Last week, the BepiColombo spacecraft achieved a major milestone on the road to Mercury orbit insertion, releasing the two orbiters from the transfer module that had carried them across hundreds of millions of kilometers. The mission consists of two distinct spacecraft: the European Space Agency’s Mercury Planetary Orbiter, known as the MPO, and the Japan Aerospace Exploration Agency’s Mercury Magnetospheric Orbiter, the MMO. SwRI led the design and fabrication of NASA’s Strofio instrument, which is mounted on the MPO. When the orbiters separated from the transfer module, the protective covers were removed from Strofio, exposing the instrument’s sensors to space for the final leg of the journey. The start of the science mission is expected early next year.

Strofio is a neutral mass spectrometer specially designed to sample the sparse exosphere that surrounds Mercury, measuring the particles that emanate directly from the planet’s surface. The instrument is remarkably small for the science it promises: it is roughly the size of a basketball and weighs just over seven pounds. Despite its modest dimensions, Strofio is expected to give scientists a substantially better understanding of the surface composition and the space environment of Mercury, the smallest of the solar system’s rocky planets and the one that orbits closest to the Sun. The instrument is part of the SERENA suite of instruments aboard BepiColombo, a collection of sensors dedicated to studying the exchange of particles between the planet’s surface and its tenuous atmosphere.

Getting a spacecraft into orbit around Mercury is one of the most demanding feats of orbital mechanics ever attempted, and BepiColombo will be only the second mission to accomplish it, following NASA’s MESSENGER spacecraft, which operated at Mercury from 2011 to 2015. The difficulty stems from a fundamental property of the inner solar system: as a spacecraft travels toward the Sun, solar gravity relentlessly accelerates it. Without intervention, a probe falling sunward would arrive at Mercury’s vicinity moving far too fast for the planet’s weak gravity to capture it into orbit. Multiple gravitational assists from Earth, Venus and Mercury itself are therefore needed to bleed off that excess energy and slow the craft down.

The consequences of this celestial braking maneuver are striking. Rather than the four to five months a simple flyby mission would require, an orbiting mission to Mercury takes almost ten years along a looping trajectory to achieve the precise speed needed for orbital insertion. Mission planners and commentators have long noted the counterintuitive comparison: it is in some respects easier to orbit Jupiter, a giant planet hundreds of millions of kilometers farther away, than Mercury. The BepiColombo mission, launched in 2018, is expected to enter orbit around Mercury in November 2026 and will achieve its final science orbit in early 2027, closing out an eight-year voyage across the inner solar system.

Dr. Frederic Allegrini, a staff scientist at SwRI, will help analyze the data that Strofio collects during its mission. He emphasizes the challenges posed by Mercury’s close proximity to the Sun, which make the orbital sampling approach not just convenient but essential. The surface of Mercury is so hot that landing a probe there and having it survive long-term is a formidable engineering challenge, he noted. A lander that could endure the extreme thermal environment would be fantastic from a scientific standpoint, but it would provide compositional information for only one location on the entire planet. Instead, with Strofio, the team will measure the surface composition by sampling the planet’s exosphere from orbit, an approach that yields more global knowledge of the surface rather than data from a single landing site.

The physics behind this technique is central to why Strofio exists at all. Mercury’s gravity is too weak to sustain a full, dense atmosphere like those of Earth, Venus or Mars. The planet can support only a tenuous outer atmosphere, called an exosphere, in which particles are so sparse that they rarely collide with one another and instead follow ballistic trajectories under gravity. This exosphere is not static; it is constantly replenished by processes that liberate material from the surface. Strofio will be sampling particles from the surface that get kicked up by mechanisms such as meteorite impacts and the solar wind, the continuous stream of charged particles flowing outward from the Sun. Because the exospheric particles originate at the surface, measuring their composition from orbit provides an indirect but planet-wide chemical census of the ground below.

Allegrini describes Mercury as an interesting object, kind of an oddball compared to the other planets. The planet is much denser than would otherwise be expected, and its core is disproportionately large relative to its size, a combination that has puzzled planetary scientists for decades. The team expects Strofio data to answer a significant number of scientific questions related to the planet’s surface composition, and they also suspect that some surprises await in the measurements. Any unexpected species detected in the exosphere could point to previously unrecognized surface minerals, unusual space-weathering processes, or exotic interactions between the solar wind and the airless terrain.

Allegrini will be joined in the analysis effort by Jared Schroeder, a postdoctoral researcher at SwRI, and together they will comb through the Strofio data looking for clues about Mercury’s origin story. Schroeder said the team is hoping to clear up some of the mysteries around the planet, including the composition of Mercury’s surface and how it interacts with the exosphere. The material that Strofio will measure comes directly from the surface, which makes the instrument’s observations a uniquely direct window onto the planet’s makeup. Understanding that composition, in turn, feeds into broader questions about how the innermost planet formed and evolved, and whether its outsized core reflects an unusual formation history, the loss of an early crust and mantle, or some combination of processes that shaped the young solar system.

The operational backbone of the mission spans multiple institutions on two continents. Mission operations for Strofio are based out of the University of Michigan, but SwRI will receive the data for scientific analysis, keeping the Texas-based institute at the center of the instrument’s science return. The BepiColombo spacecraft itself carries a name rich with history: it honors Giuseppe Colombo, the twentieth-century Italian scientist and mathematician who performed some of the first detailed studies of Mercury. Colombo’s calculations showed how to get a spacecraft into a solar orbit that would repeatedly encounter Mercury by using a gravity assist maneuver with Venus, an insight that underpins the very trajectory strategy the mission has now executed over its long, looping voyage. As the two orbiters settle into their final paths around the scorched little planet early next year, the basketball-sized spectrometer that spent nearly a decade in transit will finally get its chance to taste the exosphere and, through it, read the chemistry of Mercury itself.

Subject of Research: The Strofio neutral mass spectrometer aboard BepiColombo's Mercury Planetary Orbiter and its study of Mercury's exosphere and surface composition

Article Title: SwRI’s Strofio is gearing up to study Mercury’s surface

Article References: SwRI’s Strofio is gearing up to study Mercury’s surface. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Mercury, BepiColombo, Strofio, Southwest Research Institute, exosphere, mass spectrometry, Mercury Planetary Orbiter, planetary science, surface composition, solar wind, gravitational assists, ESA

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