“We found that cosmic rays behave very similarly with respect to the Sun at Mars as they do at Earth, and are strongly influenced by the solar cycle,” adds Elise. “As the Sun grows more active and hosts more sunspots, we see fewer cosmic rays, as our star deflects more of them. However, this ‘anti-correlation’ is seen around 5.5 months later – it isn’t immediate – and the reason for this time lag remains an intriguing open question.”

Comparing the EDAC measurements from Mars Express and Rosetta also showed that cosmic ray counts increase by around 5% per ‘astronomical unit (AU)’, with one AU being the distance between the Earth and the Sun.

In situ data, especially science data, is rare in much of the Solar System, and observations of the radiation around other planetary bodies are relatively sparse. Although spacecraft do not carry out routine science observations as they cruise through space en route to their destination, they are always collecting housekeeping data.

“This study emphasises the immense value of archiving this kind of data, and is a great example of using a spacecraft itself as a scientific instrument,” says ESA planetary scientist Olivier Witasse, co-author. “This approach allows us to do science without a spacecraft’s core research instruments even being switched on – a particularly relevant and exciting option for long interplanetary cruises, when instruments often lie dormant as they await the mission ahead.

“We can potentially use any and all spacecraft in this way, not just those equipped with particular sensors. This opens up a new realm of possibility for both current and forthcoming ESA missions to discover even more about the space environment.”

The range of distances to the Sun covered by EDAC observations is expanding with ESA’s Gaia, BepiColombo and upcoming Juice (JUpiter ICy moons Explorer) missions.