Fat Man was the codename of the atomic bomb dropped on Nagasaki on August 9, 1945.
The United States used Fat Man three days after Little Boy was dropped on Hiroshima. The weapon was carried by the B-29 bomber Bockscar and detonated over Nagasaki shortly after 11 a.m. local time. Its intended target was Kokura, but cloud cover and smoke led the crew to attack the secondary target, Nagasaki.
Fat Man used plutonium and an implosion mechanism. Conventional explosives surrounding the plutonium core compressed it rapidly, producing the nuclear chain reaction. This design differed substantially from Little Boy’s uranium gun-type mechanism.
The Nagasaki bombing occurred on the same day that the Soviet Union began its offensive against Japanese forces in Manchuria. Japan’s leaders faced both the atomic attacks and the collapse of their hope that the Soviet Union might mediate a favorable peace.
Fat Man is sometimes mistakenly identified as the Hiroshima bomb. The names are distinct: Little Boy was used at Hiroshima, while Fat Man was used at Nagasaki. The earlier Trinity test device, The Gadget, was another plutonium implosion weapon but was detonated in New Mexico.