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Nephelometer (SN)

NSSDCA ID: 1978-078E-02

Mission Name: Pioneer Venus Small Probe (North)
Principal Investigators:Prof. Jacques E. Blamont
Principal Investigators:Dr. Boris Ragent


This experiment consisted of a nephelometer to measure the energy backscattered from cloud particles. It used a solid state pulsed focused light beam emitter to illuminate the clouds. The altitude history of the backscattered signal indicated the presence and vertical extent of clouds along the trajectory. Comparisons with the measurements from the small probes indicated the spatial variability of the cloud structure. The light was at a wavelength of about 9000 A. The experiment weighed about 1.1 kg and used about 2.4 W of power.

The nephelometer comprised a solid-state pulsed focused gallium arsenide laser light emitting diode (LED) at 900 nm wavelength and detectors, which operated through a sapphire window mounted in the pressure vessel of the probe. The window was 1.3 cm in diameter with a clear aperture of diameter 1.2 cm. It allowed transmission of wavelengths from 320 to 1100 nm. The detectors consisted of three solid state photodiodes, to detect backscattered light, the ultraviolet background, and the visible background. The transmitter and detectors had lens barrels to collimate the light, mounted so that the emission and reception barrels were offset by an angle of 5 degrees to each other (175 degree backscatter measurement), tilted upwards by 4 degrees above the plane perpendicular to the axis of symmetry of the probe. The instrument was mounted 9.7 cm above the probe equator. The lens plane of the nephelometer was 7.4 cm from the exterior surface of the window.

The LED illuminated the atmosphere through the window mounted on the pressure vessel, and the scattered LED light and background solar light were focused through a Fresnel lens onto the detectors that measured the intensity. Calibration targets were mounted on the aeroshell. A fiber optics light pipe, shielded from direct reflections, viewed the front surface of the window through which the LED transmitted light, and conducted some of the reflected light to monitor the state of the window and the condition of the LED.

The nephelometer was designed to measure the atmosphere outside the region that was aerodynamically disturbed by the probe, over a distance from approximately 20 cm from the lens plane to beyond 2 meters. The cross-section measured was approximately 1.0 x 2.3 cm at 35 cm from the lens plane, and a circular cross-section with a diameter of 3.3 cm at 45 cm, and 5.9 cm at 70 cm.

The measurements, including the backscattered light, calibration data, temperature, channel noise, and window condition, were compressed and prepared for telemetry by an electronic subsystem. The nephelometer protective window cover was deployed and measurements started at about 61 km altitude.

Alternate Names

  • PioneerVenusSmallProbeNorth/SN
  • SN

Facts in Brief

Mass: 1.1 kg
Power (avg): 2.4 W

Funding Agency

  • Unknown (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. Jacques E. BlamontPrincipal InvestigatorCNRS, Service d'Aeronomie
Dr. Boris RagentPrincipal InvestigatorNASA Ames Research Center

Selected References

  • Ragent, B., et al., Pioneer Venus sounder and small probes nephelometer instrument, IEEE Trans. Geosci. Rem. Sens., GE-18, No. 1, 111-116, doi:10.1109/TGRS.1980.350292, Jan. 1980.
  • Ragent, B., and J. Blamont, Preliminary results of the Pioneer Venus nephelometer experiment, Science, 203, No. 4382, 790-792, doi:10.1126/science.203.4382.790, Feb. 1979.
  • Blamont, J., and B. Ragent, Further results of the Pioneer Venus nephelometer experiment, Science, 205, No. 4401, 67-70, doi:10.1126/science.205.4401.67, July 1979.
  • Ragent, B., and J. Blamont, The structure of the clouds of Venus: Results of the Pioneer Venus nephelometer experiment, J. Geophys. Res., 85, No. A13, 8089-8015, doi:10.1029/JA085iA13p08089, Dec. 1980.
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