Sooner or later, say astronomers, an asteroid will be discovered on a collision course with Earth. Humanity will then begin all-out planning to prevent an impact. But while there are already plenty of ideas about how to shove asteroids out of Earth's way, nobody knows whether any of them would work.

To change that, two groups in the United States and Europe have been developing separate plans for robotic missions to visit nonthreatening asteroids, try to deflect their orbits, and watch how they respond.