France and Britain appear willing to put the entire ExoMars Mars exploration program at risk with their refusal to endorse the start of work on the first of the two planned ExoMars missions, a communications relay orbiter slated to launch in 2016. The two governments are withholding support pending clarity on the design and cost of the second mission, a joint European-U.S. rover to be launched in 2018 that will rely on the orbiter to communicate with ground controllers. It’s hard to blame them for taking that position since each is being asked to commit a substantial sum to ExoMars, but the European Space Agency (ESA) does not have the luxury of being able to wait to begin work on the 2016 orbiter, without which the rover cannot function on the martian surface.

How ESA wound up in this predicament is an example of the pitfalls of internationalizing large-scale space missions. ExoMars at one time was a European endeavor that had already grown dramatically in cost and scope when NASA came aboard two years ago to make it even bigger; ExoMars became part of a broader U.S.-European campaign expected to culminate in a Mars sample-return mission sometime after 2020.

ESA and NASA had agreed to a plan under which each would build a rover for launch in 2018 aboard a U.S.-supplied rocket. But NASA earlier this year informed ESA that it would be unable to fund its own rover, leading to new negotiations and an agreement to jointly build a single rover that would collect and cache soil samples for retrieval and return to Earth by a future mission.

ESA Director General Jean-Jacques Dordain had hoped to secure funding during a June 29-30 meeting of the agency’s Industrial Policy Committee to begin full-scale development of the orbiter, which in addition to serving as a relay for the rover would carry an entry, descent and landing demonstrator package, along with a trace-gas detector. Work on the orbiter must begin now to keep it on track for its 2016 launch on a U.S. rocket.

But France and Britain balked for reasons stemming from the fact that the design details of the trans-Atlantic rover are yet to be determined. France sees in the uncertainty the potential for cost growth and also wants assurances that any problems encountered in the orbiter’s development are not addressed using funds budgeted for the rover, which it has made a priority. Britain fears NASA encroachment on its role in building the rover without a corresponding decrease in its financial contribution to ExoMars.

Mr. Dordain wanted the Industrial Policy Committee, ESA’s check-writing body, to release the ExoMars funding despite not having France and Britain on board and was counting on a signal of NASA’s commitment to make that happen. But NASA Administrator Charles Bolden, in a June 29 letter, did not deliver, saying only that he would do whatever he could to make that commitment by Sept. 15, when the U.S. space agency might have a better idea of its funding for 2012. As a result, Mr. Dordain was forced to pull ExoMars from the meeting agenda, and ESA now intends to use an existing contract vehicle to do the minimum work necessary to keep the 2016 orbiter on track.

The situations unfortunate, but there are no villains here. France and Britain have every right to protect their equities and are justifiably concerned that the program they signed up for will turn out to be something different. NASA, for its part, is facing a budgetary environment that was unforeseen when it signed on as an ExoMars partner; Mr. Bolden’s refusal to commit June 29 to the 2018 mission was validated by a spending bill, subsequently proposed by the U.S. House Appropriations Committee, that would provide NASA with nearly $2 billion less than the $18.7 billion the agency requested for next year. It remains to be seen what the Senate will propose in its version of the bill; NASA’s 2012 budget picture may or may not be clear by Sept. 15.

It would also be wrong to blame trans-Atlantic collaboration, which in the case of ExoMars helped ESA close a funding gap that otherwise would have forced the agency to scale back the mission. More generally, international collaboration can provide political stability when the going gets tough on large, expensive space missions. For example, NASA’s commitments to its international space station partners — and vice versa — helped keep the project alive during its trouble-plagued development years.

But none of this changes the fact that time is fast running out to get started in earnest on the ExoMars orbiter and still have confidence of making the 2016 launch window. If ESA misses that window there’s not much point in doing the rover. While providing a trickle of funding to keep the orbiter on track, Mr. Dordain is looking ahead to the next opportunity to secure the funds necessary to award a full-scale development contract — a meeting of the Industrial Policy Committee scheduled for Sept. 29-30. It is far from certain whether ExoMars will have come into better focus by then, but ESA nevertheless must find a way to get things moving, even if that requires a leap of faith by France and Britain. If ESA and NASA cannot make ExoMars happen, the prospects for tackling new frontiers in space exploration will be dimmed considerably.