Neptunium

Neptunium is a silvery radioactive metal and the first transuranium element, meaning heavier than uranium. Edwin McMillan and Philip Abelson made it in 1940 at Berkeley by hitting uranium with neutrons. Since uranium was named after Uranus, they named the next element after the next planet, Neptune. Tiny traces also form naturally in uranium ores.

Neptunium builds up in spent nuclear fuel. Its most stable form, neptunium-237, has a half-life of about 2 million years. Its main use is as the starting material for plutonium-238: neptunium placed in a reactor absorbs a neutron and turns into it. Plutonium-238 is the long-lasting heat source that powers deep-space probes and Mars rovers.