What is the ideal power source for robots working in space environments besides solar power?

Answer 1

Radioisotope thermoelectric generators. These are ideal for conditions where sunlight is weak or nonexistent.

Answer 2

Solar is great until you get too far from the sun or need compactness without large-area photoelectric collectors. So space engineers have turned to RTGs, radiothermal generators, which derive power through the natural decay of radioactive isotopes, especially plutonium-238 (see ref.). These are not "ideal", being essentially "dirty" (in the bomb sense), but are simple and reliable. They cannot detonate as they are not atomic piles which need active regulation, and they are encased in very tough armor to minimize the likelihood of Earth contamination in case of a crash on launch. Still there has been and probably always will be fear associated with them in the mind of the public, not entirely unfounded.

Answer 3

Nuclear. In space, pretty much only solar and nuclear work at all. Solar is much cheaper but does not provide high power, especially far from the Sun, like at Saturn.