NASA has documented the changes that space forces on a body, from decreasing vision to increasing heart problems. But how do the scientists know if the astronaut's heart changed shape because of the distance away from Earth, or if it was genetically destined to in the first place? That's where identical twin astronauts come in handy. By measuring the earthbound one against the one in orbit, they can paint a fuller picture of how we fare in space. Scroll down for more on space and the human body.