The 10-foot 8-inch (3.25 metres) long, five-foot (1.52 metres) diameter, 10,000-pound (4545 kg) weapon detonated at an altitude of about 1,800 feet (550 m) over the city. It was dropped from the B-29 bomber Bockscar, piloted by Major Charles Sweeney. The bomb had a yield of about 20 kilotons, or 8.4×1013 joule = 84 TJ (terajoule), slightly more than the bomb known as "Little Boy" dropped on Hiroshima three days earlier. Due to Nagasaki's hilly terrain, the damage was somewhat less extensive than that in relatively flat Hiroshima. An estimated 40,000 people died in the nuclear explosion at Nagasaki.
"Fat Man" was an implosion type weapon using plutonium. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive. Numerous detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the capsule, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear explosion.
This mechanism was necessary for a plutonium weapon in contrast to a uranium weapon (like "Little Boy") because the gun mechanism used in "Little Boy" (firing two sub-critical masses together into one super-critical mass) would have been impractical. Plutonium has a higher spontaneous neutron emission rate than uranium, and so two masses fired together would begin chain reactions before they formed a supercritical mass, resulting in a 'fizzle', or explosion with no fissile component. It is theoretically possible to build a plutonium gun-type device, but it would need to be 19 feet long in order to allow the sub-critical masses to be fused into a critical mass before a fizzle occurs. The mass of a plutonium gun-type device would have been beyond the payload of the B-29.
Earlier there had been one test explosion with this type of weapon, on July 16, 1945, at the Trinity site, due to worries about how the mechanism would perform in practice. In the end, it gave somewhere around 20 kt, 2 to 4 times the expected yield.
Schematic cross-section of Gadget; some boundaries are approximate. From left to right (ouside inward):