It's a nuclear propelled cruise missile (https://en.wikipedia.org/wiki/9M730_Burevestnik https://en.wikipedia.org/wiki/9M730_Burevestnik), conceptually similar

It's a nuclear propelled cruise missile (https://en.wikipedia.org/wiki/9M730_Burevestnik https://en.wikipedia.org/wiki/9M730_Burevestnik), conceptually similar to SLAM (https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missile https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missil...). An RTG makes little sense for earth orbit, because there's plentiful sunlight, and solar panels can be used. RTGs are only really suited to missions far away from the sun.