This is a technique that would (less effectively) alter the trajectory of an asteroid in order to keep it from hitting the Earth. Like other methods, it would only work if enough lead time were available, meaning, for our own defense, we need to have a comprehensive detection grid up and running ASAP. The only thing to recommend this gravitational tractor would be if the asteroid were rotating, as most of them probably are -- rotation would make it more difficult to attach a plasma engine that would point in one direction.
The orbit of the asteroid would have to be known to a LOT of decimal places...a tiny error at the orbit of Jupiter would result in a BIG ERROR on Earth. Of course, this could also be used as a weapon...a really good one, too, because by the time it got close anything you could think of to deter it would be about as effective as an umbrella.
I was thinking about the rotation problem the other night while watching a show about this sort of thing and noticed they never talk about the fact that they’re tumbling through space.
To me that leads me to think that maybe some sort of mass driver or low thrust propulsion might be better because they would only push when the asteroid is oriented correctly.
OTOH a large enough gravitational tractor parked at the right distance could probably overcome the complex moving gravitational field of the asteroid.
Well, Bruce Willis can relax. No Armageddon for us! Cool.