Nasa is preparing a three-month manned mission to an asteroid that is scheduled to pass close to Earth in 2030.

The space agency has prepared a plan, seen by The Guardian, for a mission to the 1.1 million ton 2000SG344 asteroid.

The asteroid caused concern in 2000 when the chance of it hitting Earth on its next visit in 2030 was raised to 500-1.

"An asteroid will one day be on a collision course with Earth and it makes sense, after going to the moon, to start learning more about them," said Rob Landis, an engineer at Nasa's Johnson Space Centre.

"Our study shows that it makes perfect sense to do this soon after going back to the moon."

Landis's report calls for the forthcoming Orion spacecraft to be lifted in sections into orbit using Nasa's planned Ares booster, and for two astronauts to make the three-month trip to rendezvous before landing on the asteroid's surface.

Nasa wants to use the trip to analyse the effects of long space flight on humans, and to test devices for converting interstellar ice into drinking water, breathable oxygen and even hydrogen for fuel.

The mission would be an important testing platform for a future planned trip to Mars, and would also augment plans for deflecting a potentially catstrophic asteroid strike on Earth.

"Near-Earth objects are a potential collision hazard and it may one day be necessary to deflect an asteroid from a collision course with Earth," said Ian Crawford, a planetary scientist at Birkbeck College in London.

"Having the capability in your back pocket to deflect an asteroid might be a good insurance policy for the future.

"For that, you want to know what they are made of, how to rendezvous with them, and whether you risk getting hit by debris if you fire something at it."