By Paul Rincon
Science reporter, BBC News
The European Space Agency (Esa) has pushed back the launch of its rover mission to Mars from 2011 to 2013.
ExoMars could now be supported by an orbiting craft above
The decision will not significantly delay the mission's arrival at Mars.
But it does reflect a growing will to push for an upgrade to the ExoMars project which could raise its cost from roughly 500m euros to 800m euros.
It would involve launching the rover on an Ariane 5 rocket instead of a Russian Soyuz, releasing enough mass to send an orbiting spacecraft along for the ride.
As the mission now stands, ExoMars would launch on a Soyuz-Fregat vehicle from Baikonur Cosmodrome, but the Soyuz is capable only of launching the rover and a carrier module to get it to Mars.
The more powerful Ariane 5, which launches from Kourou in French Guiana, could carry a rover and an orbiter to communicate and relay data with Earth.
An Esa review also decided more time would be needed to get key technologies ready for the mission. These include airbags, supersonic parachutes, descent control and stability systems and the rover's locomotion and navigation system.
Should Esa opt for an Ariane 5 launch, it would have to negotiate with ministers from member states in order to secure the extra money.
"From the point of view of a scientist or an engineer, a mission with an Ariane 5 makes much more sense," said Marcello Coradini, coordinator for Solar System missions in Esa's science directorate.
"However, that implies political negotiations over the budget, in which the results are not guaranteed.
"So we must be able to design a mission which is reliable with a Soyuz-Fregat, at the same time showing that a mission with an Ariane 5, in terms of science, reliability and fundamentals of the mission, pays back in terms of the extra investment."
The move to 2013 will allow more time for negotiating a new budget.
One of the concerns with launching on a Soyuz-Fregat is communication between the rover on Mars and controllers on Earth. The current mission scenario envisages ExoMars using a US spacecraft called Mars Reconnaissance Orbiter (MRO) for data relay.
However, officials have no guarantee that MRO will still be operational by the time ExoMars arrives at the Red Planet. Upgrading the carrier module to an orbiter would sever the mission's reliance on the US spacecraft for telecommunications.
"We find ourselves in this very difficult situation of having a mission scenario that relies 100% on a Nasa satellite without a 100% guarantee the satellite will be there," Dr Coradini told BBC News.
Another issue concerns the mass available for scientific instruments on the rover.
One way to increase this mass is to move to an Ariane launcher. Another way is to use novel "vented airbags" to cushion the rover's landing. These inflate like pillows under the rover and can support heavier payloads than "classical" or "bouncing ball" airbags.
But vented airbags, which are currently used for military purposes, are likely to need further refinement by engineers before they can fly on the mission.
"Launch in 2011 with a Soyuz-Fregat and using classical airbags would make available a total mass of about 8kg for the instruments of the rover," said Dr Coradini.
The Ariane 5 would have the power to launch an upgraded mission
"The scientific community does not consider that a realistic number. They say that a minimum mass to make the mission meaningful from the scientific point of view is 13-15kg."
Some of the additional mass would likely be given over to a stationary geophysics package.
The previous launch date in May-June 2011 would have brought ExoMars to the Red Planet in June 2013.
Esa is now looking at a launch window in 2013 opening on 7 December and closing in January 2014. It is expected to arrive between June and September 2014 depending on the launch date.
With a launch in 2011, ExoMars would have spent two years in a heliocentric orbit waiting for Martian dust storms to settle down so that it could land.
Dr Coradini said that by missing out these two years in space, the spacecraft would be spared extensive bombardment by charged particles originating in the Sun and outside the Solar System which could damage delicate scientific instruments.