Apollo’s Mysterious “Music,” Explained

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. Read More: Air & Space Magazine
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