While its comforting to think that the government has ways of stopping ICBMs, there is a hidden problem of 'how effective is it really?'
First off, the laser is currently not designed to take out ICBMs, but merely tactical ballistic missiles (TBMs), which have a much shorter range then ICBMs and a thinner shell (the exception being liquid-fueled ICBMs), meaning they are more likely to be able to weaken the skin in order to cause the structural failure of the missile that would result in it tearing itself apart mid-flight.
Secondly, the laser must be near the launch site, within hundreds of kilometres of the launch site, otherwise it is completely ineffective against the intended missile. The laser must also target the same area of the missile long enough to melt the outer layer of the ICBM, causing a critical structural failure in flight. Even then, once the missile is gone, we still have the problem of an armed, out of control nuclear warhead.
Thirdly, the laser is also rendered useless against MIRVs, Multiple Independently targetable Re-entry Vehicles. Each warhead within the rocket is, in essence, a smaller rocket, capable of guiding itself over long distances. Suddenly, you've got 4 missiles to contend with, each capable of surviving the destruction of the carrier missile.
Fourthly, and my final post before I decide I should get to sleep, the laser is in a 747. We're talking about one of the biggest planes in the sky right now. Those aren't just going to fly around unnoticed. Even if they fly/build in scramblers or jammers, it wouldn't be too hard to look up and see one. Until it can be built into a smaller scale, it isn't effective given the carrier sky.
Before I go, I know it may seem odd for someone to know this much about ballistic missile defences. The only reason I know most of this is that I recently did a project on missiles and missile defence, otherwise, I wouldn't know much about the subject at all.
