There are different kinds of nuclear power, and these types of small power-trickle devices have been around for decades. (Mostly in spacecraft, I think?)
Yup. All the US mars rovers use radioisotope generators that generate electricity by using the decay heat.
But some satellites actually had real fission reactors on board. The US had the experimental SNAP-10A satellite and the soviets used a small fission reactor in their RORSAT satellites to power a radar so they can detect NATO naval fleets.
