Metal
A class of materials with free electrons that conduct heat and electricity well and show luster, ductility, and malleability; most elements in the periodic table are metals.
In space vacuum, two clean pieces of the same metal can fuse on contact without heat. On Earth, oxide films and grime block this bond; in vacuum, bare atoms meet directly and act as one solid. This is cold welding, so spacecraft parts need coatings or anodizing.
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