Ask Dr. SETI ®
A New Paradigm for the Twenty First Century
by Bob Krekorian - Former NASA SETI Signal Detection Analyst
The NASA Kepler Mission and its implications for the search for extraterrestrial intelligence (SETI) is an unfinished story. What a great time to be alive. The Kepler Mission makes one reflect on what Galileo did when he first turned his telescope to the night sky and forever changed the way we view our place in the universe.
Sometime this year, the following could very well be the new reality regarding exoplanet detections. With over a year of data from the Kepler telescope that has now been downloaded and analyzed, Earth-like planets have been found in orbits inward of 0.3AU, some with small eccentricities and all with no nearby gas giants as one would expect. A reasonable argument can be made that Earth-like planets will also be found in and around what is called the habitable zone. It will take several more years of observations to unequivocally verify this assertion. Thus for SETI, there is good news and bad news. The good news is that there are other Earth-like planets out there, some with orbital parameters that might harbor life. The bad news is that the number looks to be modest, indicating the SETI search space may have to be extended to perhaps three thousand light years for success to happen.
Using stellar population statistics, extrasolar planet detections and results thus far from the Kepler Mission, an estimate can be made of the volume of SETI search space that has to be explored to have a reasonable chance to make a contact. The estimate is based on the number of F,G and K stars (singles and widely separated binaries) with favorable stellar properties (metallicity,variability,age,etc) that has an Earth-like planet in the habitable zone with the gas giants located in the outer regions of their planetary system. One star in ten thousand (strong constraints) is likely to make the cut and have a chance as a habitat for life. Under less restrictive constraints, the number could rise to one star in a thousand.
At radial distances of one, two and three thousand light years from us, it is estimated then to be one, ten and one hundred thousand suitable stars (n) with good Earths (strong constraints) and ten times that many when the constraints are loosened. The probability of success (PS) after sampling n stars is PS = 1 - exp(-np) where p is the probability of a good Earth having a technological civilization. When np = 1, PS = 0.64.
For those of us who believe that SETI is important and want to see it succeed, there is a further realization that requires them (the extraterrestrials) to have constructed an interstellar beacon whose prime purpose is to make contacts with others in the galaxy. No beacon, no bacon. What can we say about such a beacon, should one exist? Any mature technological civilization that has reached the level of beacon aspirations would have long since moved all its astronomical operations into the more logical and benign environment of space, just as we are beginning to do.
Okay, space is the place to put the beacon and the extraterrestrial engineers have decided to place their prize interstellar contact transmitter into an orbit in their star system someway inward of the habitable zone and preferably as close to their star as feasible. If we had such a beacon, it would be in an heliocentric orbit between the orbits of Earth and Venus. Here it can take advantage of an inexhaustible supply of energy (their host star) and thus sustain itself indefinitely. In this configuration, the beacon rotates synchronously as it orbits its star so that a strip of declination is illuminated at its orbital opposition and conjunction nodes.
For potential contactees, the result is a gain of two orders of magnitude in received power over an omni-directional antenna and elimination of the Doppler drift accelerations from both planetary rotation and orbital motion. Assuming a continuous wave or a pulse signal, the signal types expected, detection is a much easier proposition, since the signal trajectory does not cut across detection channels in the time-frequency plane during the observation period. When the assumption is made that the signal remains stationary in a channel, coherent detection (match filtering) methods can be used to significantly improve detection sensitivity. Finally, one of the biggest challenges facing potential SETI recipients like us or anybody is the monumental real time signal processing (computer operations per second) effort that is required, if all possible signal drift paths resulting from planetary rotation are considered.
Transmissions from an omni-directional beacon implies that information will be forthcoming at all times, year after year. What could they not tell us in one day per one hundred that would require this full time downloading script? All things considered, the beacon has been in existence and will be in existence for eons. And as we decipher the first message, just imagine the world-wide excitement and anticipation as we wait for the next transmission to arrive in say three months. Meanwhile, work on figuring out their modulation scheme, doing anti-cryptography and understanding the message content would keep us quite busy between duty cycles.
There is an additional means to increase the information rate in a communication channel and that is by taking advantage of the Shannon-Hartley theorem. The theorem relates channel capacity (bits per second) to bandwidth and signal-to-noise ratio (SNR). Increasing bandwidth increases channel capacity but at the cost of introducing more noise into the system (lower SNR). There are trade-offs here between bandwidth, SNR and an opportunity to do something on our end of the interstellar communication link. SNR is directly proportional to telescope collecting area, so once we find the signal, larger collecting areas can be built to recover SNR and take advantage of the high data rates. This puts some of the burden of maximizing channel capacity on the receiving party, maybe the entrance fee required to join the galactic telecommunication federation.
How are we to find the beacon signal if these assumptions are correct? The former NASA SETI project consisted of a targeted search (observe one star at a time for one thousand seconds) and an all sky survey (briefly examine all points on the celestial sky). Both search strategies would fail under the stellar orbital beacon hypothesis. The beacon designers would recognize all this and ask themselves, why build it in the first place if nobody can find their transmission? But an extraterrestrial beacon implies motivation to make contacts, hence there must be an interstellar roadmap that is obvious to every emerging technological civilization like us based on the fundamentals of physics, astrophysics and mathematics, especially number theory, to connect one to the broadcast. We stand on the shoulders of the great contributors to science and mathematics who came before us in our quest to make a contact. See my guest editorial, The Case for Extraterrestrial Beacons, that is on the SETI League website. Yahoo [krekorian beacons] will get it.
The NASA Astrobiology Institute addresses three basic questions. How does life begin and evolve? Does life exist elsewhere in the universe? What is the future of life on Earth and beyond? A successful SETI effort would clearly answer the second and give immeasurable insight into answering the first and third. Should NASA decide to have a twenty first century SETI project which would support President Obama's vision of NASA again inspiring the nation, I hope it is far different than the one I was with in the twentieth century. A detection could happen in our lifetime (a rendezvous with destiny) if the right people and organization (NASA) got behind it.
Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.
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this page last updated 6 March 2010
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