Solar energetic particles are among the most hazardous phenomena in the solar system. Unlike the steady wind of charged particles that flows constantly from the Sun, these episodic bursts can accelerate electrons, protons and heavier ions to a substantial fraction of the speed of light, driven by solar flares and the shock waves ahead of coronal mass ejections. At Earth, our planet’s strong dipolar magnetic field and thick atmosphere protect life and infrastructure from the worst of these events, although intense storms can still disrupt satellites, radio communications and power grids. At Mercury, the picture is radically different. The planet has only a weak magnetic field, roughly one percent of Earth’s surface field strength, and essentially no atmosphere, so any particle that penetrates its magnetosphere strikes bare rock directly.

BepiColombo is ideally placed to study this environment. The mission, a collaboration between the European Space Agency and the Japan Aerospace Exploration Agency, is en route to orbit Mercury with two scientific spacecraft, the ESA-led Mercury Planetary Orbiter and the JAXA-led Mercury Magnetospheric Orbiter. To reach its destination, the combined stack has been performing a series of planetary flybys, using gravity assists at Earth, Venus and Mercury to shed energy and adjust its trajectory. During one of its close Mercury flybys, the spacecraft passed through the planet’s magnetospheric environment at a time when the Sun was unusually active, turning a routine trajectory-correction maneuver into an unplanned natural experiment.

As the spacecraft approached the planet, its particle and field instruments registered the signature of a solar energetic particle event in progress. Outside the magnetosphere, the sensors recorded intense fluxes of energetic electrons arriving from the Sun. But as BepiColombo crossed into the region dominated by Mercury’s magnetic field, the picture changed dramatically. The fluxes of solar energetic particles dropped sharply, revealing that the planet’s magnetosphere, despite its modest strength, was acting as a genuine shield, carving out a protective cavity in the otherwise hostile radiation environment. This planetary shielding effect had been predicted by models, but direct in situ evidence of it during a major solar particle event had remained scarce.

The shielding, however, is not complete, and the second half of the story is arguably more important. The measurements showed that while most of the incoming particle population was deflected around the planet, a significant fraction of the energetic particles found their way to low altitudes and precipitated onto the surface. Charged particles can enter a magnetosphere along open magnetic field lines that connect the polar and near-polar regions to interplanetary space, and they can also be scattered into loss orbits by wave-particle interactions and by the complex, asymmetric structure of Mercury’s field, which is offset from the planet’s center and strongly compressed on the sunward side.

This surface precipitation matters because it is a primary engine of space weathering on Mercury. When energetic protons and electrons strike the regolith, they sputter atoms out of surface minerals, break chemical bonds and alter the optical and chemical properties of the soil. Over geological time, this bombardment contributes to the darkening and modification of Mercury’s surface that scientists observe remotely. It also plays a role in generating the planet’s tenuous exosphere, the wispy envelope of atoms knocked loose from the ground, including species such as sodium, calcium and magnesium that telescopes have long observed glowing around the planet. Quantifying how many energetic particles actually reach the surface during a storm therefore places a direct constraint on how fast these processes operate.

The event also carries implications for understanding Mercury’s magnetosphere as a system. Unlike Earth’s relatively stable dipole, Mercury’s magnetosphere is small, comparable in scale to the planet itself, and highly dynamic. Its boundary, the magnetopause, sits close to the surface, and during periods of high solar wind pressure it can be pushed down until the planet’s dayside field lines are compressed against the crust. Reconnection between the interplanetary magnetic field and Mercury’s own field opens the magnetosphere to the solar wind, funneling energy and particles into the magnetotail and down to the surface. Observing the system during a solar energetic particle event, rather than during quiet conditions, reveals how this delicate balance responds to extreme forcing.

The BepiColombo observations provide a benchmark for testing magnetospheric models under exactly these extreme conditions. Simulations of Mercury’s space environment must reproduce both the shielding of the bulk of the solar energetic particle population and the leakage that allows a subset of particles to reach the surface. Getting this right is essential not only for Mercury science but also for the broader problem of planetary habitability and radiation environments. Mercury represents an end-member case: a planet with a magnetic field too weak to fully protect it and no atmosphere to absorb what gets through. Studying how much shielding such a field provides informs our understanding of exoplanets around active stars, where close-orbiting rocky worlds may face relentless particle bombardment.

There is also a practical dimension for the mission itself. BepiColombo’s cruise phase, with its series of flybys, exposes the spacecraft to the full spectrum of solar activity near the innermost planet, and the solar energetic particle event during the flyby subjected the spacecraft’s electronics to elevated radiation doses. Understanding the particle environment around Mercury is therefore a matter of operational survival as well as science. When the two orbiters separate and begin their science phase in orbit around the planet, they will encounter these storms regularly, and the flyby measurements offer an early, detailed preview of what they will face.

For now, the flyby observations stand as a vivid demonstration that even a small planetary magnetosphere leaves a measurable imprint on a solar storm, deflecting most of the incoming radiation while simultaneously channeling a damaging fraction of it down to the airless ground. As BepiColombo continues its journey and ultimately settles into orbit, scientists expect many more opportunities to watch Mercury’s magnetic shield at work, transforming a fleeting flyby measurement into a long-term record of how the innermost planet weathers the Sun’s worst moods.

Subject of Research: In situ observations of planetary shielding and surface precipitation of solar energetic particles in Mercury's magnetosphere during a BepiColombo close flyby.

Article Title: Planetary shielding and surface precipitation of solar energetic particles during BepiColombo’s close Mercury flyby

Article References: Kilpua, E. K. J., Vainio, R., Grande, M., Edwards, L., Laurenza, M., Esko, E., Lehtolainen, A., Palmroos, C., Gieseler, J., Oleynik, P., Ho, G., Lawrence, D. J., Liu, S. J., Massetti, S., Heyner, D., Pump, K., Sanchez-Cano, B., & Huovelin, J. (2026). Planetary shielding and surface precipitation of solar energetic particles during BepiColombo’s close Mercury flyby. Nature Astronomy. https://doi.org/10.1038/s41550-026-02914-6

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02914-6

Keywords: BepiColombo, Mercury, solar energetic particles, magnetosphere, planetary shielding, space weathering, surface precipitation, solar storms, ESA, JAXA, exosphere, space weather