Electric Lunar Craters
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When NASA discovered water frozen in the dark bottom of a south pole lunar crater last year, it renewed visions of astronauts one day supporting habitats with resources mined from the Moon’s surface. But a computer study of the strange effects the solar wind creates as it blows unimpeded across the Moon raises another vision—one of hundreds of volts of static electricity lurking in potentially resource-rich craters just waiting to zap an astronaut.
Findings by NASA’s Lunar Science Institute indicate that before man or machine can safely enter lunar polar craters for scientific or practical purposes —such as extracting water or minerals —the agency will have to master ways for them to be grounded.
The question of how to safety discharge static electricity in space is an old one, notes Goddard Space Flight Center scientist William Farrell, lead author with six others from Goddard, the Lunar Science Institute, the University of Maryland-Baltimore and the University of California-Berkeley of a paper outlining the institute’s findings in the Journal of Geophysical Research.
Farrell has been studying the effects of the solar wind on hard bodies in space for decades, including studies early in the space shuttle’s development program of the plasma wake it throws off. Like satellites, the shuttle becomes an electrically conducting instrument and is grounded by the solar plasma that flows past it.
Similarly, astronauts or robotic rovers on the Moon are not grounded by touching its surface as they might be on Earth. Farrell jokes that the Moon’s surface has the electrical conductivity of candle wax. Fortunately, the solar wind is a good conductor and ever present for satellites, and usually for strolling astronauts. But the institute’s study raises concerns about what might happen to astronauts and spacecraft should they descend into the cold depths of lunar craters whose high sides shield their bottoms from the wind.
The Earth’s magnetosphere is a buffer to the solar wind. The wind flows around this magnetic field like water in a stream moving past a boulder.
The solar wind easily penetrates to the rocky surface of the Moon, where it can create mischief, largely because it stirs up solar dust. Apollo astronauts were alternately intrigued and annoyed by the wind. They saw auroras above the skyline believed to be dust particles swept from the lunar surface.
But static electricity also turned lunar dust into a “sticky and extremely abrasive” hazard of silicate particles that clung to everything, including their spacesuits, notes Farrell. Like satellites, astronauts discharge static electricity through the plasma atmosphere that surrounds them. The question is, what happens when that atmosphere lacks the safety of a surrounding plasma?
The Moon tilts only slightly toward the Sun, so the solar wind flows fairly evenly over its poles. But as it does so, the wind does not flow evenly into the bottoms of the deep craters at the poles. Instead, the institute’s computer model shows its low-mass electrons easily flowing over the jagged edges of the craters and into their bottoms, creating a negatively charged cloud. The ions, a thousand times heavier than the electrons, want to follow. But it is much harder for them to negotiate the steep rim. Most do not make it. Those that do create an electron/ion separation effect, or ambipolar electric field. It is at its most extreme on a crater’s leeward edge—along the inside crater wall—and at the crater floor nearest the solar wind flow.
On this leeward edge, the electron cloud “can create an unusually large negative charge of a few hundred volts relative to the dense solar wind flowing over the top,” says Farrell.
The situation is complicated by the extreme cold in the permanent shadows of these craters, many more than a mile deep, where temperatures can be -400F or lower.
Cold is one reason they are attractive for research and, possibly, for resources. They are so cold that they have frozen water, hydrogen and various volatiles that could have been there since the Moon’s formation more than 4 billion years ago.
“As lunar rocks get colder, they lose their conductivity,” says Farrell. “Below 100K (-280F), their conductivity is just 1 in 10 billion.”
So astronauts or robotic rovers that venture into the craters will find a highly charged atmosphere made worse by cold that inhibits the already low conductivity of the lunar surface.
As they shuffle along, the astronauts will be like people rubbing their feet on a woolen rug in the winter. Farrell says they may have to wait 100 sec. to discharge the static electric forces they build up after a single step.
As for robots, the crater’s high concentration of static electricity is likely to short out their electronics.
The Apollo missions raised a lot of questions about the role dust plays on the Moon that NASA’s 2012 Lunar Atmosphere and Dust Environment Explorer mission is designed to tackle. Its results should produce a more complex computer model for studying the conductivity issues raised by the Lunar Science Institute’s work.
Posted by: Soderman/NLSI staff
Source: Michael Mecham/NASA/ http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=space&id=news/awst/2010/05/24/AW_05_24_2010_p62-220989.xml&headline=Solar%20Wind%20Poses%20Hazards%20On%20Moon