By Jonathan Amos and Paul Rincon
Science reporters, BBC News
NEW EXOMARS MISSION CONCEPT
Now likely to leave Earth in 2018; primary aim is to search for life
Current proposal is to use a US Atlas rocket to launch ExoMars
US also to look after the rover's entry, descent and landing
Hardware likely to be the same as for US 2013 rover, Curiosity
ExoMars would be targeted at key methane hotspots on the planet
It will have the capacity to drill 2m into the Martian ground
Esa states still have to sign off the plan and a budget
Europe's flagship robotic rover mission to Mars now looks certain to leave Earth in 2018, two years later than recently proposed, the BBC understands.
The ExoMars vehicle is intended to search the Red Planet for signs of past or present life.
The delay is the third for the mission originally planned to launch in 2011.
While the switch will disappoint many people, officials say the change will open up a greatly expanded programme of exploration at the Red Planet.
The European Space Agency (Esa) will now join forces at Mars with the US space agency (Nasa). The two organisations believe they can achieve far more by combining their expertise and budgets.
The basis for this approach was agreed at bilateral discussions in Plymouth, UK, last month.
Since then, scientists and engineers on both sides of the Atlantic have been working up the basic architecture for a series of missions in 2016, 2018 and 2020 (launch opportunities to Mars come up roughly every two years).
The plan, or baseline, for this programme - including its implications for ExoMars - is now starting to emerge.
It would see the agencies launch a European orbiter to the Red Planet in 2016. Its main aim would be to track down the sources of methane recently detected at Mars. The presence of methane is intriguing because its likely origin is either present-day life or geological activity.
Confirmation of either would be a major discovery.
Europe's Mars Express orbiter made the first methane detection from orbit
The American Atlas rocket used for this mission would also have capacity to carry sufficient mass to put some sort of static lander on the surface. The European orbiter would act as its data relay to Earth.
The 2018 launch opportunity would be taken by ExoMars, again launching on a US Atlas rocket. This mission window is actually one of the most favourable in terms of planetary alignment for many years, and that makes it possible to send a very heavy surface mission.
The proposal on the table currently is that ExoMars should be joined by a slightly smaller rover in the class of the US Spirit and Opportunity vehicles that are on the surface today.
ExoMars and its smaller cousin could be targeted at the Methane sources identified by the 2016 orbiter.
The 2020 launch opportunity would probably then be taken by a network of instrumented static landers.
Both Esa and Nasa will have tight finances going forward and will have to constrain their ambitions accordingly.
Heavy rovers like Curiosity can be landed with a skycrane
European ministers pledged sufficient monies at their major triennial gathering last year to take the budget for ExoMars to 850m euros. Esa officials believe the proposals they are formulating with Nasa can broadly match the cost requirements and the technological goals of both parties.
For Europe, the primary goals are to land, to rove and to drill on Mars. However, under the plan outlined above, these objectives could not all be achieved during the ExoMars opportunity.
In 2018, it is likely the entry, descent and landing (EDL) of Europe's rover would be handled by the Americans, using the "skycrane" system they have designed for their big 2013 rover known as Curiosity.
If Europe really does want to do EDL, the option is open for it to take responsibility for the 2016 surface package of instruments.
Esa's director-general, Jean-Jacques Dordain, has promised to report to his member states in the autumn with firm proposals for a re-scoped Mars exploration programme.
Two months of intensive discussions will now take place in those member states, and in European industry which will be responsible for building the spacecraft systems.
If financial contributions to the mission from Esa member states were to change substantially, the space agency might have to re-visit the balance of industrial work allocated to different countries through the process of "juste retour".
Esa has rules that govern the distribution of industrial contracts
Esa's rules of juste retour ensure the work which returns to a member state reflects the financial contribution it makes to a programme.
One senior European space executive called at the recent Paris air show for the whole ExoMars industrial programme to be re-opened to competition.
The ExoMars rover was originally conceived as a small technology demonstration mission.
It was approved in 2008 and should have been launched in 2011. Then, as ambitions grew and the design was beefed up, the launch was put back.
At first, it was shifted to 2013. Last year, a decision was taken to move it even further back, to 2016, because of budget concerns.