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Return-path: <usenet-space-news-request@arc.nasa.gov> Received: from mailhub.arc.nasa.gov by delphi.com (PMDF V4.2-11 #4520) id <01H6TTMIDJ4094JHU4@delphi.com>; Thu, 23 Dec 1993 18:04:19 EDT Received: from news.arc.nasa.gov by mailhub.arc.nasa.gov with SMTP (PP); Thu, 23 Dec 1993 14:55:51 -0800 Received: by news.arc.nasa.gov id AA15654 (5.65c/IDA-1.4.4 for usenet-space-news@arc.nasa.gov); Thu, 23 Dec 1993 14:00:07 -0800 Date: Thu, 23 Dec 1993 21:34:31 +0000 (GMT) From: klaes@verga.dnet.dec.com (Larry Klaes) Subject: Electronic Journal of the ASA (EJASA) - December 1993 Sender: digester@news.arc.nasa.gov To: sci-space-news@Pa.dec.com Message-id: <2fd2hr$a65@nntpd.lkg.dec.com> Organization: Digital Equipment Corporation Content-transfer-encoding: 7BIT Newsgroups: sci.space.news Path: ames!dont-send-mail-to-path-lines Followup-To: sci.space Nntp-Posting-Host: verga.enet.dec.com Approved: sci-space-news@ames.arc.nasa.gov Lines: 774 Keywords: EJASA, SETI THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC Volume 5, Number 5 - December 1993 ########################### TABLE OF CONTENTS ########################### * ASA Membership and Article Submission Information * Detectability of Extraterrestrial Technological Activities, Part 1 - Guillermo A. Lemarchand ########################### ASA MEMBERSHIP INFORMATION The Electronic Journal of the Astronomical Society of the Atlantic (EJASA) is published monthly by the Astronomical Society of the Atlantic, Incorporated. The ASA is a non-profit organization dedicated to the advancement of amateur and professional astronomy and space exploration, as well as the social and educational needs of its members. ASA membership application is open to all with an interest in astronomy and space exploration. Members receive the Journal of the ASA (the JASA is a hardcopy sent through United States Mail and is not a duplicate of this Electronic Journal) and the Astronomical League's REFLECTOR magazine. Members may also purchase discount subscriptions to ASTRONOMY and SKY & TELESCOPE magazines. For information on membership, you may contact the Society at any of the following addresses: Astronomical Society of the Atlantic (ASA) P. O. Box 15038 Atlanta, Georgia 30333-9998 U.S.A. asa@chara.gsu.edu (For ASA issues) klaes@verga.enet.dec.com (For EJASA issues) ASA BBS: (404) 321-5904, 300/1200/2400 Baud or telephone the Society Recording at (404) 264-0451 to leave your address and/or receive the latest Society news. ASA Officers and Council - President - Eric Greene Vice President - Jeff Elledge Secretary - Ingrid Siegert-Tanghe Treasurer - Mike Burkhead Directors - Becky Long, Tano Scigliano, Bob Vickers Council - Bill Bagnuolo, Michele Bagnuolo, Don Barry, Bill Black, Mike Burkhead, Jeff Elledge, Frank Guyton, Larry Klaes, Ken Poshedly, Jim Rouse, Tano Scigliano, John Stauter, Wess Stuckey, Harry Taylor, Gary Thompson, Cindy Weaver, Bob Vickers ARTICLE SUBMISSIONS Article submissions to the EJASA on astronomy and space exploration are most welcome. Please send your on-line articles in ASCII format to Larry Klaes, EJASA Editor, at the following net addresses or the above Society addresses: klaes@verga.enet.dec.com or - ...!decwrl!verga.enet.dec.com!klaes or - klaes%verga.dec@decwrl.enet.dec.com or - klaes%verga.enet.dec.com@uunet.uu.net You may also use the above addresses for EJASA back issue requests, letters to the editor, and ASA membership information. When sending your article submissions, please be certain to include either a network or regular mail address where you can be reached, a telephone number, and a brief biographical sketch. Back issues of the EJASA are also available from the ASA anonymous FTP site at chara.gsu.edu (131.96.5.29). Directory: /ejasa DISCLAIMER Submissions are welcome for consideration. Articles submitted, unless otherwise stated, become the property of the Astronomical Society of the Atlantic, Incorporated. Though the articles will not be used for profit, they are subject to editing, abridgment, and other changes. Copying or reprinting of the EJASA, in part or in whole, is encouraged, provided clear attribution is made to the Astronomical Society of the Atlantic, the Electronic Journal, and the author(s). Opinions expressed in the EJASA are those of the authors' and not necessarily those of the ASA. No responsibility is assumed by the ASA or the EJASA for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use of operation of any