Current long-range spacecraft – like the US-European Cassini probe now orbiting Saturn – roam too far from the Sun to use solar power so instead carry plutonium bricks to fuel their engines. As the radioactive plutonium decays, it generates heat that produces an electric current between two different types of metal.
This system uses no moving parts – an advantage since these can fail – but the bricks are large, heavy, and difficult to produce. And these engines yield efficiencies of just 7%.
So NASA is funding research into Stirling engines, which use temperature differentials between reservoirs of gas to create electricity. Conventional Stirling engines are an old technology, invented in 1816 as a safer alternative to steam engines.”