Deep-space missions are powered by a RTG system. A radioisotope thermoelectric generator (RTG, RITEG) is a generator that obtains its power from radioactive decay. The heat released by the decay of a radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples (see "how did Georg do it?").
RTGs are systems quite suitable as power sources for satellites and space probes and also for remote facilities where the use of solar cells is not feasible (for instance, lighthouses built by the former Soviet Union inside the Arctic Circle). RTGs are able to give few hundred watts for very long periods. The problem is to guarentee a safe use of radioisotopes.