methods, products, instructions, or ideas contained in the material herein. This Journal is Copyright (c) 1993 by the Astronomical Society of the Atlantic, Incorporated. DETECTABILITY OF EXTRATERRESTRIAL TECHNOLOGICAL ACTIVITIES Guillermo A. Lemarchand [1] Center for Radiophysics and Space Research Cornell University, Ithaca, New York, 14853 ================================================================== 1 - Visiting Fellow under ICSC World Laboratory scholarship. Present address: University of Buenos Aires, C.C.8-Suc.25, 1425- Buenos Aires, Argentina ================================================================== ================================================================== This paper was originally presented at the Second United Nations/European Space Agency Workshop on Basic Space Science Co-organized by The Planetary Society in cooperation with the Governments of Costa Rica and Colombia, 2-13 November 1992, San Jose, Costa Rica - Bogota, Colombia ================================================================== Introduction If we want to find evidence for the existence of extraterrestrial civilizations (ETC), we must work out an observational strategy for detecting this evidence in order to establish the various physical quantities in which it involves. This information must be carefully analyzed so that it is neither over-interpreted nor overlooked and can be checked by independent researchers. The physical laws that govern the Universe are the same everywhere, so we can use our knowledge of these laws to search for evidence that would finally lead us to an ETC. In general, the experimentalist studies a system by imposing constraints and observing the system's response to a controlled stimulus. The variety of these constraints and stimuli may be extended at will, and experiments can become arbitrarily complex. In the problem of the Search for Extraterrestrial Intelligence (SETI), as well as in conventional astronomy, the mean distances are so huge that the "researcher" can only observe what is received. He or she is entirely dependent on the carriers of information that transmit to him or her all he or she may learn about the Universe. Information carriers, however, are not infinite in variety. All information we currently have about the Universe beyond our solar system has been transmitted to us by means of electromagnetic radiation (radio, infrared, optical, ultraviolet, X-rays, and gamma rays), cosmic ray particles (electrons and atomic nuclei), and more recently by neutrinos. There is another possible physical carrier, gravitational waves, but they are extremely difficult to detect. For the long future of humanity, there have also been specula- tions about interstellar automatic probes that could be sent for the detection of extrasolar life forms around the nearby stars. Another set of possibilities could be the detection of extraterrestrial artifacts in our solar system, left here by alien intelligences that want to reveal their visits to us. Table 1 summarizes the possible "information carriers" that may let us find the evidence of an extraterrestrial civilization, according to our knowledge of the laws of physics. The classification of techniques in Table 1 is not intended to be complete in all respects. Thus, only a few fundamental particles have been listed. No attempt has been made to include any antiparticles. This classification, like any such scheme, is also quite arbitrary. Groupings could be made into different "astronomies". TABLE 1: Information Carriers |- | Radio Waves | Infrared Rays |- | Optical Rays | Photon Astronomy| Ultraviolet Rays | | X-Rays Boson | | Gamma Rays Astronomy | |- | Graviton Astronomy: Gravity Waves |- |- | Neutrinos |- |- Fermions| Electrons |- | Atomic | | Protons | Cosmic | Microscopic| |- | Rays | Particles | Heavy Particles |- Particle | |- Astronomy | |- | Macroscopic Particles| Meteors, meteorites, | or objects | meteoritic dust |- |- |- | Space Probes Direct | Manned Exploration Techniques | ET Astroengineering Activities in the Solar System |- The methods of collecting this information as it arrives at the planet Earth make it immediately obvious that it is impossible to gather all of it and measure all its components. Each observation technique acts as an information filter. Only a fraction (usually small) of the complete information can be gathered. The diversity of these filters is considerable. They strongly depend on the available technology at the time. In this paper a review of the advantages and