As rare-earth metals become less common, and as nuclear non-proliferation treaties proliferate, it may get harder to send probes into deep space. Preparing for that eventuality, the European Space Agency is stockpiling smoke-detector parts for a possible new fuel source.
Imagine you're driving across the Mojave Desert, and somewhere in the middle of absolutely nowhere you realize that the next gas station is further away than your car can travel on its current supply of gasoline. What next? That's the problem NASA mission planners are facing as the agency's supply of plutonium-238, the fuel used to power deep space probes like Cassini and surface scouts like the upcoming Mars Science Laboratory, are dwindling.