#6 Metal

In space, if unprotected pieces of metal touch each other, they stick together permanently.

This doesn't happen on Earth, because the oxygen in our atmosphere forms an extremely thin film of oxidized metal on every exposed surface. The oxidization layer acts as a barrier that conveniently prevents chunks of metal from sticking to other chunks of metal.

In the vacuum of space, however, there is no oxidation layer. If the atoms of two metal objects come in contact with each other, what you suddenly have is one continuous metal object, and a lot of explaining to do to your mission commander.

This is an issue on the space station. Metal tools used outside the station have to be coated with plastic or other materials that will not stick.

If you consider the Universe as a whole, metal objects sticking together at the drop of a hat is the norm. It's only in special places like Earth, with our highly caustic oxygen-rich atmosphere, that we can carry around bare pieces of metal to hit each other with and not have to worry about how inconveniently sticky they are.

Update: While this effect is real, it probably isn't an issue for NASA. If you brought a metal tool with you into space, it'd retain the protective oxidation layer it had on Earth, and you wouldn't have any problems unless you went to a lot of trouble to remove it. However, the process of cold welding is sometimes used in industrial applications.

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