The future is looking brighter for missions to the solar
system's dark corners. Plutonium needed to power the spacecraft
that cannot rely on solar power has been created in the US for
the first time in 25 years – albeit in small quantities. Some
destinations, such as the outer solar system or the polar
regions of Mars, receive too little sunlight for ambitious
missions to use solar panels there. Instead, heat from the decay
of the radioactive isotope plutonium-238 is needed to generate
electricity. Unfortunately, stores of the stuff are running out.
The US stopped producing the isotope – which had been generated
as a by-product of cold-war nuclear weapons programmes – in
1988. Its remaining stocks have been estimated at just 16.8
kilograms. "Without a restart of plutonium-238 production, it
will be impossible for the United States, or any other country,
to conduct certain important types of planetary missions after
this decade," concluded a US National Research Council report in
2011. Now, however, the US has