Explanation: How would you change the course of an Earth-threatening asteroid? One idea - a massive spacecraft that uses gravity as a towline - is illustrated in this dramatic artist’s view of a gravitational tractor in action. In the hypothetical scenario worked out in 2005 by Edward Lu and Stanley Love at NASA’s Johnson Space Center, a 20 ton nuclear-electric spacecraft tows a 200 meter diameter asteroid by simply hovering near the asteroid. The spacecraft’s ion drive thrusters are canted away from the surface. The steady thrust would gradually and predictably alter the course of the tug and asteroid, coupled by their mutual gravitational attraction. While it sounds like the stuff of science fiction, ion drives do power existing spacecraft and a gravitational tractor would work regardless of the asteroid’s structure or surface properties.