I think it's not just the SNR involved, but the time. With a round-trip of nearly 9 hours, a TCP-style "I'm sorry, I missed that?" response is hugely impractic

I think it's not just the SNR involved, but the time. With a round-trip of nearly 9 hours, a TCP-style "I'm sorry, I missed that?" response is hugely impractical. The higher the penalty for re-transmission, the more effort goes into getting it right the first time.