disadvantages of each "physical carrier" is examined, including the case that the possible ETCs are using them for interstellar communication purposes, as well as the possibility of detection activities of extraterrestrial technologies. Classification of Extraterrestrial Civilizations The analysis of the use of each information carrier are deeply connected with the assumption of the level of technology of the other civilization. Kardashev (1964) established a general criteria regarding the types of activities of extraterrestrial civilizations which can be detected at the present level of development. The most general parameters of these activities are apparently ultra-powerful energy sources, harnessing of enormous solid masses, and the transmission of large quantities of information of different kinds through space. According to Kardashev, the first two parameters are a prerequisite for any activity of a supercivilization. In this way, he suggested the following classification of energetically extravagant civilizations: TYPE I: A level "near" contemporary terrestrial civilization with an energy capability equivalent to the solar insolation on Earth, between 10exp16 and 10exp17 Watts. TYPE II: A civilization capable of utilizing and channeling the entire radiation output of its star. The energy utilization would then be comparable to the luminosity of our Sun, about 4x1026 Watts. TYPE III: A civilization with access to the power comparable to the luminosity of the entire Milky Way galaxy, about 4x10exp37 Watts. Kardashev also examined the possibilities in cosmic communica- tion which attend the investment of most of the available power into communication. A Type II civilization could transmit the contents of one hundred thousand average-sized books across the galaxy, a distance of one hundred thousand light years, in a total transmitting time of one hundred seconds. The transmission of the same information intended for a target ten million light years distant, a typical intergalactic distance, would take a transmission time of a few weeks. A Type III civilization could transmit the same information over a distance of ten billion light years, approximately the radius of the observable Universe, with a transmission time of just three seconds. Kardashev and Zhuravlev (1992) considered that the highest level of development corresponds to the highest level of utilization of solid space structures and the highest level of energy consumption. For this assumption, they considered the temperature of solid space structures in the range 3 Kelvin s T s 300 K, the consumption of energy in the range 1 Luminosity (Sun) s L s 10exp12 L(Sun), structures with sizes up to 100 kiloparsecs (kpc), and distances up to Dw 1000 mega- parsecs (mpc). One parsec equals 3.26 light years. Searching for these structures is the domain of millimeter wave astronomy. For the 300 Kelvin technology, the maximum emission occurs in the infrared region (15-20 micrometers) and searching is accomplished with infrared observations from Earth and space. The existing radio surveys of the sky (lambda = 6 centimeters (cm) on the ground and lambda = 3 millimeters (mm) for the Cosmic Background Explorer (COBE) satellite) place an essential limit on the abundance of ETC 3 Kelvin technology. The analyzes of the Infrared Astronomical Satellite (IRAS) catalog of infrared sources sets limitations on the abundance of 300 Kelvin technology. Information Carriers and the Manifestations of Advanced Technological Civilizations Boson and Photon Astronomy Electromagnetic radiation carries virtually all the information on which modern astrophysics is built. The production of electromagnetic radiation is directly related to the physical conditions prevailing in the emitter. The propagation of the information carried by electromagnetic waves (photons) is affected by the conditions along its path. The trajectories it follows depend on the local curvature of the Universe, and thus on the local distribution of matter (gravitational lenses), extinction affecting different wavelengths unequally, neutral hydrogen absorbing all radiation below the Lyman limit (91.3 mm), and absorption and scattering by interstellar dust, which is more severe at short wavelengths. Interstellar plasma absorbs radio wavelengths of kilometers and above, while the scintillations caused by them become a very important effect for the case of ETC radio messages (Cordes and Lazio, 1991). The inverse Compton effect lifts low-energy photons to high energies