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Using laser beams
and electric fields, NASA researchers are probing the curious
behavior of moondust.
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November
21, 2005: Each morning, Mian Abbas enters his
laboratory and sits down to examine--a single mote of dust.
Zen-like, he studies the same speck suspended inside a
basketball-sized vacuum chamber for as long as 10 to 12
days.
 The microscopic object of his rapt
attention is not just any old dust particle. It's moondust.
One by one, Abbas is measuring properties of individual dust
grains returned by Apollo 17 astronauts in 1972 and the
Russian Luna-24 sample-return spacecraft that landed on the
Moon in 1976.
Right: Mian Abbas and his vacuum
chamber. [More]
"Experiments on single grains are helping us understand
some of the strange and complex properties of
moondust," says Abbas. This knowledge is important.
According to NASA's Vision for Space Exploration, astronauts
will be back on the moon by 2018--and they'll have to deal
with lots of moondust.
The dozen Apollo astronauts who walked on the moon
between 1969 and 1972 were all surprised by how "sticky"
moondust was. Dust got on everything, fouling tools and
spacesuits. Equipment blackened by dust absorbed sunlight and
tended to overheat. It was a real problem.
Many researchers believe that moondust has a severe
case of static cling: it's electrically charged. In the lunar
daytime, intense ultraviolet (UV) light from the sun knocks
electrons out of the powdery grit. Dust grains on the moon's
daylit surface thus become positively
charged.
Eventually, the repulsive charges become so strong that
grains are launched off the surface "like cannonballs," says
Abbas, arcing kilometers above the moon until gravity makes
them fall back again to the ground. The moon may have a
virtual atmosphere of this flying dust, sticking to astronauts
from above and below.
Or so the theory goes.
But do grains of lunar dust truly become positively
charged when illuminated by ultraviolet light? If so, which
grains are most affected--big grains or little grains? And
what does moondust do when it's charged?
These are questions Abbas is investigating in his
"Dusty Plasma Laboratory" at the National Space Science and
Technology Center in Huntsville, Alabama. Along with
colleagues Paul Craven and doctoral student Dragana Tankosic,
Abbas injects a single grain of lunar dust into a chamber and
"catches" it using electric force fields. (The injector gives
the grain a slight charge, allowing it to be handled by
electric fields.) With the grain held suspended literally in
mid-air, they "pump the chamber down to 10-5 torr
to simulate lunar vacuum."
Next comes the mesmerizing part: Abbas shines a UV
laser on the grain. As expected, the dust gets "charged up"
and it starts to move. By adjusting the chamber's electric
fields with painstaking care, Abbas can keep the grain
centered; he can measure its changing charge and explore its
fascinating characteristics.
 Right: A single grain of moondust
hangs suspended in Abba's vacuum chamber. [More]
Like the Apollo astronauts, Abbas has already
discovered some surprises--even though his experiment is not
yet half done.
"We've found two things," says Abbas. "First,
ultraviolet light charges moondust 10 times more than theory
predicts. Second, bigger grains (1 to 2 micrometers across)
charge up more than smaller grains (0.5 micrometer), just the
opposite of what theory predicts."
Clearly, there's much to learn. For instance, what
happens at night, when the sun sets and the UV light goes
away?
That's the second half of Abbas's experiment, which he
hopes to run in early 2006. Instead of shining a UV laser onto
an individual lunar particle, he plans to bombard dust with a
beam of electrons from an electron gun. Why electrons? Theory
predicts that lunar dust may acquire negative charge
at night, because it is bombarded by free electrons in the
solar wind--that is, particles streaming from the sun that
curve around behind the moon and hit the night-dark
soil.
When Apollo astronauts visited the Moon 30+ years ago,
they landed in daylight and departed before sunset. They never
stayed the night, so what happened to moondust after dark
didn't matter. This will change: The next generation of
explorers will remain much longer than Apollo astronauts did,
eventually setting up a permanant outpost. They'll need to
know, how does moondust behave around the clock?
Stay tuned for answers from the Dusty Plasma
Lab.
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Authors: Trudy E. Bell
& Dr. Tony
Phillips | Editor: Dr. Tony Phillips
| Credit: http://science.nasa.gov/
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