The mysterious sound in space: What China’s first astronaut heard during Shenzhou 5

As the first Chinese astronaut ever sent into space in 2003, Yang Liwei made history with his ride on Shenzhou 5. However, beyond the accomplishment, what he heard during that flight was quite disturbing, a strange sound of knocking, which seemed to come from somewhere outside of the spaceship.Yang Liwei told China Daily that the knock sounded “just like someone was knocking on the shell of the spaceship. For example, when you use a hammer to knock on the shell of an iron bucket.” The sound originated not from within the ship but could not be traced.This unusual event rapidly became one of the most popular and mysterious cases in space research. Even though outer space is usually quiet, there are some cases when astronauts hear some sounds in space.While many people assume sound travels in space, that assumption couldn’t be further from the truth since sound waves require molecules for transmission. However, spacecraft themselves are intricate mechanisms that are subject to vibrations, changes in temperature, and stress.The study done by NASA on spacecraft noise reveals that changes in temperatures can make the material expand and contract causing knocking sounds. These sounds are commonly referred to as thermal noises since they usually take place due to the large difference in temperatures experienced when in orbit.While experts later argued that what Yang heard was probably the result of the shell of his spacecraft expanding and contracting as a result of heating and cooling processes, it is Yang’s claim that the sounds were rhythmic and came from outside the spacecraft that makes his story credible.In a conversation with China Daily, Yang said,"It didn't come from inside or due to any equipment... it felt like something outside."The case of Yang Liwei is not alone in terms of astronauts experiencing unusual sounds. There are a number of other reports of similar experiences by various astronauts belonging to different space programmes. Though in most cases there were logical explanations for such experiences that referred to technical aspects of space flights, some peculiarities of astronauts' perception should be noted.It was found by specialists from different space agencies that space flight could distort an individual's perception to a great extent. The effects of isolation, pressures, and strange surroundings were thoroughly studied by NASA.However, even with all the existing explanations of such phenomena, Yang Liwei's case is considered interesting by many people and scientists alike, since his story illustrates how much the process of space exploration is a combination of technology and humanity.Here is how the astronaut described his experience years later in conversations with Chinese space authorities:"Even now, I cannot confirm what caused the sound."