On August 9th, three days later, a B29 named Bockscar, callsign Dimples 77, dropped a plutonium-based Fat Man bomb on Nagasaki.
Japan surrendered unconditionally on August 15th.
The Manhattan Project
In 1941 the British government offered the American government information developed by the British scientific project codenamed Tube Alloys which established that an aerial atomic fission bomb was feasible.
President Franklin D Roosevelt authorized the creation of a secret development project which was hidden in the Manhattan District of the US Army Corps of Engineers under the direction of General Leslie Groves. This became known as the Manhattan Project. Groves brought in a theoretical physicist, Robert Oppenheimer, to head the scientific design team.
at the Trinity test site Alamogordo NM
It took almost four years to develop a successful design. Although most attention in popular literature and media is given to Oppenheimer and his scientific team, the project was very much a massive industrial-scale engineering project driven by Groves. Natural uranium was mined in Canada and then uranium was refined into fissile uranium 235 and plutonium 239 in massive processing sites at Oak Ridge, TN and Hanford, WA. The project employed over 100,000 people at its peak, almost none of whom had any idea of what they were working on.
Because the science was brand new, much of the work was trial and error and deliberately duplicative. For example, because no one knew how to how to enrich uranium in volume, Groves had four different enrichment processes working in parallel. Similarly, Oppenheimer developed two completely different bomb designs, not knowing which (if either) would work.
The Bombs
An atomic bomb requires the fissile material to be compressed into a very small volume to produce an explosive reaction (in layman’s terms, the atoms must be so crushed together that they can emit neutrons into other atoms which emit more neutrons, etc.). The Little Boy bomb was a gun-type design in which, inside the bomb, a uranium bullet was fired into a uranium target, with the impact crushing the atoms together. In the Fat Man bomb, conventional explosive was fired inwards in an implosion to crush a ball of plutonium 239 into a fraction of its normal volume.
Both bombs required only a few pounds of fissile material and 99.9% of the weight was conventional explosive triggers and the casings.
The bombs’ explosive power was measured in kilotons, a term coined to mean the equivalent of a 1,000 tons of convention explosive, and these atomic bombs had the explosive power of about 20 kilotons. The heaviest conventional bomb in World War II was the British Grand Slam which weighed 10 tons. Thus the first atomic bombs were 2,000 times more powerful than the largest conventional bombs.
The Decision
President Roosevelt, who had supported the Manhattan Project since its outset, died in April 1945, 3 months before the first atomic bomb was successfully tested. Thus President Harry S Truman had to make the decisions on whether and how to use the bomb after being president for only three months.
At this stage of the war, the war in Europe had just ended with the surrender of Nazi Germany, but the war against Japan was far from over. US forces had been battling the Japanese since the bombing of Peral Harbor in December, 1941. It had been a slow and brutal process fought over vast distances, island by island, and in July 1945 US forces were still more than a thousand miles from Tokyo. General Douglas MacArthur was planning Operation Downfall, an invasion of Japan, which was expected to take at least another year to complete and would cost at least half a million American casualties.
Despite many promises, Stalin had not declared war against Japan, and the British were exhausted from the war in Europe and southeast Asia and could offer little help. Therefore the US faced a battle far more costly and far more difficult than the invasion of Europe in 1944.
It was hoped that dropping an atomic bomb on a Japanese city would be so devastating that the Japanese would surrender. But that was far from certain. B29s were already firebombing Japanese cities. For example, in March 1945 the USAAF conducted Operation Meetinghouse, which incinerated 16 square miles of Tokyo and killed approximately 100,000 civilians.
Truman was at the Potsdam Conference in Germany with Churchill and Stalin when a bomb was successfully tested on July 20th. (Churchill knew about the bomb under the Anglo/American Hyde Park Agreement; Stalin knew about the bomb because his spies had successfully penetrated the Manhattan Project.)
The Allies issued the Potsdam Declaration demanding immediate unconditional surrender or ‘The alternative for Japan is prompt and utter destruction.’ Japan made no response and Truman gave the order to use the two existing bombs, Little Boy and Fat Man, a few days later.
The Mission
The mission to drop the atomic bombs was given to the B29 Super Fortresses of the 509th Composite Group. The missions were flown out of Tinian in the Mariana Islands, the closest islands to Japan with long enough airfield runways to support B29s. The selected target cities were the closest, in southern Japan. Hiroshima was bombed on August 6th. The target for August 9th was Kokura, but it was overclouded upon arrival and Nagasaki, the alternate target, was bombed instead.
B29s were the newest and most advanced bombers in World War II, and the only USAAF aircraft capable of carrying heavy loads long enough distances. Unfortunately, the B29s were rushed into service before they had been fully developed, and they had numerous teething troubles. In particular, their Wright R3350 radial engines were subject to severe overheating.
The August 6th Hiroshima mission went smoothly and according to plan; the August 9th mission, by contrast, was beset with trouble and the Bockscar B29 almost crash-landed on Okinawa after bombing Nagasaki.
The aftermath
Including immediate deaths and deaths over the next 90 days, approximately 150,000 died in Hiroshima and 80,000 in Nagasaki. Although the Nagasaki Fat Man bomb created a greater explosion (estimated 22 kilotons versus 16 kilotons for Little Boy) its effects were reduced because it fell over a valley and most of the city was protected from the blast by the surrounding hills.
Russia finally declared war on Japan at midnight on August 8th, two days after Hiroshima and a day before Nagasaki, and invaded Japanese-held Manchuria one hour later with a force of over one million men. This meant that Japanese defeat was inevitable and close.
Historians continue to debate whether the atomic bombings, or the prospect of occupation of some of Japan’s home islands by Russia, was the most important factor in Emperor Hirohito’s decision to surrender on August 15th.
In the meantime, unknown to Hirohito, no more bombs were available until the end of August (two more Fat Men) and Truman had called for a hold on additional bombings for humanitarian reasons.
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