UK technology helps BepiColumbo on its way to Mercury

Any long-duration space mission is a deeply impressive enterprise. However, a journey to the inner solar system — where the radiation from the sun is at its most intense and the gravitational forces are extreme — tests the ingenuity of space technologists to the limit.

For this reason, the BepiColumbo mission, due to set off on a mind-boggling six-year, seven-billion-kilometre voyage to Mercury in 2014 is one of the most challenging unmanned European space missions ever to be embarked upon.

The €665m (£584m) project, led by the European Space Agency (ESA) and relying on a significant contribution from UK-developed space technology, is intended to achieve a scientific first by delivering two probes to Mercury’s orbit. One of these, the Japanese-developed Mercury Magnetospheric Orbiter (MMO), will monitor the magnetic field around the planet, while the other, the European-developed Mercury Planetary Orbiter (MPO), will construct a detailed map of the planet’s entire surface. The mission is expected to learn significantly more about Mercury than previous missions to the solar system’s innermost planet on NASA’s Mariner and Messenger spacecraft, which conducted flybys and did not enter orbit.

However, before they can begin probing Mercury’s secrets, they have to get there and this, claims EADS Astrium’s Dr Jerry Bolter, who is heading up the UK involvement in the mission, is not going to be easy. ‘Mercury is just about the most difficult planet to get to,’ he said. ‘As you approach the sun, you get faster and faster, so we have to spend a lot of our time in reverse gear.’

To achieve this, the two orbiters — mounted on top of a transfer module — will perform a series of carefully choreographed flybys of the Moon, Earth, Venus and Mercury itself. The gravitational forces encountered during these flybys, and the thrust provided by a UK-developed solar electric propulsion system, will slow the spacecraft down just enough so that it can be captured by the relatively weak gravitation field of Mercury. An additional chemical propulsion system is used to make correction manoeuvres during the planetary flybys.

The module, the chemical propulsion system and the structure of the MPO are currently being developed by EADS Astrium engineers in Stevenage, while the £23m contract for the solar electric propulsion system was recently awarded to Qintetiq.

Using electricity generated by solar panels covering the module, this system consists of four ion thrusters that work by accelerating the inert gas xenon. Electric propulsion systems are proving increasingly popular for long-duration missions and satellites, but this is the first time that solar energy has been used as the primary power source and it is hard to imagine a more elegantly appropriate source of fuel. ‘As you get nearer the sun, you need more thrust, but as you approach the sun, you have more power because you’re getting closer to it,’ said Bolter.

Once the composite spacecraft feels the first tug of Mercury’s gravity — around 100,000km and two months away from the planet — the transfer module is jettisoned and a third, more conventional chemical propulsion system drives the two probes, still attached to each other, towards their orbital starting points.

The Japanese MMO probe will separate first and the rocket will then drive the MPO to a lower operational orbit.

However, with the main purpose of the mission now under way, the other big challenge comes to the fore: the heat. This is the hottest part of space that any European mission has ever flown to.

Not only do the orbiting probes have to cope with the heat from the sun — which can reach 470°C — they also have to tolerate similarly searing temperatures from the surface of Mercury itself. Ensuring that the inside of the probe remains at a working temperature not much higher than 20°C requires a further touch of nifty cosmic choreography that involves flipping the spacecraft over every half orbit of Mercury around the sun so that the radiator panel on the probe is always pointing

away from the sun. ‘There are times when the spacecraft has to sit between the sun and Mercury,’ said Bolter. ‘So not only do you have the sun on one side, but you also have this rather hot rock at quite close proximity radiating infrared to you.’ The MPO is designed so that one face is always protected from the sun and the other five faces of the cube — protected by a specially developed multi-layer insulation system — are allowed to point at the sun at some point during the orbit.

The two probes will have very different orbits. The MPO will be never be more than 1,500km from the surface of the planet and will get as close as 400km. The MMO will also get this close, but will stray up to 12,000km away form the surface in its efforts to monitor Mercury’s magnetic field.

The MPO, meanwhile, will be equipped with a suite of instruments — including Leicester University’s MIXS device — that is being developed to look at various different wavelengths from infrared to ultraviolet. This will allow scientists to map the planet’s mineralogy and elemental composition and determine whether or not the interior of the planet is molten.

The probe is also equipped with an accelerometer, which will be used to explain some of the peculiar features of Mercury’s orbit. ‘If you apply Newtonian physics to its orbit, it doesn’t quite work,’ said Bolter. ‘You have to go to Einstein’s theory of gravitation to actually confirm why it’s on the orbit it is, so the accelerometer on board will actually confirm Einstein.’

BepiColombo is a large and costly mission and one of the ‘cornerstones’ of the ESA’s long-term science project. However, in an era when spending on space science is under greater scrutiny than ever, does it warrant this status? Bolter believes so. ‘We’re going to learn something about one of the most mysterious planets in the solar system — we don’t really know why it’s there, we don’t know why it’s as dense as it is. It could help us perhaps understand the origins of the solar system,’ he said.