[Updated Sept 3, 2013: It is reported on SatWatch that the 216 MHz US Space Fence, used to detect orbital objects, was turned off on September 1, 2013 at 0000 UT.]
Tonight is overcast and stormy. Thunder is shaking the balance springs in the windows. The sky can only be heard, not seen. I will not see the peak of the Perseid meteor shower this year.
On a clear, moonless August night in my childhood I saw an extremely bright meteor streaking across the sky, and I could swear that it made a noise as it plunged into the upper layers of Earth’s atmosphere. Not anything thunderous, rather a crackling, sizzling noise like a sparkler on the Fourth of July. Perhaps my mind and ears were playing tricks with me, recalling the sound of fireworks. Besides, wouldn’t one expect something burning up while traveling upwards of 100,000 mph to sound off, to make itself known like the lightning?
I had read claims by others, amateur astronomers like myself, of hearing this strange crackling noise after observing a shooting star. I chalked it up to wishful thinking. Our hobby was one of silent observation. We would ask questions of the stars, with never an audible reply. We dearly wanted to hear the things we saw. That night I received a response, and it was exhilarating.
It takes the displacement of air molecules and the propagation of the pressure waves onto our ear drums for us to experience sound, and there simply is not enough air at the upper altitudes to allow for it. For the most part, grain and pea sized meteors and other loose pieces of the universe that fail to overcome our planet’s gravitational grip become mute, fleeting flashes of light, too small to survive re-entry, too high to make a sound.
We know of the exceptions. In 1908, an asteroid or portion of a comet was large enough to survive it’s fiery fall to within five miles of the ground. The air blast from that altitude flattened trees for over 800 square miles. It was estimated to be around 200 feet in diameter. More recently, in February of this year, a smaller fifty-five foot diameter meteor re-entered in the Ural mountain region of Russia. The resulting main blast that originated six miles high blew out windows, collapsed walls, and injured hundreds of people, mostly due to flying glass. Scientists now believe that this chunk of rock separated from a larger 600 foot mass that remains in space.
Then there’s the dinosaur issue. We know how that one turned out.
Fortunately for us, these occurrences are relatively rare. The fact of the matter is that 24/7, things are falling from the sky without a sound. While we go about our noisy everyday lives, most of the catastrophic events in the universe occur in total silence.
But there are ways of making the sky talk. By transmitting an extremely high power signal up through the atmosphere, and listening to the reflections that return to our receivers, we can hear the ghostly echoes from these grains of star stuff as they make their final descent.
The government has an app for that.
In 1958, the U.S. Naval Research Laboratory began construction of a radio space surveillance and tracking system that now consists of nine locations stretching from California to Georgia. Signals transmitted up into space from three transmitter sites bounce off of things that cross the path of this fence of electromagnetic energy, and are reflected back to multiple receiving points. Operation was turned over to the U.S. Air Force Space Command in 2004. This system is designed for, and used to calculate and verify orbits of man made objects such as satellites and orbital debris. But these same signals can, and do regularly bounce off of the ionized gas trails left behind by falling meteors.
The NAVSPACUR fence, as it is called, was the nation’s earliest space detection system. Although computing power has increased immeasurably since it’s inception, the basic theories of radar and direction finding have not changed. And the same VHF radio frequencies have been in use since 1958. While the radios may have been a little more specialized back in the day, today’s receivers that can tune in on these frequencies are inexpensive and readily available to anyone. Amateur radio operators and space enthusiasts tune in regularly.
The first time I listened in, I was amazed at what I heard. Every fifteen seconds or so, something was causing a ping, chirp, or a sharply rising and falling tone in my ears. There were certainly a lot of man made objects passing by. The distinctive short chirps were heard regularly, as the passing satellites reflected the signal back like a mirror, some louder than others, some lasting longer. But the sounds that intrigued me most were the haunting, reverberating echoes that sometimes lingered for several seconds. Like tiny audible snowflakes, no two seemed the same in tone, duration, or strength. At times the signal from one of the transmitter sites would find it’s way directly to my antenna, creating a continuous tone that would make it hard to hear the reflections. But soon it would fade back into obscurity, once again leaving the airwaves open for the incoming visitors.
Between local midnight and dawn, things get busy. During this period, it’s as if you’re running into a driving rainstorm with a headwind. At that time of night your location on Earth is turning into the direction of its orbit around the Sun, and the number of pings detected increases. During major meteor showers, Earth’s atmosphere practically scoops them up like ice cream, and the resulting symphony is nothing short of impressive.
Near-Earth orbit is a crowded place. Each month five million detections of satellites from the NAVSPACUR fence are processed by computers in Dahlgren Wva, where a database of orbiting objects is continuously updated to provide accurate predictions and tracking. This information helps the International Space Station avoid collisions, and assists other agencies worldwide by providing alerts to potential collisions. It also aids in determining locations and times of satellite re-entries. In between all this work, the little blips of random dust and grain that this system provides a voice to go largely unnoticed. They just happen to be in the right place at the right time, announcing their presence using a big government microphone.
Sadly, these voices from the heavens may soon go silent. General William Shelton, commander of Air Force Space Command, has ordered the entire project to be closed by October 1, 2013. A “sequestered budget environment” was mentioned as a prime factor in the decision by Andy Roake, a spokesman for Air Force Space Command. It has been suggested that the system represents 40 percent of the Air Force’s space detection capability. It is the longest ranging part of the detection space detection network, with a capability of providing data on objects out to 14,000 miles.
None of that makes a difference to me. What I do know is that with the closing of the VHF radio fence, the music from space will stop. The stars will continue to fall, and the Perseids will peak next year in August as they always have. But they may no longer sing to us on their way down. They will only announce themselves on clear, moonless nights.