NASA has decided that putting a lump of rock from an asteroid into orbit around the Moon is better than trying to hook a whole asteroid.

The ARM – Asteroid Redirection Mission – has settled on what the space agency called “Option B”, sending a spacecraft to a near-Earth asteroid big enough to have boulders on the surface and using robotic grapples to retrieve a rock and put it in a stable orbit around the moon.

“Option A” would have been to find a suitable asteroid less than 10 metres in diameter for redirection.

Reporting from the NASA press conference, Spacenews quotes associate administrator Ross Lightfoot as saying that settling on a boulder rather than grabbing a whole asteroid will give the agency more options.

“I'm going to have multiple targets when I get there”, Lightfoot said. “That's what it boils down to.”

So far, NASA says it's identified three possible candidates for the mission, known as Itokawa, Bennu, and 2008 EV5.

It expects to identify more candidates as the years go by, noting that in the three years of its asteroid initiative, it's increased the detection of near-Earth asteroids by 65 per cent.

The ARM target will be finalised by 2019, NASA says, based on size, rotation, shape and orbit.

As part of the preparation for an eventual mission to Mars, the ARM spacecraft will be the test-bed for a number of technologies, including an advanced Solar Electric Propulsion (SEP) ion drive.

While SEP will be slower than lighting a fire under a bloody big rocketTM, it needs a lot less propellant.

The ARM project will also offer a test-bed for “planetary defense techniques”, including using the small gravity of a nearby spacecraft to disturb an asteroid's orbit enough that something on a potential Earth-impact path would pass us by.

As NASA notes, “effectiveness of this manoeuvre is increased, moreover, if mass is moved from the asteroid to the spacecraft by the capture of a boulder”.

NASA plans to send a 24-25 day crewed mission to the captured lump of rock in the mid-2020s, launching an Orion spacecraft on the Space Launch System.

The agency hopes to get $US50 million for the ARM project in its 2016 budget.

NASA's 2014 video explaining the ARM is below. ®

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