 In a somewhat sterile environment, you try to keep it sterile, but bacteria can grow in there. In the environment, you can have micrometeorites that can break through your spacecraft. Also, you have changes in circadian rhythm. Instead of every once a day of a sunrise, you have it every 90 minutes, for example, if you're in low Earth orbit. So you have this environment that's dramatically different from what the human body normally experiences. And so this is one of the key factors, I think, that drives some of the stress on the body.