Editing Lunar Prospector (section)

=== Neutron Spectrometer (NS) ===
[[File:Lunar prospector instrument lp ns fs lg.gif|thumb|''Lunar Prospector'' Neutron Spectrometer (NS)]]
[[File:78783main epistaS.jpg|thumb|''Lunar Prospector'' Neutron Spectrometer data showing excess hydrogen at the Moon's south pole. Magenta and dark blue show highest hydrogen concentrations.]]
Based on the ''Lunar Prospector'' Neutron Spectrometer (NS) data, mission scientists have determined that there is evidence for [[Lunar water|lunar water ice]] in the polar craters of the Moon,<ref>{{cite web|title=Neutron Spectrometer Results |url=http://lunar.arc.nasa.gov/results/neures.htm |publisher=NASA |access-date=July 14, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080531054648/http://lunar.arc.nasa.gov/results/neures.htm |archive-date=May 31, 2008 }}</ref> an estimated 3 billion [[tonne]]s (800 billion US gallons).

The neutron spectrometer was a narrow cylinder colocated with the Alpha Particle Spectrometer at the end of one of the three radial ''Lunar Prospector'' science booms. The instrument had a surface resolution of {{convert|150|km|abbr=on}}. The neutron spectrometer consisted of two canisters each containing helium-3 and an energy counter. Any thermal neutrons colliding with the helium atoms give an energy signature which can be detected and counted. One of the canisters was wrapped in [[cadmium]], and one in [[tin]]. The cadmium screens out thermal (low energy or slow-moving) neutrons, while the tin does not. Thermal neutrons are [[cosmic ray|cosmic-ray]]-generated neutrons which have lost much of their energy in collisions with hydrogen atoms. Differences in the counts between the two canisters indicate the number of thermal neutrons detected, which in turn indicates the amount of hydrogen in the Moon's crust at a given location. Large quantities of hydrogen would likely be due to the presence of water.

The NS was designed to detect minute amounts of water ice which were believed to exist on the Moon. It was capable of detecting water ice at a level of less than 0.01%. For the polar ice studies, the NS was slated to examine the poles to 80 degrees latitude, with a sensitivity of at least 10 ppm by volume of hydrogen. For the implanted hydrogen studies, the NS was intended to examine the entire globe with a sensitivity of 50 ppmv. The Moon has a number of permanently shadowed craters near the poles with continuous temperatures of {{convert|-190|C|F}}. These craters may act as cold-traps of water from incoming comets and meteoroids. Any water from these bodies which found its way into these craters could become permanently frozen. The NS was also used to measure the abundance of hydrogen implanted by [[solar wind]].