Would this even work? It seems like airbags would behave differently in space than what he's thinking of. The only way to deflect a rock in space is to push off of it with some other mass. He explicitely states that they'd try to get the mass of the airbag to be as low as possible, and just use air to deflect the rock. Having a mile-wide airbag would work on earth because it would push against the atmosphere, but that wouldn't happen in space. In my humble opinion, the asteroid would just catch on the airbag and push it along, with very little change in course.