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August
27, 2007: Most people appreciate lunar eclipses for
their silent midnight beauty. NASA astronomer Bill Cooke is
different: he loves the explosions.
On Tuesday morning, Aug. 28th, Earth's shadow will settle
across the Moon for a 90-minute total eclipse: full
story. In the midst of the lunar darkness, Cooke hopes
to record some flashes of light--explosions caused by meteoroids
crashing into the Moon and blasting themselves to smithereens.
 Right:
A lunar meteoroid strike, artist's concept.
"The
eclipse is a great time to look," says Cooke, who heads
up NASA's Meteoroid Environment Office (MEO) at the Marshall
Space Flight Center. The entire face of the Moon will be in
shadow for more than two hours, offering more than 7 million
sq. miles of dark terrain as target for incoming meteoroids.
Lunar explosions are nothing new. Cooke's team has been monitoring
the Moon since late 2005 and they've recorded 62 impacts so
far. "Meteoroids that hit Earth disintegrate in the atmosphere,
producing a harmless streak of light. But the Moon has no
atmosphere, so 'lunar meteors' plunge into the ground,"
he says. Typical strikes release as much energy as 100 kg
of TNT, gouging craters several meters wide and producing
bursts of light bright enough to be seen 240,000 miles away
on Earth through ordinary backyard telescopes.
"About
half of the impacts we see come from regular meteor showers
like the Perseids and Leonids," says MEO team-member
Danielle Moser. "The other half are 'sporadic' meteors
associated with no particular asteroid or comet."
Below:
A Moon-map of lunar meteoroid impacts observed by the MEO
group since Dec. 2005. [Larger
image] [More]
The
MEO observatory is located on the grounds of the Marshall
Space Flight Center in Huntsville, Alabama, and consists of
two 14-inch telescopes equipped with sensitive low-light video
cameras. Moser and colleague Victoria Coffey will be on duty
Tuesday morning.
During
the eclipse, they hope to catch an elusive variety of meteor
called Helions.
"Helion
meteoroids come from the direction of the sun," Cooke
says, "and that makes them very difficult to observe."
They streak across the sky most often around local noon when
the sun's glare is too intense for meteor watching.
Wait
a minute. Meteors from the sun? "The sun itself is not
the source," he explains. "We believe Helion meteoroids
come from ancient sungrazing comets that laid down trails
of dusty debris in the vicinity of the sun."
 No
one can be certain, however, because Helion meteoroids are
so devilishly difficult to study. Astronomers see them only
in small numbers briefly before dawn or after sunset. Attempts
to study Helions via radar during the day have been foiled,
to a degree, by terrestrial radio interference and natural
radio bursts from the sun—both of which can drown out meteoroid
"pings."
Right:
The MEO observatory at the Marshall Space Flight Center. [Larger
image]
Enter
the eclipse.
During
the eclipse, the Man in the Moon (the face we see from Earth)
will be turned squarely toward the sun—"perfect geometry
for intercepting Helion meteoroids," says Moser. "And
with Earth's shadow providing some darkness, we should be
able to see any explosions quite clearly."
"Watching
Helion meteoroids hit the Moon and studying the flashes will
tell us more about their size, velocity and penetration,"
she adds. That, in turn, will further the MEO's goal of estimating
meteoroid hazards to spacecraft and future Moon-walking astronauts.
No
one has ever seen a lunar impact during an eclipse, "but
there's a first time for everything," Cooke says. Stay
tuned to Science@NASA for results.
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Author: Dr.
Tony Phillips | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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