 When you get into the vacuum of space, you can't use air to push your propellant out. You're in a vacuum. Air ain't going to help it. You're going to have... You've got to have something that's behind it to push it. And the only thing they had to push it was helium. You know, that's an inert gas that's not going to cause an explosion. You know, it's one of those things. I mean...