Caught in a solar storm: BepiColombo watches particles rain onto Mercury

In September 2024, the joint ESA and JAXA spacecraft BepiColombo swooped just 165 kilometers above Mercury, and by sheer luck, the Sun was erupting at that exact moment. Its Finnish-built SIXS instrument caught energetic protons and electrons punching through Mercury’s feeble magnetic shield and raining down onto the surface. The results, published in Nature Astronomy by a team led by Emilia Kilpua, offer a rare close-up of how a nearly unprotected planet weathers a solar beating. And BepiColombo is now finally arriving: its two orbiters are set to slip into Mercury’s orbit later this year.

A planet with barely any armor

Mercury is the closest planet to the Sun and one of the strangest. It does have a magnetic field, but a weak one, roughly a hundred times feebler than Earth’s, wrapped around a small and jittery magnetosphere. It also has essentially no atmosphere. That combination means the solar wind, and the far more violent bursts from solar eruptions, can couple almost directly to the ground. When the Sun throws a tantrum, Mercury has very little to hide behind.

A lucky flyby

BepiColombo has looped past Mercury six times during its eight-year cruise, using the planet’s gravity to brake its way toward orbit. Its fourth pass, on 4 September 2024, brought it to just 165 kilometers above the surface, far closer than its eventual science orbit will be. As luck would have it, a major solar particle eruption was underway at that very moment, and the SIXS detector, an instrument built in Finland to measure energetic particles and X-rays, had a front-row seat to the storm.

Shielded, but not enough

Here is what the data show, and it cuts two ways. On one hand, SIXS recorded clear signs of planetary shielding: the solid body of Mercury and its weak magnetic field cast a kind of shadow, blocking some particles and carving dropouts into the measured fluxes, with the pattern depending on the particles’ type, direction and energy. On the other hand, that shielding was far from complete. During the eruption, large numbers of highly energetic protons and electrons still rained onto the surface across a wide area, in a sustained way rather than a brief flash.

Why a bombarded surface matters

Now the implications, framed as such. When these particles strike Mercury’s bare surface, they knock atoms and molecules loose and trigger X-ray emission, processes that over geological time could reshape the planet’s weathering and mineralogy, and that also feed the thin veil of gas surrounding it. That link to the surface is an inference the team draws from the particle measurements, one to be tested directly once BepiColombo is in orbit and several instruments can observe together. There is a bonus, too: Mercury during a solar storm resembles what Earth might face if an extreme storm ever squeezed our own magnetosphere hard enough, so these readings double as a natural laboratory for the most severe space weather. After an eight-year journey, BepiColombo’s two orbiters are due to enter Mercury’s orbit in the coming months, and the real work is only beginning.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: Emilia K. J. Kilpua et al., Planetary shielding and surface precipitation of solar energetic particles during BepiColombo’s close Mercury flyby, Nature Astronomy (2026). DOI: 10.1038/s41550-026-02914-6