The US space agency is planning for a robotic spaceship to capture a small asteroid and park it near the Moon for astronauts to explore, a top senator disclosed.
The plan would speed up by four years the existing mission to land astronauts on an asteroid by bringing the space rock closer to Earth, senator Bill Nelson said.
The robotic ship would capture the 500-ton, 25ft (450 metric-ton, 7.6-metre) asteroid in 2019.
Then using an Orion space capsule, now being developed, a crew of about four astronauts would nuzzle up next to the rock in 2021 for spacewalking exploration, according to a government document obtained by the Associated Press.
Mr Nelson said this would help Nasa develop the capability to nudge away a dangerous asteroid if one headed to Earth in the future. It would also be training for a future mission to send astronauts to Mars in the 2030s, he said.
Mr Nelson, chairman of the senate science and space subcommittee, said President Barack Obama is putting 100 million dollars (£65 million) in planning money for the accelerated asteroid mission in the 2014 budget that comes out next week. The money would be used to find the right small asteroid.
"It really is a clever concept," Mr Nelson said at a news conference in Florida, the state where Nasa launches take place. "Go find your ideal candidate for an asteroid. Go get it robotically and bring it back."
While there are thousands of asteroids that size out there, finding the right one that comes by Earth at just the right time to be captured will not be easy, said Donald Yeomans, who heads Nasa's Near Earth Object programme that monitors close-by asteroids. He said once a suitable rock is found, it would be captured with the space equivalent of "a baggie with a drawstring. You bag it. You attach the solar propulsion module to de-spin it and bring it back to where you want it".
Mr Yeomans said an asteroid of that size is no threat to Earth because it would burn up should it inadvertently enter Earth's atmosphere. The mission Mr Nelson described is perfectly safe, he said.
The government document said the mission, with no price tag at the moment, would inspire because it "will send humans farther than they have ever been before".