An astronaut is read as courage, exploration and the will to go where few have gone. Being one in a dream points to ambition, and sometimes to feeling far from everyone else.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.