Little Boy (“little boy” in French) is the code name of the A-bomb which was dropped on Hiroshima in Japan on August 6, 1945 at 8:15 a.m., when the bomber left at 2:45 a.m. from the island of Tinyan. The bomb was dropped by the US Army's B-29 Enola Gay bomber. It was the first atomic bomb used offensively; the second was Fat Man, dropped on Nagasaki three days later.
The weapon was developed during World War II as part of the Manhattan Project, and derived its explosive power from enriched uranium. With a length of 3 m and a diameter of 71 cm, it had a mass of 4,400 kg. It contained just over 64 kg of uranium 235, of which “only” 700 g entered into fission (ie 1.1%).
Little Boy caused the second artificial nuclear explosion in history after the Trinity atomic test.
The bombing of Hiroshima
Embarked in the B-29 Enola Gay bomber piloted by Lieutenant-Colonel Paul Tibbets, the bomb was armed in flight at 9,600 meters above the city, then dropped by bomber Thomas Ferebee. The detonation took place at an altitude of 580 meters, directly above the Shima hospital, in the heart of the city, about 300 meters southeast of the point initially targeted: the Aioi Bridge. With a power of 13 to 16 kt (the data varies on this subject), it was less powerful than Fat Man, which was launched on Nagasaki (between 21 and 23 kt). However, the damage and the number of victims were much greater, Hiroshima being on flat ground, while the hypocenter of Nagasaki was in a small valley. There were 70,000 deaths directly linked to the explosion and the shock wave. Many of the survivors would later meet a tragic end, largely due to the effects of radiation (radiation during the explosion, fallout, "black rain", contamination of food and water) and the effects thermics of the bomb (burns) with serious consequences on newborns.
The technology used by Little Boy, known as the “insertion A bomb”, was very simple: an explosive charge projected a block of uranium 235 onto another block to reach the critical mass allowing fission to start. It was therefore decided to use this weapon without prior testing. However, it turned out that the implosion technology used during the test at Trinity Site and then at Nagasaki was more efficient: it required fewer fissile materials and made it possible to use plutonium 239.
Moreover, this insertion bomb was extremely dangerous to handle, because the two masses of uranium could have come into contact accidentally, during a plane crash for example. None of the other five Mark I bombs, built on the Little Boy model, were therefore used by the US Army.
Development of the uranium bomb
Development of the first prototypes and experimental work began as early as the spring of 1943, as the Los Alamos National Laboratory became operational as part of the Manhattan Project. The effort was not continuous, the engineers having focused on the plutonium bomb, more complex but also more powerful. Experts thought that assembling a uranium bomb would be almost a formality, as soon as the plutonium model was finished.
Another track than the plutonium bomb
Until the spring of 1944, scientists worked on the Thin Man project (opposite of Fat Man, designating the implosion bomb). Testing at the Muroc, California base showed that the Thin Man prototype had good ballistics and did not wobble, unlike the two Fat Man prototypes. These underwent a pendulum movement during the descent with deviations that could have