Analysis of 3GM High Accuracy Accelerometer data collected during JUICE lunar earth gravity assist

Abstract. The JUpiter ICy moons Explorer (JUICE) mission, launched in April 2023 by the European Space Agency (ESA), is designed to investigate Jupiter and its largest icy moons, Ganymede, Callisto, and Europa, with a focus on assessing their potential habitability and investigating subsurface oceans. During its eight-year interplanetary cruise to the Jovian system, JUICE is scheduled to perform several flybys. The first of these, the combined Lunar-Earth Gravity Assist (LEGA), took place in August 2024. The spacecraft is equipped with a High Accuracy Accelerometer (HAA) that is part of the Gravity and Geophysics of Jupiter and the Galilean Moons (3GM) radio science instrument. During LEGA operations, HAA collected two hours of scientific data centered across the Moon's closest approach. We present here a detailed analysis of the HAA calibrated measurements that show a strong agreement with predicted non-gravitational accelerations, including those related to spacecraft deformation caused by the Moon's gravity gradient and thermoelastic displacements of the solar arrays during penumbra transitions. Additionally, unexpected dynamic responses were observed, including structural vibrations excited by the movement of the steerable telescope of the Submilimetre Wave Instrument (SWI) and a distinct outgassing event detected shortly after crossing the lunar terminator. The outgassing, likely involving sublimated water ice on the spacecraft, resulted in a measurable velocity change of 0.7 ± 0.1 mm s−1 along the -Z spacecraft axis and a consequent mass loss of a few grams. This direction coincides with the normal direction of the spacecraft's most exposed surface to the Moon illuminated surface. The JUICE orbital reconstruction derived from radio tracking data collected by the Deep Space Transponder (DST) confirmed a consistent velocity variation, supporting HAA findings. These in-flight observations are essential for instrument calibration, characterization of the spacecraft's dynamic environment, and refining operational strategies.