PARIS — NASA and the European Space Agency (ESA) have agreed to create a joint program to guide a common trans-Atlantic Mars exploration program but details of what missions will be launched remain unclear.

Following a June 29-30 meeting in Plymouth, England, between ESA Science Director David Southwood and Edward Weiler, NASA’s associate administrator for science, the two agencies said in a joint July 8 statement they had created a Mars Exploration Joint Initiative (MEJI) to design missions for 2016, 2018 and 2020.

The missions will include “landers and orbiters conducting astrobiological, geological, geophysical and other high-priority investigations, and leading to the return of samples from Mars in the 2020s,” according to the July 8 statement.

Conspicuously absent from the statement was any reference to ESA’sExoMarslander and rover, a roughly $1 billion mission ESA governments tentatively approved for launch in 2016. ESA and NASA officials said before the June 29-30 meeting that ExoMars could be the first step in a tight U.S.-Europe Mars exploration program, with NASA handling the ExoMars launch aboard an Atlas 5 rocket that would also carry NASA’s planned Mars Science Orbiter.

The eight-day delay in producing a statement about the bilateral meeting, and the statement’s lack of detail, fueled speculation among European government and industry officials that ExoMars’ science package might be split into two missions, for launches in 2016 and 2018. How that would be done while sticking within the current ExoMars budget of around 850 million euros ($1.18 billion) is unclear.

NASA officials declined to discuss what the Mars agreement means in terms of concrete missions.

“The Plymouth meeting was the beginning of a series of discussions scheduled between NASA and ESA to further develop the initiative,” NASA spokesman Dwayne Brown said in a July 8 e-mail response to questions. “It simply would be premature and inappropriate to add any further explanation or speculation at this time.”

Even before the meeting in Britain, NASA officials had sought to lower expectations for an Atlas 5 ExoMars launch in 2016, saying the European rover may be too heavy for the rocket, and that the relative position of Mars and Earth might argue against a 2016 launch.

Southwood said he would reserve comment on the MEJI initiative with NASA until he has briefed ESA member states – particularly France, Germany and Italy — on the different options and their financial consequences.

Germany, Italy and France have all said their backing for ExoMars is contingent on the program remaining within its current budget.

“With NASA we’ve agreed a terrific opportunity in MEJI, but the options within that context require our member states to deliberate and discuss among themselves,” Southwood said July 7. He said a joint effort permits both agencies to relax some of the constraints of their individual Mars exploration plans. “After all, both sides have financial boundary conditions framed by roughly flat budgets foreseen ahead. In practice, in ESA we have to draw up a proposal for the autumn.”

ESA governments in November 2008 told Southwood to tailor the ExoMarslander and rover mission to fit into the existing budget and make a formal program presentation by late this year. But the science package agreed upon then used up the available budget, forcing ESA to seek a partnership with Russia or NASA for a launch.

Leaving ExoMars’ exact design open for so long has already caused Astrium Satellites, which lost the ExoMars prime contractor role to ThalesAlenia Space, to call for the program to be placed back into industrial competition. “We are designing innovative approaches and concepts for ExoMars,” Astrium Satellites Chief Executive EvertDudok said in a June 16 press briefing.

Thales Alenia Space officials have resisted this. One official said: “If Astrium wants to re-open the ExoMars contract because ExoMars has been modified, shouldn’t we also re-open the BepiColombo contract?” BepiColombo is an ESA science mission to Mercury that has been redesigned following the discovery that the satellite being built by Astrium will be much heavier than forecast, causing a sharp reduction in the satellite’s launch costs.

Peter B. de Selding was the Paris bureau chief for SpaceNews.