Image caption The idea was to send two rovers together - one from the US (Max-C), and one from Europe (ExoMars)

America and Europe are looking now at flying just one rover to Mars in 2018.

The US and European space agencies (Nasa and Esa) had planned to land two vehicles together on the Red Planet to perform a kind of tandem mission.

One rover would have investigated below the surface with a drill; the other was to have collected interesting rocks for later return to Earth.

But cost concerns have prompted Nasa and Esa to consider combining these roles into a single vehicle.

The idea follows high level discussions between the partners in California.

It was discussed by Esa member states on Thursday and received broad support. A lot of technical details still need to be worked out, however.

We expect to see a very strong involvement on the part of the UK in the development of this rover Dr David Parker, UK Space Agency

Minimise change

The new rover would be larger than either of the vehicles in the paired concept - known as ExoMars (Esa) and Max-C (Nasa).

One suggestion is that the new vehicle would be built in Europe and take a mix of European and US instruments.

Nasa would provide the rocket to get it into space and the "mothership" to carry it to Mars.

Critically, this new vehicle's placement on the surface of the planet would use the equipment planned for the American "Curiosity" rover due for launch this year.

Image caption MSL-Curiosity will be lowered to the surface of Mars by a rocket-powered skycrane

The 900kg Curiosity - also called the Mars Science Laboratory - will be lowered on to Mars with tethers slung from a rocket-powered "sky-crane".

Making as few changes as possible to this landing architecture for 2018 is considered an imperative if costs on the whole venture are to be constrained.

Despite the reorganisation, the goals of the 2018 mission opportunity would stay broadly the same.

These would be to look for signs of past or present life by digging down into the soil; and packaging, or caching, rocks that can be picked up and despatched to Earth laboratories by a subsequent mission.

In what are acknowledged to be difficult financial times, America and Europe are both having to reassess how they explore the Solar System.

An influential group of US planetary scientists issued a report last month in which they said America's plans at Mars were too costly and, unchecked, could damage other space projects.

They called for the $3.5bn (£2.2bn) expected to be spent on the US side of the 2018 paired concept to be reduced by $1bn. If that saving could not be made, the group argued, the mission should be shelved.

Europe, too, is under pressure to control expenditure. It has been trying to fashion a Mars rover mission since its member states approved such a project in 2005.

But there has always been a struggle to match the ambitions for the vehicle to the funds available.

Going as part of a double-rover mission with the Americans appeared the most viable solution - until now.

Italy and the UK

The changes being proposed by senior Nasa and Esa officials will mean upheaval in Europe and the US.

But while the Americans had not progressed very far with the design of their Max-C rover, European industry has already presented its preliminary design for ExoMars. It was ready to start building the vehicle.

One key outcome of the Californian bilateral discussions is that the orbiting spacecraft the two agencies plan to send to Mars together in 2016 will go ahead.

Image caption European industry had advanced to the point where it was ready to start building ExoMars

This satellite, which will map methane and other traces gases in the Martian atmosphere, will be needed to relay the data from whatever rover concept arrives at the Red Planet at the end of the decade.

Member states of the European Space Agency got their first opportunity to discuss the one-rover idea at an exploration programme board meeting in Paris on Thursday.

The two European nations with most at stake are Italy and the UK.

The former, by promising to invest the most money, leads the 2016 and 2018 campaigns. The latter, as the second largest contributor, has taken primacy on the development of the European rover itself.

But there are other nations in the project, and their concerns will need to be satisfied before the one-rover initiative can fully take shape.

Dr David Parker, director of space science and exploration for the UK Space Agency, said after the programme board: "It's got lots of questions and uncertainties at this stage; but I'd certainly say that if we can make this happen with one rover, all the European instruments onboard and the American caching system - it could be quite a mouth-watering prospect, in scientific terms.

"We expect to see a very strong involvement on the part of the UK in the development of this rover," he told BBC News.

At the last Esa Ministerial Council Meeting in 2008 - the European organisation's key policy and budget-setting forum - member states allocated 850 million euros (£750m) to their ExoMars project.

It is widely acknowledged that this sum is not sufficient to carry through both the 2016 satellite and the European side of the 2018 endeavour - neither as a single large rover concept nor as a smaller vehicle paired with an American robot.

Jonathan.Amos-INTERNET@bbc.co.uk