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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: Science@NASA
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