Decaying Reactors (RTG)s

Developed in the 1950s, radioisotope thermal generators, or RTGs, power spacecraft such as Cassini (illustrated) as well as historic spacecraft including Pioneer 10 and 11, the Viking and Voyager missions, and experiment 'packages' on the Apollo moon landers. Reliable RTG power allowed some missions to continue for decades (Illustration: NASA/JPL)"

We owe the wonderful images of the outer solar system taken by the Galileo and Cassini spacecraft to the power generated by Cold War surplus nuclear isotopes. But those leftovers are expected to run out in 2018, and no good alternatives are ready, warns a new report by the US National Research Council.

The crucial isotope is plutonium-238. It can't be used in weapons or reactors, but as the atoms decay, they emit alpha particles, or helium nuclei, that easily convert their energy to heat.

Nearly half a century ago, NASA developed radioisotope thermal generators (RTGs) that convert the heat from radioactive decay into electricity. That makes them the only long-term source of power available when sunlight is too weak to use solar cells.

Plutonium 238 production

But the weapons-production reactors that produced plutonium-238 were shuttered two decades ago. The existing inventory is running out, and most of it is needed for pending missions including the Mars Science Laboratory and the planned Europa orbiter.

With eight years needed to start producing 5 kilograms of plutonium-238 per year, the report argues that funding be added to the 2010 budget for the project. Existing reactors could produce the isotope but at least $150 million in new equipment would be needed to process it into a usable form.