T he joint BepiColombo missionOpens in a new tab of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) is settling into orbit around Mercury after an eight-year journey. In early September, BepiColombo's two orbiters, the Mercury Planetary Orbiter and the Mercury Magnetospheric Orbiter, separated from the transfer module. The orbiters will enter Mercury's orbit in November and separate from each other in December.
During its eight-year journey, the spacecraft has already flown past Mercury at close range six times. During its fourth flyby in September 2024, BepiColombo came remarkably close to the surface, at a distance of just 165 kilometers. During the flyby, electrons and protons accelerated by a solar eruption bombarded the planet. The SIXS particle and X-ray detectorOpens in a new tab , designed and built in Finland for BepiColombo, detected how these particles penetrated the planet's magnetic field and rained down onto its surface.
These measurements from the SIXS instrument have now been published in the prestigious Nature Astronomy journalOpens in a new tab .
"The fourth flyby was truly unique," says Emilia Kilpua , principal investigator of SIXS and professor of space physics at the University of Helsinki.
"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."
SIXS revealed that during such an eruption, a large number of highly energetic particles reach Mercury's surface across a wide area. The planet has no significant atmosphere to protect it.
The high-energy particles that rain down on the surface knock atoms and molecules loose from Mercury's surface and generate X-ray radiation. These processes provide insights into Mercury's surface composition and how the planet has evolved. When in orbit, several instruments can study these processes together.
"Mercury's magnetic field is weaker than Earth's, and its magnetosphere is much smaller than Earth's," says Rami Vainio , co-Principal Investigator (co-PI) of SIXS and professor of space physics at the University of Turku.
The conditions at Mercury resemble those that would occur on Earth when a powerful solar storm compresses its magnetosphere.
"SIXS's observations help us assess how destructive particle radiation would penetrate Earth's near-space environment and atmosphere during the most powerful space storms," Vainio says. The SIXS observations will also be used in the Center of Excellence in Space Resilience (2026–2033), funded by the Research Council of Finland, in which Kilpua's and Vainio's research groups participate.
BepiColombo is a joint mission of the European and Japanese space agencies. The spacecraft was launched in October 2018. SIXS, designed and built in Finland, measures X-ray radiation and high-energy particles from the Sun. Finland is also strongly involved in the MIXS instrument, which measures X-ray radiation coming from Mercury's surface. In total, BepiColombo's two orbiters carry 16 scientific instruments.
Mercury is the planet closest to the Sun, making it exceptionally strongly affected by solar activity. BepiColombo is only the third spacecraft to study Mercury at close range, and Europe's first mission to the planet. Previously, only two American spacecraft, Mariner 10 and MESSENGER, have studied the planet..
Nature Astronomy
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Planetary shielding and surface precipitation of solar energetic particles during BepiColombo’s close Mercury flyby
8-Sep-2026
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