Scientists who plan to lasso an asteroid and park it above Earth for astronauts to explore say they have identified several potential targets.
Nasa is still working on plans for the ambitious mission, in which a robotic spaceship would be used to grab a rock or capture one inside a giant inflatable bag before it is dragged towards the Moon.
However, a team at the US space agency has already begun scanning the sky for suitable candidates.
They include one measuring just six metres long that passed within 7,600 miles of Earth as it whizzed by in 2011.
Known as 2011 MD, the asteroid has an unusually low density, suggesting it has a structure similar to that of a pile of rocks.
Other, larger targets may need to be broken down into boulder-sized pieces before they can be scooped up and placed into lunar orbit.
Nasa executive Lindley Johnson said about 10 rocks would pass close enough to Earth to be examined in the early 2020s.
Associate administrator John Grunsfeld added: "Observing these elusive remnants that may date from the formation of our solar system as they come close to Earth is expanding our understanding of our world and the space it resides in.
"Closer studies of these objects challenge our capabilities for future exploration and will help us test ways to protect our planet from impact."