The nonsense that we can not destroy or divert an incoming asteroid or comet is usually broadcast by

1) well meaning anti-nuclear activists who fear mankind's ability to create nuclear charges more than they do the threat of impact
2) Mars idiots ("space cases") like Eby who think that a manned Mars mission will cost between $2-5 billion per flight, and don't want to see resources diverted from the location of their escapist fantasies

To my knowledge no one from any nuclear laboratory has ever made this statement.

As for group 1, the hazard is way worse than most of them ever thought, so we're going to have to use nuclear charges at some point, or suffer the consequences - though even without nuclear charges we can still limit the damages through the simple technique of evacuating the impact area - and for smaller impactors (75 meters or so), it may turn out that conventional explosives are enough - Deep Impact will tell...

These folks concerns are not baseless, and are well founded, and a real effort is going to have to be made to see that nuclear charges (and technologies) for planetary defense are never used for harmful purposes - in the US, this is a function of our State Department and arms control agencies -

We also have to worry about a small impactor setting off a nuclear exchange - and at the current impact rate, this problem is not hypothetical - the chances of one of similar to Tunguska hitting during Justin's lifetime is very nearly 1.

Deep Impact will provide us with information on comet density so that when (and that's "when", not "if") we (mankind as a whole) have to stop one of these things we will be able to...

EP