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Bus Neutral Mass Spectrometer (BNMS)

NSSDCA ID: 1978-078A-03

Mission Name: Pioneer Venus Probe Bus
Principal Investigator:Dr. Ulf Von Zahn


The Bus Neutral-particle Mass Spectrometer (BNMS) experiment obtained measurements which provided information on the origin and evolution of Venus' atmosphere, the present energy balance and dynamics of the upper atmosphere, and the interaction of the upper atmosphere with solar radiation and the interplanetary medium. A magnetic deflection, double-focusing mass spectrometer was flown to measure the number densities of upper atmosphere neutral molecules in the mass range 1 to 46 atomic mass units.

The BNMS was a double-focusing magnetic deflection mass spectrometer comprising an ion source, electric and magnetic analyzers, and a multiple ion detecting system. Prior to atmospheric entry, a pyro fired, removing a spring-loaded covering from the opening into the instrument and allowing the ambient atmosphere to enter. The particles were ionized by an electron beam of 56-eV electrons at 100 microamps, which were emitted from a heated 51 micron diameter W-Re filament. The beam was collimated by a pair of magnets and collected on the opposite side by an identical filament. The ionized particles pass through focusing electrodes and an entrance slit and through an electrostatic analyzer consisting of two separated curved plates, with a radius of curvature of 58.9 mm, a positive charge on one plate and a negative charge on the other. Thye then pass into the magnetic analyzer, a permanent magnet of Alnico 700 with a flux density of 0.5 T. Three exit slits, for three different ion trajectories, lead to spiraltron electron multipliers. The upper two slits lead to the low mass (1 - 8 amu) multiplier, and the lower slit to the high mass (12 - 46 amu) multiplier. There is also a grid collector in front of the high mass multiplier, and two electrometers, one (total ion current) at the entrance to the magnetic analyzer, and one (high-mass) behind the grid collector.

The analyzer operates by peak stepping at 0.1 sec per mass peak. The electrometers are auto-ranging, from 0.01 to 8 pico A for the total ion current, and 0.001 to 0.8 pico A for the high-mass. The electrometer output signals were digitized into 10 bit words, with 2 bits for range information. The total ion current electrometer was sampled every 0.5 sec, and the high-mass every 0.1 sec. For the electron multipliers, the voltages could be selected in the range from 1800 to 4400 V in 8 steps. The pulses go through voltage amplifiers and discriminators, and transferred into 24-bit shift registers. The 8 most significant bits and 4 bit exponent are extracted, giving one 12-bit log base 2 data word for each channel, read out by telemetry every 0.1 sec. The data processing unit combine the data from the electrometers, multipliers, adds housekeeping data, and ends up with two serial bits streams sampled by the telemetry every 0.1 sec and 0.5 sec respectively.

Two days before entry, a known gas mixture was released into the apparatus as a calibration source. The data rate was fixed at 512 bits/sec. The instrument made measurements from about 700 km altitude to 130 km, where the bus ceased operations. Below 215 km, the BNMS operated in a fly-through mode, where only high-energy ions were sampled, about 25% of the time. For the last 50 seconds of measurement, a titanium pump was used to maintain the necessary pressure. The total instrument mass was 6.5 kg, it used 4.6 W power without the titanium pump, and 22 W when the pump was on.

Alternate Names

  • BNMS
  • PioneerVenusProbeBus/BNMS
  • urn:nasa:pds:context:instrument:pvmp.bus.bnms

Facts in Brief

Mass: 6.5 kg
Power (avg): 4.6 W

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. NierOther InvestigatorUniversity of Minnesota
Prof. Donald M. HuntenOther InvestigatorUniversity of
Dr. Ulf Von ZahnPrincipal InvestigatorUniversitat

Selected References

  • von Zahn, U., et al., The Pioneer Venus bus neutral gas mass spectrometer, IEEE Trans. Geosci. Rem. Sens., GE-18, No. 1, 122-125, doi:10.1109/TGRS.1980.350294, Jan. 1980.
  • von Zahn, U., et al., Venus thermosphere: In situ composition measurements, the temperature profile, and the homopause altitude, Science, 203, No. 4382, 768-770, doi:10.1126/science.203.4382.768, Feb. 1979.
  • Colin, L., Ed., and D. M., Ed. Hunten, Pioneer Venus experiment descriptions, Space Sci. Rev., 20, No. 4, 451-525, doi:10.1007/BF02186463, June 1977.
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