Direct Measurement of Neutral Gas during Hypervelocity Planetary Flybys

Fausch, Rico G.; Wurz, Peter; Cotting, Björn; Rohner, Urs; Tulej, Marek (2022). Direct Measurement of Neutral Gas during Hypervelocity Planetary Flybys. In: 2022 IEEE Aerospace Conference (AERO) (pp. 1-12). IEEE 10.1109/AERO53065.2022.9843767

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Planetary flybys at hypervelocity provide the opportunity for sensitive, in situ chemical composition analysis performed with mass spectrometers to identify both simple and complex species in search of the origin and evolution of life in the Solar System. Current mass spectrometric instrumentation handles the fast flow of neutral gas (and ions) streaming into the instrument with an open or closed ion source system. In an open source system, the particles enter the ion source directly without contact with any element, are ionized there, and then are deflected into the mass analyzer section by electrostatic means. The advantage of an open source is that there is no particle alteration by surface interactions, but the electrostatic deflection limits the upper velocity of incoming particles to about 5 km/s. In a closed source system, the particles enter an antechamber through a small hole first. Many collisions with the chamber wall thermalize them before they enter the actual ion source as thermalized gas. The advantages of a closed source system include the large velocity range and the increased sensitivity resulting from the ram pressure enhancement. Its disadvantage is that the particles may fragment, or chemically alter as they hit the wall leading to complicated fractionation patterns in the recorded mass spectrum. Thus, the larger the molecules are the more difficult, or even impossible, inferring the original molecule becomes. Here we present a novel ion optical system for a mass spectrometer that directly measures the atmospheric species at orbital velocities with an open source system, but without ion deflection into the mass analyzer section to overcome the disadvantages of both techniques and unite their advantages. Furthermore, the mass analyzer features a selectivity for the velocity of the analyzed gas allowing for the first time for sensitive mass spectrometric analysis of the atmospheric gas, while reliably knowing the influence of the background gas resulting from spacecraft outgassing. As a first realization, we built an ion optical system for 1U of a CubeSat (CubeSatTOF). Our results demonstrate that the sample acquisition system preserves the chemical nature of molecules even when relative encounter velocities of up to 20 km/s are required. Its mass range is about m/z 1 to 300, thanks to its mass resolution exceeding 300 (FWHM), designed for an application of analyzing Earth's exosphere. In addition, we designed a larger system (OpenTOF) with a mass range of about m/z 1 to 1,000 and a mass resolution of up to 1,000 (FWHM) for flybys of Enceladus, Io, Europa, Ganymede, Venus, and others, capable of analysis of complex (bio) molecules.

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