I would not rely on others to decide this for me. What will matter to you is what satisfies you most about the work you are doing. True, it is impossible for you to know for sure right now which master's degree would open the doors to the "most sastisfactory" work environment. But you might ask yourself which areas of space science are the most interesting to you now? Best way is to try different laboratories doing different kinds of work. If you cannot work as an "apprentice" then at least meet and talk to a variety of folks already doing the kinds of jobs you would select from. They can tell you about their experiences, which may help you decide.
Military test pilots do a lot of shuttle flying... But yeah. I always wanted to become an astronaut too. Sadly, I don't think that's ever gonna happen. I just hope I'll make enough money to buy a trip from SpaceX some day.