Plutonium was first produced in 1940 and 1941 at Berkeley by Glenn Seaborg, Edwin McMillan, Joseph Kennedy and Arthur Wahl, who made it by bombarding uranium with deuterons. It was named after the planet Pluto, then reckoned the ninth and outermost planet. Plutonium is a dense, silvery, intensely radioactive metal, notorious as a fissile material of nuclear weapons and useful as a long-lived power source for spacecraft. A few kilograms of plutonium form the core of a fission weapon, while the isotope plutonium-238, warm from its own decay, has powered spacecraft and instruments far from the Sun for decades, including the Voyager probes and Mars rovers; the metal must be handled with great care for its radioactivity and toxicity.
Plutonium exists in two chemically identical but practically very different guises: plutonium-239, bred in reactors and fissile enough to sustain the explosive chain reaction of a weapon, and plutonium-238, a different isotope that simply generates steady heat from its own decay and powers spacecraft rather than bombs. The world's first plutonium-fuelled detonation was not Nagasaki but the Trinity test in the New Mexico desert on 16 July 1945, a secret trial three weeks earlier that confirmed the implosion-type weapon design before it was used in combat.