Finally got around to playing this, and I've run into something that makes me think I'm missing something.

I've done the lunar unmanned orbiter, and there seem no further missions for it. So I decided to go ahead and getting the one manned capsule (Mercury) started. But it won't let me do it because I have a "program already opened". I don't recall having to do anything special to "close" a program before; is there some reason I'd need to here?