Going to the Moon takes days. A mission to Mars could be months. As technology improves, spending years in zero gravity could become a possibility. And when you put people together in such close quarters for such long periods of time — things are bound to happen. But for all we know about space and how to survive there, we know practically nothing about sex 200 miles up. Tests with animals and insects make it appear more difficult, but there is almost zero research with humans — and if space really is the final frontier, it's probably time to start finding out.