The Pioneer Venus Bus Neutral Gas Mass Spectrometer

The neutral gas mass spectrometer aboard the Pioneer Venus Multiprobe Bus employs a semi-open ion source with electron impact ionization of the ambient gas, a double-focusing mass analyzer with Mattauch-Herzog geometry, and two direct current electrometers as well as two counting multipliers for ion detection. The mass per charge range is 1 to 46 amu/e, and the mean sampling speed is 10 mass peaks/s. An ion getter pump and a titanium sublimation pump are connected to the analyzer section of the instrument providing differential pumping capability between the ion source and the analyzer section. This makes possible instrument operation up to 10-2 mbar ion source pressure. The dynamic range is over 8 orders of magnitude. Additional features are an in-flight calibration system and periodic use of the flythrough mode of ion source operation. The instrument power consumption is 4.6 W without titanium pump, and 22 W with the latter. Total instrument mass is 6.5 kg.