No one doubts that space has still many well-kept secrets. One of these secrets was brought to light after 40 years in a documentary of Science Channel entitled NASA’s Unexplained Files.

The documentary revealed that in 1969 the crew of Apollo 10 heard a strange whistling sound, which sounded like music, when they were outside the range of terrestrial radio.

The transcripts of the dialogue between the astronauts, Thomas P. Stafford, John W. Young and Eugene A. Cernan, were released in 2008, but audio of the discussion, and the sounds that the astronauts were referencing, is only just being made public.

Astronauts were at first afraid of talking about this weird experience, since they thought that no one would believe them.

One of the crew says: “You hear that? That whistling sound? Whooooooooo!”

“It sounds like, you know, outer space-type music,” another replies.

Later, one says: “I tell you, that music is really weird.”

“No one will believe us,” replies a colleague.

However, a NASA technician on the TV show explains that the “radios in the two spacecraft [the lunar module and the command module] were interfering with each other.” CNN says.

But, this explanation is disputed by the ponderous TV voiceover and astronaut Al Worden, who says on the show that “logic tells me that if there was something recorded on there, then there’s something there.”