NASA is on a flight path to replant astronauts on the Moon, looking to sustain a human presence on that cratered, airless orb on a "go-as-the nation-can-afford-to-pay" basis. That approach is seen as letting people step back onto the lunar surface no later than 2020.
Space engineers have honed in on one possible site for a lunar outpost: the Moon's south pole.
It's a tactical setting on the rim of Shackleton Crater, a feature some 12 miles (19 kilometers) in diameter.
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There's real estate here that basks in near-perpetual sunlight. Also, it's a region that is a doorway into the depths of always dark, Sun-deprived, territory.
What's possibly lurking there in that super-cold darkness is water ice — portrayed by some researchers as the gold standard for future exploration on and from the Moon. Yet there is considerable debate about this resource. If there, such a raw storehouse might be processed into usable oxygen and hydrogen.
Experts have been chipping away at the water ice on the Moon issue for years.
According to NASA Moon outpost thinkers, there are five key reasons for building up an encampment near a lunar pole:
Polar sites have plenty of sunlight, which lessens concerns about energy storage. It would be possible to operate a polar outpost on solar power. While not highlighting it as such, NASA's go-solar tactic also doubles as a non-nuclear, perhaps more politically correct approach.
The environment at the poles of the Moon is relatively benign, making it easier to design a habitat. Temperatures at the poles vary no more than about 50 degrees Celsius all year round, while temperatures at the equator can vary 250 degrees Celsius from day to night.
At the Moon's south pole there is "ample evidence," NASA planners point out, of enhanced hydrogen — an important natural resource for future development for energy generation, propellant production and other potential uses.
The poles can teach robotic and human explorers volumes about the Moon. This landscape is among the most complex of regions, yet very little is known about them.
To land equipment and scientific payloads near the lunar south pole, specifically, as opposed to another location, will require less propellant and could be more cost effective.
Not in critical path
NASA's notional positioning of a base at the Moon's south pole was not solely driven by the thought of driving over and diving into a reservoir of potential water ice. On the other hand, if it's truly there, so much the better.
There were many reasons for choosing the polar regions of the Moon for the outpost location, said Jerry Sanders, In-Situ Resource Utilization (ISRU) Project Manager at NASA's Johnson Space Center in Houston, Texas.
One of them is the possibility of water/ice and other possible cometary volatiles, said Sanders. He noted with interest that the Stardust sample collection mission to comet Wild 2 found methyl and ethylamines, which not only has hydrogen, but carbon and nitrogen as well.
"The presence of large quantities of readily assessable water would make long term settlement of the Moon much more attainable," Sanders explained to SPACE.com. "But for an initial outpost, oxygen alone from the regolith [the Moon's topside layer of dust and rocky debris] provides most of the benefits of early ISRU ... oxygen for extravehicular activity and life support backup."
Sanders underscored a message that was made clear from an early December space exploration meeting in Houston, Texas during which NASA's lunar outpost proposal was rolled out in detail.
"Because ISRU has never been demonstrated, it cannot be in the 'critical path' of outpost success…be it water or oxygen from regolith," Sanders said. "However, as ISRU capabilities and processes come online and are shown to be valid, ISRU products will help reduce the logistics required from Earth with the goal of use in propulsion and surface exploration activities."
Measurements taken by NASA's robotic Lunar Prospector that orbited the Moon in 1998-1999 reported hydrogen enhancement over large shadowed areas — exciting some scientists about the prospect of large quantities of water ice tucked away in always dark nooks and crannies.
The projection is that billions of gallons of water could be locked up in the form of water ice.
That finding seemed to back an earlier data point from the U.S. Defense Department's Clementine Moon probe.
Scientists on that project reported that Clementine data suggested the presence of ice at the bottom of a permanently shadowed crater near the Moon's south pole.
Of late, the Advanced Moon Imaging Experiment on the European Space Agency's (ESA) SMART-1 lunar orbiter searched for light, shadow and ice at the Moon's south pole before it was purposely plowed into the Moon in early September 2006.
The ESA craft made long repeated camera exposures to see inside the shadowed areas, using imagery skills that could reveal patchy ice surface layers inside the crater.
"We still do not know if this hydrogen is due to enhanced trapping of solar wind, or to the water ice brought on the Moon by the bombardment of comets and asteroids," said Bernard Foing, ESA's SMART-1 project scientist. "We need to analyze all remote sensing data sets consistently. Future lander and rover missions to the Moon will help in the search and characterization of lunar polar ice, both on the surface and below the subsurface," he explained.
However, astronomers recently utilized Earth-based radio-telescopes to scan the 1.3 mile (2 kilometer) wide inner edge of the Shackleton crater ridge. The report is that they were not able to identify a distinctive signature of ice deposits pigeon holed within that south pole region.
Covering all your bases
Shackleton crater as the locale for a lunar outpost is not an X marks the spot, done deal, said Ben Bussey, senior staff scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
"It might be the place ... but it might not," Bussey told SPACE.com. "What you're doing is covering all your bases for planning a lunar outpost."
Bussey said that there are places near the rim of Shackleton which have a lot of illumination that is potentially constant. "And you know you are close to areas of permanent shadow that we know have enhanced hydrogen ... and may or may not have ice ... depending on who you believe."
India's Chandrayaan-1 lunar orbiter, to be lofted in early 2008, has a good shot at further identifying possible water ice-laden spots with a U.S.-provided low-power imaging radar, Bussey advised — one of two U.S. experiments on the Indian Moon probe.
"The idea is that we find regions of interest with Chandrayaan-1 radar. We would investigate those using all the capabilities of the radar on NASA's Lunar Reconnaissance Orbiter, Bussey added, a Moon probe to be launched late in 2008.
Still, going to and touching the ice via robotic landers is key, Bussey said. "From orbit, we're identifying what we hope is a resource. But to characterize the absolute nature of the resource, you have to touch it physically and do the sort of measurements you can only do on the spot."
For Bussey, a verdict of ice would be a windfall if it's really there.
"You don't have to have ice at the poles to make that a good place for the initial lunar outpost. If you happen to have big resources of ice…well, that's just icing on the cake so to speak. Pardon the pun," Bussey concluded.
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