In October 2003, during China's first manned space mission, astronaut Yang Liwei reported hearing a strange knocking sound outside his Shenzhou 5 spacecraft. More than two decades later, the origin of the sound remains officially unexplained, representing one of the more persistent mysteries of modern spaceflight.
The incident, which was later reported by astronauts on two subsequent missions, has prompted several scientific theories but no definitive conclusion has ever been reached.
Key Takeaways
- In 2003, Chinese astronaut Yang Liwei heard a mysterious knocking sound during the Shenzhou 5 mission.
- The sound was described as a wooden mallet hitting a metal bucket.
- Astronauts on the Shenzhou 6 (2005) and Shenzhou 7 (2008) missions reported hearing the same sound.
- Despite multiple theories, including thermal contraction and space debris, the exact cause has never been confirmed.
A Historic Flight and an Unsettling Sound
On October 15, 2003, China launched its first manned spacecraft, Shenzhou 5, with astronaut Yang Liwei as its sole crew member. The mission marked a significant milestone for the country's space program, making it the third nation to independently send a human into orbit.
During the 21-hour flight, however, Yang experienced something unexpected. He reported hearing a distinct knocking noise that seemed to come from outside the capsule. The sound was not constant but occurred intermittently, adding to the perplexing nature of the event.
Yang described the sound as being similar to "someone knocking the body of the spaceship just as knocking an iron bucket with a wooden hammer."
He checked inside the spacecraft for any potential source but found nothing that could explain the noise. The experience left him puzzled, as the vacuum of space should not transmit sound in the way it does on Earth.
Why Sound in Space is Unusual
Sound waves require a medium, such as air, water, or a solid object, to travel. The near-perfect vacuum of space lacks this medium, meaning sound cannot propagate as it does on Earth. Any sound heard by an astronaut must originate from direct contact with the spacecraft or be transmitted through the structure of the vehicle itself.
A Recurring Phenomenon on Subsequent Missions
The mystery of the knocking sound deepened when it was not limited to a single mission. In 2005, the crew of the Shenzhou 6 mission also reported hearing the same type of sound. This recurrence suggested the noise was not a one-time anomaly specific to Yang Liwei's flight.
Three years later, in 2008, astronauts aboard the Shenzhou 7 mission again confirmed hearing the knocking. With the phenomenon now documented across three separate flights over a five-year period, it became clear that there was a consistent, physical cause, even if it remained unidentified.
The consistency of the reports from different crews on different spacecraft ruled out auditory illusions and pointed toward a mechanical or environmental source linked to the Shenzhou vehicle's design or its interaction with the space environment.
Contextualizing Anomalous Sounds in Orbit
Unexplained sounds are not entirely unique in the history of space exploration, but most have eventually been explained. For example, during the 1969 Apollo 10 mission, astronauts heard an eerie whistling sound while orbiting the far side of the moon, out of radio contact with Earth.
This "space music" remained a mystery for years until declassified transcripts and analysis revealed it was likely caused by radio interference between the lunar module's and the command module's VHF radios. More recently, the crew of the Boeing Starliner spacecraft heard a pulsing noise, which engineers later identified as audio feedback between the capsule and the International Space Station.
Other Space Sounds: While the Shenzhou knocking remains a mystery, NASA has successfully recorded and identified other sounds in space, such as the low-pitched hum emitted by a black hole, which was captured by observing pressure waves in surrounding gas clouds.
Unlike these other cases, the knocking sound reported by the Shenzhou crews has resisted a simple explanation. The fact that the source could not be pinpointed after three separate occurrences makes it a particularly intriguing case.
Scientific Theories and an Enduring Mystery
Upon returning to Earth, Yang Liwei discussed the sound with technicians and experts, but no one could provide a definitive answer. Over the years, several plausible theories have been proposed to explain the phenomenon. According to experts, any sound would have to be caused by something physically interacting with the spacecraft's hull.
The leading theories include:
- Thermal Expansion and Contraction: As the spacecraft moves between direct sunlight and the cold shadow of Earth, the extreme temperature changes could cause parts of its outer structure to expand or contract. This movement could produce sharp, knocking-like sounds as different materials shift against each other.
- Minor Space Debris: The spacecraft could have been struck by very small particles of space debris or micrometeoroids. While larger impacts would be damaging, tiny particles could create audible pings or knocks without causing significant harm to the vehicle.
- Escaping Air or Gases: Small amounts of air or other gases escaping from objects or components within the spacecraft's unpressurized sections could cause vibrations in the hull, which might be perceived as a knocking sound from inside the cabin.
Despite these viable explanations, none have been conclusively proven. The fact that the sound was so distinct and was described similarly by multiple astronauts suggests a specific, repeatable mechanism. For now, the knocking sound of the Shenzhou missions remains an unsolved curiosity—a reminder that even in our advanced age of space exploration, there are still mysteries waiting to be solved just beyond the atmosphere.
