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Nov.
23, 2007: At Cape Canaveral, not far from the launch
pad where the space shuttle lifts off, there's a ragged hole
in a chain link fence. Its message: Watch out for flying
boulders.
"The
powerful exhaust of the shuttle's solid rocket boosters blasts
concrete out of the flame trench below the engines,"
explains physicist Phil Metzger of the Kennedy Space Center
(KSC). "On some launches, boulders of concrete up to
a half meter wide are blown out as far as a half a kilometer
away, traveling fast enough to shatter concrete light poles
and punch through chain link fences."
Right:
Metzger examines a fence in boulder-range of the shuttle's
launch pad. [Larger
image]
This
is no problem as long as people and equipment are kept at
a safe distance, easily done. But, Metzger wonders, what if
all this was happening on the Moon?
NASA
is returning to the Moon in the next decade with plans to
establish a durable outpost. There will be habitats, rovers,
supply depots and mining equipment. Ships will be coming and
going, landing and blasting off--and kicking up debris that
might fly a lot farther than boulders at Cape Canaveral. Metzger
is researching this problem as part of his work at KSC's Granular
Mechanics and Surface Systems Lab.
"Boulders
are no concern," he says. Lunar spacecraft will be far
less massive than the space shuttle and they won't need such
a powerful kick to escape lunar gravity. Movies made by six
Apollo spacecraft of their landings and takeoffs showed nothing
larger than gravel being rolled away by exhaust gases from
the landing rockets.
Instead,
Metzger is sweating the really small stuff--"moondust."
Here
on Earth, no one pays much heed to dust or sand blasted out
by a rocket launch because "atmospheric drag rapidly
slows the lightweight particles so they fall harmlessly to
the ground a few meters from the blast," he explains.
But
on the Moon? "There is no atmosphere to slow tiny particles."
Small grit can travel enormous distances at high speeds, scouring
everything in its path.
Above: Artist's concept of a lunar lander.
[More]
This
isn't just theory. In November 1969, the Apollo 12 Lunar Module
(LM, pronounced "lem") landed about 200 meters from
Surveyor 3, a robotic probe that had landed on the Moon in
April 1967. The Apollo 12 astronauts walked over to Surveyor
3 to photograph it and to retrieve some pieces for return
to Earth. Right away, they noticed that most of Surveyor 3,
which at launch was pristine white, had darkened to brown--a
result of two-and-a-half years' exposure to extreme lunar
conditions.
But
the side of Surveyor 3 facing the LM had been sandblasted
back to white. In fact, "every bolt, cable, or bracket
blocking the spray of fine grit from Apollo 12 left permanent
shadows etched onto Surveyor," Metzger says. From examining
the returned artifacts, scientists later calculated the sandblasting
resulted primarily from finest dust particles only 1 to 10
micrometers (0.00004 to 0.0004 inch) across.
The
scoured surfaces were also pocked with hundreds of microscopic
impact craters ranging from 30 to 60 micrometers (0.001 to
0.002 inch) across caused by particles of about the same size
traveling at high speeds. Moreover, fine grit had been driven
into tiny cracks and crevices, including inside Surveyor's
camera.
This
evidence concerns Metzger because in a future lunar outpost,
high-speed fine grit could scour the reflective coating off
thermal control blankets, roughen the surfaces of windows
and other optics, compromise the surfaces of solar panels,
and penetrate connectors or other mechanisms on digging machines
or spacesuits, causing friction and even mechanical failure.
Right:
The touchdown of Apollo 12 near Surveyor 3 provides key data
on the sandblasting effects of lunar landers. [Larger
image]
Well,
why not just land far enough away that speeding sand and dust
ceases to be a problem?
Answer:
You can run, but you can't hide. Dust particles accelerated
by a rocket's exhaust could theoretically travel all the way
around the Moon!
Metzger's
team has analyzed how the impact craters formed on Surveyor
3 and finds that the particles must have been traveling at
least 400 to 1,000 meters per second. "In fact, they
may have been traveling as fast as the exhaust gases of the
lunar lander—that is, at 1 or 2 kilometers per second."
Particles
speeding horizontally at 1.7 kilometers per second will travel
literally halfway around the Moon. Boost that speed to 2 kilometers
per second, and the projectiles can completely circle the
Moon. If no mountains got in the way, grit accelerated by
a rocket landing could zip entirely around the Moon "and
land back at the rocket's feet," says Metzger.
Now,
Metzger is helping other teams of NASA engineers figure out
how to mitigate the effects of lunar landings and takeoffs.
One strategy might be to locate spaceports in places where
mountains and hills serve as natural dust blockers. Artificial
berms or other ingenious structures might offer a solution,
too.
Says
Metzger, "we're working on it." Stay tuned for updates
from Science@NASA.
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Authors: Trudy E. Bell, Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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