Space.com NASA has set up a "rapid response system" to pick the best candidates for its ambitious asteroid-capture mission. The space agency aims to use a robotic spacecraft to haul a near-Earth asteroid into a stable lunar orbit, where astronauts would visit it in the future. It's not as easy as just picking a space rock and going, however. Many asteroids are too big to be moved easily or are in unstable orbits. Others are too distant for telescopes to figure out what they're made of, which could make them unsuitable candidates. "There are other elements involved, but if size were the only factor, we'd be looking for an asteroid smaller than about 40 feet (12 meters) across," Paul Chodas, a senior scientist in the Near-Earth Object Program Office at NASA's Jet Propulsion Laboratory (JPL) in California, said in a a statement.