PIONEER VENUS LARGE PROBE NEUTRAL MASS-SPECTROMETER

A neutral particle mass spectrometer has been constructed to measure the composition and abundance of the atmospheric gases of the planet Venus. This instrument is being flown on the Pioneer Venus Multiprobe and will sample the planet's atmosphere from an attitude of 67 km to its surface. The mass spectrometer is a single focusing magnetic sector field instrument capable of scanning the mass range from 1-208 amu with a sensitivity of better than 1 ppm relative to the dominant gas, CO//2. It possesses a unique inlet system which is capable of sampling gases from an environment of 770 K and 100 atm. Operation of the instrument is under the control of a microprocessor. Through this powerful device, a highly efficient peak stepping and data compression program is effected permitting transmission to earth of the entire mass spectral output in only 64 seconds at a data rate of 40 bits/s resulting in an excellent altitude resolution of variations of atmospheric composition. An isotope ratio measurement cell purges an atmospheric sample of active gases yielding an enriched rare gas sample for isotopic ratio analysis. To maintain an ultraclean vacuum in the mass analyzer tube, a combination of ion pumping and chemical gettering is employed. Instrument weight is 11 kg, volume is 10650 cm**3, and power is 14 W.