The magic ingredient inside the guts of NASA’s Curiosity Rover currently zipping around on Mars is even rarer than most people realize.

To create the battery that powers Curiosity, NASA used Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs) via the Department of Energy. The uber-batteries are essentially powered by plutonium-238, which is a byproduct of creating nuclear weapons. Most of what NASA uses was left over from the Cold War. The only problem? According to Space News, only 37 pounds of what NASA has available to use is still viable.

With the current supply, it's estimated that NASA could build three more batteries. Once they use that up, no more nuclear batteries. NASA already has one of those final three batteries slotted to power another Mars rover mission, dubbed Mars 2020, though the other two have not been assigned. But that proposed submarine probe for Saturn’s moon Titan could certainly be one option for those remaining batteries.

Along with powering the Curiosity mission, nuclear MMRTG power sources have also been used for Voyager and New Horizons. The power source is so handy because it lasts an extremely long time and helps put off heat to keep the components from freezing out in the vacuum of space.

So, what happens when we run out? NASA actually launched an alternative nuclear battery program, the Advance Stirling Radioisotope Generator (ASRG), which would be four times more efficient and generate the same amount of power. But NASA pulled the plug on the development team last year, so you mark that option off the list. The Department of Energy has recently tried to restart production on some plutonium-238, but it’ll take time — and keeping up with the demand of future NASA missions is going to remain a major concern.

But hey, considering all the NASA budget cuts, its not like there are a ton of funded projects just waiting for a battery. So, yay?

(Via Popular Science, Space News)