Are there any fields that merges health and space science?
Neither of these fields is my area, so I'll mention an orthogonal path: you could become an expert in something else (e.g., statistics, game theory, programming -- right now the hot thing is machine learning / deep learning, but who know where we'll be in 10+ years), then you can apply that toolkit to both space programs and biomedical programs (whether through a university or through scientific consulting). Of course, it's an uncertain path, particularly if there is no "something else" that you are also passionate about.