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January
23, 2007: If you stare at the Moon long enough, you
start seeing things. "82 things to be exact," says
Bill Cooke, leader of NASA's Meteoroid Environment Group.
That's how many "transient phenomena" the group
has video-taped since they started monitoring the night side
of the Moon in Nov. 2005.
 "In
107 hours of observing, we've tallied 20 lunar meteors + at
least 60 Earth-orbiting satellites + one airplane + one terrestrial
meteor = 82 in all."
Right:
The NASA Meteoroid Environment Group's lunar observatory at
the Marshall Space Flight Center in Huntsville, Alabama. Inset
is one of two 14-inch telescopes simultaneously trained on
the Moon during the group's observing sessions. [larger
image]
This
is the first systematic count of lunar night-side phenomena.
"It gives astronomers an idea of what to expect when
they undertake a lunar monitoring program from Earth."
Cooke's
prime target is lunar meteors--flashes of light that occur
when meteoroids hit the Moon's surface: video.
"Of the 20 lunar meteors we've seen so far, about half
come from well-known meteor showers such as the Leonids and
Geminids. The other half are random meteoroids that take us
completely by surprise." NASA is preparing to send astronauts
back to the Moon and the agency is understandably interested
in how often this happens.
"Everything
else we've seen is just a coincidence, something flying in
front of the Moon while we happen to be watching." Leading
this category are Earth-orbiting satellites and pieces of
space debris. This Orbcomm A4 communications satellite is
a typical example:
Above:
An Orbcomm communication satellite passes in front of the
Moon on Nov. 17, 2006: video.
NORAD
tracks more than 10,000 Earth-orbiting objects wider than
10 cm. "Some of them are bound to cross in front of the
Moon while we're watching," he says. Objects like Orbcomm
are easy to identify as satellites. Tumbling space debris,
on the other hand, can be trickier: "A sudden glint of
sunlight from a flat surface looks an awful lot like a lunar
meteor flash," he explains. "So we have to be very
careful. When we see a flash of light on the Moon, we always
double-check that there was no piece of space junk passing
by at that exact moment."
Back
in days of Apollo, astronomers who monitored the Moon didn't
have this problem. "There were very few satellites in
Earth orbit, and a lunar transit was rare," he says.
"But now we see one or two every night."
Here's
a mystery: "Can you identify this
object?" laughs Cooke. "Airplanes are my favorite."
So
far, they have detected only one terrestrial meteor—that is,
a meteoroid disintegrating in Earth's atmosphere: video.
This may seem puzzling. During a typical meteor shower, novice
sky watchers see dozens of shooting stars. Why has NASA counted
only one? "The telescope's field of view is too narrow,"
explains Cooke. The human eye is much better for terrestrial
meteor watching.
He's
more interested in the Moon, anyway. Exploding meteoroids,
tumbling satellites and jet airplanes: "It's a great
show." What's next? "We plan to keep watching, so
stay tuned."
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Author: Dr. Tony
Phillips | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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