The eerie “whistling sounds” were recorded during the mission’s flight around the dark side of the Moon.

The classic B-movie sci-fi sounds of the eerie, otherworldly theremin were first coined by the great film and television composer Bernard Herrmann, most notably in his 1951 score for the film The Day the Earth Stood Still. Little did Herrmann know how close his orchestration was to actual space music. Transcripts and recordings from the 1969 Apollo 10 lunar mission, recently released by NASA, have revealed that during the astronauts’ journey across the dark side of the moon, a mysterious “whistling space music” was heard through their radio headsets.

It may sound like science fiction, and indeed, the three men who boldly went into lunar orbit that day in 1969 were so baffled by the occurrence they decided not to report it directly to NASA at the time. However after sitting in NASA’s archives for almost 50 years, the recordings of the celestial whistling, heard by astronauts Thomas Stafford, John Young and Eugene Cernan, have now been released to the public for the first time.

The extraordinary tapes have become the subject of a new series on the Discovery Channel: NASA’s Unexplained Files. During the astronauts’ journey around the far side of the Moon, their contact with Earth was temporarily cut. It was while they were out of direct radio communication with NASA that the three astronauts began to hear the strange sounds. “You hear that? That whistling sound?” Cernan asks his Apollo crewmates, before adding, “It even sounds like outer-space-type music.” After hearing the space song for an hour, it abruptly ceased.

The transcript of the astronauts’ confused conversation was released in 2008, but the actual recording of the sounds has only now been made available to the public. NASA have refuted excited claims by UFO enthusiasts that the strange “music” is evidence of alien contact. An engineer from the US space agency has surmised that the noises are more likely to be radio feedback between the three astronauts, as their onboard radio headsets were in close proximity within the cramped confines of the lunar module. However, Michael Collins, who piloted the Apollo 11 mission, said he too heard “eerie woo-woo sounds” as he flew solo around the dark side of the Moon, while Buzz Aldrin and Neil Armstrong made their historic journey to the surface.

This isn’t the first time space music has been released by NASA. Radio frequencies recorded by the Voyager probes, produced by our solar system’s gas giants – Saturn, Jupiter, Neptune and Uranus – have been adjusted to human ranges of hearing by NASA scientists, offering a strange yet beautiful soundscape of growls, hisses and hums: truly the music of the spheres.