Slitherine

I know this has probably been asked before, but I haven't found a thread on it.

Why does the GSA mode have an observatory, when the NASA and Soviet modes do not?

As I recall, originally all modes had it. It was supposed to be where you went to look at missions in progress over the seasons, but that function was never implemented and seems to have been taken over by the Vehicle Assembly Building. So they probably just forgot to take it out of GSA mode.