If two pieces of metal touch in space they permanently stick together. This is not happening on Earth because the oxygen in our atmosphere on every exposed surface forms an extremely thin film of oxidized metal. The oxidization layer acts as a barrier which conveniently prevents metal chunks from sticking to other metal chunks.

There is no oxidation layer in the space vacuum. When in the space atoms of two metal objects come into contact with each other they permanently stick together. This is an issue on the space station. Metal tools used outside the station are coated with plastic or other materials so that they will not stick.

If you think about the World as a whole, the norm is metal artifacts that stick together at the drop of a hat. Even in unique situations like Earth, with our extremely caustic oxygen-rich atmosphere, do we bring bare metal bits around to reach each other and not have to think about how uncomfortably sticky they are.

For NASA, those effects are not a concern. If you took a metal device into space with you, it would maintain the protective oxidation coating it had on Earth, so you would not have any issues unless you had a lot of trouble extracting it.

In industrial applications, this method, called cold welding, is often used. Cold welding, also termed as cold pressure welding or contact welding, is a solid-state welding process in which two metals are fused together without fusion or heating at the interface of the two welded sections. This ensures that the joint and joining cycle does not have a liquid or molten phase present. The cold-welding process works by forcing two metals or materials to fit together. In general, when two metals are pressed together, they don't weld one to another. It is mainly due to the fact that on the surface of the two materials there is an oxide layer or thin barrier which prevents them from diffusing together.

Cold welding however overcomes this by pre-preparing the metals. This preparation includes washing and rubbing up the metals to such a high degree that it eliminates this surface shield or oxide layer. By degreasing and wire brushing the metal before welding it gives a suitable clean surface where the metals can be forced together with the appropriate amount of force and thus welded together.

Hope you have understood why two pieces of metal touch in space Permanently stick together? and science behind…...