The ESA/JAXA BepiColombo spacecraft made another flyby of its eventual target, Mercury. This is one of a series of Mercury flybys, as the spacecraft completes a complex set of maneuvers designed to deliver it to the innermost planet’s orbit. Its cameras captured some fantastic images of Mercury.
BepiColombo will eventually enter orbit around Mercury in November 2026. However, Mercury is a challenge to visit because of its proximity to the Sun and the Sun’s overwhelming gravity. To eventually orbit Mercury, the spacecraft is performing six gravity-assist flybys of the Solar System’s innermost planet. This is the 4,100 kg spacecraft’s fourth flyby.
The images are a bonus. The spacecraft’s monitoring cameras captured them, and those cameras are there to keep an eye on the spacecraft itself. But in this situation, they were able to image Mercury and some prominent craters. As BepiColombo approached and passed by Mercury, different monitoring cameras were able to capture images.
The closest approach during the recent flyby was on September 4th. BepiColombo—named after Italian scientist Giuseppe “Bepi” Colombo—came within about 165 km of Mercury’s surface. This was the first time that the spacecraft had a view of the planet’s south pole.
Even though Mercury is so close, it’s seldom visited. BepiColombo is only the third spacecraft to visit the small planet after NASA’s Mariner 10 mission in 1974/75 and Messenger mission from 2011 to 2015. Its proximity to the Sun is a complex challenge.
“BepiColombo is only the third space mission to visit Mercury, making it the least-explored planet in the inner Solar System, partly because it is so difficult to get to,” said Jack Wright, ESA Research Fellow, Planetary Scientist, and M-CAM imaging team coordinator.
“It is a world of extremes and contradictions, so I dubbed it the ‘Problem Child of the Solar System’ in the past. The images and science data collected during the flybys offer a tantalizing prelude to BepiColombo’s orbital phase, where it will help to solve Mercury’s outstanding mysteries,” said Wright.
The next flyby is only a few months away, on December 1st, 2024. The final one is on January 8th, 2025.
BepiColombo is actually two orbiters in one. Once it enters Mercury’s orbit, it’ll separate into the ESA’s Mercury Planetary Orbiter (MPO) and the JAXA-built Mercury Magnetospheric Orbiter (MMO) or Mio. The Mercury Transfer Module is the spacecraft that delivers the pair of orbiters.
There’s a lot we don’t know about Mercury, where it originated, and how it evolved so close to its star. The spacecraft will study Mercury physically, its form, interior, structure, geology, composition, and abundant craters. It’ll also study the planet’s exosphere.
Instead of an atmosphere, Mercury has an exosphere, a region consisting of atoms blasted off the planet’s surface by the Sun and by impacts. The exosphere is dynamic and changes according to how the solar wind interacts with the surface. Studying it is an opportunity to study planetary evolution and space weather. Understanding the exosphere is also critical to future missions, especially any potential landers, because it can affect spacecraft operations.
BepiColombo will also study Mercury’s magnetosphere and magnetic fields. Mercury’s global magnetic field is extremely weak, only about 1% as strong as Earth’s. This is mysterious since the planet seems to have a large iron core.
Mercury’s magnetosphere is also an object of interest. The powerful solar wind shapes it and prevents it from rising much above the surface. The magnetosphere is also very dynamic and quickly responds to changes in the solar wind, making it a natural laboratory to study the physics of magnetospheres. Its weakness also challenges our understanding of how planetary dynamos function.
BepiColombo was initially scheduled to reach Mercury’s orbit in December 2025. However, a problem firing its thrusters during a maneuver in April 2024 added 11 months to the mission. The revised orbital insertion will be in November 2026.
Once it reaches the rapidly moving Mercury, we’ll start to learn more than ever about this sometimes overlooked planet.