Before I move on to Blog in Space III: Across the Universe, I want to briefly touch upon the Wow! Signal. The Wow! Signal is the most famous signal in the history of the SETI program - a still unexplained signal that appears to be a non-terrestrial (non Earth) artificial signal received for 72 seconds on 15 August 1977.
However, it was a one time event - no radio telescope has ever been able to pick up the signal again. Some feel that indicates it is not a signal from an alien sentient race after all.
But why can't an alien signal sometimes be a one time transmission?
There is one good terrestrial example I will discuss in the upcoming Blog in Space III post: the one time transmission of the Beatles song "Across the Universe" into space. This transmission was aimed at Polaris - it was not a omni directional transmission, nor was it a continuous transmission. Any listener on Polaris over 43o years from now will have a small window of opportunity to catch the signal. If the Polarians tried to pick up the signal again, they'll be out of luck, scratching their heads, or whatever it is Polarians scratch when they are puzzled - if they scratch when puzzle, as they debate the signal's nature.
The Wow! Signal could be a one time signal sent out by a sentient race, their "hello, we are here" signal. Why don't they keep sending out their hello? Why didn't we keep sending "Across the Universe"? Their own SETI program could be nascent like ours, with their radio telescopes mostly tied up with other scientific research, not leaving much time for broadcasting signals into deep space.
The Wow! Signal could also be an errant communication transmission to a moving target - say, oh, I don't know, an interplanetary or interstellar space ship or deep space probe. Such communication transmissions would most likely not be a continuous stream. They could be repeated at regular intervals, but since we don't know what the intervals are, and since we do not have radio telescopes constantly trained on the Wow! Locale (the area of the sky the Wow! Signal came from), it would be a great stroke of luck to hear another signal from the Wow! Locale on one of the short sporadic times we are listening for it.
The Wow! Signal could be a transmission from aliens to one of their deep space probes - a signal that is sent out very infrequently, only when needed for course corrections, turning on or off a science instrument or experiment, or to transmit program updates to fix a problem the probe developed.
Another thing to consider is if this was an alien transmission that was not aimed at us but at a moving space craft or space probe, the likelihood that the space craft or probe is on a direct line between the aliens and our solar system is rather minute. Most likely the space craft or probe is moving at an angle to us, especially for a craft or probe within their own system. The probe may flyby other planets to get a gravity assist to help boost its velocity, or the probe may flyby other planets as part of its long term mission. The chances for the alien's probe to happen to line up between the aliens and Earth right when they are transmitting a signal to the probe (or, conversely, for the alien's planet to line up between the Earth and the probe which is transmitting a signal to the alien's planet) are minute at best.
But wait, there's more! Another complication is that our planet is circling our sun which is itself moving through space while the alien's planet is circling a star which is also moving through space. Think about a probe circling Mars. As Mars travels in its orbit, and we in ours, the angle of the transmission beam between the Earth and Mars changes, due to the fact that each planet's orbit is different in size and each planet travels at a different speed around the Sun. Sometimes the Earth and Mars are on the same side of the Sun, and other times on opposite sides of the Sun. Because of this, the background stars change as well.
And even if the alien's space craft or probe was sent toward our Sun, because of their planet's orbit, the transmission signals sent to the craft would be sent from varying angles. This would be, of course, most pronounced early in the craft's journey to our Sun as the angular distance would be the greatest. Only occasionally would the alien planet, their space craft, and the Earth line up. As the craft nears the Sun, the angular distance would decrease (compare how the distance between the rails of a train track appear close up to the distance between the rails appear at the horizon). In that case, we would actually be in some luck (unless the craft is an invading alien army) - as the craft gets closer, our chances of relocating their transmission signals increases.
Don't count the Wow! Signal out as an alien signal just because we detected it only once.
References:
Alexander, Amir. "The "Wow!" Signal Still Eludes Detection." Planetary News: SETI. 17 January, 2001. Web. 10 August 2008. <http://www.planetary.org/news/2001/0117_The_Wow_Signal_Still_Eludes.html>.
"A Gravity Assist Primer." Jet Propulsion Laboratory. Web. 10 August 2008. <http://www2.jpl.nasa.gov/basics/grav/primer.html>.
Kawa, Barry. "The Wow! Signal." Big Ear Radio Observatory. 6 September 2006. Web. 10 August 2008. <http://www.bigear.org/wow.htm>. Reprinted from the Cleveland Plain Dealer Sunday Magazine section, 18 September 1994.
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