It seems like it should be pretty easy to get to Mercury. The little rocky planet is so much closer to Earth than distant destinations like Jupiter, where we’ve successfully sent multiple spacecraft. Plus, it doesn’t have a crushing atmosphere like our nearest neighbor Venus.
But, in fact, it’s actually really difficult to reach the innermost planet of our solar system—which makes it that much more impressive that the ESA and JAXA’s BepiColombo mission has almost reached Mercury, recently completing its final flyby of the planet before entering orbit next year.
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Reaching Mercury is such a challenge because “the gravitational pull of the Sun is very strong near Mercury, which makes it difficult for spacecraft to slow down enough to enter orbit around the planet,” explains Lina Hadid, staff scientist at CNRS in France and principal investigator of one of BepiColombo’s instruments.
“One needs a lot of fuel with a very big rocket for launch, or one can use the help of the planets and do planetary flybys,” adds Johannes Benkhoff, ESA BepiColombo Project Scientist. “We needed nine flybys (one at Earth, two at Venus, and six at Mercury) to slow down our spacecraft and bring it into a good position to enter the orbit of Mercury.”
Sequence of 89 images taken by the monitoring cameras on board the European-Japanese BepiColombo mission to Mercury, as the spacecraft made a close approach of Venus on August 10th, 2021. Credit: ESA/BepiColombo/MTM
Plus, the situation doesn’t get any easier once you’re near Mercury. There’s no atmosphere to help a spacecraft slow down like there is at Venus or Mars. And so close to the sun, a spacecraft has to withstand extreme heat and radiation.
Only two missions have reached Mercury before BepiColombo: NASA’s Mariner 10 flybys in the 1970s and MESSENGER, which orbited the small world from 2011 to 2015. Mariner 10 showed us that Mercury’s surface is cratered and barren, lacking any atmosphere but possessing a huge metal core. MESSENGER revealed dormant volcanoes, water ice hiding in shadowed craters, and an off-center magnetic field in the planet.
There’s still a lot about this curious world that scientists have yet to figure out. Why is Mercury so unusually dense? Why does it have a magnetic field, but the Moon doesn’t? How does the solar wind change the planet’s geology?
Bepi-Colombo aims to address these questions with thorough observations of the planet’s surface over two years once it enters orbit in November 2026. “This will allow for in-depth mapping and long-duration studies that will provide a more complete picture of Mercury’s geology, internal structure, and plasma environment,” says Hadid.
The mission is actually two satellites in a trench coat masquerading as one until they reach their destination: the Mercury Planetary Orbiter (MPO) and Mercury Magnetospheric Orbiter (MMO). “These two spacecraft, with different orbits and perspectives, will allow us to study Mercury from multiple vantage points simultaneously,” says Hadid. The satellites include multiple spectrometers and cameras to image Mercury’s surface at different wavelengths of light, plus magnetometers, accelerometers, dust counters, and more to paint a full picture of the environment around the planet.
Even though the official mission doesn’t start until it enters orbit next year, BepiColombo has already done some neat science on its journey. “We learned a lot about the solar wind, the magnetospheric environment around Mercury and Venus, and we could test our experiments to be better prepared for the nominal mission,” says Benkhoff.
This most recent and final flyby passed over Mercury’s North pole, providing the team with an opportunity to spot some permanently shadowed craters and observe ions around this key part of Mercury’s magnetic field. Bonus: there are some incredible images of the planet from this flyby, from the closest vantage point any spacecraft has had around Mercury.
“Mercury is a surprise. Every flyby was different,” adds Hadid. “We are ready to make exciting observations after the orbit insertion in November 2026!”
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