The US space agency has released the archived audio tapes of a conversation between Apollo 10 astronauts about an "outer-space-type music" as their craft flew on the far side of the moon in 1969, a couple of months before Neil Armstrong set foot on the Moon, media reports said.
Although the text from the transcript of the discussion was released in 2008, the audio recording is only just being made public and will be played in an upcoming Science Channel series "NASA's Unexplained Files", CNN reported.
Since it is not possible to receive radio signals on the far side of the Moon, the musical dance of frequencies and noise made astronauts Eugene Cernan and John Young wonder whether "that whistling sound" could be an "outer-space-type music".
"That music even sounds outer-spacey, doesn't it? You hear that? That whistling sound? Whoooooo," says Cernan in the recording.
Young answers in the affirmative, saying: "I wonder what it is."
So, what could possibly be the explanation?
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In a statement released last week by NASA, Cernan said: "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."
In the promo of "NASA's Unexplained Files", astronaut Al Worden says that "logic tells me that if there was something recorded on there, then there's something there", but according to cbsnews.com, Apollo 11 command module pilot Michael Collins provided a detailed explanation about the noise in his 1974 book "Carrying the Fire: An Astronaut's Journeys", which he co-authored with Charles Lindbergh.
As Collins waited for Neil Armstrong and Buzz Aldrin to catch up after blasting off from the lunar surface, he too heard "a strange noise" in his headset - "an eerie woo-woo sound", Collins wrote.
"Had I not been warned about it, it would have scared hell out of me," he further wrote.
"Stafford's Apollo 10 crew had first heard it, during their practice rendezvous around the moon. Alone on the back side, they were more than a little surprised to hear a noise that John Young in the command module and Stafford in the LM (lunar module) each denied making," Collins added.
After returning to Earth, the Apollo 10 crew "gingerly mentioned it in their debriefing sessions, but fortunately the radio technicians (rather than the UFO fans) had a ready explanation for it", Collins noted.
"It was interference between the LM's and command module's VHF radios. We had heard it yesterday when we turned our VHF radios on after separating our two vehicles, and Neil said that it 'sounds like wind whipping around the trees'. It stopped as soon as the LM got on the ground, and started up again just a short time ago. A strange noise in a strange place," he wrote in the book.