A U.S. Air Force 8-36 droned through the sky 35,000 ft. above Yucca Flat, Nev. just before dawn one morning last week, and slowly opened its barn-sized bomb-bay doors. Forty-two seconds later, at 4:15 a.m., the desert below exploded into noonday brilliance. For five miles around, acres of Joshua trees, cactus and sagebrush burst into flame. A sturdy frame house ten miles from the explosion collapsed. In San Francisco, 600 miles to the west, people saw the incandescent flash; in Pasadena, 250 miles southwest, they heard the explosion as a rumble in the distance.
The first of the day's two...