By Seth Borenstein

NASA is planning for a robotic spaceship to lasso a small asteroid and park it near the moon for astronauts to explore, a top senator said Friday.

The ship would capture the 500-ton, 25-foot asteroid in 2019. Then using an Orion space capsule, 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.

Senator Bill Nelson, Democrat-Florida, said 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.

Nelson, who is chairman of the Senate science and space subcommittee, said Friday that US President Barack Obama is putting US$100 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," Nelson said in a press conference in Orlando. "Go find your ideal candidate for an asteroid. Go get it robotically and bring it back."

This would be the first time ever humanity has manipulated a space object in such a grand scale, like what it does on Earth, said Robert Braun, a Georgia Institute of Technology aerospace engineering professor who used to be NASA's chief technology officer.

"It's a great combination of our robotic and human capabilities to do the kind of thing that NASA should be doing in this century," Braun said.

Last year, the Keck Institute for Space Studies proposed a similar mission for NASA with a price tag of US$2.6 billion. There is no cost estimate for the space agency's version. NASA's plans were first reported by Aviation Week.

While there are thousands of asteroids around 25-feet, 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."

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NASA to lasso asteroid, bring it closer