SwRI Strofio will measure the "exosphere" of Mercury

This colorful view of Mercury was made using images from the basic color map imagery campaign conducted during MESSENGER's main mission. Source: NASA / Johns Hopkins University / Applied Physics Laboratory / Carnegie Institution of Washington

The Space Shuttle BepiColombo of the European Space Agency will be launched to Mercury with a unique payload designed and built by the Southwest Research Institute: an instrument called Strofio, which will study the faint exosphere of Mercury. Strofio's measurements, which are part of the SERENA range of instruments, will help us better understand the surface of the planet and the history of the smallest rocky planet orbiting the Sun.

"An exosphere is different from an atmosphere," said Stefano Livi, a scientist at the institute that runs the Strofio experiment, funded by NASA's Discovery Mission of Opportunity program. "Mercury does not have enough gravity to maintain a proper atmosphere. Instead, it is surrounded by a fine particle environment, without collision. Particles escape from the Mercury surface and briefly populate this exosphere before returning to the surface or drifting into the interplanetary space. "

The proximity of Mercury to the Sun makes it difficult to observe from Earth. It is also difficult for spacecraft to reach and survive in a hostile environment. The BepiColombo mission includes two satellites: Mercury Planetary Orbiter from ESA and Mercury Magnetospheric Orbiter (MMO) from the Japan Aerospace Exploration Agency (JAXA), who will study Mercury and help us better understand the formation of our solar system. Onboard an MMO, Strofio will study the interactions between the exosphere and the magnetosphere of Mercury and the surface of the planet.

To understand these interactions, Strofio must identify the particles escaping from the Mercury surface. Since the exosphere is very thin, particle sampling is particularly difficult. In fact, the amount of particles is so rare that the environment can not be easily imitated in a typical vacuum chamber. The scientists had to tackle this problem while developing and evaluating the super sensitive Strofio instrument. The facilities at the University of Bern, Switzerland, could create the necessary parameters and allow SwRI, within a few months, to analyze and demonstrate the operation of the experiment in the Mercury environment.

"Strofio is innovative in its ability to detect the rare population of static particles in the Mercury exosphere," Livi said. "We had to rethink and rearrange the typical spectrometer designs."

Each captured particle is analyzed in a rotating field. When and where each particle arrives at the detector determines the mass and the composition. Strofio uses detection algorithms to improve the sensitivity of the instrument and improve its identification. The instrument also had to meet strict limits of mass and configuration associated with the spacecraft. The instrument the size of a basketball weighs a little more than 7 pounds.

"Strofio, it's like one of my kids is leaving for Mercury," Livi said. "It's exciting, I'm looking forward to the 2025 horizon when we'll see chemical particles come off the planet, and in addition to answering many questions, I'm also waiting for surprises."

BepiColombo is the first time that ESA and JAXA have joined forces for a major mission in space science. NASA frequently collaborates on ESA missions, including the Rosetta mission on the Comet 67P and the JUpiter ICy Moons Explorer mission (JUICE), which is scheduled for release in 2022.