In 2003, during China’s first human spaceflight, Yang Liwei found himself in a strange situation. While orbiting about 300 kilometers above Earth in the Shenzhou 5 capsule, he heard a knocking sound—like someone tapping a metal bucket.
Despite thorough checks and years of investigations, no one has been able to explain the origin of that sound, and it wasn’t just Yang who experienced it. Other astronauts on subsequent missions reported similar noises.
The Sound in Space
In space, we’re usually told that sound can’t exist without air. Professor Goh Cher Hiang from the National University of Singapore emphasizes that sound requires a medium—like air or water—to travel. However, Yang’s sound defied this basic principle, leaving experts puzzled and the phenomenon unresolved.
After Yang’s mission, he returned and tried to recreate the sound but couldn’t. His spacecraft showed no signs of damage, and the systems were running normally.
Multiple Reports Heighten the Mystery
What makes this mystery more intriguing is that astronauts from the Shenzhou 6 and Shenzhou 7 missions also heard the same rhythmic knocking. Even Yang warned future crews not to panic if they heard it—he’d experienced it too, and it hadn’t been harmful.
This pattern across multiple missions shifted the sound from a mere anecdote to a genuine phenomenon.
Theories Explored
Engineers have proposed several explanations, none of which have fully convinced anyone. One idea is thermal stress, caused by extreme temperature changes in space—solar heat followed by the cold of the Earth’s shadow. Yet, Yang described the sound as rhythmic, not random.
Another theory involves micrometeoroid impacts. Tiny debris fragments could create knocking sounds, but again, there was no visible damage to the capsule.
Some suggested a psychological factor, such as auditory hallucinations triggered by the stress of being in space. However, no other astronauts have widely reported similar experiences in comparable conditions.
Connections to Other Missions
This isn’t the first time unexplained sounds have emerged in space. In 1969, Apollo 10 astronauts reported a high-pitched “whistling” sound while orbiting the Moon, which NASA later attributed to radio interference. But some experts remain skeptical.
More recently, NASA’s Juno spacecraft captured electromagnetic emissions around Jupiter’s moon Ganymede, which were converted into sounds. However, these noises weren’t acoustic signals but rather data transformed for human interpretation.
Yang’s experience remains unique. He heard a sound directly, with no instruments or sensors recording it.
Conclusion
The mysterious knocking sound has yet to change procedures within China’s space program. Officials haven’t issued new safety guidelines or modified capsule designs based on Yang’s experience. Rather, it’s treated as an intriguing enigma that future crews might encounter.
Space remains full of mysteries, and Yang Liwei’s story is just one of many that remind us how much we have yet to learn about the cosmos.