in collisions with relativistic electrons, while gamma and X-ray photons lose energy by the direct Compton effect. The radiation reaching the observer thus bears the imprint of both the source and the accidents of its passage though space. The Universe observable with electromagnetic radiation is five- dimensional. Within this phase, four dimensions - frequency coverage plus spatial, spectral, and temporal resolutions - should properly be measured logarithmically with each unit corresponding to one decade (Tarter, 1984). The fifth dimension is polarization, which has four possible states: Circular, linear, elliptical, and unpolarized. This increases the volume of logarithmic phase space fourfold. It is useful to attempt to estimate the volume of the search space which may need to be explored to detect an ETC signal. For the case of electromagnetic waves, we have a "Cosmic Haystack" with an eight- dimensional phase space. Three spatial dimensions (coordinates of the source), one dimension for the frequency of emission, two dimensions for the polarization, one temporal dimension to synchronize trans- missions with receptions, and one dimension for the sensitivity of the receiver or the transmission power. If we consider only the microwave region of the spectrum (300 megahertz (MHz) to 300 gigahertz (GHz)), it is easy to show that this Cosmic Haystack has roughly 10exp29 cells, each of 0.1 Hz bandwidth, per the number of directions in the sky in which an Arecibo (305- meter) radio telescope would need to be pointed to conduct an all-sky survey, per a sensitivity between 10exp(-20) and 10exp(-30) [W m-2], per two polarizations. The temporal dimension (synchronization between transmission and reception) was not considered in the calculation. The number of cells increase dramatically if we expand our search to other regions of the electromagnetic spectrum. Until now, only a small fraction of the whole Haystack has been explored (w 10exp(-15) - 10exp(-16)). TABLE 2: Characteristics of the Electromagnetic Spectrum (All the numbers that follows each 10 are exponents.) ================================================================== Spectrum Frequency Wavelength Minimum Energy Region Region [Hz] Region [m] per photon [eV] ================================================================== Radio 3x106-3x1010 100-0.01 10-8 - 10-6 Millimeter 3x1010-3x1012 0.01-10-4 10-6 - 10-4 Infrared 3x1012-3x1014 10-4-10-6 10-4 - 10-2 Optical 3x1014-1015 10-6-3x10-7 10-2 - 5 Ultraviolet 1015-3x1016 3x10-7-10-8 5 - 102 X-rays 3x1016-3x1019 10-8-10-11 102 - 105 Gamma-rays r3x1019 s10-11 r105 ================================================================== Radio Waves In the last thirty years, most of the SETI projects have been developed in the radio region of the electromagnetic spectrum. A complete description of the techniques that all the present and near-future SETI programs are using for detecting extraterrestrial intelligence radio beacons can be found elsewhere (e.g., Horowitz and Sagan, 1993). The general hypothesis for this kind of search is that there are several civilizations in the galaxy that are transmitting omnidirectional radio signals (civilization Type II), or that these civilizations are beaming these kind of messages to Earth. In this section we will discuss only the detectability of extraterrestrial technological manifestations in the radio spectrum. Domestic Radio Signals Sullivan et al (1978) and Sullivan (1981) considered the possibility of eavesdropping on radio emissions inadvertently "leaking" from other technical civilizations. To better understand the information which might be derived from radio leakage, the case of our planet Earth was analyzed. As an example, they showed that the United States Naval Space Surveillance System (Breetz, 1968) has an effective radiated power of 1.4x10exp (10) watts into a bandwidth of only 0.1 Hz. Its beam is such that any eavesdropper in the declination range of zero to 33 degrees (28 percent of the sky) will be illuminated daily for a period of roughly seven seconds. This radar has a detecta- bility range of leaking terrestrial signals to sixty light years for an Arecibo-type (305-meter) antenna at the receiving end, or six hundred light years for a Cyclops array (one thousand dishes of 100- meter size each). Recently Billingham and Tarter (1992) estimated the maximum range at which radar signals from Earth could be detected by a search similar to the NASA High Resolution Microwave Survey (HRMS) assumed to be