TEGs on space probes are powered (heated) by the radioactive decay of plutonium 240. since the probe is going into deep space the plutonium doesn't pose a hazard. some older satellites were powered the same way, but a russian satellite crashing into the canadian arctic kind of made everybody re-think it's use on orbital platforms. solar heated TEG arrays and photoelectric panels are what's primarily used in orbit these days since sunlight is abundant here, but in short supply in deep space... EDIT (correction): plutonium 238, not 240. plutonium 238 has a relatively short half life (78 years) and so it's alpha decay happens at a rate high enough to generate a lot of heat in a small package, and so is ideal as a heat source for a TEG.