Scientists at the U.S. National Research Council have issued a report warning that our current supply of nuclear spacecraft fuel will be completely exhausted as early as 2018.
The fuel, mainly consisting of plutonium-238 (pictured at right), is used primarily to power exploratory vehicles such as the Apollo moon landers and the Cassini-Huygens robotic spacecraft. Due to plutonium-238's tendency to shed alpha particles during decay -- which offer relatively large amounts of energy in the form of heat -- vehicles powered by the isotope can function for decades using only small amounts of the element.
Though it's almost impossible, due to p-238's unique decay patterns, to construct weaponry from the element, our surplus of the isotope was originally spawned from plants designed to construct nuclear arms. Given that these plants have almost entirely been decommissioned since the end of the Cold War, we no longer have a simple, cost-effective way to create more of the fuel.
What does this mean for you, personally? Unless you're an astronaut, not much, but it serves as yet another barrier to our exploration of space.
Combined with the repeated budget cuts NASA has seen during the past few government regimes, this is a damning blow in our quest to explore (and eventually colonize) other planets -- assuming scientists are unable to find another, relatively inexpensive way to procure fuel for their fancy toys.
That said, we still have a few billion years before the Sun decides to expand and incinerate our planet, so there's no need to start biting your nails just yet. For the moment you're better off fretting over more immediate threats, such as the legions of robots bent on destroying humanity or the armies of resurrected dinosaurs currently devising plans to eat everyone you love.