On-orbit, self-contamination of a spacecraft is a concern facing instrument and spacecraft designers. While on the Earth, gases adsorb onto spacecraft surfaces. These gases are later released when placed in the vacuum of space. The rate at which the emitted gases are returned to the spacecraft by collisions with other gaseous molecules is known as the return flux. Models predicting the amount of gas released by a spacecraft that is returned to itself do exist, but these models have had very limited experimental testing. We describe a flight experiment designed to provide a test of these models and the analysis of the data obtained by that experiment. The experiment flew on a 1996 space shuttle mission and provided in-situ testing of the return flux models. Analysis of the limited data obtained by the experiment has determined the return flux is primarily due to collisions with the ambient atmosphere and not collisions with other gases released by the spacecraft. Limited measurements of the ambient atmosphere were also made.
The Cassini Orbiter Ion and Neutral Mass Spectrometer (INMS) is designed to measure the composition and density variations of low energy ions and neutral species in the upper atmosphere of Titan, in the vicinity of the icy satellites and in the inner magnetosphere of Saturn where densities are sufficiently high for measurement. The sensor utilizes a dual radio frequency quadrupole mass analyzer with a mass range of 1-99 amu, two electron multipliers operated in pulse-counting mode to cover the dynamic range required and two separate ion sources. A closed ion source measures non-surface reactive neutral species which have thermally accommodated to the inlet walls such as N2 and CH4. An open ion source allows direct beaming ions or chemically active neutral species such as N and HCN to be measured without surface interaction. The instrument can alternate between these three different modes. Characterization and calibration of each of these three modes is done using a low energy ion beam, a neutral molecular beam and a neutral thermal gas source. An onboard flight computer is used to control instrument operating parameters in accordance with pre-programmed sequences and to package the telemetry data. The sensor is sealed and maintained in a vacuum prior to launch to provide a clean environment for measurement of neutral species when it is opened to the ambient atmosphere after orbit insertion. The instrument is provided by NASA/Goddard Spaceflight Center, Code 915. Operation of the instrument and data analysis will be carried out by a Science Team.
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