Nice report <https://arstechnica.com/features/2020/10/the-space-operating-systems-booting-up-where-no-one-has-gone-before/> going into some details on the proprietary VxWorks (popular with NASA) and the open-source RTEMS (originating from the US military, now heavily used by ESA). Also mentions SpaceChain, which was developed in order to implement a BitCoin network in space. This is based on Sylix, which comes from the Chinese military, but is now open-source as well. And there is some use of Linux, but only in situations where hard-realtime constraints can be guaranteed by lower-level custom hardware. Summary: VxWorks and RTEMS are pretty much identical in basic features and maturity/reliability, but RTEMS does have an edge in configurability.