The European Space Agency (ESA) seems to have gotten tired of waiting for NASA to commit to its share of the joint 2016/2018 Mars missions that were planned to lay the groundwork for an eventual delivery of samples of Mars to Earth.

Space News is reporting that, to save the two-mission plan, ESA Director-General Jean-Jacques Dordain has formally invited Russia in as a full partner. He's asking the Russian space agency, Roscosmos, to provide its Proton rocket to launch the European Mars telecommunications orbiter and a small lander in 2016.

NASA has told ESA that its budget will not allow it to commit to launching the 2016 mission, and the hoped-for 2018 launch of the ExoMars rover on an Atlas 5 is not confirmed. If Russia accepts ESA's partnership offer, the rover will launch on a Proton.

This is good news for proponents of international cooperation. Three leading space agencies representing many nations will be exploring Mars together. By its refusal to commit to the original NASA-ESA plan, the U.S. seems willing to cede leadership to the Europeans, who have invited Russia into the game.

It's disappointing news to those who hoped the U.S. would contribute its considerable and multifaceted resources to half of both missions.

Without its originally promised participation, the fate of both the 2016 and 2018 missions will remain uncertain, despite the Russian partnership. The exploration of Mars is not easy or cheap, and ESA faces some difficult choices as it tries to keep both missions on track.

Over the past few days, Planetary Society Members bombarded the White House with messages urging that the U.S. commit to its partnership with ESA. We know that their message was heard in the Office of Science and Technology Policy and in the halls of NASA (an unofficial source reports NASA was "agog" at our Members' response.)

But the Office of Management and Budget, which seems to be the source of the roadblock, did not respond to the outpouring of support for Mars exploration.

The road to Mars has always been difficult. The way is littered with failed spacecraft and broken plans. But we won't give up.

The Planetary Society will be calling on its considerable resources - its Members - to keep up the struggle to explore Mars. In the coming weeks, we will target other powers-that-be to demonstrate that the people of Earth do want to explore Mars and that we will keep up the pressure until it is done.