September 15, 2026 — Southwest Research Institute hardware will soon enter orbit around Mercury as part of the BepiColombo mission that includes two spacecraft — the European Space Agency’s (ESA) Mercury Planetary Orbiter (MPO) and the Japan Aerospace Exploration Agency’s (JAXA) Mercury Magnetospheric Orbiter (MMO). SwRI led the design and fabrication of NASA’s Strofio, an instrument aboard MPO.
Last week, the BepiColombo spacecraft completed a major milestone ahead of Mercury orbit insertion and the start of the science mission early next year, releasing the two orbiters from the transfer module. When the orbiters separated from the transfer module, the protective covers were removed from Strofio.
Strofio is a neutral mass spectrometer specially designed to sample the sparse exosphere around Mercury, measuring the particles emanating from its surface. The basketball-sized instrument weighs just over 7 pounds and promises scientists a better understanding of the surface composition and space environment of Mercury — the smallest rocky planet of the solar system orbiting closest to the Sun.
Orbiting Mercury with a spacecraft is extremely challenging and complicated — and BepiColombo will be only the second mission to do so, following NASA’s MESSENGER spacecraft (2011 to 2015). As the spacecraft travels toward the Sun, solar gravity forces it to pick up speed. Multiple gravitational assists from Earth, Venus and Mercury are then needed to slow down the craft. Rather than the four to five months it would take for a flyby mission, an orbiting mission takes almost 10 years in a looping trajectory to achieve the precise speed needed to enter orbit around the rocky planet. It is easier to orbit Jupiter than Mercury even though Jupiter is much farther away.
The BepiColombo mission, launched in 2018, is expected to enter orbit around Mercury in November 2026 and will achieve its final orbit in early 2027. SwRI Staff Scientist Dr. Frederic Allegrini will help analyze the data that Strofio collects during its mission. He notes the challenges with orbiting Mercury, particularly its close proximity to the Sun.
“The surface of Mercury is so hot that landing a probe there and having it survive long-term is challenging,” Allegrini said. “Studying the composition of Mercury’s surface from a landing site would be fantastic but would provide that information for only one location on the entire planet. Instead, with Strofio, we will measure the surface composition from sampling the planet’s exosphere from an orbit, which also gives us a more global knowledge rather than that from a single landing site.”
Mercury’s gravity is too weak to sustain a full atmosphere and can only support a tenuous outer atmosphere called the exosphere. Strofio will be sampling particles from the surface that get kicked up by processes such as meteor impacts and the solar wind.
“Mercury is an interesting object — kind of an oddball compared to the other planets,” Allegrini said. “It is much denser than you would expect, and its core is disproportionately large. We expect Strofio data to answer a lot of scientific questions related to its surface composition, and we also suspect some surprises.”
Allegrini along with Jared Schroeder, a postdoctoral researcher at SwRI, will be analyzing Strofio data to look for clues about Mercury’s origin story.
“We are hoping to clear up some of the mysteries around Mercury,” Schroeder said. “For instance, what is the composition of Mercury’s surface and how does it interact with the exosphere? The material that Strofio will measure comes directly from the surface.”
Mission operations for Strofio are based out of the University of Michigan, but SwRI will receive the data for analysis. Strofio is part of the SERENA suite of instruments aboard BepiColombo.
The BepiColombo spacecraft was named in honor of Giuseppe Colombo, a 20th Century Italian scientist and mathematician, who did some of the first detailed studies of Mercury. His calculations showed how to get a spacecraft into a solar orbit to encounter Mercury by using a gravity assist maneuver with Venus.
For more information, visit Planetary Science or contact Mike W. Thomas, +1 210 522 2255, Communications Department, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166.