Personally I don't see how effective SAMs, which are generally designed to shoot down aircraft and slower moving theatre range missiles, could be effective against something like an ICBM even in the ascent phase. Compare a ballistic missile to two very similar systems: satellite launch rockets and the Space Shuttle. The Shuttle breaks the sound barrier in under 1 minute, and it is at around 28 miles altitude and traveling several times the speed of sound at 2 minutes when the boosters separate. An ICBM is basically a small rocket (by spacefaring standards), and indeed, several satellites have been put into earth orbit by russian ICBMs (which was one reason why Sputnik caused so much panic in the west), so the comparison isn't completely inaccurate. If anything, it'd be *harder* to shoot down an ICBM with a SAM because it would almost definately accelerate faster than the shuttle, both since it's lighter than the shuttle, and since something like the shuttle has to purposely limit its acceleration to keep the crew from being crushed into salsa. To have a chance at shooting down an ICBM with a SAM, you'd have to be very close, and detect a launch and react to it within less than a minute. Remember the ICBM, unlike a plane or a terminal stage scud, is accelerating too, so you'd have to not only overcome that acceleration, but the massive headstart in terms of speed and distance.

Trying to shoot down an ICBM in terminal stage with a SAM is basically impossible. At this point, you have a MUCH smaller radar cross section to work with (by now it's no longer a whole rocket, but probably just what's under the nose cone), and the warhead is traveling at near orbital speeds. The shuttle orbits at more than 18,000 mph, which is something like mach 23. Even using a HIGHLY conservative estimate of half the shuttle's orbital speed, an ICBM warhead would be traveling at mach 10-15, well beyond the patriot's maximum speed of mach 5.

The reason we're looking at things like airborne lasers and interceptor missiles is they give us a much larger window to act, as interceptor missiles also travel at orbital speeds, and no matter how fast a missile is, it can't outrun a laserbeam.

I guess in theory, if we could convince say, North Korea to let us set up patriot missile batteries all within 10 miles of their launch sites, and then say, had a computer system setup to do automagic tracking and launching of the patriots, we might have a chance at shooting down an ICBM in the ascent phase with a SAM. Personally, I don't let my computer upgrade without me involved in the loop, much less let it shoot off hundreds of pounds of explosives traveling at supersonic speeds, so my money is on OMG LASER.