Lyric discussion by savethemooses 

Cover art for Spaceman lyrics by Harry Nilsson

I s'pose this could be a metaphor for when you actually get to be with a person you had a huge crush on and finding it to be more than you can handle, or boring, or what have you (A la Sam and Cindy from "Freaks and Geeks").

Or it could just be about wanting to be a spaceman.