Chinese astronaut Yang Liwei experienced something unusual and eerie on his first flight to space. Liwei heard a weird knock on his spaceship, which was China's first manned spacecraft Shenzhou-V.
This incident occurred in 2003 and he recently revealed that the knock felt like "someone knocking the body of the spaceship just as knocking an iron bucket with a wooden hammer," BBC reported.
"It neither came from outside nor inside the spaceship," Liwei shared further.
This eerie knocking made Liwei nervous, he even peeped out of the porthole of his spaceship to find out a reason behind the knock, but he couldn't find anything.
Even today, Liwei can't figure out the cause of the knocking sound, even the sound experts failed in explaining what it was.
This mysterious instance raised quite some curiosity. Who or what could have caused the knocking in the space still remains unknown.
No sound can travel to space as there is no medium, hence space is expected to be soundless.
"The travelling of sound requires a medium - be it air particles or water molecules or metal, solid atoms," Professor Goh Cher Hiang, an expert in space engineering at the National University of Singapore, was quoted as saying by the BBC.
"If it is knocking, there could be something physical hitting the spacecraft carrying the astronaut," he added.
Some examples related to sound travelling would be underwater sonar, thunder sound which travels through air, or even a solid musical instrument which provides a mean for sound to travel.
Wee-Seng Soh, Hiang's colleague, came up with an explanation. He said the knocking sound could 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."
The Chinese media revealed that a similar sound was heard by many other Chinese astronauts who went on space missions between 2005 and 2008. Liwei even told other astronauts about the instance to avoid them from worrying during the mission like he did.
This phenomenon is now considered to be normal by Liwei.
Check out some strange sounds captured by NASA's Juno Space Probe in here: