Making Meteor Showers Sing

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For centuries skywatchers have reported hearing a sound as a meteor visibly passed overhead. Professional astronomers, however, consistently dismissed the possibility of a meteor being seen and heard simultaneously based on the physics of sound and light. Particles from asteroids or comets that enter the Earth’s atmosphere is traveling at very high speeds and the friction generated by moving against air particles heats these meteors. Consequently, most meteors vaporize and create the sought after shooting star streaking across the sky. Yet sound travels much more slowly than light does. Disintegrating meteors visible to an observer on Earth’s surface are typically 60 miles above the planet’s surface. Thus a meteor would not possibly be heard until approximately five minutes after having been sighted. Given this reasoning, when a fireball passed over England in 1719, astronomer Edmond Halley (who calculated the orbit of the eponymous Halley’s Comet) concluded that reports of “hearing it hiss as it went along, as if it had been near at hand” had to be “the effect of pure fantasy”. Yet the phenomenon continued and reports from meteor listeners accumulated.

Astronomers now understand that observers reporting hissing, sizzling or buzzing sounds during meteor showers are not delusional but rather are likely experiencing electrophonic sounds. In addition to releasing electromagnetic radiation in the visible portion of the spectrum, meteors release very low frequency (VLF) radio waves, which travel at the speed of light. Humans cannot directly hear these radio waves which oscillate at audio frequencies between a few kilohertz and 30 kilohertz. But in the presence of a physical object acting as a transducer, VLF radiation is converted into sound waves that a human ear can detect. Colin Keay, a physicist at the University of Newcastle in Australia during the 1970s, showed in a laboratory study that radio waves can induce low-frequency currents and rustling sounds in ordinary objects, even wire-framed eye glasses. He hypothesized that when the magnetic fields in the glowing trail of a meteor are permeated by Earth’s magnetic field a potential source of energy for VLF waves is created. Researchers tested Keay’s hypothesis and found that distinct VLF electromagnetic pulses were produced during the Leonid meteor shower of November 1999.

Recently Dave Prochnow described how a person could hack an old stereo receiver in order to cause an audible spike in signal reception when a meteor passes overhead. In this case, the hacker is using the radio and a good FM antenna to pick up the signal of a distant FM radio station whose strength has been augmented when reflected by the ionized trail of a meteor. Although mechanistically very different from Keay’s documentation of electrophonics, this hack attempts to achieve the similar phenomenon of “hearing” meteors. In fact, radio engineers regularly monitor for “meteor echoes” by detecting TV signals that are reflected from meteor trails and “radio meteors” by detecting radio signals that bounce off the ionized gases produced by disintegrating meteoroids. In both cases, listeners can hear a brief “ping” on the receiver’s speaker when a meteor passes by with the correct geometry. In addition to detecting the many meteors that are too dim for the human eye to see, radio observing is advantageous because meteors can be detected 24 hours a day and even when skies are cloudy. The International Meteor Organization and the North American Meteor Network list dozens of meteor showers that are monitored almost exclusively by radio observations and not detected by their visual counterparts. Often these are “daylight meteor showers”. Radio observation, however, provides no information to the listener regarding where the meteor came from. Experienced visual observers can discriminate the direction or constellation from which a meteor emanates but radio detection cannot.

Prochnow’s published instructions appear in the November 2014 issue of Popular Science under the title of “Listen in on a Meteor Shower: How to Repurpose Your Old Radio to Listen to Meteor Showers”. In addition to a stereo receiver and an FM Yagi antenna, the hacker needs to have downloaded Radio-SkyPipe II software on a computer. Making meteors sing seems to be a good candidate for a collaborative project between Astronomy Club and STEM Club. Let’s hope it’s coming soon to a Science Center roof near you.