Orbital Mass Spectrometer

Description

The objective of this composition experiment was to use the measured concentrations to study the sources, sinks, and transport mechanisms of the lunar ambient atmosphere. The flight neutral magnetic mass spectrometer was similar to that flown on Apollo 15 (71-063A-13). It was mounted at the end of a retractable boom, which, when fully extended, measured 7.3 m. This distance was expected to be beyond the outgassed molecular cloud. Control of the experiment functions and the boom motion was provided by a set of command mudule switches that were operated by a crew member according to the mission time line or by instruction from the ground controller. A scoop mounted on the top of the package was the gas inlet plenum. This inlet was oriented along the spacecraft velocity vector for maximum ram effect when ambient measurements were obtained, and it was oriented in the wake direction to determine background spectra and instrument outgassing. For this flight, the inlet structure was fitted with a thermally controlled inner plenum, which was heated to approximately 250 deg c for 1 hr before operation to outgas the structure. Subsequently, the temperature was kept at 70 deg c during data collection. Two mass ranges, 12 to 28 and 28 to 67 amu, were scanned simultaneously because this analyzer had two collectors. Ions of a given mass, when focused on one of the collectors, were counted for a period of 0.1 sec, and then the accumulated count was telemetered. Formation of the ions at the junction of the gas inlet plenum and analyzer was accomplished by an electron beam with 70-eV energy. The flight instrument was calibrated in a molecular beam facility to determine the absolute sensitivity factors. For most gases, one count corresponded to 260 molecules/cc. An important exception was neon, for which one count corresponded to 1100 atoms/cc in the lunar atmosphere. These sensitivity numbers were applicable only when the inlet faced in the direction of motion. Owing to a boom malfunction approximately 200 hr after launch, the mass spectrometer wad jettisoned before transearth injection. Some preliminary results and more experiment detail can be found in 'Apollo 16 - lunar orbital mass spectrometer experiment,' R. R. Hodges, J. H. Hoffman, and D. E. Evans, in the Apollo 16 Preliminary Science Report, July 19, 1972.