BepiColombo's Mercury Flyby Reveals How Solar Storms Penetrate Planetary Magnetospheres

Eight years into its journey, the BepiColombo spacecraft has spent the better part of a decade circling Mercury, collecting data on a world that remains among the least understood in our solar system. The mission, a partnership between the European Space Agency and Japan's space agency, has made six close passes at the planet closest to the sun. On the fourth of those approaches, in September 2024, something rare and valuable happened: the spacecraft found itself in exactly the right place at exactly the right moment to witness how a solar storm tears through a planetary shield.

BepiColombo came within 165 kilometers of Mercury's surface on September 4th, 2024. At that same instant, the sun unleashed a burst of high-energy charged particles—a solar eruption that sent waves of radiation outward into the solar system. Aboard the spacecraft, an instrument called the Solar Intensity X-ray and Particles Spectrometer, or SIXS, was watching. Built by engineers at the University of Helsinki, SIXS detected something crucial: a large volume of those solar particles penetrated Mercury's magnetic field and struck the planet's surface across a wide area. When those particles collided with atoms and molecules on the ground, they generated X-rays—a signature that SIXS could measure and record.

Emilia Kilpua, a space physics professor at the University of Helsinki and the lead author of a new study published in Nature Astronomy, called the moment extraordinary. "The spacecraft came much closer to the surface than it will be on its final orbit, and we were lucky that a major particle eruption occurred on the Sun at exactly that moment," she said. That convergence of proximity and timing gave researchers something they rarely get: a direct measurement of how solar radiation batters a world with a weak magnetic field.

Mercury's magnetic field is far smaller and weaker than Earth's. Its magnetosphere—the bubble of magnetic protection that surrounds the planet—is correspondingly diminished. This makes Mercury a natural laboratory for understanding what would happen to Earth if a truly catastrophic solar storm struck. When the sun unleashes its most violent outbursts, it can compress Earth's magnetosphere, driving radiation deeper into near-space and toward the upper atmosphere. The SIXS observations of Mercury offer a window into that scenario. Rami Vainio, a space physics professor at the University of Turku and co-principal investigator for SIXS, explained the stakes plainly: the data helps researchers assess how destructive particle radiation would penetrate Earth's near-space environment and atmosphere during the most powerful space storms.

That assessment matters because solar storms are not theoretical threats. Major solar events have damaged satellites, knocked out electrical grids, and disrupted other critical infrastructure both in space and on the ground. Understanding how radiation behaves when it encounters a weak magnetic field—as it does at Mercury—gives scientists tools to predict what could happen to Earth and to prepare defenses. The findings from this flyby are already being fed into the Center of Excellence in Space Resilience, a program designed to keep commercial and scientific operations in low Earth orbit safe even under the most extreme space weather conditions.

Meanwhile, BepiColombo itself is approaching the next phase of its mission. The two orbiters that make up the spacecraft—the Mercury Planetary Orbiter and the Mercury Magnetospheric Orbiter—separated from their transfer module earlier this month. In November, they will enter Mercury's orbit. By December, they will separate from each other and begin their independent investigations. The data from that September flyby, captured in a moment of rare alignment between spacecraft position and solar violence, will inform those investigations and the broader effort to understand how planets shield themselves from the sun's most dangerous moods.