Wif he wants to go to Mercury, he has to brake properly first. The BepiColombo mission of the European Space Agency ESA and the Japanese space agency JAXA currently has to take this rule of thumb to heart. It was launched in 2018 to be only the third mission to make its way to the planet closest to the sun, into whose orbit it is to swing in 2025. Since its launch, it has slowed down once in the Earth’s gravitational field, twice in that of Venus and once in that of Mercury itself. On Thursday she passed Mercury for the second time – and sent photos of its surface.
The first image released by ESA a few hours after the image was taken at 11:49 a.m. German time shows the planet’s surface rich in geological features. The probe looked at the crater landscape from a distance of 920 kilometers. Five minutes earlier, it had come as close as 200 kilometers to Mercury. However, BepiColombo was still above the night side of Mercury. The still very oblique incidence of sunlight allows the geological features to stand out particularly clearly.
The images come from one of the three cameras on the Mercury Transfer Module (MTM), which carries the mission’s two satellites, the Mercury Planetary Orbiter (MPO) and the Magnetospheric Orbiter, to Mercury. Both are supposed to orbit the planet on different high polar orbits. The cameras of the transfer module are positioned in such a way that they have a solar panel and the MPO in view. The image of camera 2 also shows parts of the support rod for the magnetometer and an antenna of the MPO. The resolution of these cameras is still moderate at 1024 x 1024 pixels. The mission’s high-resolution scientific cameras will remain safely stowed during the voyage and will not be operational until Mercury orbit.
The image shows large impact craters. Visible behind the magnetometer rod, the multi-ring crater is about 200 kilometers across. To the right of the girder is a cliff that is also around 200 kilometers long and around two kilometers high. This month it was given the name “Challenger Rupes” after a 19th-century expedition ship. Some pictured craters are still quite young. A small crater emitting bright rays is visible in the upper right of the image. It’s material that was ejected on impact. These structures and the faults that can also be seen can provide clues to the inner structure and in particular the tectonics of Mercury – a topic that is also high on the scientific agenda of the main mission from 2026.
Finally, Mercury still holds some secrets, because as seen from Earth, it is always quite close to the Sun in the sky, making it the hardest to observe of all the planets with ground-based astronomy. The Mariner 10 probe passed it three times in 1974 and 1975, and the Messenger probe also flew by it between 2011 and 2015. The latter mission in particular raised a number of questions. For example, it turned out that the chemical composition of the surface deviated significantly from expectations. There are also geological features that are not known from other planets, such as the so-called “hollows” – depressions that are probably caused by outgassing on the surface and can be found in one of the craters on the top right of the current image. The surface also shows tectonic and volcanic activity. Mercury seems to have had a very active geological past. Understanding all this better was the motivation for sending BepiColombo on its way. The probe now has four more flybys to come. The next one will take place in a year.