NASA Administrator Mike Griffin told a House Appropriations subcommittee March 5 that the U.S. inventory of plutonium-238 - the radioactive material essential for building long-lasting batteries known to the experts as radioisotope power systems - is running out quickly.

"Looking ahead, plutonium is in short supply," Griffin told lawmakers during the first of two days of hearings on the U.S. space agency's 2009 budget request.

It's really unfortunate they way manned exploration is sucking up all of the money at NASA. Project Prometheus which was to develop fission reactors for space probes got killed which would have completely obviated this problem.

SPACE.com -- Plutonium Shortage May Thwart Future NASA Missions to Outer Planets