US and Europe plan new spaceship
Europe and the US could be building a spaceship together later this decade.
It is one of the ideas being considered as Europe ponders the next evolution of its ATV orbital freighter.
The sophisticated robotic vessel is used to transport up to 7.5 tonnes of supplies to the space station, but only three more units are in production.
Europe is now looking to develop a derivative of the ship and a joint venture with the Americans on a future vessel is being discussed.
The European Space Agency's Director General, Jean-Jacques Dordain, said the new concept must leverage the capabilities of the existing vehicle, such as its automatic rendezvous and docking technology, but that its precise role was up for debate.
He wants the broad concept laid out by the autumn so that he can present it to Esa member states for consideration.
"We shall work with the US space agency in a way that I can present in October a proposal on a new vehicle that in my view should be derived from the ATV, but which for the first time will be embedded into a common vision between Nasa and Esa," he told BBC News.
"I cannot tell you what type of vehicle it will be because this is not something I can define myself; this is something we have to define together - Nasa and Esa.
"What I have in mind is a part that will be built by Europe and a part by the United States; and which together can make a transportation vehicle."
The Automated Transfer Vehicle is the biggest spaceship ever built in Europe. Although unmanned in flight, once attached to the International Space Station astronauts can move freely around inside it.
At launch, it weighs some 20 tonnes, and its primary purpose is to haul fuel, water, air, food and equipment to the space station.
Its colossal load forms the basis of the "subscription fee" Europe pays to belong to the orbiting project.
But with only three more freighters set to fly beyond the vessel already attached to the international outpost, Esa thinks it imperative that Europe define a follow-on as soon as possible so that the industrial expertise gained on ATV can be maintained and developed.
While the new concept ship is still something of a blank sheet of paper, it will have to fit with what the ISS partners hope to achieve at the station in the coming decade and the 2020s.
First and foremost, the ISS will be a busy platform for doing science experiments in microgravity. But the partners also want to see the station become a testbed for the technologies and techniques humans will need when they push out beyond low-Earth orbit to explore destinations such as asteroids and Mars.
It is in this context that the future ATV-derived vehicle could play some kind of role - manned or unmanned.
Potential projects it could get involved in might include the assembly at the ISS of the components required to make a deep-space craft. This craft might even undertake a brief foray beyond the station; and the ISS partners have already discussed the idea of repeating the 1968 Apollo 8 mission that saw three astronauts loop behind the back of the Moon.
"People are thinking about ideas that have increasing degrees of complexity," explained Esa's ISS programme manager, Bernardo Patti.
"First, perhaps it is a check-out mission of an exploration vehicle to the ISS. The second is to make a sortie that goes well beyond low-Earth orbit - with a crew or without a crew. And then having an Apollo 8 type of visit around the Moon makes perfect sense," he told BBC News.
Whatever concept emerges from the discussions between Esa and Nasa, its adoption will have to fight for funding in an extremely tight fiscal environment.
Germany is the lead nation in Europe on the ISS project. Its contribution accounts for some 40% of the Esa part of the station programme, and the head of the German space agency (DLR), Professor Jan Woerner, told the BBC he did not think the finances were available currently to proceed with a post-ATV ship.
Professor Woerner's own preference would be for Europe to develop a capability to safely re-enter the Earth's atmosphere - something the present ATV cannot do; it burns up as it falls back to Earth.
"Of course all the numbers have been defined within the financial crisis and my hope is that in three years' time or so, we will have new numbers, new chances, new possibilities - and that we can raise this question again."