Nearly 300 years before the first space mission, physicist Isaac Newton presented what is now commonly known as his third law of motion: for every action, there is an equal and opposite reaction. For example, if 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 rollerblades.) This law applies in space as well as on Earth.

On Earth, gravity and friction provide a solid footing from which it is possible to counteract opposing forces. Tightening a loose bolt is generally an easy task. In space, by contrast, the conditions of "weightlessness" can make even the simplest of repairs a challenge. Without a handhold or other means of stabilizing themselves, astronauts attempting to tighten bolts will actually be pushed in the direction opposite their effort. The astronauts will spin instead of the bolts!

This problem required an engineering solution. After going through a design process that included a lot of trial and error, NASA engineers developed new strategies and equipment to help overcome the stability challenge presented by space's microgravity environment. Their solution: equip spacecraft with footholds and handholds. These allow space-walking astronauts to anchor themselves to the spacecraft and thus gain some control over the various forces they experience while doing work. In addition, scientists revised astronaut training methods to include underwater training. This helps astronauts learn to control their bodies more effectively while carrying out various tasks under conditions of "weightlessness."

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