NASA narrowed its hunt for an asteroid to capture to three candidates, it said this week.
NASA narrowed its hunt for an asteroid to capture to three candidates, it said this week.
The asteroids fit the requirements of being between 7m and 10m in size, scientists said at a conference in San Diego, California.
"We have two to three which we will characterise in the next year and if all goes well, those will be valid candidates that could be certified targets," senior scientist at the Nasa Near-Earth Object Programme Office, Paul Chodas, said.
The plan is to send a robotic spacecraft to capture the asteroid and drag it into orbit around the Moon.
Once there, astronauts could visit the asteroid and take samples of it back to Earth for study.
The spacecraft used for travel there and back would be the Orion multipurpose vehicle, which is still being built, as well as a new deep-space rocket launcher.
The Nasa programme aims to break new ground by increasing the space agency's capabilities beyond a low Earth orbit, where the International Space Station circles the earth.
US President Barack Obama has proclaimed the project would be a key step on the way to sending humans to Mars by the 2030s.
Obama's 2014 budget for Nasa asked for $100-million (almost R1-billion) for the project. But overall costs could be as high as $2-billion.
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