PARIS — European Space Agency (ESA) governments on Dec. 17, 2009, gave final approval to a two-part Mars exploration program to be conducted with NASA, confirming their commitment to spend 850 million euros ($1.2 billion) on missions in 2016 and 2018, ESA Director-General Jean-Jacques Dordain said.

ESA has estimated that it will need 1 billion euros for what it calls the ExoMars mission, but that figure includes the costs of operating the orbiter, lander and rover associated with the mission. Dordain said the missing 150 million euros will be solicited from a meeting of ESA government ministers in late 2011 or early 2012.

In an interview, Dordain said ESA, with its ExoMars hardware financing now firmed up, will spend the coming 12 months working out the precise details of the two missions with NASA. By late 2010, he said, NASA and ESA should be ready to sign a final accord assigning each side’s roles and responsibilities.

ESA’s hardware will include a Mars orbiter and lander to be launched on a NASA-supplied Atlas 5 rocket in 2016, and a rover vehicle to be launched, also on a NASA-provided Atlas 5, in 2018.

Unlike some ESA programs, the ExoMars financing will not include a 20 percent margin for cost overruns.

“We have agreed that this is 1 billion euros and not one euro more,” Dordain said. “If costs rise, it will require a unanimous approval by the participating nations.”

Dordain said ESA has divided the ExoMars package into four separate funding pools — for the orbiter, the lander, the rover and mission operations — and that cost growth in one element will not be financed by any of the others.

Several ESA governments had feared that the 2018 mission, whose development-cost peak will occur after that of the 2016 mission, would have its budget raided should cost overruns occur in elements of the 2016 mission. Dordain said this could not occur given the way ExoMars development has been structured.

The German delegation, which, with a commitment of 89.6 million euros, is the fourth-largest contributor to the ExoMars mission after Italy, Britain and France, resisted approving the program because of the still-uncertain source of the operations budget.

“We were the bad guys on this one,” said Johann-Dietrich Woerner, chairman of the German Aerospace Center, DLR. “From our view, if you do not operate and control your mission, you don’t really have a mission. But after some heavy discussions, we agreed to approve it and then hope that either the mission’s overall costs come down, or that we will resolve the operations budget in 2012.”

ESA Science Director David Southwood said the agency’s billion-euro cost estimate includes sufficient margins so that if some components cost more than planned, the overall program cost will remain under the approved ceiling.

ThalesAlenia Space of France and Italy will remain overall contract leader given Italy’s first-place position among ExoMars government backers, Southwood said, while Astrium Ltd. of Britain will have prime responsibility for the rover, and OHB Technology of Germany will be lead builder of the Mars orbiter.

Some elements of the program’s industrial organization remain to be determined, Southwood said in a Dec. 18 interview. As is often the case with ESA programs, he said, the trick is to ensure that industrial work for companies in smaller program contributors such as Belgium, Spain and Switzerland matches these governments’ contributions.

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