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2008-05-22 10:49:25
Tony Phillips
Science@NASA
SPACE.comWed May 21, 11:02 AM ET
Not so long ago, anyone claiming to see flashes of light on the Moon would be
viewed with deep suspicion by professional astronomers. Such reports were filed
under "L" ... for lunatic.
Not anymore. Over the past two and a half years, NASA astronomers have observed
the Moon flashing at them not just once but one hundred times.
"They're explosions caused by meteoroids hitting the Moon," explains Bill
Cooke, head of NASA's Meteoroid Environment Office at the Marshall Space Flight
Center (MSFC). "A typical blast is about as powerful as a few hundred pounds of
TNT and can be photographed easily using a backyard telescope."
The impactor was a tiny fragment of extinct comet 2003 EH1. Every year in early
January, the Earth-Moon system passes through a stream of debris from that
comet, producing the well-known Quadrantid meteor shower. Here on Earth,
Quadrantids disintegrate as flashes of light in the atmosphere; on the airless
Moon they hit the ground and explode.
"We started our monitoring program in late 2005 after NASA announced plans to
return astronauts to the Moon," says team leader Rob Suggs of the MSFC. If
people were going to be walking around up there, "it seemed like a good idea to
measure how often the Moon was getting hit."
"Almost immediately, we detected a flash."
That first detection "I'll never forget it," he says came on Nov. 7, 2005,
when a piece of Comet Encke about the size of a baseball hit Mare Imbrium. The
resulting explosion produced a 7th magnitude flash, too dim for the naked eye
but an easy target for the team's 10-inch telescope.
A common question, says Cooke, is "how can something explode on the Moon?
There's no oxygen up there."
These explosions don't require oxygen or combustion. Meteoroids hit the moon
with tremendous kinetic energy, traveling 30,000 mph or faster. "At that speed,
even a pebble can blast a crater several feet wide. The impact heats up rocks
and soil on the lunar surface hot enough to glow like molten lava hence the
flash."
During meteor showers such as the Quadrantids or Perseids, when the Moon passes
through dense streams of cometary debris, the rate of lunar flashes can go as
high as one per hour. Impacts subside when the Moon exits the stream, but
curiously the rate never goes to zero.
"Even when no meteor shower is active, we still see flashes," says Cooke.
These "off-shower" impacts come from a vast swarm of natural space junk
littering the inner solar system. Bits of stray comet dust and chips off old
asteroids pepper the Moon in small but ultimately significant numbers. Earth
gets hit, too, which is why on any given night you can stand under a dark sky
and see a few meteors per hour glide overhead no meteor shower required. Over
the course of a year, these random or "sporadic" impacts outnumber impacts from
organized meteor showers by a ratio of approximately 2:1.
"That's an important finding," says Suggs. "It means there's no time of year
when the Moon is impact-free."
Fortunately, says Cooke, astronauts are in little danger. "The odds of a direct
hit are negligible. If, however, we start building big lunar outposts with lots
of surface area, we'll have to carefully consider these statistics and bear in
mind the odds of a structure getting hit."
Secondary impacts are the greater concern. When meteoroids strike the Moon,
debris goes flying in all directions. A single meteoroid produces a spray
consisting of thousands of "secondary" particles all traveling at bullet-like
velocities. This could be a problem because, while the odds of a direct hit are
low, the odds of a secondary hit may be significantly greater. "Secondary
particles smaller than a millimeter could pierce a spacesuit," notes Cooke.
At present, no one knows how far and wide secondary particles travel. To get a
handle on the problem, Cooke, Suggs and colleagues are shooting artificial
meteoroids at simulated moon dust and measuring the spray. This work is being
done at the Vertical Gun Range at NASA's Ames Research Center in Mountain View,
CA.
Meanwhile, back at the observatory, the team has upgraded their original
10-inch (25 cm) telescope to a pair of telescopes, one 14-inch (36 cm) and one
20-inch (51 cm), located at the Marshall Space Flight Center in Alabama.
They've also established a new observing site in Georgia with a 14-inch
telescope. Multiple telescopes allow double- and triple-checking of faint
flashes and improve the statistical underpinnings of the survey.
"The Moon is still flashing," says Suggs. Indeed, during the writing of this
story, three more impacts were detected, bringing the total to 103.
Potential Danger: Moon Hit By More Space Rocks Than Thought Video Player: Moon
2.0: Join the Revolution Video Player: Moon meets Rover Original Story: 100
Explosions Recorded on the Moon