On this day in 1945, a B-29 bomber piloted by Col. Paul Tibbets dropped an atomic bomb on the Japanese city of Hiroshima. That bomber, nicknamed “Enola Gay” after Col. Tibbets mother, is on display at the Smithsonian Air and Space Museum. (I’ve always wondered how Enola Gay felt about this.)
The bomb utilized U-235, an isotope of U-238. U-235 makes up only .7% of any given amount of U-238, and is fiendishly hard to separate out. Much of the money and physical plant of the Manhattan Project was devoted to separating U-235, which is fissionable, from U-238, which is not.
The Hiroshima bomb was nicknamed “Little Boy,” in contrast to the plutonium bomb tested in New Mexico on July 16, 1945, and to the plutonium bomb dropped on Nagasaki on August 9. The plutonium bomb was nicknamed “Fat Man” because of its size and rounder configuration.
Basically, Little Boy worked like this. Two sub-critical mass pieces of U-235 were separated at opposite ends of what was essentially a gun-like tube. When the bomb detonated, conventional explosives shot one piece of U-235 into the other, achieving critical mass, meaning the amount of U-235 capable of a self-sustaining chain reaction. The resulting super-fast chain reaction released energy equivalent to about 15 — 16 tons of TNT.
The bomb exploded at 8:15 am in Hiroshima—there were clocks stopped at that time all over the city. Given that it was wartime and there were so many refugees, we don’t know precisely how many people were in Hiroshima that morning, and casualty estimates vary wildly. Best guess is that about 80,000 people were killed instantly, and another 110,000 died from injuries, radiation, etc. in the four months following. No one knows how many more deaths from cancer were caused over the years following because of radiation, but it seems likely that there were many.
I’m not going to get into the arguments for and against using this terrible weapon against a city. Proponents argue it ended the war (along with the Nagasaki bomb and the entry of the USSR into the war against Japan), and saved far more lives than it took. Opponents claim that it was unnecessary because Japan was likely to surrender soon anyway, that it could have been dropped over uninhabited areas as a demonstration/threat, even that we only did it to impress the Russians. (I may write more on this topic in a few days, if I can find the time).
One thing is certain—at 8:15 am on August 6, 1945, some 76 years ago, the door of hell was opened briefly, and we got a glance inside. Since then we’ve managed to avoid walking through that door, but the threat remains. I wonder what this implies for climate change? We’ve avoided nuclear war, but haven’t done much at all to eliminate the threat. Can we muster the political will to address climate change? One would kill us instantly, the other over decades, but each represents an existential threat.