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ESA and JAXA release Mercury eyecandy, courtesy of spacecraft BepiColumbo

Fourth of nine scheduled planetary assists completed as spacecraft inches closer to releasing its orbiters

The European Space Agency (ESA) and Japan Aerospace Exploration Agency's (JAXA) joint mission BepiColombo sent back its first photos of Mercury on Friday as it completed the fourth of nine planetary flybys enroute to study the solar system's smallest and innermost planet.

The spacecraft passed a mere 199km from Mercury's surface as it received a gravity assist. Once it reached a feasible distance away for photos (1,000km), it snapped and transmitted back to Earth 1,024 x 1,024 black and white images of the cratered celestial body photobombed by the transfer module's antennas and magnetometer boom.

The pièce de résistance tweeted by the mission took place at an altitude of 2,418km at 23:44 UTC. It depicts the planet's northern hemisphere, including previously lava-flooded plain Sihtu Planitia and the Rudaki Plains that surround the Calvino crater. Viewers can also see the illuminated 166km-wide Lermontov crater, a geographical feature full of what ESA calls "hollows" where volatile elements escape to space making the area on the image appear bright.

BepiColombo launched in 2018 for a seven-year cruise and is named for Italian scientist Giuseppe "Bepi" Colombo, the man who first proposed the now common interplanetary gravity assist manoeuvre on which the craft relies. So far in its journey BepiColombo has completed one Earth and two Venus flybys in addition to the Mercury flyby that took place Friday.

The remaining five Mercury flybys will position the spacecraft to release its two science orbiters in December 2025.

At that point, ESA's Mercury Planetary Orbiter (MPO) and JAXA's Mercury Magnetospheric Orbiter (MMO) will separate from their carrier, the Mercury Transfer Module (MTM), and study the planet in separate orbits for one year, or longer if extended, to understand the planet's core and surface processes, magnetic field, and exosphere.

Scientists also hope to verify if there is water in some of the craters near the poles as they never see sunlight.

As the only planet aside from Earth with a global magnetic field and the closest planet to its parent star, Mercury is of great interest to space boffins and BepiColombo is certainly not the first attempt to understand the planet.

Mariner 10 had three flybys in 1974, but documented only 45 per cent of Mercury's surface. It's a tough object to study given that half of the atmosphere-less planet faces the sun reaching 430°C while the dark side keeps extremely cold temperatures of -185°C.

While flybys have occurred, to study the planet in this case really does require the slingshotting brainchild of the spacecraft's namesake which is assisted by the solar-electric propulsion of its ion thrusters. The strong gravitational pull from the Sun makes direct approaches to Mercury tricky – the amount of fuel required to complete a necessary braking manoeuvre wouldn't fit on the spacecraft.

For those anxiously awaiting the flyby, ESA kindly put out a 36-song playlist full of positive affirmations and planetary references, complete with one Queen/Freddy Mercury tune, thereby narrowly dodging a missed opportunity. ®

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