The civilian stockpile of the plutonium isotope used to power the radiothermal generators (RTG) that make electricity for US deep space probes has dwindled to only 35 kg (77 lb), so the first 50 g (1.7 oz) of plutonium oxide produced by the laboratory marks a major turnaround in American space capabilities.

PU-238 is an unstable isotope of plutonium with a half life of 87.7 years. As it decays into uranium-234, each gram produces about 0.5 watts of thermal power, which allows small RTGs to power spacecraft far from the Sun or on the surface of planets without the need for solar panels.

It's a system that's been very successful since the Apollo program, but since the Savannah River Plant in South Carolina stopped producing PU-238 in 1988, NASA has been relying on a stockpile of the element purchased from Russia in 1993. Only half of what remains meets specifications, so ORNL says that there's only enough left to power two or three NASA missions.