By Paul Rincon
Science reporter, BBC News, Plymouth
Teams right across Europe are involved in the ExoMars project
The US and European space agencies are to discuss the potential for mounting joint missions to Mars during a summit underway in Plymouth, UK.
Nasa's science chief Ed Weiler told BBC News that co-operation made sense given the agencies' shared science goals and the growing expense of such ventures.
America is now likely to play a major role in Europe's 2016 ExoMars mission.
But Dr Weiler said the plan for exploiting later launch opportunities was still "a blank piece of paper".
Speaking to me in Plymouth, he said: "For many decades now, we've been running our own science programmes, especially on Mars and other planets. These missions are getting so, so expensive - approaching billions of euros or dollars."
Dr Weiler admitted that the two agencies had sometimes competed to achieve scientific "firsts".
But he added: "We have very similar scientific goals, maybe we ought to consider working together jointly on all our future Mars missions, so that we can do more than either one of us can do by ourselves.
"That's one of the key subject areas for this meeting: to see if Esa and Nasa can get together and come up - not with two Mars programmes - but with a 'western hemisphere Mars programme'."
Nasa is due to launch a $2bn (£1.2bn) nuclear-powered rover called Mars Science Laboratory (MSL) to the planet in 2011, while Europe is forging ahead with its ExoMars rover mission, due to launch in 2016. But what happens on subsequent launch opportunities in 2018 and 2020 is up for discussion.
Nasa's associate administrator for science is leading a delegation of about 10 top US space agency officials, including its planetary science director Jim Green.
They will be joined at the table by a European team headed by David Southwood, Dr Weiler's counterpart in Esa.
Professor Southwood told BBC News that the discussions, in his home town of Plymouth, would cover every aspect of potential co-operation, but that Mars exploration was perhaps the "most sensitive" issue of all.
The European and US space agencies are working towards a mission - called Mars Sample Return - to collect and return geological samples from the Red Planet.
"Everything is directed around that goal on the other hand, the architecture of it - how we do it - that's where we Europeans have to learn to work with the Americans, share ideas and start setting our priorities," Professor Southwood, Esa's director of science and robotic exploration, explained.
Esa recently announced that it would have to scale back its ExoMars rover mission, which will search for signs of past or present life on the planet, dropping a major instrument package to constrain costs.
Europe is working out a possible solution, in which America would provide the launcher - an Atlas rocket. This scenario could also see the US build a carrier spacecraft that delivers the rover to Mars and provide an orbiter that relays the data back to Earth.
ExoMars will use Radioisotope Heater Units (RHUs) to heat the rover at night. Esa had been planning to source these from Russia, but would turn to the US to provide these elements under the agreement being thrashed out by the two agencies.
Quid pro quo
But in return for Nasa's considerable investment in ExoMars, European money would be put into future, American-led missions.
The US space agency has its own concerns; it is having to plan ahead with reduced money for Mars exploration, and, like Europe, is keen to discuss sharing the cost of future missions to the Red Planet.
"I can't do the kind of aggressive programme Nasa would like to have done. I'm not sure David (Southwood) can do the programme his scientists would like him to do. But, together, we have a darn good programme," Dr Weiler told BBC News.
Professor Southwood has expressed his hope that a letter of intent could be signed with Nasa on an ExoMars collaboration. But it is not clear whether the Plymouth meeting will produce such an outcome.
Nasa had been planning to launch its own orbiter mission to Mars at the 2016 opportunity. But under the prospective agreement between the agencies, this would be incorporated into a European-led mission.
The bilateral science summit is held each year, alternating between the US and Europe. The last talks were held in Annapolis, on the US east coast.
The summit will also discuss the recently agreed joint Nasa-Esa mission to Jupiter and its icy moons as well as astronomy missions to look for Earth-like planets.
Professor Wendy Purcell, vice-chancellor and chief executive of the University of Plymouth, said: "To see Nasa and Esa coming to Plymouth to make decisions is fantastic. I think it inspires the city and it inspires young people."
The university is holding a week-long exhibition featuring the rover "Bridget", a prototype for the ExoMars mission, built by EADS-Astrium.
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