The crucial point, McCaughrean said, is that scientists cannot be sure just how habitable an exoplanet is until they understand conditions on a world that orbits near a star. Hence the importance of Mercury. However, the little world – the most cratered planet in the solar system – offers other important goals for science.
Another mystery that astronomers hope to solve is the puzzle of Mercury’s magnetic field. It is the only rocky planet in our solar system, other than Earth, that has one, though it is about 100 times weaker than ours. However, studies by the US probe Messenger – which orbited Mercury between 2011 and 2015 – discovered that this field is offset from the planet’s centre by about 500 kilometres, an astonishing amount for a little planet.
“In fact, most theories about the formation of our solar system suggest that a tiny world like Mercury should not have a magnetic field at all, so we are facing a real mystery,” said McCaughrean. “We cannot claim we understand how our solar system formed if we cannot explain why Mercury possesses such an anomaly. We have real theoretical issues to resolve, and hopefully BepiColombo will let us do that.”
In addition, BepiColombo will seek out regions, in high latitudes, where its predecessor Messenger found evidence of water ice inside the unlit walls of craters. Their existence is another of the puzzles that astronomers have uncovered on this little planet.
Designing and building a craft that can achieve these goals has not been easy. BepiColombo – named after the 20th-century Italian mathematician and engineer Giuseppe Colombo – is actually a twin spacecraft: a European planetary orbiter that will study the planet and a Japanese craft that will study the planet’s magnetic field. Both will be carried to their target by a propulsion module known as the Mercury transfer module.
The mission was approved in 2000 by the European Space Agency in collaboration with Jaxa. However, the complexities of flying a probe so close to the Sun forced design delays that raised costs from several hundred million euros to the probe’s current estimated price tag of about €1.6bn. It also triggered years of delay in building the satellite. Indeed, the project came close to termination when several member states, including Britain, urged that no extra funds be made available after costs over-ran to several hundred million euros. It took weeks of complex negotiations to save the project.
The main problem facing BepiColombo was that the mission’s principal orbiter has to be flown so that it hovers over the searing hot surface of Mercury while the Sun beams down on it. “Solar radiation is 10 times the level on Mercury than it is on Earth because the planet is so close to the Sun,” said Mauro Patroncini, of Thales Alenia Space Italia, which built much of the probe. “At the same time, Mercury’s surface is so hot – about 430C – that it generates a massive flux of infrared radiation and heat of its own.” Essentially the satellite will be grilled on both sides: the Sun on one and Mercury on the other.
“We thought we could deal with that with conventional techniques when we were designing the craft, but realised in 2006 that we would have to develop new technologies, including temperature-resistant coatings and insulated instruments, to stop the craft overheating violently,” said Patroncini. “That is what caused the delays and cost over-runs.”
The danger of overheating was endorsed by ESA project manager Ulrich Reininghaus, who also stressed the problems that lay ahead for the mission. “We’re flying into a pizza oven. We had to test materials at different, very high temperature regimes, sometimes with very unwanted results.” Hence the delays in launch.
Constructing a craft that can endure such hellish conditions has proven to be a gruelling business. There were tears of frustration over the delays, and a great deal of sweat was expended in redesigning a craft that has pushed engineers to the limit of their abilities, said McCaughrean. Then there was the construction race after all the delays.
“BepiColombo has sheets of tough insulating material that has had to be sown, by hand, into position in its sun shield and other parts,” he said. “A colleague went into the probe’s assembly area one morning and found blood spattered beside it. Another member of staff had stabbed themselves with a needle overnight as they stitched up the thermal blankets. So you could easily say that blood, sweat and tears were expended in getting this mission ready for launch.”
BepiColombo is a probe of many parts: a massive sunshade, a planetary orbiter, a second orbiter that will study the planet’s magnetic field, and a transfer vehicle that will carry these various components to Mercury after the mission’s launch on an Ariane 5 rocket from Europe’s spaceport in Kourou, French Guiana, in October 2018.
The craft will weigh more than 4 tonnes, including 1.4 tonnes of propellant that will be used to power it on a complex journey round the solar system en route to Mercury. The trip will take seven years and will involve making one flyby of Earth, two of Venus and six of Mercury in order to position the craft when it reaches its destination in 2025 in a way that makes it easy to slip into orbit around the planet.
“We could fire BepiColombo straight at Mercury and it would get there in a few months, but we would have to use all our fuel decelerating to reach the planet at a reasonable speed,” said Mark McCaughrean, the ESA’s senior scientific adviser. “That is why we are taking the slow way.”
Once in orbit round Mercury, the transfer vehicle will release its two orbiters. The European craft will map Mercury while the Japanese craft will hover further from the planet and study its magnetic field. BepiColombo is designed to operate for one year, but with the option of running for a second. Privately ESA officials hope they will get up to three or four years out of the craft.