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So you thought nothing ever happens on the moon?
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December 23, 2005: NASA scientists have observed
an explosion on the moon. The blast, equal in energy to about
70 kg of TNT, occurred near the edge of Mare Imbrium (the
Sea of Rains) on Nov. 7, 2005, when a 12-centimeter-wide meteoroid
slammed into the ground traveling 27 km/s.
 "What
a surprise," says Marshall Space Flight Center (MSFC)
researcher Rob Suggs, who recorded the impact's flash. He
and colleague Wes Swift were testing a new telescope and video
camera they assembled to monitor the moon for meteor strikes.
On their first night out, "we caught one," says
Suggs.
Right:
The red dot marks the location of the Nov. 7, 2005, meteoroid
impact. Credit: NASA/MSFC/Bill Cooke. [Larger
image]
The
object that hit the moon was "probably a Taurid,"
says MSFC meteor expert Bill Cooke. In other words, it was
part of the same meteor shower that peppered Earth with fireballs
in late October and early November 2005. (See "Fireball
Sightings" from Science@NASA.)
The
moon was peppered, too, but unlike Earth, the moon has no
atmosphere to intercept meteoroids and turn them into harmless
streaks of light. On the moon, meteoroids hit the ground--and
explode.
"The
flash we saw," says Suggs, "was about as bright
as a 7th magnitude star." That's two and a half times
dimmer than the faintest star a person can see with their
unaided eye, but it was an easy catch for the group's 10-inch
telescope.
Cooke
estimates that the impact gouged a crater in the moon's surface
"about 3 meters wide and 0.4 meters deep." As moon
craters go, that's small. "Even the Hubble Space Telescope
couldn't see it," notes Cooke. The moon is 384,400 km
away. At that distance, the smallest things Hubble can distinguish
are about 60 meters wide.
This
isn't the first time meteoroids have been seen hitting the
moon. During the Leonid meteor storms of 1999 and 2001, amateur
and professional astronomers witnessed at least half-a-dozen
flashes ranging in brightness from 7th to 3rd magnitude. Many
of the explosions were photographed simultaneously by widely
separated observers.
Above:
The Nov. 7th lunar Taurid explosion, shown as a sequence
of 6 false-color video frames. Credit: Wes Swift/NASA.
Since
the Leonids of 2001, astronomers have not spent much time
hunting for lunar meteors. "It's gone out of fashion,"
says Suggs. But with NASA planning to return to the moon by
2018, he says, it's time to start watching again.
There
are many questions that need answering: "How often do
big meteoroids strike the moon? Does this happen only during
meteor showers like the Leonids and Taurids? Or can we expect
strikes throughout the year from 'sporadic meteors?'"
asks Suggs. Explorers on the moon are going to want to know.
"The
chance of an astronaut being directly hit by a big meteoroid
is miniscule," says Cooke. Although, he allows, the odds
are not well known "because we haven't done enough observing
to gather the data we need to calculate the odds." Furthermore,
while the danger of a direct hit is almost nil for an individual
astronaut, it might add up to something appreciable for an
entire lunar outpost.
 Of
greater concern, believes Suggs, is the spray—"the secondary
meteoroids produced by the blast." No one knows how far
the spray reaches and exactly what form it takes.
Right:
An artist's concept of the Nov. 7, 2005, explosion. Credit:
NASA/MSFC. [Larger
image]
Also,
ground-shaking impacts could kick up moondust, possibly over
a wide area. Moondust is electrostatically charged and notoriously
clingy. (See "Mesmerized
by Moondust" from Science@NASA.) Even a small amount
of moondust can be a great nuisance: it gets into spacesuit
joints and seals, clings to faceplates, and even makes the
air smell when it is tramped indoors by moonwalkers. Could
meteoroid impacts be a source of lunar "dust storms?"
Another question for the future....
Suggs
and his team plan to make more observations. "We're contemplating
a long-term monitoring program active not only during major
meteor showers, but also at times in between. We need to develop
software to find these flashes automatically," he continues.
"Staring at 4 hours of tape to find a split-second flash
can get boring; this is a job for a computer."
With
improvements, their system might catch lots of lunar meteors.
Says Suggs, "I'm ready for more surprises."
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Author: Dr. Tony
Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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far as they know, Suggs and Swift were the only ones
who recorded the impact of Nov. 7th—"probably because
we were the only ones looking," says Suggs. So,
unlike the lunar Leonids of 1999 and 2001, the lunar
Taurid of 2005 was not confirmed by a second or third
observer.
Nevertheless,
"we are 99% sure it was real," says Suggs.
Other
possibilities include
- a
satellite passing in front of the moon, glinting in
sunlight;
- a
cosmic ray hitting the video camera's CCD chip;
- a
meteor in Earth's atmosphere, directly between Earth
and the Moon.
"We
don't believe it was a satellite," says Cooke who,
together with aerospace engineer Heather McNamara, searched
through NORAD's catalogue of 8363 "trackable objects"
in Earth orbit. "There was no unclassified satellite
or piece of space debris in the right place at the right
time to cause the flash."
It
couldn't have been a cosmic ray. "We observed the
lunar explosion in five consecutive video frames (total
time span: 150 msec). A cosmic ray would have caused
a flash in only one frame," explains Suggs.
Right:
The light curve of the flash observed by Suggs and Swift
on Nov. 7, 2005. Credit: NASA/MSFC
And
finally, it almost certainly couldn't have been a meteor
in Earth's atmosphere. "To masquerade as a lunar
impact, a meteor in Earth's atmosphere would have to
be heading directly toward our observing site at the
Marshall Space Flight Center, head on, so that it looked
like a point rather than a streak of light," says
Suggs. "A meteoroid hitting the moon is more plausible.
Furthermore," he says, "the light curve of
our Nov. 7th Taurid has the same shape as light curves
of lunar Leonids observed in 1999 and 2001. Also, it
doesn't match the light curve of a 'point meteor.'"
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