operating somewhere in the Milky Way galaxy. They examined the trans- mission of the planetary radar of Arecibo and the ballistic missile early warning systems (BMEWS). For the calculation of maximum range R, the standard range equation is: R=(EIRP/(4PI PHImin))exp(1/2) Where PHImin is the sensitivity of the search system in [W m-2]. For the NASA HRMS Target Search PHImin = 10exp (-27) and for the NASA HRMS Sky Survey PHImin w 10exp(-23) (f)exp(1/2), where f is the frequency in GHz. Table 3 shows the distances where the Arecibo and BMEWS transmissions could be detected by a similar NASA HRMS spectrometer. TABLE 3: HRMS Sensitivity for Earth's Most Powerful Transmissions: ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ARECIBO PLANETARY RADAR (1) TARGETED SEARCH MAXIMUM RANGE (light years) Unswitched With CW detector 4217 With pulse detector 2371 Switched With CW detector 94 With pulse detector 290 (2) SKY SURVEY Unswitched CW detector 77 Switched CW detector 9 BMEWS (1) TARGETED SEARCH Pulse transmit CW detector 6 Pulse transmit pulse detector 19 (2) SKY SURVEY Pulse transmit CW detector 0.7 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ All these calculations assumed that the transmitting civilization is at the same level of technological evolution as ours on Earth. Von Hoerner (1961) classified the possible nature of the ETC signals into three general possibilities: Local communication on the other planet, interstellar communication with certain distinct partners, and a desire to attract the attention of unknown future partners. Thus he named them as local broadcast, long-distance calls, and contacting signals (beacons). In most of the past fifty SETI radio projects, the strategy was with the hypothesis that there are several civilizations transmitting omnidirectional beacon signals. Unfortunately, no one has been able to show any positive evidence of this kind of beacon signal. Another possibility is the radio detection of interstellar communi- cations between an ETC planet and possible space vehicles. Vallee and Simard-Normandin (1985) carried out a search for these kind of signals near the galactic center. Because one of the characteristics of arti- ficial transmitters (television, radar, etc.) is the highly polarized signal (Sullivan et al, 1978), these researchers made seven observing runs of roughly three days each in a program to scan for strongly polarized radio signals at the wavelength of lambda=2.82 cm. Radar Warning Signals Assuming that there is a certain number N of civilizations in the galaxy at or beyond our own level of technical facility, and considering that each civilization is on or near a planet of a Main Sequence star where the planetoid and comet impact hazards are considered as serious as here, Lemarchand and Sagan (1993) considered the possibility for detecting some of these "intelligent activities" developed to warn of these potentially dangerous impacts. Because line-of-sight radar astrometric measurements have much finer intrinsic fractional precision than their optical plane-of-sight counterparts, they are potentially valuable for refining the knowledge of planetoid and comet orbits. Radar is an essential astrometric tool, yielding both a direct range to a nearby object and the radial velocity (with respect to the observer) from the Doppler shifted echo (Yeomans et al, 1987, Ostro et al, 1991, and Yeomans et al, 1992). Since in our solar system, most of Earth's nearby planetoids are discovered as a result of their rapid motion across the sky, radar observations are therefore often immediately possible and appropriate. A single radar detection yields astronomy with a fractional precision that is several hundred times better than that of optical astrometry. The inclusion of radar with the optical data in the orbit solution can quickly and dramatically reduce future ephemeris uncertainty. It provides both impact parameter and impact ellipse estimates. This kind of radar research gives a clearer picture of the object to be intercepted and the orientation of asymmetric bodies prior to interception. This is particularly important for eccentric or multiple objects. Radar is also the unique tool capable for making a survey of such small objects at all angles with respect to the central star. It can also measure reflectivity and polarization to obtain physical characteristics and composition. For this case, we can assume that each of the extraterrestrial civilizations in the galaxy maintains as good a radar planetoid and/or