By Jonathan Amos
Science reporter, BBC News, Friedrichshafen
The European Space Agency (Esa) has signed an industrial contract to build a probe to send to the planet Mercury.
BepiColombo will launch in 2013 on a seven-billion-km flight to the innermost world, arriving in 2019.
The 350m-euro (£260m) deal with EADS Astrium will lead to the production of major spacecraft components in Germany, Italy, France and the UK.
BepiColombo will be one of Europe's most sophisticated scientific missions to date, Esa says.
"One of the key questions of planetary science is to understand the evolution of our Solar System," explained Dr Johannes Benkhoff, Esa's project scientist on the mission.
"And for that, Mercury is a candidate where we need to go. It is a planet of the extremes. It has huge temperature variations, it is the planet with the highest density and it has a very harsh radiation environment."
The signing comes in the same week as the US has passed by Mercury with its Messenger probe, the first spacecraft to visit the planet in more than 30 years.
Researchers hope that by following hard on the heels of the Americans, BepiColombo can help tie down the answers to the big questions that still remain over how this oddball world came into being.
The mission is a joint endeavour with the Japanese.
Closest planet to the Sun; smallest in Solar System
Visited by Mariner 10 in 1970s; by Messenger this week
Diameter: 4,880km, about one-third the size of Earth
Second densest planet in Solar System; 5.3x that of water
Caloris basin is largest known feature (1,300km in diameter)
Possibility of water-ice in permanently shadowed craters
Huge iron core takes up more than 60% of the planet's mass
Surface temperatures swing between 425C and -180C
Has an extremely thin atmosphere (exosphere)
Only inner planet after Earth with global magnetic field
Europe will produce a Mercury Planetary Orbiter (MPO) that will be equipped with 11 scientific instruments. Flying in a polar orbit, it will study Mercury for at least a year, imaging the planet's surface, generating height profiles, and collecting data on Mercury's composition and wispy atmosphere.
Japan will be responsible for the Mercury Magnetospheric Orbiter (MMO). It will investigate the planet's magnetic field with its five on-board instruments.
Like Earth (but unlike Venus and Mars) Mercury has a global magnetic field.
How so small a world, which appears - at the surface at least - to be utterly inactive, can produce this field is a major puzzle to planetary scientists.
Just as Messenger is a major technological advance on the Mariner 10 spacecraft which flew past the planet in the 1970s, so BepiColombo intends to improve still further the quality of the science return by introducing even more innovative approaches.
"Messenger paves the way for us," Dr Benkhoff told BBC News. "But unfortunately the Messenger will investigate in detail only one quarter of the planet; and so BepiColombo, because we are going into a roundish, much closer orbit, will be able to investigate the planet as whole."
The mission will provide:
- The most comprehensive, highest resolution global coverage using its advanced suite of instruments
- The first thermal infrared images, to determine surface composition and create global temperature maps
- The first global three-dimensional (stereo) imagery of the surface features of Mercury
- The best ever measurements of Mercury's gravity environment, which will help test Einstein's theories (as classical Newtonian mechanics do not adequately describe the way the planet's orbit behaves in the deep gravity well produced by the Sun)
- The first double-point studies, with two orbiters sampling the environment in two locations at once
BEBICOLOMBO MISSION TO PLANET MERCURY
BepiColombo employs a Fregat booster (1) to give it the initial impetus to move away from Planet Earth
The mission then relies on a dual propulsion module (2) to control its passage into the inner Solar System
(3) The European Mercury Planetary Orbiter carries 11 instruments, one of which is supplied by Russian scientists
For the cruise phase, the spacecraft stack uses a sunshield (4) to protect Japan's Mercury Magnetospheric Orbiter (5)
At Mercury, the Japanese orbiter dispenses with its sunshield and spins to prevent surfaces overheating
The MMO and MPO also separate from each other at Mercury and go into different polar orbits
Total length of mission stack components is five metres; launch mass is three tonnes (50% of that is fuel)
But the close orbit to Mercury and the proximity to the Sun mean engineers face a number of major challenges. The biggest by far is the thermal environment.
BepiColombo will be baked directly by the Sun, receiving some 14,000 watts per square metre; about 10 times what a spacecraft in orbit around Earth would receive.
With some surfaces being roasted to temperatures in excess of 350C, BepiColombo will need multi-layer insulation and a highly efficient radiator to keep scientific instruments and electronics operating at normal temperatures.
Some parts of the spacecraft, though, cannot be hidden away.
"One example is the solar array which must be kept below 250C because that is the limit that can be withstood by the solar cells and their electronics," explained Dr Rainer Best, EADS Astrium's BepiColombo project manager.
"So you actually employ a trick; you have an array that comprises 60% mirrors and only 40% active cells. Mirrors will reflect the heat. We will also incline the array so the Sun is not perpendicular to it."
Esa admits however that it still has to prove some material technologies for flight and this work will continue over the next few years using a vacuum chamber that can simulate the blinding heat of the Sun.
The mission is named for Giuseppe (Bepi) Colombo, a 20th Century researcher from the University of Padua, Italy, who completed many pioneering studies on Mercury.
Although the industrial contract is worth 350m euros, the overall cost of the mission is much higher. Once operational and launch costs are included, together with the costs of the instruments, which are borne by national governments, and the Japanese contribution - the overall value of BepiColombo is about 965m euros.
Both orbiters, together with their dual solar-electric and chemical propulsion system, will be launched on a Russian Soyuz-Fregat rocket from the Kourou spaceport in French Guiana.
BEPI'S EUROPEAN ORBITER
Europe's main unit is the Mercury Planetary Orbiter
MPO will operate in tight 400 by 1,500km orbit; 2.3-hour period
Seeks comprehensive, high-resolution global coverage
Will study surface and internal composition of the planet
Systems will experience high temperatures and radiation
Time in orbit: One year nominal plus a year's mission extension
The contract signature ceremony took place here at Astrium's Friedrichshafen facility, in the presence of the local German state of Baden Wurttemberg's prime minister, Dr Guenther Oettinger.
Astrium Germany's core partners on the manufacturing of BepiColombo are Thales Alenia Space in Italy and the Astrium UK division.
About a quarter of the value of the Esa contract will be passed through Britain.
It will be responsible for the structure of the entire spacecraft including the launch vehicle adapter.
"The structure forms the whole backbone of the assembly," said Dr Jerry Bolter, Astrium UK's project manager.
"It carries the instruments, it carries the heavy tanks; it enables the entire suite of instruments and equipments to survive the rigours of launch. And then later on, it maintains the relative pointing between the different elements of the science spacecraft. We believe it is one of the most crucial [parts of the mission]."
The UK will also do the complex mission analysis that will require numerous swing-bys of the Earth, the Moon, and Venus in its six-year flight plan; and also the dual propulsion system. The solar-electric engine work is subcontracted to Qinetiq.