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Lunar ProspectorA lunar orbiter launched Jan. 6 1998 by NASA as part of its Discovery program. Its primary mission was to conduct a thorough investigation of the Moon from a low polar orbit. The investigation included mapping the composition of the Moon's surface and pinpointing lunar resources, making measurements of magnetic and gravity fields, and detecting radon outgassing events in the hope of partly accounting for the Moon's tenuous atmosphere. Following a 105-hour cruise to the Moon, the probe went into a nearly circular orbit taking it once around the satellite every 118 minutes at an altitude of 100 km. The Lunar Prospector carried five instruments, but it had no onboard cameras, unlike Clementine, an earlier Moon-mapping mission which had concentrated on equatorial and middle latitudes. Instead, it sought to collect its data using a gamma-ray spectrometer and a neutron spectrometer, a magnetometer and electron reflectometer, and an alpha particle spectrometer. In a part of the mission known as the Doppler Gravity Experiment, investigators used Doppler tracking of S-band radio signals to characterize the spacecraft's orbit and map the Moon's gravity field and topographical crustal structure. The Lunar Prospector's neutron spectrometer returned data that seemed to support the existence of water ice locked up in permanently shadowed polar craters, a situation suggested by analysis of Clementine's results. In December 1998, project scientists lowered the Lunar Prospector's orbit to 40 km and later to 30 km. On July 31 1999, the probe was guided into a polar crater and deliberately crashed in the hope of throwing up water in the impact plume, but none was observed.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006