The ones I speak of had actual reactors with moving parts. Most of them were Soviet, one was American (a research one, the soviet ones were active radar sats with a shelf life of only a few months). https://en.wikipedia.org/wiki/US-A (The soviets called them US-A for some weird reason, lol). There were 33 of them, 3 of which have already crashed to earth.

But yeah RTGs are very nasty stuff too. They are much easier to secure against breaking apart on re-entry though (although dropping a concentrated plutonium source into a random place is not a great idea either obviously).

I don't think any are used on current earth-orbiting sats though: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...