I figure Sherlock wasn't about to leave until he had figured out if he was right because. John, really, how are you supposed to have any fun in Space when there's unsolved case hanging over your head that you'll never get another chance to solve. Priorities!

And yes, common sense dictates that Sherlock Holmes is a requirement in any new colony. At least in his mind. In reality, it was probably Mycroft pulling some strings and internally rolling his eyes that his brother is going to be late to the bloody apocolypse.

I like your theory! Sherlock would be a very good Htichhiker because he wouldn't panic, though I don't know if he would keep very good track of his towel. And all his Guide entries would be 'don't go there, it's boring'.