Telecom satellites already require tons of specialised integrated circuits because general purpose CPUs are too complex and power hungry for the amount of bandwidth the satellites process. This SpaceX constellation will certainly have custom designed electronics, even if they don't use any rad hardened ICs.
That'd be a really bad idea. Linux is too complex to trust, and lacks WCET making it unsuitable for hard realtime.
There's open source options, but Linux isn't among them. I'd look at seL4 for this purpose.
Also, I think the Falcon 9 uses VxWorks for at least some of its realtime control:
Can anyone who works at SpaceX chime in?
> One of the areas they focus on is scheduler performance. They do not have hard realtime requirements, but do care about wakeup latencies, he said.
It sounds like their hard real time controls are on non-Linux OSs.