov. 17, 2006 10:56:34 UT
Location: 79.2 E, 36.7 N
Peak magnitude: 8
Mass
~ 235g
Diameter ~7.7 cm (3 inches)
Striking power ~ 5.9x108 Joules
(273 lbs TNT)
|
Flash
#2
Time:
Nov. 17, 2006 10:46:27 UT
Location: 76.7E, 42.6N
Peak magnitude: 9.4
Mass ~ 110 g
Diameter ~ 6.0 cm (2.3 inches)
Striking power ~ 2.8x108
Joules (130 lbs TNT)
|
Previous
stories about lunar impacts:
A
Meteoroid Hits the Moon -- (Science@NASA) a sporadic
meteoroid hit Mare Nubium, the lunar Sea of Clouds,
resulting in the best video yet of a lunar explosion.
The
Sky is Falling -- (Science@NASA) NASA researchers
are mining old Apollo seismic data for clues to lunar
meteoroid impacts
An
Explosion on the Moon -- (Science@NASA) A piece
of Comet Encke hit Mare Imbrium and exploded like 70
kg of TNT.
The
Vision for Space Exploration |
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