apollo lander moon earth nasaNASA

The crew of NASA's Apollo 10 mission in May of 1969 set the groundwork for showing that Apollo 11's moon landing would be possible. They flew around the moon and practiced separating and descending the lunar module to better understand potential issues with landing.

But they heard some mysterious sounds in the process, an eerie whistling that they couldn't understand and weren't sure how to report.

As lunar module pilot Eugene Cernan asks John Young, the command module pilot:"That music even sounds outer-spacey, doesn't it? You hear that? That whistling sound?"

That text from the transcript of their discussion was released online in 2008.

NASA transciptNASA

Now, audio from the mission has made its way into an episode of the Science Channel show "NASA's Unexplained Files." It's tough to distinguish the show's sound effects from recorded audio, but if you skip to 2:16 here you can catch some of the whistling:

Here's part two of the promo for the show:

So the question is, why — when cut off from communication with Earth — would the crew in different modules capture a mysterious audio transmission?

In the transcript, you can see that the astronauts weren't sure whether they should say anything. If NASA thought they were unstable, there was a real risk they'd be grounded after the mission, and they had no explanation for what they heard.

Decades later, the Cassini spacecraft captured similar mysterious transmissions from Saturn — but there we have a better explanation. When charged particles move through the magnetic environment around Saturn, they're distorted in ways that cause those odd sounds.

But as planetary scientist Kevin Grazier explains on the show promo, that explanation doesn't work for the moon, which doesn't have an atmosphere or magnetic field.

So what was it?

NASA's explanation is straightforward. The radios on the command module and lunar module were interfering with each other, causing a strange sound to appear, like when you hold a cell phone near a speaker.

On the show promo, astronaut Al Worden isn't so sure and argues there must have been something causing the noise, saying that "logic tells me that if there was something recorded on there, then there's something there."

But as CNN notes, Apollo 11 pilot Michael Collins reported hearing similar interference noises on that mission when the lunar module detached from the command module, something he'd been warned would happen by NASA radio technicians.

And when Apollo 11's lunar module landed, the mysterious noises stopped.

The full transcript of the mission is below.

AS10_LMApollo 10 Onboard Voice Transcription-Lunar Module

Correction: This post originally stated that the audio was just released to the public, however NASA has clarified that audio and transcripts have been available in the National Archives since the 1970s.