Spacecraft Outgassing Observed by the BepiColombo Ion Spectrometers

During the first flyby of the BepiColombo composite spacecraft at Mercury in October 2021 ion spectrometers observed two intense spectral lines with energies between 10 and 70 eV. The spectral lines persisted also at larger distances from Mercury and were observed again at lower intensity during cruise phase in March 2022 and at the second and third Mercury flyby as a single band. The ion composition indicates that water is the dominant gas source. The outgassing causes the composite spacecraft to charge up to a negative potential of up to -50 V. The distribution and intensity of the lower energy signal depends on the intensity of low energy electron fluxes around the spacecraft which again depend on the magnetic field orientation. We interpret the observation as being caused by water outgassing from different source locations on the spacecraft being ionized in two different regions of the surrounding potential. The interpretation is confirmed by two dimensional particle-in-cell simulations. The BepiColombo spacecraft is on its way through the inner solar system in a composite configuration consisting of two satellites and a propulsion unit with two large solar arrays. This configuration will only be separated after orbit insertion in December 2025. During the cruise phase and planetary flybys in the years 2021-2023 the ion spectrometers onboard the two satellites observed strong fluxes of low energy positive ions. We interpret these observations as being caused by outgassing of water from the spacecraft and a negative charging of the spacecraft caused by a high electron density surrounding the spacecraft. Around the first Mercury flyby in October 2021 all ion spectrometers observed two separate peaks in the low energy ion spectra. We explain these as being caused by water molecules being ionized by strong photon and electron fluxes in different regions of the negative potential surrounding the spacecraft. From these different potential regions ions are accelerated back to the spacecraft. Strong outgassing from the BepiColombo spacecraft was observed during first Mercury Flyby in 2021 and later during interplanetary cruiseThe gas composition is dominated by water moleculesThe ion energy spectra sometimes show a double band structure which we interpret as being caused by different ionization locations within a negative spacecraft potential