The Silent Knock: Unexplained Sound from China's First Spaceflight

During China’s 2003 inaugural manned mission (Shenzhou 5), astronaut Yang Liwei reported a rhythmic knocking inside the capsule about 300 km above Earth. Despite checks, attempts to reproduce it, and later missions (2005, 2008) reportedly hearing similar sounds, no physical source or telemetry evidence has been found. Theories include thermal expansion, micrometeoroid impacts, or perceptual distortion, but none have been confirmed. The incident remains an unresolved mystery in space history, with no official procedural changes tied to it and no acoustic signals captured by instruments.