The "music" from the far side of the Moon

In May 1969, two months before the first landing, Apollo 10 flew a full dress rehearsal in lunar orbit. When passing over the side that never faces Earth, the spacecraft lost contact with Mission Control. It was during that stretch that astronauts Thomas Stafford, John Young and Eugene Cernan heard a warbling whistle in their headsets, which one of them compared to strange music.

The recording sat forgotten in the archives for years and became famous when it aired on a TV show in 2016, presented as a revealed secret. To the Apollo program's engineers, however, there is no mystery: the high-frequency radios of the lunar module and the command module, close to each other, were interfering with one another. The far side, away from Earth's radio noise, simply made the sound clearer.