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Retarding Potential Analyzer (RPA)

NSSDCA ID: 1975-075C-14

Mission Name: Viking 1 Lander
Principal Investigator:Prof. Alfred O. C. Nier


The Viking entry science ionospheric properties experiment (one of three that were part of the entry science investigation) studied the composition, structure, and temperature of the ionosphere, which were probed during the descent of the lander capsule by means of a retarding potential analyzer (RPA) mounted flush with the front face of the aeroshell. To conserve battery power, the instrument was operated intermittently between 16,000 and 5,000 km altitude but continuously from 5,000 to 100 km. The instrument comprised a current-collecting plate with seven grids ahead of it. A fixed program of potentials was applied to the grids, and the collected currents were measured at 10-ms intervals. The instrument operated in three phases to measure energetic electrons, thermal electrons, and thermal ions.

Alternate Names

  • RPA
  • Viking1Lander/RPA

Funding Agency

  • NASA-Office of Space Science (United States)


  • Planetary Science: Atmospheres

Additional Information

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



NameRoleOriginal AffiliationE-mail
Prof. Alfred O. C. NierTeam LeaderUniversity of Minnesota
Dr. Nelson W. SpencerTeam MemberNASA Goddard Space Flight Center
Prof. William B. HansonTeam MemberUniversity of Texas, Dallas

Selected References

  • Hanson, W. B., et al., The Martian ionosphere as observed by the Viking retarding potential analyzers, J. Geophys. Res., 82, No. 28, 4351-4363, doi:10.1029/JS082i028p04351, Sept. 1977.
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