When two pieces of a similar metal touch in outer space, they instantly fuse into one! This process is called cold welding.
It is possible to weld two pieces of a similar type of metal together without using heat or chemicals. This process is called cold welding.
Cold welding happens in a vacuum, like in outer space, and it is explained very simply by Richard Feynman in 'The Feynman Lectures.'
He explains using copper as an example: "The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to "know" that they are in different pieces of copper.
When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms "know" when they are not on the same part."
Obviously cold welding can become a problem in space, as spacecraft like space shuttles are often manufactured with moving parts like bay doors that have to remain moving!
Designers must therefore design these spacecraft whilst keeping cold welding in mind to prevent future problems.
These problems can be overcome if the adjoining parts are not made of a similar metal, or if surfaces are coated to reduce contact adhesion. The easiest way to reduce the risk of accidental cold welding is obviously to reduce the number of moving parts.