Analysis of 3GM High Accuracy Accelerometer data collected during JUICE LEGA

The JUpiter ICy moons Explorer (JUICE) is an ESA L-class mission dedicated to studying Jupiter and its Galilean moons. Launched on April 14, 2023, the spacecraft will reach the Jovian system in mid-2031 after a series of Earth and Venus gravity assists. On August 20, 2024, JUICE successfully completed the first-ever Lunar-Earth Gravity Assist (LEGA), involving a Moon flyby at 777 km altitude the day before.

The spacecraft’s scientific payload is composed by 10 instruments. Among them there is the 3GM (Gravity and Geophysics of Jupiter and the Galilean Moon) radio science package, which includes a Ka band Transponder (KaT) for gravity measurements, an Ultra Stable Oscillator (USO) for dual-frequency downlink experiments, and a High Accuracy Accelerometer (HAA). The HAA calibrates non-gravitational accelerations in the frequency band [10⁻⁴–10⁻¹ Hz], primarily due to propellant sloshing. The LEGA event provided a valuable opportunity to calibrate most of the JUICE instruments by collecting data near celestial objects. The Moon closest approach (CA) occurred on August 19, 2024, at 21:14:56 UTC, was preceded by an Earth occultation, started around 20:36 UTC and ended at 21:09 UTC, during which the spacecraft experienced solar radiation pressure (SRP) drops and abrupt temperature changes.

The HAA collected data at a 10 Hz sampling rate in the -2h/+1h time interval around Moon CA, although it was switched on 48 hours before to ensure thermal stability. Active thermal control was disabled during LEGA, since previous inflight tests had evidenced the need for an optimization of thermal control setting parameters. Analysis of HAA data along the spacecraft’s +Z axis revealed gravity gradient accelerations closely matching theoretical predictions. Additional dynamic signals were also detected during the umbra phase. These included oscillations at 0.45 Hz caused by vibrations of the magnetometer boom, excited by a 72° rotation of the SWI antenna. During eclipse egress, a rapid transition from umbra to full illumination caused SRP “kicks” and temperature spikes, exciting the 0.13 Hz solar array vibration mode. These thermal snap effects are consistent with expectations from mathematical modelling, and similar trends with opposite signs have been observed during the eclipse ingress, albeit with smaller oscillation amplitudes due to a smoother illumination-umbra transition.

HAA data from other axes, while noisier and more susceptible to temperature effects, also matched the expected gravity gradient signal and revealed to be perfectly co-aligned with signals caused by vibrations induced by SWI antenna operations. Despite the absence of active thermal control during LEGA, the HAA proved effective in capturing dynamic perturbations, demonstrating its capability to support JUICE’s scientific objectives.