An old professor of mine was part of the medical research in these initial studies. I remember some of the big unanswered question about 0G were the digestive system and the vestibular system. Digestive because we had no idea if a stomach and the rest could deal with 0g and not just cause vomiting and whatnot. The vestibular (sense of balance) because the tiny crystals in your inner ear may detach in prolonged 0g away from the hair-like cells that transmit motion to your brain, causing permanent vertigo. Not a good thing for a pilot to have. There were many other effects that were unknown too, hence the space monkeys. Since they came back alright, it was seen as safe enough to send highly trained and very expensive test pilots up there too. Man, the Cold War, what a trip.