Astronauts
on the Moon and Mars are going to have to cope with an uncommon
amount of static electricity.
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August 10, 2005: Have you ever walked across a wool
carpet in leather-soled shoes on a dry winter day, and then
reached out toward a doorknob? ZAP! A stinging spark
leaps between your fingers and the metal knob.
That's
static discharge--lightning writ small.
Static
discharge is merely annoying to anyone on Earth living where
winters have exceptionally low humidity. But to astronauts
on the Moon or on Mars, static discharge could be real trouble.
Right:
Beware the door knob. [More]
"On
Mars, we think the soil is so dry and insulating that if an
astronaut were out walking, once he or she returned to the
habitat and reached out to open the airlock, a little lightning
bolt might zap critical electronics," explains Geoffrey
A. Landis, a physicist with the Photovoltaics and Space Environmental
Effects Branch at NASA Glenn Research Center in Cleveland,
Ohio.
This
phenomenon is called triboelectric charging.
The
prefix "tribo" (pronounced TRY-bo) means "rubbing."
When certain pairs of unlike materials, such as wool and hard
shoe-sole leather, rub together, one material gives up some
of its electrons to the other material. The separation of
charge can create a strong electric field.
Here
on Earth, the air around us and the clothes we wear usually
have enough humidity to be decent electrical conductors, so
any charges separated by walking or rubbing have a ready path
to ground. Electrons bleed off into the ground instead of
accumulating on your body.
But
when air and materials are extraordinarily dry, such as on
a dry winter's day, they are excellent insulators, so there
is no ready pathway to ground. Your body can accumulate negative
charges, possibly up to an amazing 20 thousand volts. If you
touch a conductor, such as a metal doorknob, then--ZAP!--all
the accumulated electrons discharge at once.
On
the Moon and on Mars, conditions are ideal for triboelectric
charging. The soil is drier than desert sand on Earth. That
makes it an excellent electrical insulator. Moreover, the
soil and most materials used in spacesuits and spacecraft
(e.g., aluminized mylar, neoprene-coated nylon, Dacron,
urethane-coated nylon, tricot, and stainless steel) are completely
unlike each other. When astronauts walk or rovers roll across
the ground, their boots or wheels gather electrons as they
rub through the gravel and dust. Because the soil is insulating,
providing no path to ground, a space suit or rover can build
up tremendous triboelectric charge, whose magnitude is yet
unknown. And when the astronaut or vehicle gets back to base
and touches metal--ZAP! The lights in the base may
go out, or worse.
Physicist
Joseph Kolecki and colleagues at NASA Glenn first noticed
this problem in the late 1990s before Mars Pathfinder was
launched. "When we ran a prototype wheel of the Sojourner
rover over simulated Martian dust in a simulated Martian atmosphere,
we found it charged up to hundreds of volts," he recalls.
That
discovery so concerned the scientists that they modified Pathfinder's
rover design, adding needles half an inch long, made of ultrathin
(0.0001-inch diameter) tungsten wire sharpened to a point,
at the base of antennas. The needles would allow any electric
charge that built up on the rover to bleed off into the thin
Martian atmosphere, "like a miniature lightning rod operating
in reverse," explains Carlos Calle, lead scientist at
NASA's Electrostatics and Surface Physics Laboratory at Kennedy
Space Center, Florida. Similar protective needles were also
installed on the Spirit and Opportunity rovers.
Right:
Electrostatic discharge points at the base of Sojourner's
antenna. [More]
On
the Moon, "Apollo astronauts never reported being zapped
by electrostatic discharges," notes Calle. "However,
future lunar missions using large excavation equipment to
move lots of dry dirt and dust could produce electrostatic
fields. Because there's no atmosphere on the Moon, the fields
could grow quite strong. Eventually, discharges could occur
in vacuum."
"On
Mars," he continues, "discharges can happen at no
more than a few hundred volts. It's likely that these will
take the form of coronal glows rather than lightning bolts.
As such, they may not be life threatening for the astronauts,
but they could be harmful to electronic equipment."
So
what's the solution to this problem?
Here
on Earth, it's simple: we minimize static discharge by grounding
electrical systems. Grounding them means literally connecting
them to Earth--pounding copper rods deep into the ground.
Ground rods work well in most places on Earth because several
feet deep the soil is damp, and is thus a good conductor.
The Earth itself provides a "sea of electrons,"
which neutralizes everything connected to it, explains Calle.
There's
no moisture, though, in the soil of the Moon or Mars. Even
the ice believed to permeate Martian soil wouldn't help, as
"frozen water is not a terribly good conductor,"
says Landis. So ground rods would be ineffective in establishing
a neutral "common ground" for a lunar or Martian
colony.
On
Mars, the best ground might be, ironically, the air. A tiny
radioactive source "such as that used in smoke detectors,"
could be attached to each spacesuit and to the habitat, suggests
Landis. Low-energy alpha particles would fly off into the
rarefied atmosphere, hitting molecules and ionizing them (removing
electrons). Thus, the atmosphere right around the habitat
or astronaut would become conductive, neutralizing any excess
charge.
Right:
Note the marsdust clinging to Sojourner's wheels. This is
indirect evidence of electrostatic charging. [More]
Achieving
a common ground on the Moon would be trickier, where there's
not even a rarefied atmosphere to help bleed off the charge.
Instead, a common ground might be provided by burying a huge
sheet of foil or mesh of fine wires, possibly made of aluminum
(which is highly conductive and could be extracted from lunar
soil), underneath the entire work area. Then all the habitat's
walls and apparatus would be electrically connected to the
aluminum.
Research
is still preliminary. So ideas differ amongst the physicists
who are seeking, well, some common ground.
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Author: Trudy E.
Bell | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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