A new television show suggests that the Apollo 10 mission encountered some sort of mysterious music or alien signal while on the dark side of the moon.

According to a story on Discovery News “The astronauts who flew the dress rehearsal for the first lunar landing reported hearing mysterious ‘outer space-type music’ while flying behind the moon in May 1969. The case of the odd, unexplained whistling noise is uncovered during an upcoming episode of the Science Channel series, ‘NASA’s Unexplained Files.’ The episode focuses on a strange event experienced by the crew members of Apollo 10.”

The Apollo 10 astronauts can be heard on the tapes mentioning the odd sound, which lasted nearly an hour.

Some reports even suggested that there was an attempt to keep the event a secret: “The Apollo 10 astronauts—Tom Stafford, John Young and Eugene Cernan—can be heard on recordings from the flight talking about the strange sound, and whether to tell NASA about it.”

NASA pointed out, however, in a post to their tumblr page, that the recordings have been publicly available at the National Archives since the early 1970s.

Apollo 10 Astronauts Heard Odd 'Music' on Far Side of Moon

Nonetheless, speculation has spurred conspiracy theorists who claim the recordings as the latest smoking gun evidence of a UFO coverup.

So what can we tell about the odd event? First, it’s clear that the sound—whatever it was—originated in the capsule as part of the mission, not outside it. In the 1660s British scientist Robert Boyle demonstrated that sound waves must be carried by a medium (such as air), and therefore cannot travel in a vacuum or near vacuum (such as space).

Sound waves on Earth are created by mechanical vibrations that cause air molecules to move back and forth at the same frequency and reach the human ear where they are turned into sound. Sometimes that sound is instantly recognizable (such as a loved one’s voice, or carbonation escaping from an opening soda can) and other times it’s not recognized at all (such as an unknown rattle or hum).

The fact that the sound was clearly coming from their headsets — and not, for example, from deep space — is an important clue.

Spectacular Apollo Photos Inspired Moon Science

Music of the Spheres?

Another curiosity is that the sound was described as “outer-space type music,” which is an artifact of a psychological phenomenon called priming. This involves how our ideas about the world influence our interpretations of audio and visual phenomenon, especially ambiguous stimuli.

What does music from outer space sound like? No one knows, since no extraterrestrial music has ever been heard or recorded. Without some basis for comparison, it’s clear what the astronauts meant was that it reminded them of composed music that had appeared in popular culture used to evoke an outer space setting, such as perhaps the theremin instrument in “The Day The Earth Stood Still” or “The Thing From Another World” or countless other radio plays, television shows such as “The Twilight Zone” and “The Outer Limits” and so on, which were broadcast decades before the Apollo 10 mission.

What the Science Channel show refers to as “an unsettling incident on the far side of the moon” was described as a “whistling sound” at the time, and others expanded into something far more significant, including “strange otherworldly music” or possibly even “alien speech.”

Moonwalkers: Stunning Photos from Apollo 11

Despite it being casually referred to as “music,” there’s no mention of the sound having any characteristics of music: no melody, rhythm, or form, for example. Calling it “music” makes it seem more mysterious, but there’s no evidence that it was anything other than as described: a constant whistling or “wooooo” sound.

So we know that the sound (not “music”) was coming from their radio headsets. What could it be?

Down to Earth

James Oberg, a space historian and 22-year veteran of Mission Control, told Discovery News in an email that the mystery “is created by concealing fundamental features of the event, such as the highly significant fact that the ‘space music’ weird noises occurred during formation flying by the Apollo ‘Command Module’ and the ‘Lunar Module’ landing vehicle, as the astronauts talked by radio from ship to ship. The noises were clearly some sort of interference between the two radios separated by tens of kilometers, and they were heard again on the following mission.”

Oberg noted that the claim that NASA and the astronauts tried to hide or cover up the mystery is false. Not only did astronaut Michael Collins report the same sound on Apollo 11, he described it in his 1974 book “Carrying the Fire: An Astronaut’s Journey.”

According to Oberg, “The noises were discussed in real time by the astronauts over radio links that were broadcast live, and in more detail by the crew during debriefings back on Earth — as they should have been, as a fundamental safety principle of examining all anomalies for indications of threatening malfunctions.”

The idea that the astronauts discussed whether or not to tell NASA about the sound is nonsensical, since all their transmissions and conversation was being recorded. The discussion was not whether to engage in a conspiracy to hide what they heard (as Oberg notes, it was being broadcast live, and in any event even if it weren’t NASA would find out sooner or later when the recordings were reviewed).

Instead, the discussion was whether the odd sound might indicate a severe enough communication problem that Mission Control should be notified. Though the Apollo 10 recordings were classified at the time (per standard protocol), they were eventually released in 2008; if NASA had something to hide, why would they have declassified and released the recording at all?

Lunar Phases: The Changing Face of the Moon

Some doubt that the sound could have been ordinary radio interference. Pilot Al Worden — who was not on the Apollo 10 mission and didn’t hear the sound first hand — is quoted in the show as believing that “The Apollo 10 crew is very used to the kind of noise that they should be hearing,” suggesting that had it been interference, the crew would have been clearly recognized it as such.

While it is true that the crew would be familiar with the sound of interference, that doesn’t mean that all interference would necessarily sound exactly the same. The fact that the astronauts didn’t recognize the sound and couldn’t identify its origin is not surprising.

Even our own voices — which should be as natural and familiar to us as our own hands and bodies — sound strange to us because we do not hear our voices as other people do. This is because our vocal chords create sounds that travel through the air to others, but through our own bodies (including bone, cartilage, and soft tissue) to us.

Video: Earth is Ever So Slowly Crushing the Moon

As Oberg noted, “No astronauts had ever experienced interference with this particular hardware, especially behind the moon, ever before.”

This was an extraordinary circumstance, and there’s no reason to think the Apollo 10 crew would have heard that type of interference before.

Different conditions create different sounds, and a low, monotone “woooo” sound coming over a radio is far more likely to be interference than, say, alien music. While providing fodder for the mystery-mongering UFO crowd (not to mention Pink Floyd fans), the fact is that the Apollo 10 mystery sound is not quite so mysterious when all the facts are examined. Cue the woooo!