Jun 28, 1999

Radio Meteor Alert

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Earth may be headed into two meteoroid swarms near the end of June. Ham radio operators can monitor the action, which may be invisible to the naked eye.


Click to listen! Data courtesy Stan Nelson
June 28, 1999: In June, 1998, meteor watchers observed an outburst of shooting stars from the constellation Boötes. The "June Boötids" are a well-known annual meteor shower that usually produce just a few visible meteors per hour, but in 1998 the hourly rate soared to over 100! Although no one knows what caused the outburst last year, meteor organizations are alerting their members to watch out for a repeat performance around June 27-28, 1999.


As if one meteoroid swarm weren't enough, the earth might be headed into two. The International Meteor Organization has issued an alert that a "resonant swarm" of meteoroids predicted by Dr. David Asher in 1993 could produce an outburst of Beta Taurid meteors in late June and early July 1999.

Above, right: Meteor enthusiast, Stan Nelson of Roswell, NM, captured from a meteor on June 27, 1999. He used an ICOM R8500 communications receiver to monitor 217 MHz transmissions from the Navy Space Surveillance Radar located in Lake Kickapoo, TX.

Both showers will be difficult to see visually. The full moon will outshine most June Boötid meteors, and the Beta Taurids are most active when the sun is above the horizon. Ham radio operators have the best chance of successfully monitoring these showers.

Data reported yesterday to Science@NASA by an experienced radio meteor observer, Stan Nelson in New Mexico, suggests that enhanced meteor activity may be ongoing as of June 27, 1999.

"My neighbor and fellow astronomy enthusiast, Russ Lockett, and I monitored 217 MHz radar returns for meteors this morning [June 27, 1999]", said Nelson. "We started at 5:15 am. MDT and noted 46 'blips' by 6:15 am. It was very active session and ... most of the returns appeared to be typical meteor signatures."


Listen to a meteor radar echo recorded June 27, 1999

It's impossible to say whether the echoes observed by Nelson were meteoroids from the Beta Taurid stream or from the June Boötid stream. They could be a mixture of both.

Beta Taurids
The Beta Taurids are an annual meteor shower belonging to a class of "daytime showers" that peak after sunrise. (The International Meteor Organization lists a about dozen daylight meteor showers that are monitored almost exclusively by radio observations.) The Beta Taurids are usually active between June 5 to July 18. They emanate from an average radiant of RA=5h18m, DECL=+21.2 deg and exhibit maximum activity around June 29 (Solar Longitude=98.3 deg). This year the radiant is just 10 degrees west of the Sun on June 28, so it is unlikely that visual observers will be able to see many of these shooting stars. The maximum hourly rate typically reaches about 25 to the eyes of radar.

Left: Artist's concept of radio meteor observing. When fast-moving meteoroids strike Earth's atmosphere they heat and ionize the air in their path. The luminous ionized trails are not only visually striking -- they also reflect radio waves. During a major meteor shower, radio signals from TV stations, RADAR facilities, and AM/FM transmitters are constantly bouncing off short lived meteor trails. Whenever a meteor passes by with the correct geometry, listeners hear a brief "blip" on the receiver's loudspeaker. more information from the North American Meteor Network.

"To identify what happens with the Beta Taurids this year, whether a swarm event or not, I would suggest radio observers should be especially alert between June 18-19 through to July 2-3 at least," says Alastair McBeath, the vice President of the International Meteor Organization. "Still later in July might be better, as some earlier results suggested a Beta Taurid maximum around July 2 or 3; this has not been found in data from the 1990s so far, however."

The Beta Taurid radiant is above the horizon between roughly 03-04h local time to about 18-19h, for northern hemisphere sites between approximately 35-55 degrees north near June 28.

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June Boötids
The June Boötids are typically active from June 27 to July 5 with a maximum that falls on the 28th. Normally it is a very weak shower. The greatest activity levels reach 1 to 2 per hour only, but the June Boötids are remembered for strong displays in 1916, 1921, 1927, and most recently in 1998. The radiant, which is near the constellation Boötes at RA=14h52m and DECL=+58 deg, is above the horizon from about noon until 6 a.m. for mid-latitude sites in the northern hemisphere. Normally the best way to observe the June Boötids would be visually, by simply going out and looking up. However, this year the bright full moon will make visual observations very difficult, so the June Boötids like the Beta Taurids are best observed by radio methods.

