U.S. Instruments on Indian Orbiter Seen Aiding Exploration
Aviation Week & Space Technology, 03/20/2006, page 55
Frank Morring, Jr., Washington
U.S. instruments on India's lunar orbiter could pinpoint future lunar base locations
As NASA gears up to return humans to the Moon, a pair of U.S.-funded instruments picked to fly on India's Chandrayaan-1 lunar orbiter next year will continue the search for water at the Moon's poles and other surface resources that might support lunar bases one day.
The U.S. instruments--an 8-kg. synthetic aperture radar and a sensitive imaging spectrometer--will work with others provided by the Indian Space Research Organization (ISRO), the European Space Agency and the Bulgarian Academy of Sciences to refine lunar maps dating back to the Apollo era.
"This data set. . . will be harvested for decades," says Brown University's Carle Pieters, principal investigator on the Moon Mineralogy Mapper (M3) that will produce surface composition maps of the lunar surface. "This will be the primary compositional database that will be used as we decide where to put these various outposts."
While the M3 instrument will be working with a hyperspectral imager supplied by ISRO to study surface composition across the global lunar surface, the miniature imaging radar instrument (Mini-SAR) will focus on the poles above 80 deg. Lat. It will map the terrain in those regions at a resolution of about 75 meters per pixel, with emphasis on the bottoms of deep craters where ice is believed to exist.
"It operates in S-band as a side-looking synthetic aperture radar, but also as a scatterometer," says Paul Spudis of the Johns Hopkins University Applied Physics Laboratory (APL), principal investigator on the instrument. Its "goal is to image the dark areas of the Moon and to look for evidence of lunar ice."
Scientists involved in Chandrayaan-1 have agreed to share data from their various national instruments across the board (AW&ST Mar. 13, p. 26). Results from the two-year mission, scheduled for a September 2007 launch, will feed directly into NASA's plans to work with international partners to explore the Moon with robots and eventually with humans, gathering mapping data that can be used to pick sites for future exploration missions.
The M3 instrument will extend the capabilities of India's hyperspectral imager (HySI), which will use 64 continuous channels with a resolution of 80 meters per pixel and a spectral resolution of 15 nm. From Chandrayaan-1's planned 100-km. lunar polar orbit, the M3 instrument should deliver 70 meters per pixel and a spectral resolution of about 10 nm.
While it was selected to map all types of mineral resources on the lunar surface, its spectroscopy should be able to identify water ice on the surface in the near-infrared wavelengths, Pieters says, even though M3 is a passive instrument relying on reflected sunlight, and the ice is believed to lie at dark bottoms of polar craters.
By adding the returns from each orbit, Pieters says, the M3 instrument should be able to collect enough scattered light for meaningful results, even from the darkest craters. Provided there actually is ice in the craters, the active Mini-SAR will provide another measurement--by capturing radar signals bounced back by imperfections in the otherwise radio-transparent ice--to produce maps of ice distribution in the craters.
To make maximum use of the bandwidth available on the spacecraft, the radar will operate when light conditions warrant switching off India's terrain-mapping stereo camera. "Chandrayaan plans to make a global 5-meter-per-pixel image map of the whole Moon, but they stop imaging for 40 days every six months because the illumination conditions are bad," says Spudis. "That's when we do the radar imaging."
That sort of coordination between Indian and U.S. controllers could prove useful if, as expected, NASA's Lunar Reconnaissance Orbiter (LRO) reaches the Moon while Chandrayaan is still operating. Spudis says if both spacecraft spend some time in the same orbit, it may be possible to operate the Mini-SAR on Chandrayaan and a higher-resolution version of the instrument on LRO jointly for bistatic imaging to get more information about the expected polar ice deposits. "The reason that's significant is you can actually unambiguously distinguish volume from surface scattering," Spudis says.
While there is no commitment yet to try the bistatic-imaging experiment, it would resolve a debate in the lunar science community over whether the radar returns would represent actual ice, or just a misreading of signals returned by surface roughness.
Ultimately, the maps produced by Mini-SAR, M3 and the other Chandrayaan-1 instruments may drive exploration-site selection far beyond the poles, Pieters says, depending on in situ resource utilization technology advances in the years to come.
