A Chinese astronaut says he might’ve been on the receiving end of an extraterrestrial knock-knock joke. Yang Liwei, China’s first man in space, says he heard something — or someone — knocking on his spaceship during his first trip to space in 2003. While it’s amusing to imagine that it was some alien wanting to come inside, the real source of the answer is probably much more mundane, though we don’t know for sure.

In a recent interview on China Central Television, Yang recalled hearing the strange sound “from time to time,” during his maiden space flight.

“It neither came from outside nor inside the spaceship, but sounded like someone is knocking the body of the spaceship just as knocking an iron bucket with a wooden hammer,” he explained, saying that it made him nervous. Yang recalled trying to peer out of the craft’s porthole to try to see what was causing the sound, but he didn’t see anything.

Once he was safely back on Earth, Yang told technicians about the sound, but they were unable to recreate it. Shenzhou 6 and Shenzhou 7 astronauts have also reported hearing it, and Yang now tells people that the knocking “is a normal phenomenon, so there is no need to worry.”

An alien saying “hello.”
An alien saying "hello."

Inverse asked NASA if American astronauts had ever encountered a knocking like this.

“As far as we know, there haven’t been any reports of such sounds,” NASA’s Kathryn Hambleton said.

The knocking could’ve been caused by something hitting the spaceship, but there are other possibilities. Wee-Seng Soh, an expert in space engineering at the National University of Singapore, told the BBC that it could have been “a result of expansion or contraction of the spaceship, especially since the temperature of the spaceship’s exterior could change considerably within the orbit.” Remember, China was still a rookie to manned spaceflight when their astronauts first started hearing the noise.

While it’s doubtful that there’s a truly shocking source behind the noise, it’s still a mystery as to what, exactly, caused it. The truth is out there — perhaps knocking on the door, wanting to come inside.

Photos via Flickr / (: Rebecca-louise :)