Airborne Laser Successfully Tracks, Hits Missile
fructose writes "The Airborne Laser managed to acquire, track, and illuminate a test missile a few days ago. According to the press release, the Boeing plane 'used its infrared sensors to find a target missile launched from San Nicolas Island, Calif ... issued engagement and target location instructions to the beam control/fire control system ... fired its two solid-state illuminator lasers to track the target and ... fired a surrogate high-energy laser at the target, simulating a missile intercept.' The sensors on board the missile confirmed the 'hit.' Michael Rinn, ABL's program director, said, 'Pointing and focusing a laser beam on a target that is rocketing skyward at thousands of miles per hour is no easy task, but the Airborne Laser is uniquely able to do the job.' The next steps will be to test the high-power laser at full strength in flight and do a complete system test later this year. Its success or failure will determine whether the project gets canceled. Looks like the Real Genius fans out there are finally living the dream."
Which is generally isn't above the clouds in the stratosphere.
Nothing, in theory. Just like there's nothing in theory that says the rocket can't have a zillion other systems designed to defeat this laser. In practice, however, the answer is weight. A rocket's weight is around 80-90% fuel with payload being from 2-5%. A small increase in payload weight leads to a great increase in the rocket's size and fuel load.
A rocket designer ends up having to make a series of compromises between the strength of the rocket itself, the payload and the range. If you want to protect your rocket you're going to have to give up payload, give up range or increase rocket size, all of which make them less useful as weapons.