Spacecraft Return Flux Considerations for Missions Targeting Detection of Organics with Mass Spectrometers

摘要

Science missions targeting the detection of organics from jet plumes originating from icy moons (i.e., Europa and Enceladus), as well as missions targeting main-belt comets (i.e., 238P/Read) and comets in the Kuiper Belt and Oort Cloud, are expected to employ next-generation mass spectrometers to measure the composition of exospheres. jet plume effluents, and outgassing emanates from discrete sources or diffuse regions. During icy moon fly-bys, when mass spectrometers would be making measurements, molecular emissions from the spacecraft (from materials outgassing and thruster operations) interact with the local exosphere (and associated jet plumes from the sub-surface oceans of the icy moons). Spacecraft molecular effluents collide with molecules from the ambient exosphere, and a fraction of the spacecraft emissions are returned to the spacecraft and its complement of science instruments. The rate at which the emitted molecules are returned to the spacecraft by collisions with other molecules (ambient scatter in the exosphere) is known as the molecular return flux. Return flux of molecular emissions from spacecraft sources (flight system and instruments) contribute to contaminant deposition on contamination sensitive instruments. Characterization of return flux is critical to the definition of requirements for materials outgassing (for the flight system and instruments) and for the definition of thruster operations. Typical spacecraft configurations have several sources of organic contamination. A major source of organic contamination is materials outgassing from solar arrays (if solar powered), the spacecraft bus, propulsion module, electronics vault, high-gain antennas, and science instruments. A second major source of contamination are thruster firings from monopropellant or bipropellant systems. Analysis results using materials outgassing rate data from typical spacecraft materials demonstrate the magnitude of the molecular return flux to the spacecraft and scie