ICBMs are kept on standby because of the perceived deterrence value. (Whether they actually provide the level of assumed deterrence isa debate for another thread; I’m just establishing this in terms of the conventional justification.) In order to prevent a large potential impactor we would need to have many months of lead time, both to get the redirect system into position to intercept the object and because of the limited amount of impulse that could conceivably be applied to it means that we’d need to make a trajectory alteration as far out as possible. Now, obviously having a system capable of launching and deploying an asteroid redirect system would be prudent, but it isn’t as if you’d need to launch it at a moment’s notice; in fact, you’d probably want to run extensive and peer-reviewed studies on the best way to apply impulse to redirect it to prevent future hazards and contingencies for breakup scenarios.
No, absolutely not, although most large asteroids are a consolidation of smaller bodies and preventing them from breaking up is a significant challenge. Although breaking it up into smaller pieces means that more or all of it will “burn up” (vaporize) in the atmosphere, that atmosphere is still a crucial part of our planet and delivering that much energy will still do tremendous damage even if no solid parts of it reach the surface. Furthermore, breaking it up means dozens or hundreds of individual pieces that would then have to be tracked and moved separately rather than redirecting one large mass, and if in a periodic orbit could repeatedly intercept the Earth. The only reason I could see for wanting to do this is if the singular object is too large to shift, but that doesn’t fundamentally change the total impulse requirements, so it remains a problem noentheless.
Here is a concept I have promoted for using nuclear weapons to generate a mediated impulse to redirect a hazardous object without breaking it up. This is obviously not trivial, and the potential for weaponizing this, particularly if it were kept on some kind of standby status, is evident, but it would make sense to develop this capability or something like it as a hedge against a future threat.
Although the SDI program only lasted for a few years, the research and development of very high throughput lasers has gone on for decades without producing something capable of actually destroying an ICBM from hundreds of miles away. And while there is the thermal blooming problem of firing through an atmosphere, inherent divergence and other issues with laser throughput are actually the more problematic issues with making such a system workable.
The laser system cited in the press release you linked to is the centerpiece of a large research facility which is developed for doing nuclear physics research. It is a pulsed laser in no way a system that could ever be developed for shooting large rocks in space for extended periods of time, with pulse durations measured in the hundreds of femtoseconds. Although we may some day be capable of building very high power throughput lasers capable of continuous operation sufficient to disintegrate or provide impulse to very large objects millions of miles away in space as portrayed in science fiction and specious asteroid redirect proposals, that is very far away from current capabilities in directed energy science. I believe @CalMeacham actually works in this field and can provide specific expertise to this proposal but I think he’ll agree that it is not feasible within the foreseeable future, and the fundamental energy requirements to do so would be prohibitive in any case.
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