The road to build the atom bomb
August 6, 2020 - On August 6, 1945, the United States dropped an atomic bomb on the Japanese city of Hiroshima. Tens of thousands of people died instantly. Three days later, Nagasaki met the same fate. The bombs killed more than 200,000 people.
The war in Europe had ended with the unconditional surrender of Nazi Germany on May 7, 1945, but fighting continued in the Asia-Pacific region. In mid-June, U.S. President Harry S. Truman approved a plan to invade and occupy Japan. The invasion would involve some 2.7 million troops and likely cost hundreds of thousands of U.S. lives.
On July 16, in the parched landscape of Jornada del Muerto -- Dead Man’s Journey -- a remote desert in Alamogordo, New Mexico, a group of engineers and physicists led by J. Robert Oppenheimer detonated the world’s first nuclear device. The “Gadget” exploded with a power equivalent to 18.6 kilotons of TNT -- four times the predicted yield -- and generated a flash of light visible from space.
Ten days later President Truman, British Prime Minister Winston Churchill, and China’s Chiang Kai-shek issued the Potsdam Declaration announcing the terms for Japan’s surrender. The declaration ended by warning of “prompt and utter destruction” if Japan failed to surrender unconditionally. Japan rejected the terms.
On July 29 and August 2, two atomic bombs -- a uranium-fuelled weapon code-named “Little Boy” and a plutonium-fuelled bomb, “Fat Man,” -- arrived on the island of Tinian in the Mariana Islands, 2,500 kilometres from Japan. Truman authorized the use of the weapons as soon as possible. However, an approaching typhoon postponed the planned deployment of Little Boy on August 1.
The 4,037kg Little Boy was carried in a B-29 Superfortress piloted by Col. Paul W. Tibbets, commander of the 509th Operations Group. At 02:45 Tinian time, on August 6, Tibbets and 11 crewmembers took off on the B-29 -- nicknamed “Enola Gay” after Tibbets’ mother -- from Tinian toward Hiroshima.
At 08:15:15 Hiroshima time, the bomb bay doors snap open, and Little Boy drops clear of its restraining hook. The bomb falls for 43 seconds. An explosive charge fires a uranium 235 projectile down the gun barrel into an enriched uranium core assembly. The core becomes supercritical. A storm of neutrons -- generated by four beryllium/polonium initiators -- triggers the chain reaction. Six hundred metres above Hiroshima’s Aioi Bridge the bomb detonates with the power of 16 kilotons.
The ferocity of the blast etches the shadows of incinerated humans on steps, pavements and walls. Nearly 13 square kilometres, over 60 per cent of the city, is destroyed. Between 70,000 and 80,000 people are believed to have been killed by the blast, and many more from injuries and radiation.
Three days later, a plutonium bomb, “Fat Man,” is dropped over Nagasaki, killing more than 50,000 people. At noon on August 15, 1945, Emperor Hirohito announces his country’s surrender.
- Hiroshima and Nagasaki Bombing Timeline (Atomic Heritage Foundation)
- ‘Now I Am Become Death’: The Legacy of the First Nuclear Bomb Test (New York Times)
- 75th Anniversary of the Bombings of Hiroshima & Nagasaki (Union of Concerned Scientists)
- Powerful Pictures Show What Nuclear ‘Fire and Fury’ Really Looks Like (National Geogtraphic)