June's Invisible Meteors
Every year in early June, Earth passes through the densest part of two interplanetary meteoroid streams. One, possibly consisting of debris from the asteroid Icarus, produces about 60 shooting stars each hour from the direction of the constellation Aries. The other debris stream produces up to 40 meteors per hour from the constellation Perseus. Sadly for star gazers, both constellations are very close to the Sun when these showers reach maximum activity. The blinding glare of the Sun makes most "Arietids" and "zeta Perseids" invisible the naked eye. Still, the few that are visible are really worth seeing!
Above: "Hey Joe, what was that?!" ... "I dunno, just keep fishing." This fanciful picture by Duane Hilton shows a bright meteor streaking across the daylight sky above Twin Lakes, CA. Unlike the Yukon fireball of January, 2000, which resulted from a 200 metric ton asteroid, the Arietid and zeta
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"The Arietid meteors are active from a radiant located in eastern Aries approximately 10 degrees west of the Pleiades," explains Robert Lunsford, Secretary General of the International Meteor Society. "This is 32 degrees east of the Sun so activity can only be seen shortly before the onset on dawn. Any activity would likely appear as 'Earthgrazers,' which are long meteors lasting several seconds shooting upward from the horizon."
Above: This star chart shows the appearance of the eastern sky at 5:00 am on June 7, 2000 from a mid-northern latitude observing site. The Arietids radiant is 15 degrees above the horizon, while the zeta Perseids radiant is a scant 4 degrees high. Although meteors can streak very far from their radiants, it is difficult to spot many shooting stars when the radiant is low. Both the Arietid and Perseid meteor rates pick up substantially later in the day when the radiants move overhead.
Arietid and zeta Perseid meteoroids strike Earth's atmosphere at about 75,000 mph. As they move rapidly through the air, these specks of space dust heat and ionize the gas around them. During major meteor showers like the Arietids and zeta Perseids, radio signals from TV stations, RADAR facilities, and AM/FM transmitters are constantly bouncing off these short-lived meteor trails. Whenever a meteor passes overhead with the correct geometry, radio listeners can hear a brief "blip" (the reflected signal of the distant transmitter) on the receiver's loudspeaker.
Right: Stan Nelson in his ham radio "shack" (above), points his radio antenna toward the Navy Space Surveillance transmitter in Texas (below).
Listening to radio meteors doesn't require ultra-sophisticated equipment. For example, just last month Stan Nelson of Roswell, NM, was monitoring the eta Aquarid meteor shower at 217 MHz when he recorded this reflection (see below) of a signal transmitted by the powerful Naval Space Surveillance Radar (NAVSPASUR in Kickapoo TX) from a meteor trail. His listening station consists of a common VHF ham receiver and an inexpensive Yagi antenna.
Nelson's audio recording contains two sounds -- the first is a staccato ping caused by an Earth-orbiting satellite racing over the NAVSPASUR radar facility. The longer blip, two seconds later, is a reflection of the radar signal from an ionized meteoroid trail. These are just two samples of hundreds of reflections Nelson recorded around the peak of May's eta Aquarid meteor shower.
Above: Stan Nelson recorded this audio signal on May 5, 2000 while listening for meteor reflections from the powerful NAVSPASUR radar at 217 MHz. The green trace on top is the envelope of his ham receiver's loudspeaker output. The blue-colored time vs. frequency plot is the signal's dynamic spectrum. Click on the image to play the WAV-format audio recording.
The Arietid and zeta Perseid showers are considered to be among the best showers of the year for radio observers. In fact, both showers were discovered by radio observers using the Jodrell Bank radio telescope in England in 1947 [ref]. The month of June comes to an end with another daytime meteor shower, the beta Taurids, which peak around June 29 with as many as 25 meteors per hour. All in all, June seems to be the best month of the year to be a radio meteor observer.
To detect radio meteors, you need to know how to tune in and when to listen. For more information about meteor echoes from NAVSPASUR, please see the Dec. 1998 Science@NASA article "The Ghosts of Fireballs Past." The North American Meteor Network's radio meteor tutorial is a good source of information about radio meteor "blips" from FM or TV stations.
|Arietids at a Glance||zeta Perseids at a Glance|
The zeta Perseid Meteor Shower -from Gary Kronk's Comets & Meteor Showers web site
The Arietid Meteor Shower -from Gary Kronk's Comets & Meteor Showers web site
International Meteor Organization -featuring a page about radio observations of meteors.
North American Meteor Network -includes tutorials about meteor watching, radio meteors, and a meteor observer's calendar
American Meteor Society