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Neil deGrasse Tyson On How To Stop a Meteor Hitting the Earth
An anonymous reader writes "Astrophysicist Neil deGrasse Tyson talks stopping extinction-level meteor hits: '...Here in America, we're really good at blowing stuff up and less good at knowing where the pieces land, you know...So, people who have studied the problem generally – and I'm in this camp – see a deflection scenario is more sound and more controllable. So if this is the asteroid and it's sort of headed toward us, one way is you send up a space ship and they'll both feel each other. And the space ship hovers. And they'll both feel each other's gravity. And they want to sort of drift toward one another. But you don't let that happen. You set off little retro rockets that prevent it. And the act of doing so slowly tugs the asteroid into a new orbit.'"
The asteroidmay not be solid rock. It could be a rubble-pile type, and there might not be anything solid-enough to apply force to in a consistent way. It might be two closely orbiting bodies of rock, in which case you can't push on one in any type of consistent direction.
The benefit of the gravity-tug approach is that if you have a body of some concentrated mass moving at you, then if you have a spaceship sit away from it and maintain a constant position relative to a point other then the asteroid, then you can act on it's entire mass consistently.
Find it early enough, and you can do this with high-efficiency ion thrusters, rather then needing inefficient chemical rockets.
Re: reactive force from retrorockets - you fire them off-angle to the asteroid so exhaust doesn't hit them. You can easily mount orthogonal engines which would carefully cancel the attraction of the asteroid without directing any exhaust at it.