In 1969, just two months before humankind stepped foot on the moon, Apollo 10 astronauts were orbiting its dark side. NASA recently released recordings of that mission, which revealed that the astronauts were taken aback by a peculiar sound they heard after they were out of range of Earth-based radio transmissions.
The strange sound came through their headsets after embarking on an hour-long trip to the moon's furthest side. The dark side of the moon was out of range of any Earthly radio signals, which means that the sound could not have come from Earth. It also means that while they were orbiting that side, they were out of contact with NASA. Lasting for the duration of their voyage around the moon's dark side, it was referred to as "weird" and "outer space-type music" by the astronauts on board. They refer to the strange sound a number of times during their voyage around the moon, all of them unsure what it was they were hearing. For a time, the group was uncertain if they should even report what they heard to NASA command.
NASA recorded the transcript of the mission once it was over and the team had returned to Earth. Those transcripts were stored in the archives and were classified until 2008. Still, the mysterious music heard wasn't revealed to the public until recently. The Science Channel is airing its third season of NASA's Unexplained Files, and the mystery sound heard will feature on an upcoming episode.
Al Worden, an astronaut aboard Apollo 15, said that those who heard the sound would have been "used to the kind of noise they should be hearing". He also said that if the astronauts reported an unusual sound "then there was something there". The episode explores the various explanations, including magnetic fields or atmospheric interference, and discounts them, providing experts who agree that the moon lacks a magnetic field and has inadequate atmosphere to cause such noise.
You can watch a sneak peak video below, and catch the series when it returns on February 23, on the Science Channel.