Hiroshima was destroyed in a flash by a bomb dropped from a propeller-driven B-29 of the U.S. Army Air Force, on the warm morning of Monday, August 6, 1945. The bomb was not chemical, as bombs until then had been, but rather atomic, designed to release the energies Einstein described. It was a simple cannon-type device of the sort that today any number of people could build in a garage. It fell nose-down for forty-three seconds, and for maximum effect never hit the ground. One thousand nine hundred feet above the city the bomb fired a lump of highly enriched uranium down a steel tube into a receiving lump of the same refined material, creating a combined uranium mass of 133 pounds. In relation to its surface area, that mass was more than enough to achieve “criticality” and allow for an uncontrollable chain of fission reactions, during which neutrons collided with uranium nuclei, releasing further neutrons in a blossoming process of self-destruction. The reactions could be sustained for just a millisecond, and they fully exploited less than two pounds of the uranium before the resulting heat forced a halt to the process through expansion. Uranium is the heaviest element on earth, almost twice as heavy as lead, and two pounds of it amounts to only about three tablespoonfuls. Nonetheless, the explosion over Hiroshima yielded a force equivalent to 15,000 tons (fifteen kilotons) of TNT, achieved temperatures higher than the sun’s, and emitted light-speed pulses of dangerous radiation. More than 150,000 people died.
Three days later, the city of Nagasaki was hit by an even more powerful device—a sophisticated implosion-type bomb built around a softball-sized sphere of plutonium, which crossed the mass-to-surface-area threshold of criticality when it was symmetrically compressed by carefully arrayed explosives. A twenty-two-kiloton blast resulted. Though much of the city was shielded by hills, about 70,000 people died. Quibblers claim that a demonstration offshore, or even above Tokyo harbor, might have induced the Japanese to surrender—and if not, there was another bomb at the ready. But the idea was to terrorize a nation to the maximum extent, and there is nothing like nuking civilians to achieve that effect.