NASA's first attempts at sending astronauts outside their spacecrafts to "walk" and perform tasks quickly turned into exercises in frustration and fear. Each time an astronaut attempted to turn a valve or bolt, he was sent hurtling in the opposite direction with little control over his trajectory. Following the early termination of several such attempts, NASA scientists set out to determine what was happening and, more importantly, how to help future astronauts overcome the effects of their own actions.

Nearly 300 years before the first space mission, physicist Isaac Newton presented what came to be known as his third law of motion, which says that for every action, there is an equal and opposite reaction. This law applies on Earth as well as in space. It means that if, for example, we push heavily against a wall, the wall pushes back with a force equal and opposite to our push. (To illustrate this force more vividly, imagine pushing against a wall while wearing roller skates.)

In space, when an astronaut applies force to open or close a valve, the valve applies the same amount of force to the astronaut. The difference is that on Earth, gravity and friction provide a solid footing from which to counteract opposing forces. Under the conditions of "weightlessness" that exist in space, astronauts lack this stability.

In the aftermath of several failed space walks, NASA engineers designed spacecraft differently in order to provide some of the missing stability. They equipped spacecraft with footholds and handholds that allowed space-walking astronauts to anchor themselves to the spacecraft and thus gain some control over the various forces they would experience while doing work. In addition, scientists revised astronaut training methods and had astronauts train underwater. These conditions helped them learn to control their bodies more effectively while carrying out various tasks under conditions of weightlessness.