Carnival of Space 163 – Interstellar Beacons

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The Carnival of Space 163 is up at the Planetary Society.

At Centauri Dreams, there is analysis of SETI detection that goes beyond the Duncan Forgan and Robert Nichol paper on detecting extraneous emissions from an extraterrestrial civilization using technology like the Square Kilometer Array.

James Benford (Microwave Sciences) has some thoughts on the issue growing out of his own work with brother Gregory on interstellar beacons and SETI reception in general. The Forgan & Nichol paper on detection of leakage radiation does neglect our continuing use of microwave beams not only for radar, but also for likely future beaming of power for space purposes, such as power transfer.

To follow the latest Benford thoughts on SETI matters, see James Benford, Gregory Benford and Dominic Benford, “Messaging with Cost-Optimized Interstellar Beacons,” Astrobiology Vol. 10, No. 5, pp. 475-490 ( preprint), and the same authors’ “Searching for Cost-Optimized Interstellar Beacons,” Astrobiology Vol. 10, No. 5, pp. 491-498

Searching for interstellar beacons

What would SETI Beacon transmitters be like if built by civilizations with a variety of motivations, but who cared about cost? We studied in a companion paper how, for fixed power density in the far field, we could build a cost-optimum interstellar Beacon system. Here we consider, if someone like us were to produce a Beacon, how should we look for it? High-power transmitters might be built for wide variety of motives other than twoway communication; Beacons built to be seen over thousands of light years are such. Altruistic Beacon builders will have to contend with other altruistic causes, just as humans do, so may select for economy of effort. Cost, spectral lines near 1 GHz and interstellar scintillation favor radiating frequencies substantially above the classic water hole. Therefore the transmission strategy for a distant, cost-conscious Beacon will be a rapid scan of the galactic plane, to cover the angular space. Such pulses will be infrequent events for the receiver. Such Beacons built by distant advanced, wealthy societies will have very different characteristics from what SETI researchers seek. Future searches should pay special attention to areas along the galactic disk where SETI searches have seen coherent signals that have not recurred on the limited listening time intervals we have used. We will need to wait for recurring events that may arrive in intermittent bursts. Several new SETI search strategies emerge from these ideas. We propose a new test for SETI Beacons, based on the Life Plane hypotheses.”

Building cost effective interstellar beacons

This paper considers galactic scale Beacons from the point of view of expense to a builder on Earth. For fixed power density in the far field, what is the cost-optimum interstellar Beacon system? Experience shows an optimum tradeoff, depending on transmission frequency and on antenna size and power. This emerges by minimizing the cost of producing a desired effective isotropic radiated power, which in turn determines the maximum range of detectability of a transmitted signal. We derive general relations for cost-optimal aperture and power. For linear dependence of capital cost on transmitter power and antenna area, minimum capital cost occurs when the cost is equally divided between antenna gain and radiated power. For non-linear power law dependence a similar simple division occurs. This is validated in cost data for many systems; industry uses this cost optimum as a rule-of-thumb. Costs of pulsed cost-efficient transmitters are estimated from these relations using current cost parameters ($/W, $/m2) as a basis. Galactic-scale Beacons demand effective isotropic radiated power > 10^17 W, emitted powers are > 1 GW, with antenna areas > km2. We show the scaling and give examples of such Beacons. Thrifty beacon systems would be large and costly, have narrow searchlight beams and short dwell times when the Beacon would be seen by an alien oberver at target areas in the sky. They may revisit an area infrequently and will likely transmit at higher microwave frequencies, ~10 GHz. The natural corridor to broadcast is along the galactic spiral radius or along the spiral galactic arm we are in. Our second paper argues that nearly all SETI searches to date had little chance of seeing such Beacons.

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