INSTRUMENT OVERVIEW
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        The Neutral Mass Spectrometer (ONMS) had the primary objective of measuring the number densities of neutral atoms and 
        molecules in the upper atmosphere of Venus, from perigee near 150 km to about 500 km, to help define its chemical, 
        dynamical, and thermal state. Combined with lower atmospheric neutral mass data from the large probe and bus probe, it 
        would also help determine the profile of atmospheric mixing. The instrument was housed in a 4.5 cm diameter tube, 16 cm 
        long, with a small cylinder attached perpendicularly holding a getter pump, all held inside a shielded cylindrical 
        electronic support structure inside the orbiter bus. One end of the tube was mounted against the exterior wall of the bus, 
        connected to a cylindrical chamber mounted on the outside to hold the orifice and ion source.
    
        The experiment used a quadrupole mass spectrometer with three ion-source operating modes and three mass-scanning modes. 
        A knife-edged orifice, located on the instrument platform of the orbiter 27 degrees from the spin axis, allows gas into a 
        chamber holding the ion source. The ion source could be operated alternately in open and closed configurations to increase 
        accuracy. In open source mode, only free-streaming particles, with large kinetic energies due to the relative velocity of 
        the orbiter (~10 km/s at periapsis), are measured. Open source mode is used to measure noble and non-reactive gases. In the 
        closed source mode all the particles measured are surface-reflected particles, basically inflowing gas stagnated in the 
        source chamber. This mode is used for measurement of chemically active gases. There was also a "flip-flop" mode, 
        alternating between the modes.
    
        The quadrupole analyzer consisted of 7.5 cm long rods with a field radius of 0.2 cm. Gas atoms and molecules were ionized 
        and separated by the quadrupole filter according to their mass, using stepping of applied AC and DC voltages. Ions exiting 
        the analyzer were deflected into the a secondary electron multiplier for charge conversion and amplification. A pulse detector 
        counted the pulses, and an electrometer amplifier measured the current. These are proportional to the particle density.
    
        An adaptive mass scan was used to reduce the bit rate required for a given information-return rate. The resolution was 0.0001 
        for adjacent masses, and the mass range was 1 to 46 amu. It could scan continuously through the entire mass range or to scan 
        any combination of 8 masses within that range. The kinetic energy of the ionizing electrons could be chosen to be 70 or 27 eV 
        to allow discrimination of constituents of equal mass. The maximum average vertical spacing of the sample points was approximately 
        400 m at 500 km altitude an horizontal spacing along the orbital path was 2 km. Sampling was either equally spaced in time over 
        a spin period or restricted to 45 degrees around the ram position.
    
        Approximately two days after orbit insertion, a metal-ceramic breakoff caps was removed by activation of a pyrotechnic device 
        to expose the ONMS. Vertical and horizontal density variations of the major neutral constituents of the upper atmosphere of 
        Venus were detected and measured to define the dynamic, chemical, and thermal states of the upper atmosphere. Important constituents 
        measured were He, O, O2, CO, CO2 and/or N2, and A. It was also possible to study H, D and/or H2, C, and NO.