Everyday billions of space rocks crash into the earth’s atmosphere and disintegrate before they reach the ground. This produces two main effects—one we can see with our eyes, the other we cannot.
The effect which we can see—provided the meteor is large enough—is the actual breaking up of the rock as it slams against the air, heats the air molecules and the heat melts the rock. As it burns and falls through the atmosphere, the meteor leaves a trail of glowing particles in its wake which we call the shooting star. Although this event appears to happen only a few thousand feet up in the sky, most meteors break up at altitudes between 80 to 100 kilometers.
A meteor. Photo credit: Sergiu Bacioiu/Flickr
The second effect, the one which we cannot see, is ionization. As the meteor vaporizes into its constituent molecules, these molecules travelling at high velocities collide with air molecules and physically tear the molecules apart causing them to become ionized. Meteorites even as small as a grain of sand can create columns of ionized air tens of kilometers long. And because the atmosphere is constantly bombarded by such grain-sized meteorites, ionization trails can be found in the upper atmosphere more or less continuously.
What if we use this ionization trails to reflect radio waves for long distance communication? As a matter of fact, we already use the ionosphere—situated between altitudes of 60 and 1,000 km—to reflect radio waves of short wavelengths to reach audiences beyond the curvature of the earth. The ionosphere can reflect radio waves of frequencies between 3 to 30 MHz. Anything higher passes right through the ionosphere. But ionization densities in meteor trails is higher than that in the ionosphere, which allows radio waves of much higher frequencies to be reflected back to the earth.
Noted Japanese physicist Hantaro Nagaoka was the first to make the connection between meteors and radio reflection in 1929, although his premise was that meteors could disrupt radio propagation. It was American radio pioneer Greenleaf Whittier Pickard and Bell Labs researcher A. M. Skellett, who first suggested that meteor showers could instead enhance radio communication. Greenleaf Pickard observed that bursts of long distance radio propagation occurred at times of major meteor showers, while Skellett theorized a relationship between the kinetic energy of a meteor and the ionization of the ionosphere.
The first significant attempt to use meteor scatter for communication was JANET carried out by the Canadian Defence Research Board in the early 1950s. The JANET project used a 90 MHz carrier signal to send out short bursts of data over distances exceeding 2,000 km. The system was operational until about 1960. Another major deployment was COMET (Communication by Meteor Trails) operated by NATO, with stations located in Netherlands, France, Italy, West Germany, the United Kingdom, and Norway.
Example of Meteor Burst Communications. Image credit.
A typical Meteor Burst Communication (MBC) link consist of a master station and one or more receiving stations. The master station sends out a continuous probing signal into the atmosphere, while the receiving stations waits and listens for the probe. At some point, a meteor will appear and the signal will be reflected back to the receiving station. The receiving station will then signal back to the master station using the same meteor trail that a communication link is available. The master station acknowledged back and a “handshake” is established. Now both stations have a small window, usually just a fraction of a second long, to send and receive data before the meteor trail disperses and the channel closes. The master channel then goes back to transmitting the probe until the next path is found. Fortunately, because meteors are so frequent they do not have to wait long. According to a declassified report from NSA, communication windows 100 milliseconds-long become available once every 17 seconds, 200 millisecond-windows can be found every 35 seconds, and 400 millisecond-windows every two and a half minutes. Longer duration windows, such as 1.6 seconds, however, occur only once every two days.
The major advantage of MBC is they are immune to disruption. A satellite can be blown up and communication cables can be cut, but meteor trails are beyond the reach of humans. An MBC signal also has a relatively small footprint, which makes eavesdropping difficult unless the attacker is located very close to the receiving stations. Because of these reasons, MBCs are still used by the military for certain applications.
MBC is also used by the United States’ Natural Resources Conservation Service’ SNOTEL system to collect information about snowfall and other related climatic data, to forecast yearly water supplies, predict floods, and for general climate research.
