Fat Man was the atomic bomb detonated over Nagasaki on August 9, 1945.
The United States used the plutonium-based Fat Man in the second wartime atomic bombing. The device was an implosion-type nuclear weapon developed at Los Alamos Laboratory. It was carried by the B-29 bomber Bockscar, piloted by Major Charles Sweeney, and exploded over Nagasaki on August 9.
Fat Man differed technically from Little Boy, the uranium gun-type bomb used against Hiroshima three days earlier. Its rounded shape inspired its codename. The names were part of a wider Manhattan Project naming system: “Thin Man” referred to an earlier long, gun-type design, while “The Gadget” was the plutonium device tested at Trinity in New Mexico.
Fat Man was not the last nuclear weapon associated with the war. Improved Fat Man-type devices were later used in the 1946 Operation Crossroads tests at Bikini Atoll, although those tests were conducted after Japan’s surrender.