Plutonium

Plutonium is a silvery radioactive metal made almost entirely inside nuclear reactors, when uranium-238 absorbs neutrons. Glenn Seaborg and colleagues first produced it at Berkeley in 1940–41, naming it after Pluto, following neptunium.

Plutonium-239 splits easily, which made it the core of the first nuclear test in 1945 and of the bomb dropped on Nagasaki. It is also recycled into reactor fuel. Plutonium-238 is different: it decays steadily, giving off heat for decades. Radioisotope generators turn that heat into electricity for spacecraft too far from the Sun for solar panels, such as Voyager and New Horizons, and for the Curiosity and Perseverance rovers on Mars. Plutonium is highly toxic if inhaled.