China's first astronaut, Yang Liwei, reported hearing a strange 'knocking' sound during his 2003 mission aboard Shenzhou 5, a phenomenon that has puzzled engineers for years. The sound, described as 'like someone hitting an iron bucket with a wooden hammer', was heard repeatedly throughout his 21-hour flight.
Yang, who orbited Earth 14 times before landing in Mongolia, said the noise seemed to come from outside the capsule, despite the vacuum of space where sound cannot travel. He inspected the spacecraft thoroughly but could not identify the source.
Subsequent Chinese missions in 2005 and 2008 also reported similar knocking sounds. Yang had warned later crews not to be alarmed if they heard the noise. Engineers have since examined the spacecraft but found no explanation.
Potential theories include micro-debris impacts, thermal fractures from extreme temperature changes, or mechanical vibrations from onboard systems. However, no conclusive evidence has been found, leaving the 'knock-knock' as one of spaceflight's enduring mysteries.
