Being a solo astronaut on a space mission must be pretty lonely, but the last thing you’d probably expect is a knock at the door.
That’s what one astronaut recalled being puzzled by whilst in orbit alone for 21 hours.
Yang Liwei became China’s first man in space when he took his maiden flight in 2003 but in a more recent interview, he revealed that he could hear a disturbing “knocking sound” against the door.
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He said that he heard “someone knocking the body of the spaceship just as knocking an iron bucket with a wooden hammer”.
“It neither came from outside nor inside the spaceship”, he added.
Understandably, the unexplained sound made Yang nervous and he recalled looking out of the porthole but failed to see anything that could have explained the knock.
Since returning to Earth, the astronaut has tried to work out what it was, even attempting to recreate the sound for experts to help figure it out but nothing has worked.
The source of the sound still remains a mystery and it’s a particularly distressing one considering that Yang was alone in space.
There is also no medium for sound to travel above the atmosphere so space is usually meant to be silent.
Speaking to the BBC, Professor Goh Cher Hiang, who is an expert in space engineering at the National University of Singapore, said: “The traveling 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.”
However, it turns out Yang wasn’t the only one to ever hear the strange knocking sound.
According to reports made by Chinese media, the same noise has also been heard by other Chinese astronauts in 2005 and 2008.
This also isn’t the only occurrence of astronauts claiming to have heard an unexplained noise while on a space mission.
During a test flight for the 1969 moon landing, astronauts orbited the rock and while they were cut off from radio contact with Earth while on the far side of the moon, they heard a strange sound.
The noise is still unexplained and was classified information until recent years. The noise was recorded and the audio clip was made public in 2016.
However, it’s likely that the sound was due to radio interference as the crew lost connection with Earth, rather than anything extraterrestrial trying to make contact.