NASA and the European Space Agency (ESA) have laid the foundation for a joint Mars exploration program with the potential to yield scientific discoveries and technical achievements that would not be possible had the two sides continued pursuing separate, parallel paths. Managed properly, the program could culminate in a Mars sample return mission, a holy grail of sorts for planetary scientists that today remains just beyond the horizon of the technically and financially achievable.

The partnership took an important early step with the selection, announced Aug. 2, of five instruments for its first mission, the ExoMars Trace Gas Orbiter scheduled to launch in 2016. NASA will supply four of those instruments, along with a communications payload that will relay signals from future landers, and a launch aboard an Atlas 5 rocket. Europe will supply the spacecraft platform, one instrument and a small lander whose primary objective is to demonstrate a European-built entry, descent and landing system.

The ExoMars Trace Gas Orbiter came from the restructuring of ESA’s ExoMars mission, which in its last incarnation was to feature a European lander equipped with a drilling device and rover. The billion-dollar mission proved to be a financial bridge too far for ESA, leading to discussions that eventually brought NASA on board. Plans now call for a separate mission featuring more capable landers built by NASA and ESA, to be launched in 2018 by an Atlas 5 rocket and placed on the surface by the system designed for NASA’s upcoming Mars Science Laboratory mission. The U.S. rover on the 2018 mission will have the capability to quarantine martian rock and soil samples for return to Earth at a later date.

Teaming up with ESA has implications for NASA’s Mars Exploration Program, under which the U.S. space agency has been sending robotic probes to the red planet at every opportunity — such windows open up every 26 months or so. Most significantly, NASA will have to discontinue the Mars Scout program of medium-class missions following the Mars Atmosphere and Volatile Evolution (MAVEN) mission, slated to launch in 2013. But NASA officials point out that the agency’s Discovery line of scientist-led planetary probes will now be reopened to Mars missions, with the first launch opportunity coming as early as 2017.

Critics note that there is no guarantee that a Mars mission will be selected for the Discovery opportunity, meaning that after MAVEN, two Mars windows could come and go without seeing a U.S.-built spacecraft launched to the red planet.

This concern seems misplaced and a bit shortsighted. To begin with, NASA and the science community have enjoyed a golden era of Mars exploration and discovery over the past decade with a slew of successful missions involving orbiters, landers and rovers. To cite just one example, Spirit and Opportunity, the twin Mars Exploration Rovers, far outlived their expected lifetimes, with the latter craft continuing to move about on the martian surface returning useful scientific data. NASA is gearing up for the 2011 launch of its most ambitious Mars mission yet: the $2.5 billion Mars Science Laboratory, a nuclear-powered rover the size of a car that will carry the biggest and most advanced sensor package ever to land on the red planet.

Moreover, NASA has the science lead on the 2016 ExoMars Trace Gas Orbiter with an instrument package expected to cost U.S. taxpayers in the neighborhood of $100 million — a bargain compared with the cost of a full mission. This will allow ESA to focus on the Mars exploration enabling technologies with which NASA already has a wealth of experience. Yes, it’s possible that NASA could accomplish more in the relatively near term by continuing the Mars Scout program, but once ESA masters the basics of Mars orbit capture and landing, the two agencies, pooling their substantial resources, can take on more-ambitious projects, eventually including the ever-elusive Mars sample return mission.

It has become increasingly clear in the last several years that neither NASA nor ESA has the financial resources to go it alone on a Mars sample return mission, with key enabling capabilities yet to be developed. Prominent — and perhaps the most challenging — among them is a Mars ascent vehicle to get samples off the surface of the red planet for the journey back to Earth.

NASA officials have solicited industry concepts for a Mars ascent vehicle and hope to fund concept development work starting in 2012 or 2013. Given its resources and experience in planetary exploration, NASA is the logical choice to take the lead in this aspect of Mars sample return mission development. But ESA clearly has much to offer, and these capabilities will only grow as the joint program progresses; it’s never too early to begin thinking seriously about how best to divvy up responsibilities for the other enabling technologies.

The combined technical and financial resources of Europe and the United States make a Mars sample return mission feasible. It will take a shared long-term commitment and a corresponding plan that leverages the strengths of NASA and ESA, while recognizing that both have political and economic constituencies that must be accommodated, to make it a reality. Coming to terms on the ExoMars Trace Gas Orbiter represents an early yet significant milestone in that process.