The “shooting stars” from the tau Herculids meteor shower may be visible later this month, but you might want to listen to them instead.
Excitement among meteor enthusiasts grows as we get closer to the highly anticipated meteor explosion which could be produced by a concentrated trail of dusty debris from the core of comet Schwassmann-Wachmann 3 (SW 3) late Monday evening until early Tuesday morning (May 30-31).
Even if you can’t see the show well because of clouds or light pollution, you can “observe” the meteor shower otherwise: by listening to it on the radio!
Related: Guide to the 2022 meteor showers: Dates and viewing tips
Under certain conditions, meteors can reflect radio waves in the same way that the ionosphere propagates transmissions between very distant amateur radio operators. The ionosphere generally reflects frequencies below 30 megahertz (MHz), but is transparent at higher frequencies, such as the FM broadcast band (88–108 MHz).
These high frequency (short wavelength) radio signals generally pass through the atmosphere unimpeded in straight lines; they cannot follow the curvature of the Earth to reach a listener beyond the horizon. Yet when certain layers of the upper atmosphere become ionized, they can send signals back to the ground very far away. The lowest such layer, 60 to 70 miles (96 to 112 kilometers), is called the E layer of the ionosphere, and it is the altitude at which most meteors are observed.
Thus, as a meteoroid vaporizes as it passes through earth’s atmosphere, it briefly ionizes air molecules along its path. Forming an expanding column or cylinder several miles or more in length, these ions can scatter and reflect radio waves, the same way a high-altitude jet reflects sunlight and leaves a glowing trail against the sky. which darkens after sunset. But because ion trails scatter quickly, the reflected radio waves usually only last a few seconds.
Tiny particles tend to vaporize at the bottom of the E layer. Large particles, on the other hand, begin to ignite higher up. And predictions for the particles emitted by comet SW 3 suggest that a majority of them will be large. Such meteors produce ionization of longer duration and, as they begin to “light up” higher, they may reflect signals from more distant emitters.
On the ground, the presence of the meteor is signaled by the momentary improvement in FM reception from a distant station.
How to Listen to Meteors on the Radio
For this radio method to work, find a frequency on which no nearby FM station is broadcasting. You’ll have a better chance of success sweeping the low frequency end of the FM band, below 91.1 MHz. Why there? Because that’s where the low-power, mostly college-run stations are located, and they’re generally free of local interference from high-powered commercial stations. In fact, unless you live in a very sparsely populated part of the country, your chances of finding an interference-free frequency above 91.1 MHz are pretty low, so you’ll need to tune to a distant station on a frequency clear below 91.1 MHz. .
Atlas MF, published from 1970 to 2010, provided listings of all FM stations in North America, with the unique feature of frequency-by-frequency maps. Bruce Elving, editor of FM Atlas, was a longtime proponent and expert on all things FM. He passed away in 2011, but in tribute to his love and dedication to FM radio, the 21st and final edition of FM Atlas (2010) is available for free, courtesy of AmericanRadioHistory.com. You may also see a complete list of AM and FM stations in the 2010-2011 edition of the M Street Directory.
What do meteors look like?
Normally, when you are listening to an “empty” radio frequency, you only hear a hiss. But as meteors pass through the atmosphere, a distant or silent station will abruptly “explode” for a fraction of a second to several seconds. You might also hear what initially sounds like a “pop” or hiss, then as the ionization trail dissipates, the station will quickly fade. Because of their height, meteors best reflect signals from stations 800 to 1,300 miles (1,300 to 2,100 km) from you.
When should you listen to meteors?
The best time to listen is when the radiant is 45 degrees above the horizon, as seen from a point halfway between you and the transmitter. At the predicted peak time for Tuesday morning’s potential meteor explosion, parts of Maine and the Canadian provinces of New Brunswick, Nova Scotia and Prince Edward Island will have the radiant close to this preferred altitude, while eastern New York, New England and southern Quebec will not be far behind, at around 50 to 55 degrees.
Also, it is preferable to tune to a station located in a direction perpendicular to the radiant. Because the radiant SW 3 will be close to the bright orange star Arcturus in the Constellation Bootswhich will be toward the western part of the sky, the best listening directions will be north and south of you.
Most meteors are heard but not seen
If you watch for meteors while watching your radio, most of the time you’ll hear a “ping” of reception, but you won’t see a corresponding meteor trail in the sky. Remember that most meteors you hear are about halfway between you and the radio station – about 400 to 650 miles (650 to 1,050 km). They therefore occur either near the horizon or just below it. In the 1970s, members of the Nippon Meteor Society in Japan, who made extensive recordings of radio meteors, noted that only 20–40% of meteors heard on radio were simultaneously observed visually.
What if you can’t find a clear frequency?
Especially in large metropolitan areas, finding a clear or empty FM frequency can be nearly impossible, even below 91.1 MHz. In many ways, finding a clear frequency seems to go hand in hand with trying to find dark skies free of light pollution. You’ll probably have a much better chance in rural areas or the countryside.
But if you can’t find a clear FM frequency, don’t despair. You can still listen to the meteors on livemeteors.com. A Yagi antenna in the Washington, DC metro area constantly picks up 55 or 61 MHz analog TV signals in Ontario reflected from meteor trails. When a meteor passes overhead – ping! — there is an echo. It’s the next best thing to having free access to a giant government radar!
Good luck and good listening!
Joe Rao is an instructor and guest speaker at New York’s Hayden Planetarium (opens in a new tab). He writes on astronomy for natural history review (opens in a new tab)the Farmers Almanac (opens in a new tab) and other publications. Follow us on twitter @Spacedotcom (opens in a new tab) and on Facebook (opens in a new tab).