A sonification of data recorded by the Italian Spring Accelerometer (ISA) aboard the BepiColombo spacecraft as it neared Earth ahead of the April 2020 flyby.

The data in the recording were obtained on 9 April as the spacecraft approached the planet from the distance of 256 393 km to 129 488 km. Eight Hours of measurements are condensed into a minute of audio. The original frequency of the dataset, inaudible to humans, had to be enhanced by the team from Italy’s National Institute for Astrophysics (INAF) in order to create the audio track.

Carmelo Magnafico, one of the ISA team members, compares the recording to the sound conducted through the rail when a train is approaching.

“What you are hearing is actually the sound of BepiColombo,” says Carmelo. “The vibrations caused by external and internal factors are transmitted into our accelerometer. It’s the same principle as when you put your ear on the rail to hear whether the train is coming.”

The ISA accelerometer consists of three spring-mass systems sensitive to the smallest deviations from the “freefall” state in the spacecraft motion dynamic. The instrument is so sensitive that it can detect the so-called solar radiation pressure, a tiny force that photons in the sunlight exert on the spacecraft as they hit its surface.

In another recording, obtained as the spacecraft flew through Earth’s shadow with no direct visibility of the Sun, the scientists could distinguish the moment when the sunrays stopped exerting pressure on the spacecraft’s solar panels.

“When the spacecraft enters the shadow and the force of the Sun disappears, we can hear a slight vibration,” explains Carmelo. “The solar panels, previously flexed by the Sun, then find a new balance. Upon exiting the shadow, we can hear the effect again.”

Hearing a clear effect of such a subtle influence showed the instrument is capable of registering the smallest differences in motion.

“This is an extraordinary situation,” says Carmelo. “Since we started the cruise, we have only been in direct sunshine, so we did not have the possibility to check effectively whether our instrument is measuring the variations of the force of the sunlight. This is a proof for us that the instrument is quite well-calibrated because the jump in the acceleration we measured is in line with our expectations.”

Once at Mercury, the ISA team will study the gravitational field of the smallest planet of the Solar System and its variations due to solar tides. Mapping local gravity anomalies will help scientists to better understand the internal structure of Mercury, a perplexing planet with a surprisingly large core and an unexpected magnetic field.

The video accompanying the audio sequence consists of images captured by three ‘selfie’ cameras aboard BepiColombo’s transfer module. The images correspond with the time when the audio recording was obtained.