Russia is seeking to renegotiate a deal for supplying NASA with plutonium-238, dealing a setback to planning for deep-space missions. Since U.S. production was halted in the late 1980s, U.S. stockpiles of the rare material have been steadily dwindling, forcing expensive purchases from Russia. The Department of Energy plans to resume domestic production, but that effort could take six years and $150 million.

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