Launched on May 18, 1969, the Apollo 10 lunar mission sent three men, Thomas P. Stafford, John W. Young, and Eugene A. Cernan, into orbit around the moon, the first mission in history to do so. When their command module, nicknamed "Charlie Brown", orbited to the far side of the moon, communication with Earth was impossible, since the moon readily blocked radio transmissions. However, in place of mission control’s far away voice, the crew instead picked up on a mysterious transmission, according to a new documentary.

According to the new Science Channel series, "NASA’s Unexplained Files," the astronauts aboard Apollo 10 discussed a strange whistling sound heard over their headsets, described by lunar module pilot Eugene A. Cernan as "outer-space-type music". This "music" was heard during two passes around the far side of the Moon, and was heard on both the command module and lunar lander’s systems. The noise abruptly stopped at one point, then resumed some time later.

Two theories have been put forward to explain the phenomena: one is that it’s a natural transmission, caused by charged particles passing though a magnetic field, much like the sounds recorded by the Cassini space probe as it orbited Saturn. However, planetary scientist Kevin Grazier says this is unlikely, since the Moon has no magnetic field to produce this effect, and any transmissions caused by Earth’s magnetic field would be blocked by the Moon itself.

The other theory, as explained by space historian and former JPL employee Andrew Chaikin, was that the spacecraft’s two VHF radio systems were interfering with one another, producing the sound. Apollo 15 astronaut Al Warden doubts that this is the case, as the Apollo 10 crew would be familiar with the sound such interference would make.

It should be pointed out that the sound displayed in the video is the recording made by the Cassini space probe of charged particles passing through Saturn’s magnetic field, and appears to be a stand-in for the actual sound. The sounds the Apollo 10 crew are describing are not audible in the original recordings.