Nasa is aiming to launch a rocket to an asteroid in five years and grab a boulder off of it – a stepping stone and training mission for an eventual trip sending humans to Mars.

The space agency Wednesday unveiled details of the $1.25bn plan to launch a solar-powered unmanned spaceship to an asteroid in December 2020.

The ship would spend about a year circling the large space rock and pluck a 13-foot (4 meter) boulder off its surface using robotic arms. It would have three to five opportunities to grab the rock, said Robert Lightfoot, Nasa's associate administrator.

The smaller rock would be hauled near the moon and parked in orbit around the moon. Using a giant rocket ship and the Orion crew capsule that are still being developed, two astronauts would fly to the smaller rock in 2025 and start exploring. Astronauts aboard Orion would dock with the robotic ship, make spacewalks, climbing around the mini-asteroid to inspect and document, and even grab a piece to return to Earth.

The smaller rock might not even be big enough for the two astronauts to stand on; it would have to fit in the cargo bay of the now-retired space shuttles.