A few weeks ago, I wrote on this blog about an 1967 MIT study called Project Icarus. The study, performed by MIT students as an exercise in systems engineering, assumed that the mile-wide asteroid Icarus would not merely pass Earth at a distance of four million miles on June 19, 1968, but would instead strike in the Atlantic Ocean east of Bermuda. The resulting big splash would have inundated ocean-front property around the world and cast debris high into the atmosphere, cooling our planet for several years.
I've been thinking about this as a point of departure for an alternate history. What if Icarus actually had been found in early 1967 to be 18 months from a collision with Earth? And, what if the U.S. - probably the only human agency at the time capable of intervening - had made every effort to destroy and/or deflect it?
Go read. I suspect the PoD that he has and hissubsequent events would knock history off its course: dude, millions of people just got mashed! more than he has it. I also think that the effects of the impacts, even a shattered, rubble pile Icarus would be more than he thinks. However, it is worth a read.