Ooh, just occurred to me... River Song does not shoot the Doctor dead by the shores of Lake Silencio in 2011.
Very simple leap of logic, but when she's invited to the lake, she has no idea why, and when the young-Doctor comes out of the bathroom she presumes that it's a joke ("cold, even by your standards,") and asks if he knows who Jim the Fish is.
More specifically, it means that if she did do it, she doesn't remember. Which is possible - the astronaut was introduced in an episode where the villains were playing memory games. But it's a spanner.
Not that this is new news, it wouldn't make sense if it were her, anyway. The astronaut is in a space-suit, and we never get to see their face. If you think of it as a magic trick, why would you veil the identity of the killer, if not because you want their identity concealed? It'd be a bit like hiding a ball under one of three cups, tapping the top, and then revealing it to be under the same cup. You might as well get rid of the cups entirely if that's the plan, they're just vessels for disappointment.