Training to be an astronaut is not for the faint of heart. Your training includes being strapped into a contraption that whirls you around at high speeds until you're on the verge of losing consciousness. You get to ride on a plane -- affectionately referred to as the "vomit comet" -- that nosedives into a freefall to simulate microgravity. Or, you might also get to spend six hours in an underwater tank wearing a 650-pound spacesuit. Luckily, it wouldn't feel like 650 pounds, because you'd be in a state of neutral buoyancy.