NASA ISRO and ESA requires new and advanced medicines for there astronauts to survive in the tough condition of other planet and space and also it requires some skilled doctors always as anything can happen anytime in the amount of risk they take every time.NASA recruit for doctors time to time but a fresh MBBS graduate have a least chance to be recruited by NASA. You need to have at least 5 year experience or you can go for MD or MS after your MBBS in advanced medicines. Then your chances of selection will be high.Go to their site and apply for the same as there is no fixed time for their recruitment. You should always leave an updated resume of yours in their database so that they may consider you whenever there will be any opportunity available.
Piyush Mishra, B.A(hons)applied psychology Amity university Lucknow.
Source: Quora Digest