"At the moment, though, the method of choice for saving Earth from Apophis is deflection...There are a few big ideas out there. One has several spacecraft landing on Apophis, drilling through the surface and pumping out what's inside. NASA has actually done something like this successfully with its Deep Impact mission, which crashed an impactor into a comet with the purpose of revealing the comet's composition. With Apophis, the point would be to pump the material out into space with enough force to push Apophis in the opposite direction, throwing it off course. Scientists are also talking about sending a spacecraft into the asteroid's orbit to fly next to it. This "gravity tractor" spaceship would essentially alter the gravity equation that keeps Apophis on its path, pulling on the asteroid until its position no longer threatens Earth...But according to Donald Yeomans of NASA's Near Earth Object Project, the simplest way to deflect Apophis is to send a spacecraft up there to just slam into it, knocking it out of the way."
Link: How Stuff Works - Apophis
Well, not quite. As Don knows well, the result of a kinetic impact deflection is uncertain. One can insure that the impact is strong enough that the original impact is averted... however because of the uncertainty (factor 2-10?) using the impact methodology alone leaves an uncertainty as to whether the NEO will now simply return in a few years for an impact. The probability is low, but real. And it can be totally eliminated by using a gravity tractor.. in combination with.. the kinetic impactor to "trim" the deflection result to absolutely insure the world that it isn't headed for a near-term return. The kinetic impactor alone can't give this assurance. Nor can nuclear. Finally, for Apophis, an unusually easy deflection, the gravity tractor (a very simple one) can do the job alone.
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