A molecular beam facility for the calibration of spaceborne neutral mass spectrometers

A new molecular beam calibration facility, the MOlecular Beam for Instrument Utilization and Simulation (MOBIUS), has been developed to enable realistic, in situ testing and calibration of spaceborne neutral mass spectrometers. The facility is based on a differentially pumped beamline featuring a high-pressure, high-temperature nozzle source and modular diagnostic capabilities. MOBIUS is specifically engineered to reproduce the high-speed, directed neutral particle fluxes encountered by spacecraft in planetary exospheres and upper atmospheres. We demonstrate the system's performance by characterizing supersonic beams generated across multiple gases. When operating with seeded gas mixtures (hydrogen-argon), the facility reliably produces neutral beams in the 3-5 km s-1 range, successfully achieving the high relative velocities required for instruments such as the Strofio mass spectrometer onboard BepiColombo. Furthermore, neutral flux is verified to be on the order of 1015 cm-2 s-1, and the resulting beam profiles are confirmed to be uniform and well-collimated. These results establish MOBIUS as a complete and necessary testbed for accurately characterizing the response of instruments under relevant in-orbit conditions.