In the vacuum of space, two pieces of the same metal will fuse together instantly upon contact.

Cold welding is a solid-state welding process that occurs without the need for heat or fusion. On Earth, the presence of oxygen and moisture causes metals to develop an oxide layer almost instantly. This microscopic film acts as a barrier that prevents the metal atoms from interacting with those of another object.In the vacuum of space, this protective layer cannot form. When two smooth, clean surfaces of the same metal touch, the atoms have no way of knowing they are part of separate pieces. Because the atoms are identical, they simply bond together to form a single crystal structure. This phenomenon was first recognized as a potential problem during the early years of the space race in the 1960s.NASA documented a famous instance of cold welding during the Galileo mission to Jupiter in 1991. The spacecraft's high-gain antenna failed to deploy fully because its ribs had cold-welded together during the long journey through the vacuum. This event highlighted the importance of using specialized coatings or different materials to prevent mechanical failures in space.Modern aerospace engineering now uses lubricants and dissimilar metals to avoid this effect. While cold welding is a hazard for moving parts, it is also used industrially on Earth in specialized vacuum chambers for manufacturing. It remains one of the most striking examples of how physical properties change when removed from Earth's atmosphere.