On the Gemini IV mission, NASA sent out Ed White as the first American astronaut to attempt a spacewalk. After thoroughly enjoying his trip outside the capsule, White returned...but the hatch wouldn't close properly. It was a real chore for the astronauts to wrench the hatch closed. In assessing what happened, NASA engineers wondered: Had the hatch undergone "cold welding?"

As Derek from Veritasium explains in the video below, "In space, if two metals come into contact, they can actually fuse together without the need for heat or melting of either piece." As he promptly continues, this is not what happened to the hatch on Gemini IV. But it has happened on space missions, and it's a real problem for metals we send into space.

Watch this video for a detailed explanation of how cold welding works, why it happens less than we might fear, and how it has actually occurred in space.

(Photo credit: NASA, via Wikimedia Commons.)