Yang Liwei, China's first astronaut, has revealed that he heard a mysterious knock while alone in a tiny spacecraft during his maiden flight in space.

Mysterious Knocking Sound

In an interview, Yang said that he recalled hearing something like somebody knocking on the body of the spaceship during his first flight in space in 2003. He compared this sound to somebody knocking on an iron bucket using a wooden hammer.

"A non-causal situation I have met in space is a knock that appeared from time to time," Yang related saying the sound just appeared suddenly sans any rhyme or reason. The astronaut added that the sound neither came from the inside nor outside of the spacecraft.

Yang admitted that he got a bit nervous and decided to move close to the porthole in an attempt to find out what's behind the mysterious sound. Nothing out of the ordinary, however, appeared and he could not find any explanation for the eerie knocking sound.

He was not able to figure out what the sound was even after he returned to Earth. He has already tried albeit in vain to recreate the sound so experts would be able to help him identify what it was. He tried to imitate the sound using different instruments but he never heard the exact sound again.

Sound Also Heard By Other Chinese Astronauts

Yang said that the sound was also heard by the Shenzhou 6 and Shenzhou 7 astronauts during the 2005 and 2008 missions but he has already assured the astronauts that the sound is a normal phenomenon and there is no reason to get worried.

What Is Behind The Knocking Sound?

Space is expected to be silent because there is no medium such as air particles, water molecules, metals or solid atoms that would allow sound to travel.

Goh Cher Hiang, from the National University of Singapore, said that it is possible that something physical could be hitting the spacecraft carrying the astronauts, which could be responsible for the knocking sound.

Wee-Seng Soh, also from the National University of Singapore, offered a different explanation. He said that the sound could have been the result of the spaceship's contraction or expansion given that the temperature of the spacecraft's exterior could possibly change within the orbit.

This is not the first time that a mysterious sound has been heard in space. Declassified NASA tapes, for instance, have revealed that the astronauts of the Apollo 10 mission heard odd music on the far side of the moon.