I think it’s wishful thinking, and always was.
I was aghast when I heard Reagan’s March 23, 1983 sppech proposing SDI. “This man must know something I don’t,” I thought. So, apparently, did a lot of other people, including members of his administration. I now know what he knew at the time – chiefly Edward Teller’s “Excalibur” laser system (atomic bomb-pumped X-ray lasers).
In truth, there never was a well-defined SDI plan – there were a lot of proposed systems, all with their pros and cons. It made for interesting and slippery debates between the pro- and anti-SDI camps, because you could criticize or promote one aspect of one system and have your opponent retailate by talking about a completely different system.
As far as I can see (and I’m not an arms control expert, but I’ve got a doctorate in Laser Physics) none of these systems was practical. And no matter what anyone says, I don’t believe that the Soviets were spooked by our “technological end run” – some of the most incisive criticisms of the system came from the Russians. (So did the revolutionary quadrupole magnet that some proposed using for neutral particle beams.) To believe that the Russian leaders didn’t believe their own experts is to attribute an astonishing degree of stupidity to them. It’s not a good idea to assume your opponent is stupid. I have yet to see a scrap of evidence that fear of SDI is one of the things that caused the fall of the Soviet Empire by making them spend themselves into bankruptcy, but this seems to be the advancing mythology.
Although there were some impressive gains for laser-based systems, I don’t think they were impressive enough to show that we had a viable system in hand. When MIRACL (the Mid Infra Red Advanced Chemical Laser) blew a strapped-down booster full of water into oblivion in one shot, it was impressive. Some critics said that stressing the tank was not necessary, but it DID simulate the strain on an in-flight booster. I wondered myself how far away the laser was – the news reports didn’t say, and for all I knew it was just off-camera. It turns out to have been a kilometer away, which is impressive, but a tiny fraction of the distance away a space-based laser would have been from its targets, Laser beams, despite what you may have heard, DO expand with distance. Once you’re more than a confocal parameter away the beam area spreads out with the square of the distance, so distance is a BIG factor.
Then there’s the issue of directing the beam. Proponents of SDI say that ablative coatings won’t help your missile – the beam’s blast effects will still damage the surface. Making the missile shiny won’t help either, as small defects will be damaged by the laser beam. So what is that beam going to do to your directing mirror, which is so much closer, subject to a smaller (and therefore more intense) laser beam, and is going to hit “hit” several times in rapid succession? We’ve had our lab lasers destroy themselves in just this way.
Then there’s the problem of supplying power to the laser – Chemical Lasers would themselves have to be big tanks of gas. Nuclear lasers violate treaties, and are untrustworthy to boot.
I suspect that “Smart Rocks” (which somewhere along the line became “Brillliant Pebbles”) won out because they avoided these problems. They were a workable relatively low-tech solution to the problem.