This would seem like an improvement on the concept, especially since the "inert mass" need not be of terrestrial origin. In addition to improving rocket efficiencies, this approach would also allow the mass to be much closer to the asteroid (indeed, I should think that having it land on the surface would probably not pose any particular problem).
I'm still unconvinced, though, of the practicality of getting any useful amount of mass into the right place to do any good. Gravity is a pretty weak force, after all, so the mass in question would have to be pretty incredibly big to do anything.
This would seem like an improvement on the concept, especially since the "inert mass" need not be of terrestrial origin. In addition to improving rocket efficiencies, this approach would also allow the mass to be much closer to the asteroid (indeed, I should think that having it land on the surface would probably not pose any particular problem).
I had not thought it through as far as that, but remember, the asteroid may be tumbling in its path, in which case at least some separation between it and the inert mass will be necessary.
I'm still unconvinced, though, of the practicality of getting any useful amount of mass into the right place to do any good. Gravity is a pretty weak force, after all, so the mass in question would have to be pretty incredibly big to do anything.
I had intended to post the following response:
"Not necessarily, because it wouldn't have to do much. Remember, we're talking here about an asteroid whose 'appointment with Earth' is many years, maybe even decades in the future. In such a case, even a very tiny change in its trajectory will result in a large change in its position at the critical moment. Even a very small gravitational acceleration, if applied continuously for long enough can amount to a substantial alteration in its velocity.
"At the kind of velocities we are discussing (~ 5 X 104 m/sec), a lateral acceleration of one micron/sec2 applied continuously for a period of 107 seconds (which is less than four months) will result in a lateral modification to the asteroid's velocity of 10 meters per second. Over a period of several years, such a change results in a very large change in the ultimate position of the asteroid."
However, I then actually tried some figures, and came up with this:
"Assume an asteroid of density 5000 Kg/m (i.e. a stony body, about five times as dense as water), of size equivalent to a sphere 1000 meters in diameter. Such a body would have a mass of approximately 2.6 X 1012 Kg. The force necessary to cause an acceleration of one micron/sec2 would thus be 2.6 X 106 Newtons. Further assume a distance between the centers of gravity of the inert mass and the asteroid to be one kilometer.
"Plugging these figures into Newton's Law of Gravitation ( F = M1M2G / d2) yields a mass of approximately 1.5 X 1010 Kg, or about 15 million tonnes."
Clearly, we can improve upon this mass both by bringing the bodies closer together (although the radius of the asteroid will limit this), and by applying an even smaller acceleration. However, it does not seem to me that it is possible to get the mass much below one million tonnes and still achieve the desired deflection. Furthermore, engines capable of exerting a force of that magnitude for a period of months do not currently exist.
Under the circumstances, I think you're right: It's a nice idea, but beyond the capabilities of our technology for at least the next several decades.