The crew of an Apollo mission to the moon were so startled when they encountered strange music-like noises coming through their headsets, they didn't know whether or not to report it to NASA.
It was 1969, two months before Apollo 11's historic first manned landing on the moon, when Apollo 10 entered lunar orbit, which included traversing the far side of the moon where all spacecraft are out of radio contact with Earth for about an hour and nobody on Earth can see or hear them.
As far as the public knew, everything about the mission went smoothly.
The Science Channel series, "NASA's Unexplained Files," discusses the overlooked recordings of an unsettling experience the three Apollo astronauts had while flying above the far side of the moon.
The recordings contain "strange, otherworldly music coming through the Apollo module's radio," Science Channel says.
The conversation between the three astronauts indicated they heard sounds like they had never heard before:
"It sounds like, you know, outer space-type music."
"You hear that? That whistling sound? Whooooooooo!"
"Well, that sure is weird music!"
The sounds lasted almost an hour, and just before the astronauts regained radio contact with Earth, they discussed whether or not to tell Mission Control what they had experienced:
"It's unbelievable! You know?"
"Shall we tell them about it?"
"I don't know. We ought to think about it."
"The Apollo 10 crew was very used to the kind of noise that they should be hearing. Logic tells me that if there was something recorded on there, then there was something there," Apollo 15 astronaut Al Worden says on the Science Channel program. "NASA would withhold information from the public if they thought it was in the public's best interest."
"You don't hear about anything like that until years after the incident occurs, and then you kind of wonder, because it's such an old memory of those things that you get concerned about if they were making something up or was there something really there? Because you never really know," Worden told The Huffington Post.
"If you're behind the moon and hear some weird noise on your radio, and you know you're blocked from the Earth, then what could you possibly think?" Worden said.
NASA issued a statement this week explaining the files have long been available to the public, and that they have an explanation for the sound on the recording. The space agency says the tapes were never lost.
"While listed as 'confidential' in 1969 at the height of the Space Race, Apollo 10 mission transcripts and audio have been publicly available since 1973. Since the Internet did not exist in the Apollo era, NASA has only recently provided digital files for some of those earlier missions.
"The Apollo 10 audio clips were uploaded in 2012, but the mission's audio recordings have been available at the National Archives since the early 1970s. As for the likely source of the sounds questioned in the television program, Apollo 10 Lunar Module Pilot Gene Cernan told NASA on Monday, 'I don't remember that incident exciting me enough to take it seriously. It was probably just radio interference. Had we thought it was something other than that, we would have briefed everyone after the flight.
"We never gave it another thought."
"We'd had a lot of incidents where guys who flew in space saw and heard things that they didn't recognize, and you wonder about all of that," Worden said. "I have a very open mind about what could've happened. It's somebody's hearsay evidence -- it's only a visual or audio event, which is hard to pin down. Recollection is one thing, but actual proof is something entirely different."
NASA has made Apollo 10 transcripts and audio transmissions available to the public for further clarification.
Science Channel's "NASA's Unexplained Files" airs on Tuesdays at 10 p.m. Check your local listings.
CORRECTION: A previous version of this post stated that the recordings in question had been "lost" and were only recently declassified, and characterized the sound on the recordings as "unexplained." This article has been updated with NASA's statement, and accordingly throughout. Similar characterizations in the videos above, produced by the Science Channel, do not address these changes.