NASA’s current asteroid-redirect mission plan calls for a robotic probe to capture a small asteroid (or one boulder from a large space rock) in deep space and tow it back to lunar orbit, where it would be visited eventually by astronauts.
But another proposal advocated in an October 2014 editorial in the journal Nature would fly a crew directly to a near-Earth asteroid, or NEA, in its native orbit.
If the ultimate goal is a mission to Mars, a NEA in its native orbit provides more experience in operating long-duration missions in deep space, says the editorial's author, MIT professor Richard Binzel. NASA’s proposed asteroid redirect mission would not take crews much farther than they’ve already been: the moon.
A manned flight to a NEA would take weeks or months, as opposed to NASA's redirect plan, which would only involve astronauts flying in space for days.
The first step toward visiting a NEA should be undertaking a comprehensive search for close-flying space rocks, Binzel says. Such a survey would find thousands of potential astronaut destinations that could serve as "stepping stones" to Mars, and would also identify many space rocks that could pose a threat to Earth down the road.