http://news.nationalgeographic.com/news/2005/11/1109_051109_asteroid_tug.html
"A spacecraft could use a gravity "towline" to alter the course of an Earth-bound asteroid, a new study by two NASA astronauts suggests.

Previous schemes to deflect an incoming space rock range from landing a spacecraft on the asteroid and pushing it off course to blowing it up with nuclear weapons.

The new plan takes a gentler approach. A spacecraft would hover above the asteroid and gradually pull it off course using nothing more than the gravitational attraction between the two bodies."