This commenter is correct. Things like radiation tolerance and power draw are much bigger concerns. I've seen a number of projects using RT systems entirely b

This commenter is correct. Things like radiation tolerance and power draw are much bigger concerns. I've seen a number of projects using RT systems entirely because they were based on earlier systems that were RT and therefore the engineers just used the same kernel despite having no RT requirements. PowerPC is popular because there are a couple companies out there that are taking these parts and running them through radiation beams to certify them for spaceflight (essentially profiling their failure modes so devs can account for them). ARM isn't popular yet as they tend to make it difficult to license designs for lower volume silicon but RISC-V in particular is gaining traction here very rapidly.