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Orbital Mass Spectrometer

NSSDCA ID: 1971-063A-13

Mission Name: Apollo 15 Command and Service Module (CSM)
Principal Investigator:Dr. John H. Hoffman


The lunar orbital science experiments package included a mass spectrometer experiment whose objective was to use the measured composition data to study the sources, sinks, and transport mechanisms of the ambient lunar atmosphere. From July 26 to August 7, 1971, the duration of the Apollo 15 flight, 90 hr of data were obtained -- 40 hr while in lunar orbit and 50 hr during transearth coast. The anaylzer was mounted on a retractable boom. When fully extended, the boom placed the spectrometer 7.3 m from the spacecraft, a distance anticipated to be beyond the outgassed molecular cloud. Control of the experiment functions and boom motion was provided by a set of five switches in the command module, which were operated by a crew member according to the mission time line or by instruction from the ground controller. Instrument weight was 11 kg, and its dimensions were approximately 30 x 32 x 23 cm. 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 when ambient measurements were obtained, and it was oriented in the wake direction to determine background spectra and instrument outgassing. The plenum contained the spectrometer ion source, which had redundant filaments mounted on either side of the ionization chamber. Several outgassing operations during flight maintained the ion source in a reasonably outgassed state. Use of a two-collector system in the analyzer permitted the simultaneous scanning of two mass ranges -- 12 to 28 and 28 to 66 amu. Mass resolution was the order of a 1-percent valley at mass 40 amu. The mass sweep was achieved by varying the applied high voltage in a series of 590 steps over the range from 620 to 1560 v with a dwell time of approximately 0.1 sec. Thirty additional steps at zero volts were used to determine background counting rate and to apply internal calibration, so that 62 sec were required to complete a mass scan. The voltage step number that determined the mass number of the ion being measured was identified by counting from step one -- a sweep start flag. Bendix electron multipliers were used as pulse amplifiers to determine the counting rate of ions passing each collector slit for each voltage step. Prelaunch experiment calibration included operation in a molecular beam facility. More details of this experiment can be found in 'Lunar orbital mass spectrometer,' J. H. Hoffman, 'Int. J. Mass Spectrom. Ion Phys.,' Vol 8, pp 403-416, 1972.

Alternate Names

  • Apollo15CSM/MassSpectrometer
  • S165
  • urn:nasa:pds:context:instrument:oms.a15c

Facts in Brief

Mass: 11 kg

Funding Agency

  • NASA-Office of Space Science Applications (United States)


  • Planetary Science: Atmospheres

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Mr. Vernon M. Dauphin, Jr.Other InvestigatorNASA Johnson Space Center
Dr. John H. HoffmanPrincipal InvestigatorUniversity of Texas, Dallas
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