LONG BEACH, Calif. — Scientists are expressing disbelief at inaction by the U.S. government over accepting more than $1 billion of funding on offer from Europe to collaborate on a Mars sample-return mission.

Speaking at the American Institute of Aeronautics and Astronautics Space 2011 meeting here, Stanford University professor and NASA adviser Scott Hubbard says the funding issue threatens the start of the project, which is aimed at a Mars landing in 2018.

“The European Space Agency is willing to put €850 million [$1.16 billion] to collaborate with us. But for reasons unknown, somewhere in the administration somebody is refusing to release the letter that would allow the head of ESA to collaborate with NASA Administrator Charles Bolden,” Hubbard says. “Why on Earth would you refuse to allow over $1 billion of funding? It borders on the irresponsible.”

The cooperative Mars sample-return mission has been given top priority in the latest National Academies decadal survey of the scientific community. The mission would be broken into three parts: obtaining the sample, boosting the sample to Mars orbit and returning it to Earth for analysis.

“It would be a series of missions. We’ve never done that before. It has been difficult, but the two teams in the trenches are making enormous progress,” says NASA Headquarters Planetary Science Division Director James Green.

Describing the priorities of the decadal survey, Cornell University’s Steven Squyres, principal investigator for the Mars Exploration Rovers, says: “We recommended in the strongest possible terms that we should fly the NASA-ESA missions only if the cost to NASA is no more than $2.5 billion. The latest estimates are way below that, which is very encouraging.”

“It would be irresponsible for us to turn down €850 million from the Europeans,” declares Tim Frei, vice president of Northrop Grumman’s space systems division.

Failure to act quickly to clear up the issue compounds a growing worry that NASA’s international image will be tarnished. “The decline in NASA’s science budget is startling and disturbing,” Hubbard says. “The reduction puts our ability to meet NASA’s exploration goals at serious risk, and our ability to be involved in international collaboration. NASA used to be a good partner; now we’re on the verge of being a bad partner.”