THE GRAVITY TRACTOR

DART is based on the idea that the best way to shift an asteroid’s course is to push it. But it’s also possible to pull it using nature’s own sciencefiction-style tractor beam: gravity.

Gravity works both ways. If you are near an asteroid, its gravity tugs on you. But yours also tugs on it. Thus, if you want to change the course of an asteroid, all you need to do is to hover above it for an extended period of time, waiting while gravity tugs it your direction. If needed, the effect can be strengthened by first plucking a boulder from the surface of the asteroid, thereby adding its mass to that of the hovering spacecraft.

The effect is small, but it adds up, and a few years ago NASA had a mission in development called ARM (Asteroid Redirect Mission) that would actually have tested it out. (The mission was shelved by the Trump administration in 2017.)

The gravity tractor’s main advantage is that it’s gentle. There’s no risk of breaking the asteroid into scattered pieces, any one of which might become a new hazard. Its disadvantage is that it is complex, slow, and needs a new type of rocket nozzle not yet created – one that wouldn’t undo the process by blasting its exhaust right at the asteroid the spacecraft is trying to tow, thereby shoving it away rather than pulling it forward.