Space.com: NASA is facing the prospect of trying to explore deep space without the aid of the long-lasting nuclear batteries it has relied upon for decades to send spacecraft to destinations where sunlight is in short supply.

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.

Though Griffin did not mention it, the U.S. Department of Energy over the winter quietly shelved long-standing plans to resume domestic production of plutonium-238. In 2005, the Department of Energy (DOE) gave public notice of its intent to consolidate the nation's radioisotope power system activities at Idaho National Laboratory and start producing plutonium-238 there by 2011.

Restarting production was projected at the time to cost $250 million and take five years. Those plans are now on hold. "DOE did not request funding in 2009 for [Plutonium-238] production, since NASA has been directed to fund any new production capabilities," Angela Hill, an Energy Department spokeswoman told to space.com. "Production may or may not resume based on NASA's decision. Based on current mission plans, DOE will only continue to provide new Radioisotope Power Systems until 2015."

NASA's 2009 budget request includes no money for re-establishing the Department of Energy's long dormant plutonium-238 production capability.