A tentative agreement being hammered out by the United States, European and Italian space agencies could give Europe a key role in a mission to bring Martian soil to earth.
But it also means British-led plans to land on Mars and dig below its surface in 2003 will have to be scaled down, though scientists say little of the fundamental science will be lost.
A meeting of the European Space Agency's scientific programme committee was expected to agree this week that the proposed ESA Mars Express mission, which will place a spacecraft in orbit around Mars, should also act as a key component of Nasa's planned mission in 2005 to return Martian soil samples to earth. Under the deal, the Mars Express orbiter would act as a relay satellite, processing data sent from a US mission from Mars to earth.
Paul Murdin, director of space science at the Particle Physics and Astronomy Research Council and and ESA committee member, said: "You are not going to solve all Mars questions in one mission. To produce science you can rely on, you need a sequence of missions. ESA's being part of that series of missions is a very good thing."
To make the link-up, Mars Express will have to alter its orbit to one nearer the planet, which will take more fuel and rockets. It also means that the weight of the lander, Beagle 2, proposed by the Open University's Colin Pillinger to accompany Mars Express and to search below the Martian surface for signs of former life, will have to be almost halved to fewer than 60kg to be accommodated.
The committee will hear that the weight of Beagle 2, whose funding is uncertain, can be cut without compromising science. "Instead of a Beagle, we would have a Chihuahua," Dr Murdin said.