Whether or not Voyager I has actually left the solar system, one thing is certain — it never would have made it far enough to have a debate if it weren't for the help of plutonium-238. The isotope has been crucial for powering spacecraft — there's 10 pounds of the stuff in the batteries of Curiosity. But supplies are very, very limited. In fact, the US scientific stockpile is down to 36 pounds. All of it spoken for. What happens to space exploration when there's no more plutonium-238? Dave Mosher investigates at Wired.