Spacecraft traveling to the outer reaches of the solar system, where there's little sunlight and temperatures reach absolute zero, typically have to get their energy from plutonium. But the supply of this fuel is running low and it's unclear if Congress will fork over the $30 million needed to restart production, according to this NPR report.

Plutonium-238 (not plutonium-239, which is used for nuclear weapons) gives off a significant amount of heat, which can be converted into electricity. The material has been used as the power source on over two dozen deep space missions including those of the Voyager spacecraft in the 1970s and the Cassini spacecraft currently imaging Saturn's rings and moons. And it's the only material that can be used for such missions.

The United States has not made any new supplies of plutonium-238 for spacecraft since the 1980s. The U.S. has borrowed from Russia, but its production stopped a while back as well.

From the NPR report:

According to McNutt [a planetary scientist at the Johns Hopkins University Applied Physics Laboratory], NASA has enough plutonium-238 for its next Mars rover, called the Mars Science Laboratory, and the next planned major mission to the outer planets.

The agency could also potentially have a relatively low-cost, Discovery-class mission that would use only a small amount of the stuff, to test a new power-generation technology that could more efficiently convert the heat of plutonium-238 to electricity.

But that's about it, and after that, NASA would be stuck, McNutt says. "It's kind of like having a car, and if all the gasoline stations are closed and are out of gasoline, and you're out of gas," he says, "you're not going to go anywhere."

NASA needs about 11 pounds of the stuff per year for its missions, and even if production of plutonium started now it would still take about eight years to satisfy demand.

NPR reports that it would likely cost in excess of $150 million to restart production, however the alternative would leave NASA and planetary scientists without the ability to explore as much of the solar system.

The decision is being left to Congress, which is deciding the future of NASA's human spaceflight program.