Characterization of the outgassing event during BepiColombo second Venus flyby using Italian Spring Accelerometer data

Abstract

The BepiColombo ESA-JAXA mission, launched on October 20, 2018, is scheduled to reach Mercury in November 2026. The Mercury Composite Spacecraft comprises three modules: the Mercury Planetary Orbiter, the Mercury Magnetospheric Orbiter, and the Mercury Transfer Module. Currently, BepiColombo is in its seven-year cruise phase, having completed one Earth flyby, two Venus flybys, and three Mercury flybys. The spacecraft is equipped with the high-accuracy Italian Spring Accelerometer, capable of measuring non-gravitational acceleration variations at frequencies between $[3\times 10^{-5},10^{-1}]\mathrm{Hz}$. Interpreting accelerometer data can be challenging due to overlapping dynamic effects. During the second Venus flyby, the accelerometer data revealed significant signatures of the gravity gradient signal induced by the planet on the proof masses. Notably, a large, unexpected acceleration spike was detected near the closest approach, lasting a few minutes. Further analysis determined that this spike was most likely caused by outgassing from the Mercury Planetary Orbiter radiator. This paper analyzes the Italian Spring Accelerometer data from the second Venus flyby, focusing on the unexpected acceleration spike. By combining the torque data from the reaction wheel with accelerometer data, the team was able to estimate the outgassing location, confirming it as the spacecraft radiator. Additionally, data from the Mass Spectrum Analyzer sensor, part of the Mercury Plasma Particle Experiment, suggest that $H_{2}O$ outgassing occurred. The estimated mass of sublimated water is approximately 2 grams.