NASA is providing a water-detecting instrument, the Neutron Spectrometer System (NSS), to the Lunar Polar Exploration (LUPEX) mission led by JAXA and ISRO. The NSS instrument detects ice under the lunar surface, and instruments like NSS can infer the presence of hydrogen by detecting interactions with particles called neutrons. The NSS instrument uses a 'gas proportional counter' to detect neutrons bouncing out of the lunar soil, features two tubes that contain a rare gas called helium-3 that is very sensitive to neutrons, and when neutrons strike the helium-3 gas atoms, the gas produces electrical pulses that can be counted to infer the presence and quantity of hydrogen. Scientists can search for water on the Moon without drilling into the surface by hunting for concentrations of hydrogen, as the water on the Moon is mostly found as molecules within lunar regolith, but there may be ice deposits below the surface of the lunar South Pole. Astronauts could use the Moon's water for breathable air, rocket fuel, and more, with the first step being to find deposits of meaningful quantities of water close to the surface to mark potential landing areas for future astronauts.
The NSS instrument will be installed on LUPEX's lunar rover planned to arrive at the Moon no earlier than 2028. NASA's support of LUPEX is part of an ongoing effort to identify and characterize lunar water and other materials that easily evaporate near the Moon's South Pole. Past missions in lunar orbit have found signs of water at the Moon's poles, but ground missions are needed to build detailed maps of location and quantity.