Apollo’s Mysterious “Music,” Explained

The Science Channel’s cable show, “NASA’s Unexplained Files” reports on mysterious “music” heard by the crew of Apollo 10 while they were orbiting the Moon—on the far side, out of radio contact with the Earth. In fact, most of the Apollo crews heard strange whoops and whines on the comm loop at one time or another during their missions. Are these strange sounds evidence for aliens hovering just beyond the astronauts’ reach? To understand how the sounds may have originated, you must understand a little about how radio works and how electromagnetic interference is created and mitigated.

The Apollo spacecraft required several different radio links of different frequencies in order to safely conduct the missions to the Moon. These included telemetry sent down for their onboard systems (to allow Mission Control to monitor the health and performance of the vehicle), voice and biomedical information from the crew, and navigational data, including information on ranging to the surface so that the spacecraft could safely approach and land on the Moon.

Radio (RF, for radio frequency) takes a very high frequency electromagnetic signal (the carrier) and varies either the amplitude (AM) or frequency (FM) of that signal in minute ways to carry information. These variations (modulation) can be either analog (sounds, such as voice and music) or digital (data) signals. The production of a radio wave generates not just a single frequency, but often additional signals at integer multiples of that frequency, a phenomenon known as harmonics. Harmonics are undesirable in communications systems, and great care is taken by engineers during the design process to minimize harmonics and also to severely limit the emission of stray radio waves (RF leakage).

When two radio signals interact, they sometimes mix in a complex manner to create a third signal—called the beat frequency. Sometimes this is done deliberately, as in normal AM radio, in which the broadcast signals are deliberately mixed with a locally generated RF signal inside the receiver to produce an intermediate frequency, which then can be amplified using a single-tuned stage. This technique (called superheterodyne)—while sounding complex—actually makes radio possible by simplifying the circuitry needed to tune all of the possible station frequencies into a single, tuned stage.

Any wire carrying RF energy can act as an antenna, both sending and receiving signals. It requires careful planning during system design to minimize this effect and most communications systems are very good—but not perfect—at isolating stray RF radiation. Thus, in a complex vehicle, with several million different parts all operating simultaneously (some continuously), the possibilities for RF interference are great.

The sounds heard by the crew of Apollo 10 were described at the time as “whistling” or as a “whooooo” sound. Other Apollo crews in lunar orbit heard similar sounds—one memorably described as a “woo-woo” sound by Apollo 11 Command Module Pilot Michael Collins. Such a description perfectly fits the way beat frequencies sound, as two different RF signals—close in frequency but separated enough to interfere on a complex basis—join and mix to produce a warbling, varied tone that is sometimes audible and sometimes not. If these sounds were caused by RF interference, they might be related to systems that are on simultaneously only part of the time, explaining their mysterious appearance and disappearance.

I was a radio amateur in my youth and remember hearing strange sounds, shrieks and beat frequencies from my equipment all the time. It never once occurred to me that it was caused by anything other than stray interference caused by the various oscillators and transmitters scattered around my radio shack. Of course, I was not orbiting the far side of the Moon at the time—supposedly the one place in the Solar System permanently shielded from Earth’s radio noise.

I am struck by the fact that a show entitled “NASA’s Unexplained Files” would feature this story, one well-known to space buffs for many years, and cast doubt that what the crew heard was both fully explained and understood. Apollo 15 astronaut Al Worden appears in the piece and makes the point that as experienced flight crew, the Apollo 10 astronauts would have recognized RF interference when they heard it, but that simply is not true. It does not imply incompetence or foolishness to recognize that we can all be misled at times by unexpected and startling physical effects, especially if we are in a completely unfamiliar environment. When flying over the lunar far side for the first time, I am sure that the very last thing the crew expected was to encounter strange noises on the comm loop. Such were not heard during the one previous flight to the Moon, the Christmas 1968 flight of Apollo 8. This was a completely new and unexpected phenomenon.

Why weren’t these noises heard by the Apollo 8 crew during their earlier flight in lunar orbit? That mission was a flight of only the Command-Service Module (CSM). The Lunar Module (LM) was flown to the Moon for the first time during the Apollo 10 mission. After the mission, it was postulated that RF interference between the separate LM and CSM Very High Frequency (VHF) transmitters caused the strange sounds. The physical manifestation of such interference could have taken many different forms, including simple static, pulsating and rhythmic noise, hums or audio squeals, squeaks and whoops. The exact form such noises take would depend very specifically on the configuration of the vehicles and the conditions of the RF equipment during flight, factors that can never be exactly reconstructed long after the fact.

I suppose that I am not surprised that the occasional dumb idea makes it onto television. However, this show is being presented as factual, and while these strange noises were certainly heard, it should not be construed to mean that some unusual or “unexplained” phenomena were operative. Moreover, in my opinion, a television service that has the temerity to call itself the “Science Channel” has an obligation not to sensationalize or present facts in a misleading and silly manner. The universe is full of wonderful, beautiful and unknown natural phenomena. We do not need ghost stories or nonsensical “musical aliens” to make space travel and space science interesting or to engage and educate the public. On the contrary, such nonsense trivializes science and dumbs down our national discourse.

Paul D. Spudis | READ MORE

Paul D. Spudis (1952-2018) was a senior staff scientist at the Lunar and Planetary Institute in Houston, Texas, and a prolific author on the subject of the moon. His books include The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources.