astroengine writes “NASA’s stockpile of the plutonium isotope Pu-238 is at a critical level, causing concern that there won’t be enough fuel for future deep space missions. Pellets of Pu-238 are used inside radioisotope thermoelectric generators (or RTGs) to generate electricity for space probes traveling beyond the orbit of Mars — solar energy is too weak for solar arrays at these distances. Blocked by a contract dispute with Russia to supply (more…)