Considering that NASA have been running short of plutonium for their RTGs, it makes sense to consider alternatives. For the inner solar system up to Jupiter, solar cells have been demonstrated on missions e.g. Juno. However, for missions beyond such as Pluto, solar cells reportedly are inadequate.
I made some quick calculations with Wolfram Alpha reaching on the order of 50 metric tons for a 1kW power budget and a 20 year mission. These figures are not precise but hopefully on the right magnitude.
Question - Will dropping launch costs (from reuse or otherwise) ever make these types of power systems reasonable for deep space missions? Or will some other technology make efficient use of the extra weight possible to launch when costs drop?