Apollo 10 astronauts heard an eerie sound on the far side of the Moon. What was it?

With its launch in May 1969, the Apollo 10 mission paved the way for Apollo 11 to land two people on the lunar surface just two months later. During Apollo 10, the crew flew to the Moon in a command module. Two of the three astronauts, Gene Cernan and Tom Stafford, then took a lunar module, the vehicle built for landing, down to less than ten miles above the surface. 

As they traveled around the far side of the Moon, the astronauts reported hearing something strange. Was it aliens or something else? Here’s what scientists say.

As the crew passed behind the Moon, they picked up an odd whistling noise. Cernan, who was piloting the lunar module, asked John Young, who was in the command module, "You hear that? That whistling sound?" The unusual music kept playing for almost an hour. Interestingly, NASA was also able to capture it on tape. Here’s the direct link to listen to the audio (starting at 1:42).

And here’s what makes it interesting. It started during the spacecraft’s approximately 60-minute trip around the Moon. At that time, the crew was out of contact with Earth. Later, Young seemed to worry about how it would be received, saying, "We're going to have to find out about that. Nobody will believe us." The astronauts did not discuss it publicly at the time. 

Sean O'Kane at The Verge suggested that they might have stayed quiet because they didn't want to look shaken, since doubts about their confidence could have gotten them grounded.

Michael Collins, who flew the command module on Apollo 11, described the same noise in his 1974 book, Carrying the Fire. In his book, he shared that the eerie whistle sound started when the two spacecraft were close together. Both of their radios were on, and then it went away when Armstrong and Aldrin landed the lunar module on the Moon.

Because he had been warned in advance, he wrote, "Had I not been warned about it, it would have scared the hell out of me."

According to the BBC, one of the “more sensible reasons” for the music was the "interference between the Lunar Module's and Command Module's VHF radios." Other theories, which link it to a magnetic field or the Moon's atmosphere, don't hold up, because the Moon has no magnetic field and not enough atmosphere to produce such sounds.

"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,” Mr Cernan told NASA.