By Jonathan Amos
Science reporter, BBC News, Vienna
British engineers are designing a Moon landing mission that would also test key technologies to take to Mars.
The MoonTwins concept would put two probes on the lunar surface - one at each pole - to do science experiments.
The work is being undertaken by the aerospace company Astrium UK at the request of the European Space Agency.
Esa plans eventually to go to the Red Planet to retrieve rocks for analysis on Earth, and the Moon is seen as a good place to develop the know-how.
"A Mars sample return mission would be very challenging and MoonTwins would help us understand some of the technology elements that would be needed," Mike Healy, director of space science at Astrium, told BBC News.
The agency expects to fly a demonstrator in the period 2015 to 2018. It will run a number of design phases before deciding on a final architecture.
In the MoonTwins (Moon Technological Walk-through and In-situ Network Science) scenario, the two spacecraft would be launched on the same rocket but would make their own way to the Earth's satellite.
In lunar orbit, they would practise coupling - a manoeuvre that would be required on any multi-stage Mars mission - before making their way down to the Moon's surface.
Again, the emphasis is on technology demonstration with each craft commanded to make a soft, controlled landing at a precise polar location.
It is also envisaged each probe would carry a seismometer. By sensing "moonquakes" they would provide scientists with a new way to study the satellite's interior.
It is expected, though, that the twins would be large enough to hold additional instruments.
"They won't have a roving capability but it's likely they would be able to do a short ascent, perhaps to hop to a new location," said Dr Healy.
The Moon is about to become a popular destination for exploration. Both the Japanese and the Chinese will despatch missions this year.
They will be followed in 2009 by the Indians and the Americans, who have two missions in preparation.
It is probable astronauts will be back on the surface by about 2020. At least one of the MoonTwins would be targeted at what could become a key location for human habitation - the so called Peak of Eternal Light.
This is close to the rim of Shackleton Crater at the south pole. The peak experiences near-continuous sunlight, making it an excellent location to site solar power units.
The crater itself has the opposite conditions - it is in permanent shadow.
"It's a good place to go because there could be hydrogen there," explained Esa's top Moon scientist, Dr Bernard Foing.
"It's an open question as to whether that hydrogen is in the form of water-ice or if it is simply solar wind hydrogen - elemental hydrogen in the lunar soil. But even if you've just got hydrogen, if you mix it with oxygen you can make water."
Step by step
Dr Foing said Esa would be looking at a number of proposals from different groups before settling on a final mission destination and design.
European space ministers would be asked to approve the final details, he added.
A Mars sample return mission, to bring Martian soil and rock back to Earth for analysis, is a high priority in Europe's space objectives.
It will be one of the most complex planetary missions ever proposed. All the stages of the mission must be faultless:
Send a spacecraft to Mars and land safelyDrill under the surface to collect a range of samplesSeal samples in a containerLaunch the samples into Martian orbitTransfer the samples to an Earth-return vehicleTravel back to EarthLand safely on EarthEnsure 100% planetary protection with no contamination of Mars or Earth - or of the samples on landing
The cost of mounting such an exercise means Europe will only do it in a partnership, most probably with the US.
The technology demonstrator will help determine which aspects of the mission will be built in Europe and which will be left to the partner.
Esa's future missions have been a topic of discussion here at the European Geosciences Union General Assembly meeting.