comet detection and analysis facility as is needed, either on the surface of their planet, in orbit, or on one of their possible moons. The threshold for the Equivalent Isotropic Radiated Power (EIRP) of the radar signal could be roughly estimated by the size of the object (D) that they want to detect (according to the impact hazard) and the distance to the inhabited planet (R), in order to have enough time to avoid the collision. One of the most important issues for the success of SETI observations on Earth is the ability of an observer to detect an ETC signal. This factor is proportional to the received spectral flux density of the radiation. That is, the power per unit area per unit frequency interval. The flux density will be proportional to the EIRP divided by the spectral bandwidth of the transmitting radar signals B. The EIRP is defined as the product of the transmitted power and directive antenna gain in the direction of the receiver as EIRP = PT.G, where PT is the transmitting power and G the antenna gain. This quantity has units of [W/Hz]. According to the kind of object that the ETC wants to detect (nearby planetoids, comets, spacecraft, etc.), the distance from the radar system and the selected wavelength, a galactic civilization that wants to finish a full-sky survey in only one year, will arise from a modest "Type 0" (w10exp13 W/Hz, Rw0.4 A.U., Dw5000 m, and lambdaw1 m) to the transition from "Type I" to "Type II" (w2x10exp24 W/Hz, Rw0.4 A.U., Dw10 m, lambdaw1 mm). Lemarchand and Sagan (1993) also presented a detailed description of the expected signal characteristics, as well as the most favorable positions in the sky to find one of these signals. They also have compared the capability of detection of these transmissions by each present and near future SETI projects. Infrared Waves There have been some proposals to search in the infrared region for beacon signals beamed at us (Lawton, 1971, and Townes, 1983). Basically, the higher gain available from antennas at shorter wavelengths (up to 10exp14 Hz) compensates for the higher quantum noise in the receiver and wider noise bandwidth at higher frequencies. One concludes that for the same transmitter powers and directed transmission which takes advantage of the high gain, the detectable signal-to-noise ratio is comparable at 10 micro-m and 21 cm. Since non-thermal carbon dioxide (CO2) emissions have been detected in the atmospheres of both Venus and Mars (Demming and Mumma, 1983), Rather (1991) suggested the possibility that an advanced society could construct transmitters of enormous power by orbiting large mirrors to create a high-gain maser from the natural amplification provided by the inverted atmospheric lines. An observation program around three hundred nearby solar-type stars has just begun (Tarter, 1992) by Albert Betz (University of Colorado) and Charles Townes (University of California at Berkeley). These observations are currently being made on one of the two 1.7- meter elements of an IR interferometer at Mount Wilson observatory. On average, 21 hours of observing time per month is available for searching for evidence of technological signals. Dyson (1959, 1966) proposed the search for huge artificial biospheres created around a star by an intelligent species as part of its technological growth and expansion within a planetary system. This giant structure would most likely be formed by a swarm of artificial habitats and mini-planets capable of intercepting essentially all the radiant energy from the parent star. According to Dyson (1966), the mass of a planet like Jupiter could be used to construct an immense shell which could surround the central star, having a radius of one Astronomical Unit (A.U.). The volume of such a sphere would be 4cr2S, where r is the radius of the sphere (1 A.U.) and S the thickness. He imagined a shell or layer of rigidly built objects Dw10exp6 kilometers in diameter arranged to move in orbits around the star. The minimum number of objects required to form a complete spherical shell [2] is about N=4 PIrexp2/Dexp2w2x10exp5 objects. This kind of object, known as a "Dyson Sphere", would be a very powerful source of infrared radiation. Dyson predicted the peak of the radiation at ten micrometers. The Dyson Sphere is certainly a grand, far-reaching concept. There have been some investigations to find them in the IRAS database (V. I. Slysh, 1984; Jugaku and Nishimura, 1991; and Kardashev and Zhuravlev, 1992). ================================================================== 