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Solar Wind Plasma Analyzer (OPA)

NSSDCA ID: 1978-051A-18

Mission Name: Pioneer Venus Orbiter
Principal Investigator:Dr. Aaron Barnes


The Solar Wind Plasma Analyzer (OPA) was a quadrispherical electrostatic analyzer (similar to the plasma instrument on Pioneers 10 and 11), with five current collectors and electrometers. The objectives were to measure solar wind conditions (velocity, density, flow direction, and temperature) outside the Venusian bow shock, inside the magnetosheath flow field, and to study the ionopausal structure. Solar-wind measurements were made during the transit to Venus, particularly to study macroscale problems and to determine average gradients. The near-planet wake region was also available for study. The instrument had a mass of 3.9 kg and used 5 W power.

The analyzer was a nested pair of plates, each with the shape of one-quarter of a sphere with a radius of 12 cm, held 1 cm apart. On one side, perpendicular to the plates, is an entrance aperture to allow ions, protons and electrons to pass between the two plates, at the other, exit, end is an array of five current collectors. The plates are charged so that a charged particle travelling between the plates is deflected into a curved path and, depending on the particle, the initial direction of travel, and the strength of the field, can strike one of the five detectors after passing through a drift tube grid and a suppressor grid. Each detector has its own electrometer amplifier. The voltage between the plates can be varied in a stepwise fashion from 50 V to 8000 V in 32 logarithmically equal steps for high-energy positive ions, and 15 steps from 3 V to 250 V, plus a step at 0.25 V, for electrons and low-energy positive ions, to cover a large range of particle energy/charge.

The energy/charge range was 50-8000 (ions) in 32 steps and 1-500 (electrons) in 16 steps. The angular range covered was +/-85 degrees elevation by 360 degrees azimuth, and the detector field of view was 15 deg by 25 deg or 15 deg by 45 deg, depending on position. The logic design was essentially that used on Pioneers 8 and 9. The instrument had two modes of operation, command-able from Earth. The scan mode finds the peak particle flux over one revolution of the spacecraft for each voltage step and identifies the spacecraft azimuth and collector that measures the maximum flux at each voltage. Then a polar scan and an azimuthal scan are made at four consecutive voltage steps, beginning with the step before the one at which the maximum flux was measured. Each polar scan measures the flux at each collector, while each azimuth scan measures the flux in 12 sectors centered on the peak flux direction. The step mode allocates about one second to each voltage step, for only the maximum flux scan.

Alternate Names

  • OPA
  • PioneerVenusOrbiter/OPA

Facts in Brief

Mass: 3.9 kg
Power (avg): 5 W

Funding Agency

  • NASA-Office of Space Science (United States)


  • Planetary Science: Fields and Particles
  • Space Physics: Heliospheric Studies

Additional Information

Questions and comments about this experiment can be directed to: Dr. Edwin V. Bell, II



NameRoleOriginal AffiliationE-mail
Prof. Devrie S. IntriligatorCo-InvestigatorCarmel Research
Mr. Harold R. CollardCo-InvestigatorNASA Ames Research Center
Mr. Darrell D. McKibbinCo-InvestigatorNASA Ames Research Center
Mr. John D. MihalovCo-InvestigatorNASA Ames Research
Dr. Ferdinand V. CoronitiCo-InvestigatorUniversity of California, Los
Dr. Robert C. Whitten, Jr.Co-InvestigatorNASA Ames Research
Dr. Aaron BarnesPrincipal InvestigatorNASA Ames Research Center

Selected References

  • Intriligator, D. S., et al., The Pioneer Venus Orbiter plasma analyzer experiment, IEEE Trans. Geosci. Rem. Sens., GE-18, No. 1, 39-43, doi:10.1109/TGRS.1980.350258, Jan. 1980.
  • Wolfe, J., et al., Initial observations of the Pioneer Venus Orbiter solar wind plasma experiment, Science, 203, No. 4382, 750-752, doi:10.1126/science.203.4382.750, Feb. 1979.
  • Intriligator, D. S., et al., Electron observations and ion flows from the Pioneer Venus Orbiter plasma analyzer experiment, Science, 205, No. 4401, 116-119, doi:10.1126/science.205.4401.116, July 1979.
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