Plutonium-238 (Pu-238) is the prime energy source used in NASA's deep space and planetary exploration technologies, such as the Mars Curiosity rover. Currently, the US does not produce Pu-238 and Russia, its only historical exporter, is no longer providing the fuel. It is estimated that a mere 36 pounds of Pu-238 remain for scientific consumption...enough for one-two more missions like the Mars Curiosity rover.
Marginal efforts have been made in Congress to explore producing Pu-238, but not at levels sustainable for continued space exploration. In other words, when the supply runs out, NASA's space exploration efforts will grind to a halt.
Space Exploration is a prime stimulus of technological and social innovation and PU-238 both, literally and figuratively, fuels this ingenuity.