He is working on a plan that employs a spaceship to deflect asteroids headed for Earth. "We were originally thinking about how you would land on an asteroid and push it," he says. "But that doesn't work well." If the surface isn't solid, you have trouble landing or keeping anything on it. Moreover, asteroids are always rotating. "If you're pushing and the thing is rotating, the pushing just cancels Lu says out, 120 Pulling the asteroid along would be much easier. "Rather than having a physical line between you and the thing you're towing, you're just using the force of gravity between them," Lu says. A nearby spacecraft would pull 125 the asteroid off course very slowly but steadily, using only gravity. And over the long distances of space, just a slight change in course could mean missing Earth by tens of thousands of kilometers. z 130 An Asteroid Bomb? The drawback to Lu's plan is that it would work only for asteroids up to a few hundred meters across that could be engaged far from Earth. If the rock is small, we could try hitting 135 it with a spacecraft. When all else fails, and for large asteroids and comets, only one strategy has a chance of working: nuclear bombs.