"Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of the two nuclear weapons to be used in warfare and was the third man-made nuclear explosion at that time. The name also refers more generically to the early nuclear weapon designs of U.S. weapons based on the "Fat Man" model. It was an implosion-type weapon with a plutonium core. [1]

Fat Man was possibly named after Winston Churchill[2], though Robert Serber said in his memoirs that as the "Fat Man" bomb was round and fat, he named it after Sidney Greenstreet's character in The Maltese Falcon. During development, however, there was little uniformity, and both it and the shotgun design, now commonly known as Little Boy, were variously called Fat Boy, Thin Man (allegedly after FDR), S-1, and "the gadget".[3]

"Fat Man" was detonated at an altitude of about 1,800 feet (550 m) over the city, and was dropped from a B-29 bomber Bockscar, piloted by Major Charles Sweeney of the 393d Bombardment Squadron, Heavy. The bomb had a yield of about 21 kilotons of TNT, or 8.78×1013 joules = 88 TJ (terajoules).[4] Because of Nagasaki's hilly terrain, the damage was somewhat less extensive than that in relatively flat Hiroshima. An estimated 39,000 people were killed outright by the bombing at Nagasaki, and about 25,000 were injured.[5] Thousands more would die later from related blast and burn injuries, and hundreds more from radiation illnesses from exposure to the bomb's initial radiations. The aerial bombing raid on Nagasaki had the third highest fatality rate[6] in World War II after the nuclear strike on Hiroshima[7][8][9][10] and the March 9/10 1945 fire bombing raid on Tokyo[11].