PARIS ? European Space Agency (ESA) governments on Dec. 17 gave final approval to a two-part Mars exploration program to be conducted with NASA, confirming their commitment to spend 850 million euros ($1.23 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 operating the orbiter, lander and rover associated with the mission. Dordain said the missing 150 million euros will be solicited during 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 spending peak will occur after that of the 2016 mission, would have its treasury raided should elements of the 2016 mission incur cost overruns. Dordain said this could not occur given the way ExoMars development has been structured.

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