Comment Re:Couldn't they have used an RTG? China syndrome (Score 1) 132
proper Stirling engines or steam turbines are not popular in space for some reason.
Stirling engines are used in space, but only when there is a compelling reason to do so. The basic argument against them is two words: moving parts.
Mechanical wear is a huge problem, and thermal management is not a small one. Depending on the spacecraft a sustainable thermal regime may have to be maintained across very different environmental conditions (full sunlight, deep shadow) and very different operational phases. Just getting lubrication to work properly under such circumstances, over a decade in the case of Rosetta/Philae, is non-trivial.
Simpler is better in space, so batteries and solar panels are always going to win over RTGs, and RTGs with thermoelectric conversion are always going to win over RTGs with Stirling engines unless there is a compelling need for the greater output power density each step up in complexity gets you.
Steam turbines are just a non-starter. Water is a dreadful substance to deal with. Highly reactive, prone to freezing, capable of going wrong in ways we can barely imagine, but would certainly discover if anyone was foolish enough to put a steam turbine on a spacecraft. It might be made to work one day, and there are probably some really clever things we are missing, but the cost of each mission is so high that extreme conservatism rules the day, and rightly so.