Doesn’t necessarily stay in space. Humans have been traveling to space since the first orbits about the Earth in the 1960s, we’ve gone to the Moon, but what keeps us from going further out into our solar system or even beyond it? Well, the obvious answers would include our technological limitations and how we can’t travel anywhere fast enough, but another limiting factor is the effects that space has on the human body and how quickly they occur.
A postmenopausal woman will lose 1-1.5 percent of the mass of her hip bone in a year if she isn’t being treated for bone loss-it only takes an astronaut 1 month to lose the same amount. If it take 260 days to get to Mars (approx. 9 months), an astronaut would lose as much as 13.5 percent of their bone mass!
And bone loss isn’t the only problem. Astronauts also lose blood (as a reaction to the bodily fluids moving towards the head- which also results in swollen faces and skinny legs), when the are under the influence of gravity again, this decrease in volume of blood can lead to a decrease in blood pressure and cause them to pass out.
Space also has effects on the immune system that cause dysregulation and depressed function.
Further Reading
Interactive NASA website (Videos included)
Effects of Spaceflight on the Human Body
How long would it take to travel to Mars?
