How strong is the force of gravity acting on an astronaut aboard the space shuttle? It's actually about 90 percent of the gravitational force he or she would experience at the surface of the Earth! It's a common misconception that there is little or no gravity acting on the shuttle because we see those guys floating around in there like they're weightless. Well, they're not. They are in fact only apparently weightless.

All you need to get some apparent weightlessness going is to go into a state of free fall. For example, when you get in an elevator on the 15th floor and push the button for the lobby, you feel lighter during the few moments when the elevator is accelerating downward. Now, if you were to take this to the extreme and get in and have someone cut the cable, then both you and the elevator would accelerate downward at the same rate of 1 G. Because you are falling at the exact same rate as the elevator, you would have no apparent weight relative to the elevator's floor and you could float around just like you were in deep space -- that is, until the elevator hit the ground.

Now let's apply this to some astronaut training. You can produce the same effect as with the falling elevator using specially designed airplanes that travel through a very steep parabolic trajectory. As these approach the top of their arc they cut back the engines and go into a state of free fall for up to 30 seconds at a time. During this period, everyone on board is in a state of apparent weightlessness, and can do all of the cool things we see in the video. This obviously has tremendous advantages over the elevator method; however, it can result in severe queasiness. That's why the NASA version of these jets is called "the vomit comet." In the movie "Apollo 13" the "weightless" scenes were actually filmed aboard one of these vomit comets. That's why they look so real.

But is the space shuttle in free-fall while in orbit? Yes, it is. The space shuttle has a constant centripetal acceleration towards the center of the Earth, due to gravity -- about 90 percent of the acceleration it would experience if it were just above the ground. The shuttle (or any orbiting object for that matter) maintains its orbit because it has a speed tangential to the orbit. This results in the shuttle and everything aboard continuously falling around the Earth. While everyone and everything aboard the ship does experience a significant gravitational pull, because they are all in free-fall together they all experience the delights of apparent (but not actual) weightlessness together. If you want some actual weightlessness, you have to get significantly farther from Earth than the space shuttle. During flights to the Moon (as in the Apollo missions) you'll get some of the real thing, or "the right stuff."

Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.