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January
3, 2007: Another meteor shower, another bunch of
lunar impacts...
"On
Dec. 14, 2006, we observed at least five Geminid meteors hitting
the Moon," reports Bill Cooke of NASA's Meteoroid Environment
Office in Huntsville, AL. Each impact caused an explosion
ranging in power from 50 to 125 lbs of TNT and a flash of
light as bright as a 7th-to-9th magnitude star.
Above:
Lunar impacts since Nov. 2005. Numbers 14-16 and 19-20 are
Geminids. Number 18 is a probable Geminid. Credit: NASA Meteoroid
Environment Group. [More]
The
explosions occurred while Earth and Moon were passing through
a cloud of debris following near-Earth asteroid 3200 Phaethon.
This happens every year in mid-December and gives rise to
the annual Geminid meteor shower: Streaks of light fly across
the sky as rocky chips of Phaethon hit Earth's atmosphere.
It's a beautiful display.
The
same chips hit the Moon, of course, but on the Moon there
is no atmosphere to intercept them. Instead, they hit the
ground. "We saw about one explosion per hour," says
Cooke.
How
does a meteoroid explode? "This isn't the kind of explosion
we experience on Earth," explains Cooke. The Moon has
no oxygen to support fire or combustion, but in this case
no oxygen is required: Geminid meteoroids hit the ground traveling
35 km/s (78,000 mph). "At that speed, even a pebble can
blast a crater several feet wide," says Cooke. "The
flash of light comes from rocks and soil made so hot by impact
that they suddenly glow."
Cooke's
group has been monitoring the Moon's nightside (the best place
to see flashes of light) since late 2005 and so far they've
recorded 19 hits: five or six Geminids, three Leonids, one
Taurid and a dozen random meteoroids (sporadics). "The
amazing thing is," says Cooke, "we’ve done it using
a pair of ordinary backyard telescopes, 14-inch, and off-the-shelf
CCD cameras. Amateur astronomers could be recording these
explosions, too."
Indeed,
he hopes they will. The NASA team can't observe 24-7. Daylight,
bad weather, equipment malfunctions, vacations—"lots
of things get in the way of maximum observing." Amateur
astronomers could fill in the gaps. A worldwide network of
amateurs, watching the Moon whenever possible, "would
increase the number of explosions we catch," he says.
 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]
To
that end, Cooke plans to release data reduction software developed
specifically for amateur and professional astronomers wishing
to do this type of work. (The release will be announced on
Science@NASA in the near future.) The software runs on an
ordinary PC equipped with a digital video card. "If you
have caught a lunar meteor on tape, this program can find
it. It eliminates the need to stare at hours of black and
white video, looking for split-second flashes."
More
data will help NASA assess the meteoroid threat as the agency
prepares to send astronauts back to the Moon. Ready to assist?
Stay tuned to Science@NASA for further instructions.
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Author: Dr. Tony
Phillips | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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