Suppressing the spacecraft background in neutral mass spectrometers using a velocity filter.

When investigating tenuous densities, in situ mass spectrometry is often limited by the background of the spacecraft. Technological advancements continue to improve overall sensitivity and mass resolution, but background gas not associated with the targeted environment continues to be the limiting factor for future exploration. In space-based applications, background gas is generated through outgassing of the spacecraft and instrument and varies depending on conditions such as illumination, temperature, and particle exposure. This is particularly difficult when studying tenuous atmospheres, such as the exosphere of Mercury for example, where the number density of a species of interest can be less than the number density of outgassing constituents from the spacecraft. To mitigate this problem, either a background suppression system is needed or the outgassing rate and its temporal variation need to be documented and, if possible, corrected for in later data analysis. Strofio is a neutral gas mass spectrometer and one of the scientific instruments on the BepiColombo mission to study the exosphere of Mercury, equipped with a velocity filter for background gas suppression. We simulated the velocity filter under Mercury-like conditions, where the spacecraft travels at 3 km/s relative to the exospheric gas, and we verified its performance in the laboratory. Our results demonstrate that a properly optimized configuration can reduce the contribution from background gas by a factor of 40, significantly improving the in situ detection of neutrals. This highlights the effectiveness of velocity-filtering techniques for studying Mercury's exosphere and other tenuous planetary atmospheres.