Astronaut panics after hearing an eerie 'knocking' in space where no sound can travel

It's lonely enough to drift alone in the great void of space, but imagine getting a knock on the door out of the blue. That is precisely what happened to China's first astronaut, Yang Liwei, on October 16, 2003, during his historic journey aboard Shenzhou 5. A strange sound, as if something—or someone—were knocking from the outside, reverberated through the spacecraft during his 21-hour orbit around Earth. Given that space is meant to be a silent vacuum devoid of air that may transmit sound waves, the unsettling sounds were shocking. It left Liwei with more questions than answers.

As reported by BBC, Liwei recalled that the sound felt like "someone knocking the body of the spaceship just as knocking an iron bucket with a wooden hammer." He added, "It neither came from outside nor inside the spaceship." Understandably, Liwei grew a bit anxious and looked through the porthole, hoping to find some logical explanation for the unsettling knock—but there was nothing visible outside. Despite his efforts to understand the source, both in space and upon his return to Earth, the mystery remains unsolved. He even tried to recreate the strange sound in hopes that experts could pinpoint its origin, but to no avail. Professor Goh Cher Hiang, a space engineering specialist from the National University of Singapore, also spoke of the puzzling incident.

He said, “The traveling of sound requires a medium - be it air particles or water molecules or metal, solid atoms. If it is knocking, there could be something physical ‘hitting’ the spacecraft carrying the astronaut.” His colleague Wee-Seng Soh offers an alternative perspective. He suggested, "[It might be] a result of expansion or contraction of the spaceship, especially since the temperature of the spaceship's exterior could change considerably within the orbit." Space scientist Liu Hong suggested that the sound was caused by slight deformations in the capsule's inner walls due to pressure changes.

According to some experts, variations in the temperature of the capsule may result in an air pressure difference between the inner and exterior walls. The enigmatic pounding sound may have been caused by microscopic material deformations as a result. Interestingly, Chinese astronauts on board Shenzhou 6 in 2005 and Shenzhou 7 in 2008 also reported hearing similar noises. Although it appears doubtful, one option is that the spacecraft was struck by space debris. As reported by IFL Science, a more likely explanation is that the spacecraft's structure expanded and contracted as a result of the drastic temperature changes in orbit.

The Apollo 10 crew described a similar experience in May 1969 when they heard an odd whistling sound while in orbit around the Moon. It was later determined to be the consequence of possible radio interference, proving that space sounds often have logical explanations. But the knocking sound is strange, especially since it appears to be unique to China's Shenzhou spacecraft. Additionally, this could suggest that the noise might have been caused by a design issue specific to this particular spaceship.