By Paul Rincon
BBC News science reporter, in Houston, Texas
Europe's rover mission to Mars faces a key review to decide if the landing vehicle will be accompanied to the Red Planet by an orbiting spacecraft.
Some on the ExoMars mission want its carrier module, which will ferry the rover to Mars, turned into an orbiter.
Upgrading the carrier to an orbiter would cut the mission's reliance on an American spacecraft for communications.
European Space Agency (Esa) officials are expected to decide on the matter in late 2006/early 2007.
The mission is currently due to launch on a Russian Soyuz craft in 2011.
Need for orbiter
An extra 80 million euros (£55m) would be required to fly a heavier orbiter on the more powerful Ariane-5 rocket.
"One thing people probably don't realise is that the carrier we presently have on ExoMars is in essence already an orbiter. It's an orbiter that doesn't go into orbit," Dr Jorge Vago, project scientist on ExoMars, told the BBC News website.
Under current plans, the ExoMars spacecraft would consist of a carrier that releases a descent module on approach to the Red Planet.
After this, the carrier would become space junk, playing no further part in the mission.
The descent module would deploy the rover and a stationary geophysics package to the Martian surface.
Here, vehicle and surface station would have to rely on a US space agency (Nasa) spacecraft - probably Mars Reconnaissance Orbiter (MRO), recently arrived at the Red Planet - for communication with Earth.
Dr Vago said it would be very cheap to convert the carrier spacecraft into an orbiter with enough capacity for 30kg of scientific instruments and - crucially - equipped to carry out data relay.
This, he said, would allow the European mission "to gain some independence from MRO" and also pave the way for "a follow-up to the excellent science Mars Express is conducting today".
Going down the route of using MRO as a relay means ExoMars would have to compete for time on the orbiter with Nasa's Mars Science Laboratory (MSL) rover, due to launch in 2009.
"On the other hand, there are a number of instruments [on ExoMars] with large US participation. There is one that is provided by the US. So the Americans have an interest in using ExoMars as well, so they can get their science done," Dr Vago explained.
ExoMars is part of the European Space Agency's Aurora programme of space exploration. Also envisaged as part of Aurora is a robotic mission to return samples of Martian rock and soil to Earth for analysis.
This had been conceived as a joint mission between Esa and the US space agency (Nasa), but this now looks in doubt because of cuts in funding for the agency's science programme.
This has hit, amongst other things, its Mars exploration projects. MSL is secure, but subsequent missions are shrouded in uncertainty.
"What we fear is that Mars Sample Return will be pushed into the future. For us, this is very important. This programme has started with ExoMars, we cannot just let it drop," said Dr Vago.
"If the Americans cut Mars Sample Return, either we go for an ExoMars 2 but with precision landing, or a technology demonstration mission, or a mission to the Moon.
"I would like to keep the focus on Mars. While I see the scientific validity of going to the Moon, I also see that we could line up with [Nasa's] lunar exploration programme.
"For the time being, I feel more comfortable with missions that have a higher scientific priority like ExoMars or Mars Sample Return."
Dr Vago outlined the current mission architecture for ExoMars here at the Lunar and Planetary Science Conference in Houston, Texas.