Space horror is a part of our popular culture. An episode of “Doctor Who” called “Listen,” featuring Peter Capaldi in the titular role, saw the Doctor investigate a supposed creature that can never been seen. His journey takes him to the end of the universe, where he meets a stranded traveler on a spaceship. Though nothing is supposed to be living outside of the ship, our heroes hear a series of three knocks that repeat every night. It is an unexplained phenomenon that both terrifies the traveler and intrigues the Doctor.
Strange sounds in space are not always the work of science fiction, however. One such incident involved a Chinese astronaut in 2003. While aboard the spaceship Shenzhou 5, he heard a noise that he could not explain at the moment, and that no one has been able to explain since. Though a few theories have floated around about what could have caused the noise, now, over two decades later, none of them have been definitively proven.
This isn’t the only strange sound that’s ever been heard out in space — NASA has released an audio clip letting listeners hear the sound a black hole makes. But in that case, targeted space research can record and identify sounds. When it comes to the astronaut aboard Shenzhou 5, there’s still the feeling of there being something unexpected and unexplained.
Read more: What Does Space Actually Smell Like? Here’s What Scientists Say
What sound did the astronaut hear in space?
A crescent moon in space with the rest hidden in shadow – Ramadanovic/Shutterstock
It was October 2003 and a historical moment for China. The country launched its first manned spacecraft carrying astronaut Yang Liwei, and his flight was scheduled to be 21 hours long. During his flight, however, he heard a knocking sound. He described the sound like a wooden mallet hitting a metal surface. The sound occurred on and off. He could not identify the source of it, though reported that it sounded like something was knocking on the outside of the ship.
Adding to the mystery, this was not the last time the sound was heard. Astronauts on Shenzhou 6 in 2005 and Shenzhou 7 in 2008 also reported hearing the exact same sound and were equally unable to identify the cause. Though not exactly the same sound, during the Apollo 10 mission in 1969, astronauts reported hearing eerie whistling while they were on the dark side of the moon; the whistling was even recorded by the ship’s systems. Most recently, the astronauts Butch Wilmore and Sunita Williams, famously trapped in space after the fiasco with their Boeing Starliner spacecraft, heard a strange pulsing noise from the ship.
In many of those cases, scientists could find a source for the strange sounds. The Boeing Starliner noise was explained as audio feedback from the Starliner and the International Space Station. The Apollo 10 noise was explained as radio interference between the lunar module and the command module. However, the knocking sounds heard on the three Shenzhou missions have still eluded explanation.
Theories on what the knocking sound in space was
Image of deep space with a galaxy full of stars – Triff/Shutterstock
The main reason that sounds in space are alarming is because sound waves have to be able to bounce off of objects to create noise. National University of Singapore professor Goh Cher Hiang explained this to the BBC: “The travelling of sound travels 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.”
Accordingly, one theory is that there could have simply been debris in space hitting the outside of the ship and causing the knocking sound. Another is that the changes in temperature from Earth to space caused the spaceship to either expand or contract just enough to cause an audible noise. A third theory suggests that the sound was air escaping from some objects on the ship during the journey, causing some small vibrations.
All told, there is no easy answer as to what Yang Liwei heard. Though all three theories are technically viable, it is fascinating that this sound happened across three separate missions and across a five-year time span. It is also interesting that, after three incidents, the source of the sound could not be clearly identified. We really are still uncovering strange sounds in space, like that time Voyager 1 heard an ominous hum from its location over 14.1 billion miles into deep space. Though some we can find an explanation for, some still remain an enticing mystery, which only helps to make space exploration an exciting field.
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