When I was a kid I was always fascinated when I watched welders at work, with their Star-Wars-like face masks and spark-spewing flame guns.
Welding is still common in the construction industry today. It remains the quickest and securest way of binding two metals together to form a seamless single unit. It’s used on everything from shipping, to building airplanes to car manufacturing and much more.
However, if you took a welding gun to the moon and tried to weld two bits of metal together you would struggle. This is because of the lack of oxygen which is a fundamental component in producing the flame. We can call this hot (or normal) welding.
It just so happens that there is another way in which metals can be bound together which do not require melting them beyond their melting point. It’s called cold welding.
How does it work? Well, in the vacuum and cold temperatures of outer space, if two pure pieces of metal made up of the same or similar elements were to collide, they would stick together. This is because the sub-atomic particles that make up both bits of metal literally jump across from one piece of metal to the other when they come into contact because the atoms (and the electrons and neutrons that make up those atoms and interact with each other) do not recognise the other piece of metal as being separate. So when they touch, the contact forms a pathway through which atoms from both separate pieces flow into each other and form a solid single piece of metal. Sort of like pouring two separate cups of water into the same bowl. Once they’re mixed, there is no telling which part of the final bowl of water came from which cup.
Why does this not happen with metal here on earth? Because the atoms from each separate piece of metal are in constant contact with atoms in the earth’s atmosphere which produce chemical reactions (such as oxidation) and this forms a barrier between the two pure metals. As a result, their atoms will not automatically bind with similar atoms of the other piece. In effect, other particles get in the way.
To avoid metals automatically fusing together in Space, astronauts are now equipped with metal tools that have a durable outer protective layer (such as ceramic) and are also advised not to bring into contact (outside of the space craft) metals that are of similar compositions to reduce the risk of cold welding.