How a spacecraft ‘feels’ a planetary flyby

Caption: A spectrogram visualising the effects of the 1-2 October 2021 Mercury flyby on the ESA/JAXA BepiColombo spacecraft, created from data recorded by the Italian Spring Accelerometer (ISA) onboard the Mercury Planetary Orbiter. Closest approach took place at 23:34:41 UTC on 1 October.

ISA recorded the vibrations and the movements of the spacecraft as it flew past the planet. These detections are not audible to the human ear but have been ‘sonified’ and matched to a frequency plot to better visualise the different events.

The first two distinct sounds correspond to the spacecraft entering and exiting the shadow of Mercury, respectively. This results in a change in solar radiation pressure on the spacecraft, and a change in photon flux striking the Mercury Transfer Module’s solar panels, causing them to flex. In turn, the spacecraft’s centre of mass changes slightly, and the attitude control system compensates. 

Another distinct sound is heard at around 00:05 UTC, corresponding to the PHEBUS instrument arriving back in its ‘parking’ position. PHEBUS has a baffle that changes the field of view by rotating around an axis, and when the baffle opens or closes from the parking bracket, it is detected by ISA.

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