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BepiColombo’s second date with Mercury
A second swingby at the solar system’s smallest planet.
On June 23, 2022, the joint European/Japanese Space Agencies’ mission to Mercury, BepiColombo, took a second swing past the solar system’s smallest and innermost planet, donating some of its orbital energy so the spacecraft can drop into an orbit closer in to the Sun.
This was only the second of six total passes of Mercury — it also did the same maneuver at Earth once and with Venus twice. On this flyby of Mercury it flew about 200 kilometers above the planet’s surface, and as it did so it took a series of images of the rugged, broiling surface.
BepiColombo is actually three spacecraft: the ESA Mercury Planet Orbiter and the JAXA Mercury Magnetospheric Orbiter, or Mio — both scientific orbiters — mated together by the Mercury Transfer Module, or MTM, which acts as the guidance and propulsion for the whole kit and caboodle. The two science orbiters are locked down until orbit is achieved, so these images were taken by the MTM’s Monitoring Cameras, or M-CAMs. There are three of these 1024x1024 cameras mounted at various places on the MTM to take images that monitor the spacecraft itself, but can also see into space beyond.
And hey, sometimes a planet happens to be there.
That spectacular shot shows the magnetometer boom (the instrument that detects the strength and direction of Mercury’s magnetic field) on the left and an antenna on the bottom right, as well as part of the spacecraft body along the bottom. But Mercury itself steals the show!
There’s a huge 200-kilometer-long scarp, or cliff, called Challenger Rupes that runs to the upper left from the bottom middle; the scarp face is 2 km high. Just above it is a huge impact crater also about 200 km wide, with multiple concentric rings characteristic of such a colossal event.
This second image was taken from about 1,400 km above the surface, after closest approach as the spacecraft was moving away. It shows a more heavily cratered area, and at least one of these craters is a candidate volcano — the one at about the 10:00 position around the circular antenna. One thing BepiColombo is going to do is look at the volcanic history of Mercury by examining lava flows and structures. This will help planetary scientists not only understand the surface better but also the processes and composition of the planet far below its surface.
It’s amazing how much it looks like the Moon, but Mercury is larger and denser. The Moon’s density is less than Earth’s, indicating much less metal in its interior, whereas Mercury has roughly the same density as Earth, meaning it has a lot of metal in it. It may have once been a bigger planet, and a giant grazing impact sheared off the lighter, rocky material making up its mantle and crust, leaving behind the denser core. This would have profoundly affected the planet’s magnetic field, too, generated deep inside it, which is why the JAXA Mio part of this mission is so important. It’ll map out the magnetic field and hopefully send back insight into what’s going on inside Mercury.
Getting to Mercury is hard. You can’t just aim your rocket toward the Sun and burn away; there’s a lot of sideways motion due to Earth’s orbital velocity. The spacecraft has to lose that energy, and to get to Mercury it has to lose a lot. So the techs planned a trajectory that passed Earth and Venus, allowing BepiColombo to donate energy to us and drop it down toward Mercury, a maneuver called a gravity assist, or more popularly (though less accurately) a slingshot. Then it has to pass Mercury six times in total to be able to insert itself into a high, loosely bound orbit. After that happens, in 2025, the two orbiters will separate and each do short engine burns to adjust into the desired orbits around the planet.
That’s made from 56 M-CAM images taken on this pass and covers about 15 minutes of real time. BepiColombo is moving fast.
Mercury is also a hard planet to see from Earth. Its smaller orbit means it’s always near the Sun in the sky, and it moves so rapidly that sometimes it’s only visible before sunrise or after sunset for a week — right now it rises less than an hour before the Sun, making it a difficult target. I’ve spotted it many times over the years, but if you live in a place with crowded or cloudy horizons it may never be visible. The best way to see it — which I suppose is true for any planet, but particularly for Mercury — is to send a spacecraft there. The ridiculously successful MESSENGER mission showed us this planet in detail undreamed of, and BepiColombo will be a terrific follow-up.