2 - The concept of this extraterrestrial construct was first described in the science fiction novel STAR MAKER by Olaf Stapledon in 1937. ================================================================== Optical Waves In the radio domain, there have been several proposals to use the visible region of the spectrum for interstellar communications. Since the first proposal by Schwartz and Townes (1961), intensive research has been performed on the possible use of lasers for interstellar communication. Ross (1979) examined the great advantages of using short pulses in the nanosecond regime at high energy per pulse at very low duty cycle. This proposal was experimentally explored by Shvartsman (1987) and Beskin (1993), using a Multichannel Analyzer of Nanosecond Intensity Alterations (MANIA), from the six-meter telescope in Russia. This equipment allows photon arrival times to be determined with an accuracy of 5x10exp(-8) seconds, the dead time being 3x10exp(-7) seconds and the maximum intensity of the incoming photon flux is 2x10exp4 counts/seconds. In 1993, MANIA was used from the 2.15-meter telescope of the Complejo Astronomico El Leoncito in Argentina, to examine fifty nearby solar-type stars for the presence of laser pulses (Lemarchand et al, 1993). Other interesting proposals and analysis of the advantages of lasers for interstellar communications have been performed by Betz (1986), Kingsley (1992), Ross (1980), and Rather (1991). The first international SETI in the Optical Spectrum (OSETI) Conference was organized by Stuart Kingsley, under the sponsorship of The International Society for Optical Engineering, at Los Angeles, California, in January of 1993. There have also been independent suggestions by Drake and Shklovskii (Sagan and Shklovskii, 1966) that the presence of a technical civilization could be announced by the dumping of a short-lived isotope, one which would not ordinarily be expected in the local stellar spectrum, into the atmosphere of a star. Drake suggested an atom with a strong, resonant absorption line, which may scatter about 10exp8 photons sec -1 in the stellar radiation field. A photon at optical frequencies has an energy of about 10exp(-12) erg or 0.6 eV, so each atom will scatter about 10exp(-4) erg sec-1 in the resonance line. If we consider that the typical spectral line width might be about 1 , and if we assume that a ten percent absorption will be detectable, then this "artificial smog" will scatter about (1A/5000A)x10exp(-1) = 2x10exp(-5) of the total stellar flux. Sagan and Shklovskii (1966) considered that if the central star has a typical solar flux of 4x10exp33 erg sec-1, it must scatter about 8x10exp28 erg sec-1 for the line to be detected. Thus, the ETC would need (8x10exp28)/10exp(-4) = 8x10exp32 atoms. The weight of the hydrogen atom (mH) is 1.66x10exp(-24) g, so the weight of an atom of atomic weight n is nxmH grams. Drake proposed the used of Technetium (Tc) for this purpose. This element is not found on Earth and its presence is observed very weakly in the Sun, in part because it is short-lived. Tc's most stable form decays radioactively within an average of twenty thousand years. Thus, for the case of Tc, we need to distribute some 1.3x10exp11 grams, or 1.3x10exp5 tons, of this element into the stellar spectrum. However, technetium lines have not been found in stars of solar spectral type, but rather only in peculiar ones known as S stars. We must know more than we do about both normal and peculiar stellar spectra before we can reasonably conclude that the presence of an unusual atom in an stellar spectrum is a sign of extraterrestrial intelligence. Whitmire and Wright (1980) considered the possible observational consequences of galactic civilizations which utilize their local star as a repository for radioactive fissile waste material. If a rela- tively small fraction of the nuclear sources present in the crust of a terrestrial-type planet were processed via breeder reactors, the resulting stellar spectrum would be selectively modified over geolo- gical time periods, provided that the star has a sufficiently shallow outer convective zone. They have estimated that the abundance anoma- lies resulting from the slow neutron fission of plutonium-239 and uranium-233 could be duplicated (compared with the natural nucleosyn- thesis processes), if this process takes place. Since there are no known natural nucleosynthesis mechanisms that can qualitatively duplicate the asymptotic fission abundances, the