Nelson's observation of 46 radio pings per hour was recorded when both the Beta Taurid and June Boötid radiants were low on the horizon from his observing site. Either or both of the two showers could have been active during Nelson's observing session. Ham radio operators can make an important contribution to scientific understanding of the Beta Taurid and the June Boötid meteoroid streams by observing these showers as often as possible through early July. For hams unfamiliar with radio meteor observing techniques a brief tutorial is given below. Results may be transmitted to Dr. Tony Phillips and they will be forwarded to the appropriate scientists and meteor observing organizations.


dynamic spectrum of
Stan Nelson's meteor echo
Left: Stan Nelson's radio meteor echo from June 27, 1999 looks much like this "L"-shaped reflection from a meteor on April 11, 1999 as presented in a frequency-time plot. The slanted, nearly vertical piece is the reflection from ionized air around the rapidly-moving meteoroid (sometimes called the "head" of the meteor). The doppler-shifted frequency of the reflection changes rapidly because the meteoroid moves across the line-of-sight of the transmitter at high speed. After the meteoroid disintegrates, all that remains is a slowly moving trail of ionized air. The velocity of the residual trail is low, typically no more than 0.02 km/s, compared to 30 - 70 km/s for the meteoroid. The reflection from the slowly moving gas does not experience a significant doppler shift, so it appears as a horizontal line (the bottom of the "L") in the dynamic spectrum.

How it's Done
If you're interested in detecting radio meteors, the procedures are relatively simple. You'll need a good commercial radio receiver and an aerial. Although meteor trains can reflect radio waves at almost any frequency, the best frequencies to try are usually between 50 and 120 MHz. Many observers use a common FM radio tunable between 88 and 108 MHz and a Yagi FM/TV antenna. During a meteor shower tune your receiver to a distant transmitter between 200 and 1000 miles away. Commercial radio stations, TV stations, and radar transmitters are all suitable if located at the correct distance. Under normal circumstances the transmitter should be difficult or impossible to detect, but when a meteor intervenes the signal hops over the horizon and a brief fragment of the transmission can be heard. Depending on the type of the transmitter it might sound like a tone, a bit of music or voice, or simply noise. Contact lasts for as long as the meteor train persists, usually from 100 milliseconds to a few seconds.

Stan Nelson's echo, above, was obtained at 217 MHz which is usually considered to be a poor frequency for meteor observations. However, the tremendous power of the Naval Space Surveillance radar (NAVSPASUR) more than compensates for its less-than-optimum transmission frequency. NAVSPASUR is an excellent transmitter for meteor observers across the southern United States. For more information about meteor observing with NAVSPASUR, please see the Dec. 1998 Science@NASA article The Ghosts of Fireballs Past. To learn more about radio meteor observing in general, see the North American Meteor Network radio meteor tutorial.

Web Links

The Radiometeor Audio Gallery - from the American Meteor Society

The June Boötids - From Gary Kronk's Comets & Meteors Web Site

The Beta Taurids - From Gary Kronk's Comets & Meteors Web Site

North American Meteor Network - radio meteor tutorial

North American Meteor Network - April 99 Newsletter featuring information about the Lyrids

North American Meteor Network - home page

Leonids Live! -site of the live webcast of the 1998 Leonids

Related Stories:

A Wild Ride to the Stratosphere in Search of Meteors -- Apr. 14, 1999. The payload from the NASA Meteor Balloon has been recovered.

Meteor Balloon set for Launch -- Apr. 9, 1999. NASA scientists prepare to launch a weather balloon designed to capture micrometeoroids in the stratosphere.

Leonid Sample Return Update -- Apr. 1, 1999. Scientists will describe initial results from a program to catch meteoroids in flight at the NASA/Ames Leonids Workshop April 12-15, 1999.

The Ghost of Fireballs Past -- Dec. 22, 1998. RADAR echoes from Leonid and Geminid meteors.

Bunches & Bunches of Geminids -- Dec. 15, 1998. The Geminids continued to intensify in 1998

The 1998 Leonids: A bust or a blast? -- Nov. 27, 1998. New images of Leonid fireballs and their smokey remnants.

Leonids Sample Return payload recovered! -- Nov. 23, 1998. Scientists are scanning the "comet catcher" for signs of Leonid meteoroids.

Early birds catch the Leonids -- Nov. 19, 1998. The peak of the Leonid meteor shower happened more than 14 hours earlier than experts had predicted.

A high-altitude look at the Leonids -- Nov. 18, 1998. NASA sci balloon catches video of 8 fireballs.

The Leonid Sample Return Mission -- Nov. 16, 1998. NASA scientists hope to capture a Leonid meteoroid and return it to Earth.

Great Expectations: the 1998 Leonid meteor shower -- Nov. 10, 1998. The basics of what the Leonids are and what might happen on November 17.


D. Asher, "Meteoroid Swarms and the Taurid Complex", in: "Proceedings IMC, Puimichel 1993", ed. P. Roggemans, IMO, 1993, pp.88-91.


meteor flash!
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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Author: Dr. Tony Phillips
Curator: Linda Porter
NASA Official: Ron Koczor