Little Boy and Fat Man

The two bombs that ended the Second World War were built on opposite bets. One was so simple its designers never bothered to test it. The other was so complicated it had to be proven in the desert first, and even then the men who built it were not certain it would work. Both were dropped within three days of each other in August 1945, and between them they killed somewhere between 100,000 and 200,000 people.

Little Boy: the simple gun

Little Boy, dropped on Hiroshima on August 6, 1945, worked like an artillery piece. Inside its casing a charge of cordite fired a hollow slug of uranium-235 down a gun barrel into a matching target of the same metal. When the two pieces slammed together they formed a critical mass, and the chain reaction ran away in a fraction of a second. The design was so straightforward that it was used in combat without a single full-scale test. It carried about 64 kilograms of enriched uranium, of which less than a kilogram actually fissioned, and released the equivalent of roughly 15,000 tons of TNT. Enriching the uranium had been the hard part. The bomb around it was almost crude.

Fat Man: the implosion problem

Fat Man, dropped on Nagasaki on August 9, could not be built the same way, and the reason is buried in the physics of plutonium. Plutonium bred in a reactor is contaminated with an isotope, plutonium-240, that throws off stray neutrons on its own. Fire two lumps of it together with a gun and those stray neutrons would light the reaction early, before the metal was fully assembled, and the bomb would blow itself apart in a weak fizzle instead of a full detonation. The gun-type route was a dead end for plutonium.

The answer was implosion. A sphere of plutonium was wrapped in precisely shaped high-explosive 'lenses' that, detonated in perfect unison, crushed the core inward, squeezing it to more than twice its normal density and holding it together long enough to burn. The engineering was brutally unforgiving, which is why this design, and not the uranium gun, was the one tested at Trinity on July 16, 1945. Fat Man needed only about 6 kilograms of plutonium and yielded some 21,000 tons of TNT, more than Hiroshima's bomb from a fraction of the fissile material.

What they opened

The human cost is hard to hold in the mind. Estimates of the dead run from about 90,000 to 146,000 at Hiroshima and 39,000 to 80,000 at Nagasaki, many killed in the first instant, many more over the following months by burns and radiation. Almost every nuclear weapon built since has followed Fat Man's implosion design, not Little Boy's gun. The simpler bomb was a historical dead end. The complicated one became the template for the tens of thousands of warheads that have shadowed the world ever since. Two bombs, three days, and the beginning of an age that has not ended.