predicted observational characteristics (if observed) could not easily be interpreted as a natural phenomenon. They have suggested making a survey of A5-F2 stars for (1) an anomalous overabundance of the elements of praseodymium and neodymium, (2) the presence, at any level, of technetium or plutonium, and (3) an anomalously high ratio of barium to zirconium. Of course, if a candidate star is identified, a more detailed spectral analysis could be performed and compared with the predicted ratios. Following the same kind of ideas, Philip Morrison discussed (Sullivan, 1964) converting one's sun into a signaling light by placing a cloud of particles in orbit around it. The cloud would cut enough light to make the sun appear to be flashing when seen from a distance, so long as the viewer was close to the plane of the cloud orbit. Particles about one micron in size, he thought, would be comparatively resistant to disruption. The mass of the cloud would be comparable to that of a comet covering an area of the sky five degrees wide, as seen from the sun. Every few months, the cloud would be shifted to constitute a slow form of signaling, the changes perhaps designed to represent algebraic equations. Reeves (1985) speculated on the origin of mysterious stars called blue stragglers. This class of star was first identified by Sandage (1952). Since that time, no clear consensus upon their origins has emerged. This is not, however, due to a paucity of theoretical models being devised. Indeed, a wealth of explanations have been presented to explain the origins of this star class. The essential character- istic of the blue stragglers is that they lie on, or near, the Main Sequence, but at surface temperatures and luminosities higher than those stars which define the cluster turnoff. Reeves (1985) suggested the intervention of the inhabitants that depend on these stars for light and heat. According to Reeves, these inhabitants could have found a way of keeping the stellar cores well- mixed with hydrogen, thus delaying the Main Sequence turn-off and the ultimately destructive, red giant phase. Beech (1990) made a more detailed analysis of Reeves' hypothesis and suggested an interesting list of mechanisms for mixing envelope material into the core of the star. Some of them are as follows: o Creating a "hot spot" between the stellar core and surface through the detonation of a series of hydrogen bombs. This process may alternately be achieved by aiming "a powerful, extremely concentrated laser beam" at the stellar surface. o Enhanced stellar rotation and/or enhanced magnetic fields. Abt (1985) suggested from his studies of blue stragglers that meridional mixing in rapidly rotating stars may enhance their Main Sequence lifetime. If some of these processes can be achieved, the Main Sequence lifetime may be greatly extended by factors of ten or more. It is far too early to establish, however, whether all the blue stragglers are the result of astroengineering activities. Editor's Note: References to this paper will be published in Part 2 in the January 1994 issue of the EJASA. Related EJASA Articles - "Does Extraterrestrial Life Exist?", by Angie Feazel - November 1989 "Suggestions for an Intragalactic Information Exchange System", by Lars W. Holm - November 1989 "Radio Astronomy: A Historical Perspective", by David J. Babulski - February 1990 "Getting Started in Amateur Radio Astronomy", by Jeffrey M. Lichtman - February 1990 "A Comparison of Optical and Radio Astronomy", by David J. Babulski - June 1990 "The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum, Parts A-F", by Dr. Stuart A. Kingsley - January 1992 "History of the Ohio SETI Program", by Robert S. Dixon - June 1992 "New Ears on the Sky: The NASA SETI Microwave Observing Project", by Bob Arnold, the ARC, and JPL SETI Project - July 1992 "First International Conference on Optical SETI", by Dr. Stuart A. Kingsley - October 1992 "Conference Preview: The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum", by Dr. Stuart A. Kingsley - January 1993 The Author - ================================================================== | Guillermo A. Lemarchand | | Universidad de Buenos Aires | | | | POSTAL ADDRESS: C.C.8 -Suc.25, | | 1425-Buenos Aires, | | ARGENTINA | | | | E-MAIL: lemar@seti.edu.ar | | | | PHONE: 54-1-774-0667 FAX: 54-1-786-8114 | ================================================================== THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC December 1993 - Vol. 5, No. 5 Copyright (c) 1993 - ASA