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

The particle and X-ray detector known as SIXS, designed and built in Finland, recorded the entire encounter. Now the measurements have been published in the journal Nature Astronomy, offering scientists their first close-range view of how solar energetic particles penetrate a planetary magnetosphere as small and feeble as Mercury’s and precipitate onto the planet itself. The results matter far beyond one small, scorched world. They provide a natural laboratory for understanding what happens when the most violent space weather strikes a magnetosphere, knowledge that directly informs how researchers assess the threat such storms pose to Earth’s own technological infrastructure and atmosphere.

Emilia Kilpua, principal investigator of SIXS and professor of space physics at the University of Helsinki, describes the flyby as an extraordinary stroke of fortune. The spacecraft came much closer to the surface than it will on its final science orbit, and a significant particle eruption occurred on the Sun at precisely that moment. Observing a powerful solar energetic particle event from a few hundred kilometers above Mercury, with instruments built to measure exactly such particles, is the kind of convergence mission scientists rarely dare to hope for. Instead of a quiet pass used mainly for steering and instrument testing, the team obtained a dataset of genuine scientific value that would have been impossible to schedule.

What SIXS revealed is striking. During the eruption, a large number of highly energetic particles reached Mercury’s surface across a wide area, sweeping over the planet rather than being funneled into narrow polar cusps alone. Mercury lacks a substantial atmosphere, so there is no cushion of air to absorb or slow the incoming radiation the way Earth’s atmosphere shields the surface. The planet’s magnetic field, moreover, is far weaker than Earth’s, and its magnetosphere is correspondingly much smaller, leaving it far more exposed to the full force of solar outbursts. The measurement therefore captures, in real time and at close range, how effectively a planetary magnetic field can or cannot shield the ground beneath it when the Sun unleashes its energy.

The physics of what happens next on the surface is central to why these observations matter. When energetic electrons and protons slam into the regolith, the loose layer of dust and rock that coats Mercury, they knock atoms and molecules loose from the surface material in a process called sputtering. The same impacts also generate X-ray radiation, as particles decelerate and excite atoms in the crust. Both the ejected particles and the induced X-rays carry chemical fingerprints of the surface composition. By measuring them, scientists can deduce what Mercury’s crust is made of and how the planet has been processed by billions of years of exposure to solar radiation and micrometeorite impacts. In orbit, several BepiColombo instruments will study these processes together, cross-referencing particle measurements from SIXS with X-ray observations of the surface from the MIXS instrument, in which Finland is also heavily involved, to build a coherent picture of the planet’s chemistry and evolution.

Rami Vainio, co-principal investigator of SIXS and professor of space physics at the University of Turku, emphasizes the comparative dimension of the findings. Mercury’s magnetic field is weaker than Earth’s and its magnetosphere much smaller, but the conditions at Mercury during this event resemble what Earth would experience if a powerful solar storm compressed its own magnetosphere. Under extreme conditions, Earth’s protective magnetic bubble can be squeezed inward dramatically, allowing energetic particles to penetrate far deeper into near-Earth space than usual, threatening satellites, disrupting radio communications, and in the most severe cases depositing energy into the upper atmosphere. Mercury during this flyby effectively demonstrated the end-member scenario, a magnetosphere overwhelmed by particle flux, showing how such radiation propagates and where it ultimately lands.

This is why the SIXS observations are expected to feed directly into space weather research focused on our own planet. Understanding how destructive particle radiation penetrates a planetary near-space environment and atmosphere during the most powerful space storms helps researchers refine models of radiation dose at aviation altitudes, satellite orbital predictions, and the behavior of Earth’s magnetosphere under stress. The data will also be used in the Center of Excellence in Space Resilience, funded by the Research Council of Finland for the period 2026 to 2033, in which both Kilpua’s and Vainio’s research groups participate. That program brings together space physicists, engineers, and resilience researchers to anticipate and mitigate the effects of extreme space weather on society, a concern that has grown as solar cycle activity has intensified and as modern infrastructure has become ever more dependent on satellites and sensitive electronics.

The broader context of the measurement is the remarkable journey that made it possible. BepiColombo was launched in October 2018 and has spent eight years traveling to Mercury, using a series of flybys of Earth, Venus, and Mercury itself to shed the orbital energy that a direct trajectory to the Sun’s neighborhood would make prohibitively expensive in fuel. In early September, the mission reached a milestone: the two orbiters, the Mercury Planetary Orbiter built by ESA and the Mercury Magnetospheric Orbiter built by JAXA, separated from the transfer module that had carried them across the solar system. The orbiters will enter Mercury’s orbit in November and separate from each other in December, beginning the dedicated science phase in complementary orbits. In total, the two spacecraft carry sixteen scientific instruments, forming the most comprehensive payload ever sent to the smallest and least explored of the solar system’s eight planets.

Mercury holds a special place in that exploration history, and a humble one. It is the planet closest to the Sun, which makes it exceptionally strongly affected by solar activity, yet only three spacecraft have ever studied it at close range. Mariner 10 flew past three times in the mid-1970s, revealing a heavily cratered world with a surprising magnetic field. MESSENGER orbited the planet from 2011 to 2015, mapping its surface, confirming water ice in permanently shadowed polar craters, and revealing an offset magnetic field whose origin remains debated. BepiColombo is only the third mission to visit and Europe’s first, carrying the ambition not merely to revisit these discoveries but to understand the planet as a coupled system: its interior, its surface, its thin exosphere, and its magnetosphere, all responding continuously to the ferocious output of the nearby Sun.

The fourth flyby measurements now published add an unexpected early dividend to that mission. Long before the orbiters begin their systematic observations, SIXS has demonstrated how solar energetic particles behave at Mercury, how widely they spread across the surface, and what that means for both planetary science and space weather forecasting. The lucky timing of the September 2024 encounter has effectively given the BepiColombo team a preview of the storm-driven physics they will study in detail from orbit, and given space weather scientists on Earth a benchmark dataset drawn from the most sun-exposed planet in the solar system. As the Sun approaches and passes through the active phase of its cycle, the lessons learned from Mercury’s bombardment will resonate directly in models that protect satellites, astronauts, and power grids far from the scorched world where they were first measured.

Subject of Research: Solar energetic particle precipitation onto Mercury's surface measured by the BepiColombo spacecraft during its close flyby

Article Title: BepiColombo measured Mercury's particle bombardment up close

Article References: BepiColombo measured Mercury's particle bombardment up close. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: BepiColombo, Mercury, solar energetic particles, space weather, magnetosphere, SIXS instrument, ESA, JAXA, solar storms, X-ray radiation, planetary shielding, Nature Astronomy