More than twenty years ago, Yang Liwei — China’s first man in space — reported hearing an eerie sound inside his capsule. The phenomenon, which seems to defy the basic laws of physics, still baffles scientists today.
In the vast silence of orbit, any unexpected noise can spark panic. That’s exactly what happened in 2003, when Yang Liwei, China’s first taikonaut, embarked on his historic mission. During his flight, he was startled by a strange knocking sound echoing through his spacecraft — a moment that would become one of spaceflight’s enduring mysteries.
“It sounded like someone hitting the hull, like striking an iron bucket with a wooden hammer,” Yang later told the BBC. “The noise came from neither inside nor outside the ship,” he said, unable to pinpoint its source.
Determined to find an explanation, he peered out the window, looking for signs of impact or damage. Could it be a mechanical issue? A structural flaw? Or something external? Nothing seemed out of the ordinary — yet the sound continued. Over time, Yang learned to live with it, eventually calling it a “normal phenomenon.”
Theories from physics and engineering
According to physics, Yang’s experience should have been impossible. “Sound waves need a medium — air, water, or solid matter — to travel,” explained Professor Goh Cher Hiang, a space engineering expert at the National University of Singapore, in an interview with Daily Galaxy.
But in the vacuum of space, there’s no medium to carry sound. By that logic, Yang shouldn’t have heard anything at all. So what really caused the noise inside his spacecraft?
Scientists have suggested several possible explanations. The simplest is a physical impact. “If it was a knock, perhaps some object struck the spacecraft,” said Professor Goh, though he acknowledged that the theory is speculative and nearly impossible to confirm.
While orbiting hundreds of kilometers above Earth, spacecraft face constant threats from micro-debris — tiny fragments moving fast enough to pierce metal. But no sign of collision was ever found during Yang’s mission.
Another theory, from Professor Wee-Seng Soh, also of Singapore, points to the violent temperature shifts that spacecraft endure. “It could be the hull expanding or contracting as it moves through extreme temperature changes,” he said. If true, the noise might simply be the sound of the ship’s materials flexing under thermal stress.
A sound shared by other astronauts
Perhaps most curious of all, Yang wasn’t alone. Chinese media later reported that other taikonauts on missions in 2005 and 2008 also heard the same mysterious knocking. Yang even warned future astronauts ahead of launch: “Don’t be alarmed if you hear it — it happened to me too.”
Over time, this so-called “phantom knock” became almost a tradition on Chinese spaceflights — no longer a cause for fear, but an accepted part of life in orbit.
Still, with no solid proof, the mystery endures. It’s a haunting reminder that exploration beyond our world remains full of unanswered questions — and that even the silence of space can have its secrets.
