Whenever two pieces of metal in outer space touch each other, they are more or less permanently stuck together. While welding usually requires heat, in this case, the vacuum of space does the trick, hence the name. You might think then, how do space shuttles accomplish anything out there? Well, typically, metals on Earth have a layer of oxidized material covering their surface that prevents this, so on shuttle missions, the risk of accidentally welding the shuttle to itself is negligible.