Fusing together the 40-plus databases to get this single snapshot requires tremendous computing power. Blowing it all up with a hypothetical nuclear bomb and watching things unfold for 36 hours takes exponentially more. When Barrett’s group at Virginia Tech simulated what would happen if the populations exhibited six different kinds of behaviors—like healthcare-seeking vs. shelter-seeking—it took more than a day to run and produced 250 terabytes of data. And that was taking advantage of the institute’s new 8,600-core cluster, recently donated by NASA. Last year, the US Threat Reduction Agency awarded them $27 million to speed up the pace of their analysis, so it could be run in something closer to real time.

The system takes advantage of existing destruction models, ones that have been well-characterized for decades. So simulating the first 10 or so minutes after impact doesn’t chew up much in the way of CPUs. By that time, successive waves of heat and radiation and compressed air and geomagnetic surge will have barreled through every building within five miles of 1600 Pennsylvania Avenue. These powerful pulses will have winked out the electrical grid, crippled computers, disabled phones, burned thread patterns into human flesh, imploded lungs, perforated eardrums, collapsed residences, and made shrapnel of every window in the greater metro area. Some 90,000 people will be dead; nearly everyone else will be injured. And the nuclear fallout will be just beginning.