How to get to Mercury

The third-ever mission to the planet Mercury is under way. BepiColombo was launched aboard the European Space Agency's most powerful rocket, the Ariane 5. The mission costs €1.6 billion and will reach Mercury – the planet closest to the Sun – on December 5th, 2025.

A real challenge is slowing the spacecraft down once it reaches Mercury, the smallest rocky planet in the Solar System. "Mercury is hard to get to, because as you launch a spacecraft towards the sun it starts to speed up," explains Prof Peter Gallagher, astrophysicist at Trinity College Dublin (TCD) and expert advisor to the ESA mission.

The Bepi mission could reach Mercury in six months, but it would then need to brake. "Once you leave Earth, at 22,500 km/h, you are going too fast to be captured by Mercury's gravity, unless you have a whopping big engine to slow you down," explains Dr Paul Byrne, a TCD graduate and now planetary scientist at North Carolina State University. Jupiter is about 10 times further away, yet missions can get there faster, he says.

To slow down, Bepi will sideswipe Earth's gravity in 2020, then swipe Venus in 2020 and 2021, before bumping off Mercury's gravity six times as it circles the Sun. When the spacecraft reaches Mercury in December 2025, it will have gone around the Sun 18 times. Once it is close to the planet, it will open up and release a planetary orbiter for close-up study of Mercury, while a magnetosphere orbiter from the Japanese space agency will sample charged particles zipping out from the Sun (the solar wind).

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‘Strange and inhospitable’

"Mercury is a very strange and inhospitable planet," says Gallagher. "Just over two-thirds the diameter of the Earth, and thought to be mainly made of iron." Nasa has gone to Mercury twice. This will be Europe's first time. The first Mercury mission was Mariner 10, launched in 1973, and the second was Messenger, launched in 2004. Dr Byrne studied volcanoes on Mars for his PhD in Trinity, launching a rare career as an Irish planetary scientist. In 2011, he joined Nasa's Messenger team as it reached Mercury, spending the next four years analysing data beamed back to Earth from the spacecraft.

Jupiter is about 10 times further away, yet missions can get there faster

This revealed a dense planet, with most of its interior structure consisting of iron, so hot that almost all of it is probably molten. The metal core gives Mercury a gravitational field twice that of the Moon, as well as a magnetic field. “A compass would work on Mercury,” says Byrne, but not on Mars. “The core of Mercury makes up about 85 per cent of its radius, which is not the case for Mars or Earth. The actual rocky part on top is only about 420km thick, which is crazy.” This thin rocky shell gives the planet a lunar-like landscape, pockmarked by asteroid impacts.

Our Solar System formed out of a disc of gas and dust, hot in the centre and cool at the edges. Particles clumped together, eventually forming planets or moons. Near the centre, rocky material that could withstand the heat formed rocky planets like Earth. Icy matter and gases coalesced in outer regions, making gas giants like Jupiter.

Presence of chemicals

One surprise from Messenger was the presence of chemicals on Mercury that, it was thought, should have boiled off in the planet's presumed hot birth. "The formation temperature of Mercury seems to be lower than what our models of how the Solar System formed predict. Either our models are wrong or Mercury formed further out," says Dr Johannes Benkhoff, BepiColombo mission scientist at the ESA.

Mercury formed from the original dust cloud about 4.5 billion years ago, presumably close the Sun, or perhaps after massive collisions of early planets. Knowing which scenario is correct matters. “Mercury holds the key to understanding the early Solar System,” Byrne says. “If Mercury tells us what the ratio of metal and rock-forming elements was in that part of the cloud, that gives us a better handle on what conditions were like as the planets formed.”

Messenger flew a long-looped orbit around the planet. Bepi will fly a more circular route and stay closer to the planet at all times. Bepi has similar instruments, but also extras such as thermal imaging and will generate better data. It could also spot any changes to the planet's surface since Messenger visited. Some scientists suggest that crewed landings will one day be possible on Mercury, but a robotic lander must come first.

Some scientists suggest that crewed landings will one day be possible on Mercury, but a robotic lander must come first

Conditions on the planet nearest to the Sun are extreme, even for machines. The side facing away from the Sun chills as low as minus 193 degrees, while the other side can swelter at 400 degrees. A lander would have to either withstand extreme temperature fluctuations, or bunker down in a darker polar area, where it would likely require powerful (and expensive) nuclear batteries. Interestingly, because Mercury is not tilted on its axis, craters near the poles never receive sunlight and seem to contain water ice, so a source of water may be present.

‘Fleet of spacecraft’

Mercury can also offer a view of worlds far, far away. “When I was watching TV shows growing up, the plucky hero would visit other planets. Yet it was not until 1995 that we had definitive evidence of planets outside our Solar System,” Byrne adds.

Initially, these were gas giants resembling Jupiter. Then, in 2014, the first Mercury-sized planet was discovered. “Mercury is a far more weird planet than we thought,” says Prof Byrne. “But is it a freak? It is a one-off in our Solar System, but perhaps this is a common type of planet [elsewhere in the Universe], which is why understanding how Mercury formed is so important.”

He argues that Mercury is just as interesting as Mars. “Personally, I’d send a fleet of spacecraft to Mercury,” he says.

This year, he helped write a White Paper setting out why Nasa should put a lander on Mercury and proposing a feasibility study to look at the challenges therein. A lander would allow sampling of rocks and other close-up analyses. “There is an amazing amount of stuff we could learn from a lander,” Byrne believes. The ESAs Dr Benkhoff agrees: The next big mission to Mercury should be a lander.