Imagine that you are alone in a spacecraft. It is your first time in space. There is nothing but the silence of the universe. And then suddenly you hear - a knocking sound!
This is exactly what happened to Yang Liwei in 2003. He was China’s first person to go into space. He says that he heard what sounded like someone knocking on the ship. But the loud banging didn’t sound like it was coming from inside the ship. Yet it also didn’t sound like it was coming from outside the ship.
Of course he got nervous. He looked out the spaceship’s little window, but he couldn’t see anything. He could never figure out where the sound was coming from. Even when he returned to earth, Yang couldn’t discover the cause of the knocking.
Professors at the National University of Singapore made guesses about the cause. One professor thought it could possibly have been something in space hitting the ship. His colleague thought that perhaps the noise was because of temperature changes. If the temperature inside the ship was very different from the outside, then the metal in the ship may have expanded or contracted.
Yang is no longer an astronaut. But other Chinese astronauts heard the sound in 2005 and 2008. So, although no one can explain it, they now consider it a “normal strangeness”. The sound of space.