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 Pu-238 and the U.S. Department of Energy that has not been granted funds to produce more of the isotope, NASA lacks enough of the radioisotope to fuel the future joint NASA-ESA mission to Europa. However, the head of the European Space Agency (ESA) has announced that they have plans to commence a new nuclear energy program to alleviate the situation."
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