Friday, May 31, 2013

Radio In Space!

In the book Lonely Planets , author David Grinspoon makes one notable error in the quote below.  Radio signals aren't traveling away from earth at twice the speed of light, they're moving at a fraction of the speed of light but that's still very very fast. The quote is long, but worth reading.
"We’ve sent out some symbolic broadcasts - scribbled a few simple messages and tossed them out there in leaky electromagnetic bottles.  We’ve attached notes to our four spacecraft (so far) that are leaving the solar system, just in case they wash up somewhere.  And of course, if anyone is really hunting for the likes of us, our presence is not a well-kept secret:  for decades, we have been leaking our sitcoms, talk shows, and ebullient commercials for Jesus, minivans, and beer.  A spherical shell of radio signals is expanding outward from Earth, its diameter increasing at twice the speed of light.  As I write, this sphere forms a ball of news, entertainment, psychobabble, and advertising 166 light-years in diameter.  In the time it took you to read this sentence, it grew by another million miles. The nearest stars are only about four light-years away. These indiscretions might have tipped off some of our closest neighbors that something is up on the third stone from the Sun.  Would they conclude from these transmissions that we are intelligent, or merely that some nutcases have stumbled upon primitive radio technology?"
Radio waves to escape Earth. That is undeniable. We employ this fact to communicate with our satellites both orbiting the planet and some near the edge of our solar system. But not every signal achieves escape velocity so to speak. Signals below about 25 MHz tend to get reflected off the ionosphere. In 1985 Woodruff Sullivan and Stephen Knowles used the Moon as reflector of Earth's RF leakage. Using the 305 meter  Arecibo radio telescope, he scanned a range of frequencies between 100 and 400 MHz and found that the many TV and radio frequencies could be detected. More here.

So the big question becomes: how far away can these signals be detected?  Even though there are no interfering stations on the 2nd adjacent, there are still numerous bodies in space that radiate interference. But even if these didn't exist we would still have to content with the inverse square law. As radio signals propagate outward from earth, the waves diffuse over that spherical distance. As the area a signal is distributed over grows exponentially the amplitude of that signal diminishes.

To give an example the power of the radio signal will be only 1/4 of it's original strength once you are twice the distance from the source. At 10x that distance, the power of the signal would only be 1/100th it's original power. By the time we are at the edge of the solar system it's already approaching zero. More here.