The true story behind Apollo 10's "outer-spacey" music encounter

Facebook and Twitter lit up this weekend with a tale seemingly worthy of "The X-Files." Did Apollo 10 astronauts really hear "music" coming from space as they orbited the moon in 1969, and did NASA try to keep it from the public? Well, not exactly.

Yes, the Apollo 10 crew heard strange noises over their radios while passing over the far side of the moon, beyond the reach of any earthly signals, and the astronauts were clearly startled, wondering where the sounds could be coming from and joking about whether they should mention it to flight controllers in Houston.

But as it turns out, there's a perfectly rational explanation that did not require the presence of aliens or ancient astronauts. The strange sounds were the result of radio interference between the VHF systems aboard the Apollo 10 lunar module and the command module.

But the crew did not know that at the time, and the astronauts were clearly caught off guard.

"That music even sounds outer-spacey, doesn't it?" asked Apollo 10 lunar module pilot Eugene Cernan, according to NASA's official transcript of mission audio. "You hear that? That whistling sound?"

"Yes," replied mission commander Thomas Stafford.

"Whooooo. Say your..." Cernan began.

Command module pilot John Young then chimed in: "Did you hear that whistling sound, too?"

"Yes. Sounds like, you know, outer-space-type music," Cernan replied.

"I wonder what it is?" Young asked.

A few minutes later, Cernan added "Boy, that sure is weird music."

"We're going to have to find out about that," Young said. "Nobody will believe us."

"Yes. It's a whistling, you know, like an outer-space-type thing.

Later, still on the back side of the moon, Cernan said "that eerie music is what's bothering me. You know that..."

Young: "God damn, I heard it, too."

Cernan: "You know, that was funny. That's just like something from outer space, really. Who's going to believe it?

Young: "Nobody. Shall we tell them about it?"

Cernan: I don't know. We ought to think about it some."

An upcoming episode of the Science Channel series "NASA's Unexplained Files" recounts the conversation and includes the mission audio. The Huffington Post reported the transcripts of the Apollo 10 mission were "classified and untouched in NASA's archives until 2008, producing an ongoing debate as to the nature and origin of the strange sounds heard by the astronauts."

The NASA History Office, however, tweeted Monday that the mission audio was never "classified" and that a full transcript was published in 1973. The tapes were converted to digital format and posted online in 2012.

"#Apollo 10 audio & transcripts were not classified, just no way to get them to the public before the internet," the History Office tweeted. It also publicized links where the information could be found online.

As for what caused the strange sounds, Apollo 11 command module pilot Michael Collins provided a detailed explanation in his book "Carrying the Fire," published in 1974.

Orbiting the moon and waiting for Neil Armstrong and Buzz Aldrin to catch up after blasting off from the lunar surface, Collins wrote "there is a strange noise in my headset now, an eerie woo-woo sound."

"Had I not been warned about it, it would have scared hell out of me," he 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 each denied making."

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 wrote.

"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."

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