Space agency chiefs from Europe, Russia and the U.S. are setting up talks aimed at finding a way to work toward a Mars sample return mission in the face of budget uncertainty in the U.S. that threatens to upend the joint effort worked out by NASA and the European Space Agency (ESA).

Although NASA will launch the $5 billion Mars Science Laboratory (MSL) mission as early as Nov. 18 to help find the best place to look for samples, and Congress came close to meeting NASA’s full funding request for planetary science in its fiscal 2012 congressional compromise — which still has to be passed and enacted — the U.S. space agency has refused to commit to providing Atlas 5 rockets to launch joint NASA/ESA missions in 2016 and 2018.

As a result, ESA has turned to the Russian space agency Roscosmos for possible launch of its planned Mars orbiter in the 2016 window. Roscosmos chief Vladimir Popovkin says, in principle, Russia will be glad to accommodate ESA, but details remained to be hammered out.

ESA spokesman Franco Bonacina says ESA is seeking Russian involvement in the ExoMars program that could include launching the 2016 leg of the mission on a Proton rocket in the absence of a firm U.S. commitment. In an unusual move, ESA Director General Jean-Jacques Dordain is organizing a trilateral meeting in December with Popovkin and NASA Administrator Charles Bolden solely to discuss Russian participation in the ExoMars program, according to Bonacina.

“So far, things are going in that direction,” Bonacina says, adding that ESA has until the end of January to decide whether or not to continue with the 2016 leg of the ExoMars mission or scrap it and press on with a joint ESA-NASA mission in 2018.

“The industrial contracts being handled now go to the end of January, so February is really the time where we have to see if we can go ahead with the mission as it is baselined or whether we have to do any kind of reshuffling or rescheduling,” he says.