What is the best route of becoming an astronaut?

I currently am a high school student and expect my grades to be somewhere between 88-92 percent in grade 11. I am considering taking the Air Force route but I foresee some problems. For instance, can a Canadian citizen apply to nasa? If not must I leave the Air Force and become an American citizen when nasa is... show more I currently am a high school student and expect my grades to be somewhere between 88-92 percent in grade 11. I am considering taking the Air Force route but I foresee some problems. For instance, can a Canadian citizen apply to nasa? If not must I leave the Air Force and become an American citizen when nasa is recruiting? Should I just take the civilian route? Lastly, what major should I take. I am highly interested in astrophysics and mathematics but nasa does faver aerospace engineering?

Thanks for the help!
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