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March
14, 2007: NASA scientist Bill Cooke is shooting marbles
and he's playing "keepsies." The prize won't be
another player's marbles, but knowledge that will help keep
astronauts safe when America returns to the Moon in the next
decade.
Cooke
is firing quarter-inch diameter clear shooters – Pyrex glass,
to be exact – at soil rather than at other marbles. And he
has to use a new one on each round because every 16,000 mph
(7 km/s) shot destroys his shooter.
Right:
Death of a shooter. This is a real photo of a pyrex marble
exploding on impact at the NASA Ames Vertical Gun Range. Photo
credit: Peter Schultz, Brown University, and NASA
"We
are simulating meteoroid impacts with the lunar surface,"
he explains. Cooke and others in the Space Environments Group
at NASA's Marshall Space Flight Center have recorded the
real thing many times. Their telescopes routinely detect
explosions on the Moon when meteoroids slam into the lunar
surface.
A
typical flash involves "a meteoroid the size of a softball
hitting the Moon at 27 km/s and exploding with as much energy
as 70 kg of TNT."
"Mind
you," says Cooke, "these are estimates based on a
flash of light seen 400,000 km away. There's a lot of uncertainty
in our calculations of speed, mass and energy. We'd like to
firm up these numbers."
That's
where the marbles come in....
Cooke
is using the Ames Vertical Gun Range at NASA's Ames Research
Center in Mountain View, CA, to shoot marbles into simulated
lunar soil. The firings allow him to calibrate what he sees
on the Moon. His work is funded by NASA's Office of Safety
and Mission Assurance.
"We
measure the flash so we can figure out how much of the kinetic
energy goes into light," he explained. "Once we
know this luminous efficiency, as we call it, we can apply
it to real meteoroids when they strike the Moon." High-speed
cameras and a photometer (light meter) record the results.
The
Ames Vertical Gun Range was built in the 1960s to support
Project Apollo, America's first human missions to the lunar
surface. The
Ames gun can fire a variety of shapes and materials, even
clusters of particles, at speeds from 0.5 to 7 km/s. The target
chamber usually is pumped down to a vacuum, and can be partially
refilled to simulate atmospheres on other worlds or comets.
Above:
A 30cm-diameter crater plus spattered dust are all that's
left after a test shot in the Ames Vertical Gun. Photo credit:
NASA. [Larger image]
Equally
important, the gun's barrel can be tilted to simulate impacts
at seven different angles from vertical to horizontal since
meteors rarely fly straight into the ground. Black powder
propels the marble, and special valves capture the exhaust
gases so they don't blow away the impact crater.
Cooke's
experiments are being run in two rounds. The first set of
12 shots in October 2006 fired Pyrex glass balls into dust
made from pumice, a volcanic rock, at up to 7 km/s. Follow-up
experiments will use JSC-1a lunar simulant, one of the "true
fakes" developed from terrestrial ingredients to
mimic the qualities of moon soil.
Knowing
the speed and mass of the projectile will let Cooke to scale
the flash and estimate the energies of the softball-size meteoroids
that hit the Moon at up to 72 km/s, more than six times the
speed of the Ames gun. But luminous efficiency is just part
of the question. A lot of the impact energy goes into shattering
and melting the projectile -- the main reason for using glass
rather than metal -- and then spraying debris everywhere.
Right:
The Ames Vertical Gun Range. Photo credit: NASA [Larger
image]
"The
ejecta kicked out from an impact can travel hundreds of miles,"
Cooke continued. "We need to know more about that if
we are going to live on the lunar surface for months at a
time." Because the moon has virtually no atmosphere to
slow down flying debris, particles land with the same speed
that launched them from the impact site.
So
you might dodge a bullet but still get caught by its shrapnel.
And the question is, Are you more likely to get cut off at
the ankles by debris spattered along the horizon, or hit from
above by material on high, ballistic trajectories?
To
gauge that danger, Cooke will measure the speed and direction
of secondary particles by the sheet-laser technique. Lenses
and mirrors spread a laser beam into paper-thin sheets of
light so flying particles are briefly illuminated several
times. The light traces then tell the size, direction, and
speed of debris particles leaving an impact.
The
technique requires a lot of image analysis, but it is cleaner
and more accurate than the older way of hanging aluminum sheets
in the chamber and counting holes.
The
answers will help determine the kinds of shielding needed
on exploration vehicles protecting humans where every day
is for "keepsies."
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Author: Dave Dooling | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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