When astronauts return to terra firma after weeks in space, they don’t need a throat-clogging pretzel to feel a little woozy. Many faint on the spot when they try to stand up for the first time back on Earth. The same thing happens to bedridden hospital patients who rise after being horizontal for days. Now, after an ambitious experiment aboard the Space Shuttle, researchers think they have a better idea of why astronauts — and the sick — hit the deck. The research is far from trivial. For men and women to travel to other planets, like Mars, “we have to understand what happens to the body,” Levine says.