Nasa is zeroing in on the asteroids it wants to capture, haul near the moon and have astronauts visit.
Officials have described a prime candidate: A tiny asteroid that whizzed about 12,250 kilometres above Earth in 2011. The 6 metre-long asteroid might be small enough to fit in a garage.
That asteroid also could be a pile of smaller rocks that fly together in formation.
The plan is to grab an asteroid with a giant claw or capture it with a giant inflatable bag. The asteroid would be parked above the moon, with astronauts exploring in a later mission.
NASA executive Lindley Johnson said there will eventually be about 10 possible rocks for capture in the early 2020s.
They have found at least six valid candidates so far.
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