he U.S. Department of Energy’s about-face on plans to resume production of plutonium-238 leaves NASA facing one of two unattractive choices: depending on Russia to fuel future U.S. deep space probes or forgoing

such missions altogether.

The move is an apparent reversal of the White House’s 2001 decision to resume domestic production of plutonium-238, which is needed for radioisotope power sources

the only known means of powering spacecraft that operate too far from the sun to harvest its energy-giving rays. Without these devices there could be no missions like Voyager 1 and Voyager 2, which have captured the imagination of millions during decades-long journeys that have brought humanity to the brink of interstellar space. Radioisotope power sources, which convert heat from decaying plutonium into electricity, also are used for unspecified national security purposes.

Of all the kinds of missions NASA conducts, perhaps none are more intriguing than those that explore the mysterious worlds in the outer reaches of the solar system. Scientists are particularly interested these days in the moons orbiting gas giants Jupiter and Saturn: Some are hotbeds of geologic activity; others are believed to harbor vast oceans beneath their icy surfaces. NASA planners are in the process of selecting one such destination for the agency’s next flagship-class outer-planet mission. Whether there will be another one after that is now in question.

The United States stopped making plutonium-238 in 1988, opting to buy it from Russia, which attached the stipulation that the material not be used for national security purposes. The administration of U.S. President George W. Bush decided to resume production in 2001, and directed the department, which traditionally has had this responsibility, to make the necessary plans. In 2005, the department said it would have a consolidated manufacturing facility up and running at the Idaho National Laboratory by 2011.

But there was no follow through: the department recently confirmed it has no money in its 2009 budget request for the effort, and said NASA must secure the necessary funds, estimated at $250 million. This clearly is news to NASA, which already is struggling to make ends meet.

Meanwhile, the U.S. stockpile contains enough plutonium-238 for only three NASA missions and these have all been identified: the Mars Science Laboratory scheduled to launch next year; the new flagship-class outer-planet mission, slated to launch in 2017; and a Discovery-class mission scheduled to fly around 2014. Ironically, the Discovery mission is intended in part to demonstrate a new type of more-efficient radioisotope power source.

NASA already is feeling the pinch: It has ruled out the use of a radioisotope power source on its latest New Frontiers-class planetary mission, scheduled for 2016.

If NASA wants to conduct additional deep space missions in the future it will have to make do without a radioisotope power source

a severely limiting constraint

or continue buying plutonium-238 from Russia, which has a small stockpile.

Under these circumstances, Russia, as the only known supplier of a very scarce commodity, would be able to name its price.

There also is the possibility that

Russia might renege

on the terms of a deal at the last minute, potentially putting at risk hundreds of millions of dollars of NASA investment in mission development.

On top of this, Russia only has about 10 kilograms of plutonium-238 and also has ceased production, according to NASA Administrator Mike Griffin. To get a sense of how far this will go, consider that NASA’s flagship-class Cassini mission now orbiting Saturn has 26 kilograms of plutonium on board. Even if NASA’s effort to develop a more efficient radioisotope power source succeeds, it is hard to imagine Russia’s stockpile supporting more than two missions of any significant scale.

The reason for the White House’s apparent change of heart regarding plutonium production is a mystery; the national security community, traditionally a far bigger consumer of plutonium-238 than NASA, usually gets its way with this administration.

But even if national security is removed from the equation, the current situation is unacceptable

not only for NASA but also for the global space science community that looks to the U.S. agency for leadership in exploration. If the White House refuses to provide funds for the Energy Department to restart plutonium-238 production, then Congress should take matters into its own hands.