“The project is planning to produce new Pu-238 at a rate that supports currently projected NASA missions,” says Rebecca Onuschak, program director for the DOE’s plutonium infrastructure. “There is no shortfall of material projected to meeting those needs, and so no remedy or action plan has been implemented.”
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The argument that if NASA had more RTG's laying around it would be doing more is a bit suspicious as that's not how space missions are planned. Not to mention, Charles Bolden just wrote a letter to Congress asking them why they can't fund the SLS properly. There's no extra money laying around.