It should be signposted 鈥淲elcome to hell鈥�. Mercury, the sun鈥檚 closest neighbour, sees blasts of radiation and extreme temperatures 鈥� and it鈥檚 where we鈥檙e headed next.
In October 2018, a probe called BepiColombo will set off on an epic seven-year voyage to orbit the scorched world. Provided it can withstand the unforgiving 350掳C after it arrives in 2025, the craft will try to unravel some of the enigmas left after observations by two previous missions to the planet.
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Mariner 10 swept past Mercury in 1974 and found, to everyone鈥檚 surprise, that it had a magnetic field, whereas Venus, Mars and the moon don鈥檛. More mysteries arose when Messenger visited a few years ago. In 2009, it confirmed that Mercury has a tenuous upper atmosphere of charged particles too sparse to constitute a true atmosphere. In 2012, it spotted what appeared to be ice buried in deep craters at the poles.
BepiColombo may confirm that the material is indeed solid ice, and work out how it survives. 鈥淚t could be that the ice is always in shadow, so sunlight never shines on it, explaining why it鈥檚 been there for billions of years,鈥� says Johannes Benkhoff of the European Space Agency, which is running the mission alongside the Japan Aerospace Exploration Agency.
Messenger also saw bright hollows in the planet鈥檚 plains. Volatile material may make these pits by lifting off from the surface and floating into space, maybe sublimed by intense heat from beneath. If BepiColombo finds that material does indeed leave the planet, it would be a 鈥渟ensation鈥�, says Benkhoff, because it would indicate geological activity on a planet thought to be inactive.
BepiColombo may also confirm Einstein鈥檚 theory of general relativity. Mercury鈥檚 warped orbit might be explained through relativistic effects, and because we can precisely pin down the craft鈥檚 position, we can see if the planet鈥檚 path obeys the theory.
But getting there will be no cakewalk. 鈥淚t鈥檚 easier to get to Pluto,鈥� says Benkhoff. The craft will have to brake against the sun鈥檚 huge gravitational pull, so must take a circuitous route involving one Earth flyby, two of Venus and six past Mercury itself.
This article appeared in print under the headline 鈥淢ission to Mercury鈥�
