Center for Radiophysics and Space Research Cornell University, Ithaca, New York, 14853 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
Republished in SETIQuest, Volume 1, Number 1, pp. 3-13.
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 electro-magnetic 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 speculations 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 (Lemarchand, 1992). 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".
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 is 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 civilizations by energy usage:
TYPE I: A level "near" contemporary terrestrial civilization with an energy capability equivalent to the solar insolation on Earth, between 10 to the 16 power and 10 to the 17 power 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 4 x 10 to the 26 power Watts. TYPE III: A civilization with access to the power comparable to the luminosity of the entire Milky Way galaxy, about 4 x 10 to the 37 power Watts.
Kardashev also examined the possibilities in cosmic communication 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.
Sagan (1973) considered that Kardashev's classification should be completed using decimal numbers to indicate a difference of one order of magnitude in the energy consumption. For example, a civilization Type 1.7 expends 10 to the 23 power Watts, while a civilization Type 2.3 expends 10 to the 29 power Watts. Sagan also suggested that, in order to be more accurate, a letter could indicate the societal information level (degree of their knowledge). According to Sagan, a Class A civilization will have 10 to the 6 power bits of information, a Class B, 10 to the 7 power bits, a Class C, 10 to the 8 power bits, and so on. Under this classification, our terrestrial civilization is Type 0.7H.
The level of the first extraterrestrial civilization that we can make contact with would be between 1.5J and 1.8K. A galactic civilization would be Type III Q, while a civilization with the capacity to control a federation of 10 to the 9 power galaxies would be Type IV Z. Other classification alternatives were suggested by Tang and Chang (1991).
TABLE 2: Characteristics of Extraterrestrial Civilizations TYPE I Characteristics: Planetary Society. Developed Technology. * understanding the laws of physics. * space technology. * nuclear technology. * electromagnetic communications. Initiation of spaceflight, interplanetary travel, settlement of space. Early attempts at interstellar communication. Starting to push planetary resource limits. Information Level: I. Energy Consumption: 10 to the 16 power to 10 to the 17 power Watts. Manifestations: Intentional or unintentional electromagnetic emissions,
especially radio waves.
TYPE II Characteristics: Stellar system society. Construction of space habitats. "dyson sphere" as an ultimate limit. Search for intelligent life in space. Long societal lifetimes (10 to the 3 power to 10 to the 5 power years). Initiation of interstellar travel/colonization. Ultimately all radiant energy output of native star is utilized. Information Level: L. Energy Consumption: 10 to the 26 power to 10 to the 27 power Watts. Manifestations: Electromagnetic. * radio waves. * optical lasers. * X-rays. * Gamma rays. Gravity Waves. Mass transfer. * probes. * panspermia. * stellar arks.
TYPE III Characteristics: Galactic Civilization. Interstellar communication/travel. Very long societal lifetimes (10 to the 8 power to 10 to the 9 power years). Effectively "the immortals", for planning purposes. Energy resources of the entire galaxy (10 to the 11 power to 10
to the 12 power stars) are commanded. Information Level: Q (Encyclopedia Galactica). Energy Consumption: 10 to the 37 power to 10 to the 38 power Watts. Manifestations: Feats of astroengineering. Exotic Communication. * neutrinos. * tachyons? * Waves "i"?
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 of 3 Kelvin to 300 Kelvin, the consumption of energy in the range of 1 solar luminosity to 10 to the 12 power solar luminosities, structures with sizes up to 100 kiloparsecs (kpc), and distances up to ~1000 megaparsecs (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 on the ground and lambda = 3 millimeters 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 (912 Angstrom), 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 can be characterized as a multi-dimensional phase space. Within this cosmic search space, several dimensions -- frequency coverage plus spatial, spectral, and temporal resolutions -- should properly be measured logarithmically with each unit corresponding to one decade (Tarter, 1984). Another dimension is polarization, which has four possible states: Circular, linear, elliptical, and unpolarized.
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 transmissions 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 MHz to 300 GHz), it is easy to show that this cosmic haystack has roughly 10 to the 29 power 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 10 to the -20 power and 10 to the -30 power Watts m to the -2 power, 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 (~10 to the -15 power to 10 to the -16 power).
RADIO WAVELENGTH RADIATION
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
TABLE 3: Characteristics of the Electromagnetic Spectrum Spectrum Frequency Wavelength Minimum Energy Region Region [Hz] Region [m] per photon [eV] _____________________________________________________________ Radio 3x10(6-10) 100-0.01 10(-8) to 10(-6) Millimeter 3x10(10-12) 0.01-10(-4) 10(-6) to 10(-4) Infrared 3x10(12-14) 10(-4)-10(-6) 10(-4) to 10(-2) Optical* 3x10(14-15) 10(-6)-3x10(-7) 10(-2) to 5 Ultraviolet 10(15)-3x10(16) 3x10(-7)-10(-8) 5 to 10(2) X-rays 3x10(16-19) 10(-8)-10(-11) 10(2-5) Gamma rays > 3x10(19) <10(-11)> 10(5) * Visible - SAK definition
System (Breetz, 1968) has an effective radiated power of 1.4x10 to the 10 power 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 detectability 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 full-up 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) (now the SETI Institute's "Project Phoenix") assumed to be operating somewhere in the Milky Way galaxy. They examined the transmission 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/(4 pi phi min))1/2
Where phi(min) is the sensitivity of the search system in [W m-2]. For the former NASA HRMS Target Search from Arecibo, phi(min) = 10 to the -27 power and the NASA HRMS Sky Survey phi(min) = ~10 to the -23 power (f)1/2, where f is the frequency in GHz. Table 4 shows the distances where the Arecibo and BMEWS transmissions could be detected by a similar NASA HRMS spectrometer.
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 sixty SETI radio projects, the strategy was planned 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 communications 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 artificial 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 centimeters.
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 the "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 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, Yeomans et al., 1992, and Ostro, 1993).
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 (expressed in units of Hertz).
The EIRP is defined as the product of the transmitted power and directive antenna gain in the direction of the receiver as EIRP = P(T).G, where P(T) is the transmitting power and G the antenna gain. This quantity has units of [W].
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" (flux density ~10 to the 13 power W/Hz, R ~0.4 A.U., D ~5000 m, and lambda ~1 m) to the transition from "Type I" to "Type II" (flux density ~2x10 to the 24 power W/Hz, R ~0.4 A.U., D ~10 m, lambda ~1 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.
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 10 to the 14 power 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 mu and 21 centimeters. 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 principal investigators 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 4 pi r(squared) S, 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 D ~10 to the 6 power kilometers in diameter arranged to move in orbits around the star. The minimum number of objects required to form a complete spherical shell is about N = 4 pi r(squared)/D(squared)~10 to the 5 power 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.
FIGURE 1: In 1959, Freeman Dyson suggested that very advanced civilizations, bound only by the presently known laws of physics, may surround their parent star with spherical shells made from dismantled planets. A representation of this idea applied to our solar system, using the mass of Jupiter to form a sphere at one astronomical unit (AU) from the Sun, is shown in this figure. The Dyson Sphere has a radius of 1.5x10 to the 8 power kilometers and is 3 meters thick.
The Dyson Sphere is certainly a grand, far-reaching concept. There have been some investigations that tried to find them in the IRAS database (V. I. Slysh, 1985; Jugaku and Nishimura, 1991; and Kardashev and Zhuravlev, 1992).
OPTICAL (VISIBLE) RADIATION
In the optical (visible) domain, there have been several proposals to use the visible region of the spectrum for interstellar communications.
2 The concept of this extraterrestrial construction was first described in the science fiction novel STAR MAKER by Olaf Stapledon in 1937.
TABLE 4: HRMS Sensitivity for Earth's Most Powerful Transmissions (Billingham and Tarter, 1992) 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 With CW detector 77
Switched With 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
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 et al. (1993), using a Multi-channel 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 5x10 to the -8 power seconds, the dead time being 3x10 to the -7 power seconds and the maximum intensity of the incoming photon flux is 2x10 to the 4 power counts/seconds.
Other interesting proposals and analysis of the advantages of lasers for interstellar communications have been performed by Betz (1986), Kingsley (1993), 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 10 to the 8 power photons sec to the -1 power in the stellar radiation field. A photon at optical frequencies has an energy of about 10 to the -12 power erg or 0.6 eV, so each atom will scatter about 10 to the -4 power erg sec to the -1 power in the resonance line. If we consider that the typical spectral line width might be about 1 Angstrom and if we assume that a ten percent absorption will be detectable, then this "artificial smog" will scatter about (1 Angstrom/5000 Angstrom)x10 to the -1 power = 2x10 to the -5 power of the total stellar flux.
Sagan and Shklovskii (1966) considered that if the central star has a typical solar flux of 4x10 to the 33 power erg sec to the -1 power, it must scatter about 8x10 to the 28 power erg sec to the -1 power for the line to be detected. Thus, the ETC would need (8x10 to the 28 power)/10 to the -4 power = 8x10 to the 32 power atoms. The mass of the hydrogen atom (mH) is 1.66x10 to the -24 power g, so the mass of an atom of atomic weight (mu) is approximately mu.mH 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 1.3x10 to the 11 power grams, or 1.3x10 to the 5 power tons, of this element into the stellar photosphere. 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 relatively 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 geological time periods, provided that the star has a sufficiently shallow outer convective zone. They have estimated that the abundance anomalies resulting from the slow neutron fission of plutonium-239 and uranium-233 could be duplicated (compared with the natural nucleosynthesis processes), if this process takes place.
Since there are no known natural nucleosynthesis mechanisms that can qualitatively duplicate the asymtotic 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.
FIGURE 2: Concept of an "artificial" blue straggler star according to Reeves (1985). In this figure, a series of hydrogen bombs or powerful laser beams are aimed at the surface of a star, creating a "hot point" and rejuvenating the unused hydrogen, thus keeping the star on the Main Sequence for a longer period of time than would be natural.
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 characteristic 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. A review of current thinking about these stars in the light of recent visible and ultraviolet Hubble Space Telescope observations assigns an explanation to stellar mergers occurring in the dense stellar environment of globular clusters (Bailyn, 1994).
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:
* 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.
* 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.
Our planet Earth's atmosphere is opaque to radiation in the ultraviolet waveband due to ozone and molecular absorption. As a result, astronomy in these wavelengths has to be carried out from above the atmosphere, preferably with artificial satellites.
The band divides rather naturally into two regions. The region 300 > lambda > 120 nanometers can be studied using techniques similar to those used in optical astronomy. At shorter wavelengths, however, it is difficult to find materials which reflect radiation at normal incidence. Rather, the incident radiation is simply absorbed by the mirror material with little or no reflection.
Perhaps the main problem for using this waveband as a detector for extraterrestrial intelligence is that at wavelengths shorter than 912 Angstrom, the Lyman limit for hydrogen, interstellar gas becomes opaque, probably due to photoelectric absorption by neutral hydrogen in the Lyman continuum. This effect is particularly important for objects localized inside our Milky Way galaxy.
In spite of all these limitations, in 1974, H. Wischnia used the facilities of the Orbiting Astronomical Satellite (OAO, also called COPERNICUS) to search for ultraviolet laser pulses around the nearby stars Epsilon Eridani, Tau Ceti, and Epsilon Indi (Tarter, 1991). The observations were carried out in ultraviolet wavelengths below 0.28 micrometers.
Of particular interest in this waveband are the Lyman series of absorption lines with wavelengths of 1215 Angstrom, 1025 Angstrom, 972 Angstrom, 949 Angstrom, and 937 Angstrom, corresponding to the various fully excited states of the hydrogen atom. The predominant line is the so-called "Lyman Alpha" of 1215 Angstrom. It is quite possible that an extraterrestrial civilization could choose to generate a signal which may lie close to, or preferably right within, the absorption region. The background radiation of our Sun (and presumably other solar-type stars) is down by a factor of 10 to the 6 power at Lyman Alpha wavelengths when compared with the output of visible light. Such a reduction offers a considerable improvement in the signal-to-noise ratio.
Elliot (1973) pointed out that X-rays are not appropriate as a means of transmitting a continuous stream of information because of their high quantum noise. As a means of sending and receiving the "first beacon signal", however, he considered that X-rays could have certain advantages. Elliot analyzed the X-ray emissions of the terrestrial nuclear explosions carried out in the early 1960s. When a nuclear weapon explodes, about seventy percent of the energy released is in the form of kilovolt X-rays. This X-ray pulse is formed in less than one microsecond. If the explosion occurs above eighty kilometers, the X-rays are not absorbed by the atmosphere and are free to propagate into space.
Other advantages of using the X-ray pulse generated by a high-altitude nuclear explosion are: (a) The pulse is short and will not be broadened or appreciably attenuated by propagation through the interstellar medium; (b) there are no stringent frequency requirements on the receiver, since the pulse covers a broad X-ray spectrum; and (c) the X-ray flux involved is much larger than that of a solar-type star, a natural source of X-rays.
Elliot estimated the distance at which the United States "Starfish" nuclear test could be detected by our present technology of X-ray detectors. Assuming that the energy of the explosion is equivalent to 1.4 megatons and that the X-ray pulse was equally intense in all directions, he found that this explosion should be detected from a distance of ~400 Astronomical Units, about ten times the radius of Pluto's solar orbit.
Supposing that all the terrestrial nuclear powers  pooled their nuclear weapons stockpiles to produce a single explosion in space (E~2x10 to the 4 power megatons). Considering that the X-ray pulse could be concentrated into a conical beam of about thirty degrees in angle with no loss of radiation, a typical terrestrial X-ray detector should be able to detect a signal from a distance of ~190 light years.
3 - In 1989 the United States and the Soviet Union had almost 55,000 nuclear warheads with a combined destructive power of 15,500 megatons (Source: Bulletin of Atomic Scientist, 1990).
PHOTO 1: The neutrino telescope of Raymond Davis, Jr. (University of Pennsylvania), deep inside the Homestake Mine, South Dakota. The large vessel contains 100,00 gallons of a chlorine compound. Once in a great while, a neutrino from the Sun hits a chlorine nucleus and converts it to argon, a gas detectable by its radioactivity.
Fabian (1977) suggested that a supercivilization (in the transition from Kardashev's Type I to Type II) could -- in the same sense of the proposals by Drake and Shklovskii -- drop material onto a neutron star. This material could, from gravitational acceleration, reach the surface at about one-third of the velocity of light. About ten percent of the rest mass energy of the matter involved in such an impact will emerge as radiation, predominantly as X-rays if the mass flow rate exceeds ~10 to the 16 power f g m sec to the -1 power, where f is the fraction of the surface onto which matter falls. Thus, to produce an X-ray flash as energetic as that suggested by Elliot (1973), ten tons of matter of any composition would have to be dropped onto a neutron star. According to Fabian (1977), such a flash will be broadband and almost omnidirectional, although some beaming may occur if the neutron star possesses a strong magnetic field. The above arguments show that an object of about one kilometer in size and 10 to the 16 power gm dropped onto a neutron star could produce an X-ray pulse (E~10 to the 36 power ergs) strong enough to be easily detectable throughout the Milky Way galaxy. Harwit and Salpeter (1973) argue that the soft gamma-ray burst observed by Klebesadel et al. (1973) may be due to comets falling onto the surface of a neutron star. If an advanced civilization could cause individual objects that might be easily found in the neutron star's neighborhood to fall onto the star in a pre-arranged pattern, this could be detected across the galaxy. GAMMA-RAY RADIATION Viewing et al. (1977) pointed out that an alternative method for SETI could be the detection of alien artifacts. Because most of the different models for interstellar propulsion (Mallove and Matloff, 1989; Mauldin, 1992) produce copious quantities of gamma rays, they proposed the search for this kind of radiation from the antimatter annihilation process. Harris (1986) showed that interstellar spacecraft as gamma-ray sources will be most easily recognized by their proper motions. Their velocities, being substantial fractions of the velocity of light, will be at least one hundred times the highest velocities characteristic of normal astronomical objects. No steady gamma-ray source is known with a proper motion as large as one degree per year. For spacecraft of sufficient size, the poor angular resolution of current gamma-ray telescopes imposes a limit on identification rather than detection. Of the Compton Gamma-Ray Observatory (GRO) instruments, EGRET has an angular resolution of 1.6 degrees at 100 MeV. The best possible resolution of BATSE is 0.5 degrees (Kurfess, 1985, Morris, 1985). In the most favorable circumstances -- purely tangential velocity close to c; observations made over the ten-year lifetime of GRO -- these resolutions will be able to detect the motion of spacecraft at distances of ~100 parsecs and ~300 parsecs, respectively. The distinctive line emission from these objects is readily detectable if the spacecraft is sufficiently massive. A consumption of ~(1 to 10)R(squared) tons per second of antimatter is necessary for detection by GRO of a spacecraft at a distance R (in parsec) (Harris, 1991b).
Harris (1991a) reported the negative results of a search for the linear alignments of burst along a potential spacecraft trajectory, using burst locations and spectra observed by the 1978-1980 interplanetary network of gamma-ray detectors.
Improvements in the measurement of gamma-ray source positions may be expected from two directions. For steady sources and very bright transients, larger detectors will achieve improved signal-to-noise ratios and hence better source positions for a given angular resolution. However, the positions of most of the gamma-ray bursts are measured more accurately from the event delay times across interplanetary distances (Harris, 1991b). Progress in this field is dependent on future space mission launch schedules.
ATOMIC MICROSCOPIC PARTICLES
Another technique of astronomical investigation relies on the observation of microscopic particles which stream toward Earth in huge numbers from different directions. They include elementary particles such as electrons, protons, and heavier and complex atomic particles. The more energetic of these particles are frequently referred to as cosmic rays. Another fundamentalnuclear particle that could be use as an "information carrier" is the neutrino. NEUTRINOS
A neutrino is a weakly interacting particle that travels at essentially the speed of the light and has an intrinsic angular momentum of 1/2 (h/4pi). Neutrinos are
produced on Earth by natural radioactivity, nuclear reactors, and high-energy accelerators. There are six types of neutrinos and three flavors of neutrinos and anti-neutrinos. Each are associated with a massive lepton that experiences weak, electromagnetic, and gravitational forces, but not strong interactions. The known leptons are electrons, muons, and taus (in increasing order of their rest masses).
Because neutrinos interact only weakly with matter, they can reach us from otherwise inaccessible regions where photons are trapped. Hence, with neutrinos, we can look inside stars and examine directly energetic physical processes that occur only in stellar interiors. Large detectors, consisting typically of hundreds or thousands of tons of material, are required to observe "astronomical neutrinos".
The neutrino burst detected from Supernova 1987A confirmed the basic theory of core collapse and refined our views of neutrino properties. Twenty neutrinos in the energy range from 6 to 39 MeV were detected over ~12 seconds (Chevalier, 1992). Given the distance to the Large Magellanic Cloud galaxy and the sensitivity of the instruments, the results were consistent with an emission of about 10 to the 58 power neutrinos in the supernova. To have a real idea about the difficulties in the detection of this particle, it was estimated that during those ~12 seconds, more than 10 to the 11 power neutrinos passed through each square centimeter of Earth. Our technology was only capable of detecting twenty of them.
There have been several proposals to use collimated neutrino beams in the energy range from 1 to 100 GeV as a potential means for telecommunication over global distances of 10 to the 3 and 4 power kilometers (Saenz et al. 1977, Ueberall et al. 1979). Similar proposals for telecommunication for not only global but also interstellar and intergalactic distances were investigated by Subotowicz (1979) and Pasachoff and Kutner (1979).
The main difficulty of all these ideas is the low cross-section of interaction of the neutrino and the very low detection efficiency. Subotowicz (1979) suggested that some advanced civilizations may deliberately shut out emergent civilizations such as ours from the conversation. Generation and detection of neutrinos is so difficult that an advanced civilization may purposely choose such a system in order to find and communicate only with their own level of development.
ELECTRONS, PROTONS, AND ATOMIC NUCLEI (COSMIC RAYS)
Highly energetic protons, neutrons, electrons, positrons, and a wide variety of heavier nucleons constitute the cloud of galactic cosmic ray particles. Anti-nucleons consisting of antimatter have been sought: During the 1980s, anti-protons were found. Unstable sub-nuclear particles are not expected to figure significantly as carriers of cosmic information. Their short lifetimes preclude travel over long distances in the Universe, unless the most extreme energies are assumed.
The energy range of galactic, or possible extragalactic, cosmic ray particles that we can hope to use as carriers of information lies between 10 to the 11 power eV and 10 to the 21 power eV. However, most of the stable cosmic rays are atomic nuclei and are highly charged, therefore studies at high angular resolution, high spectral (energy) resolution, or high time resolution are not likely to yield a great deal of "information". Due to the electromagnetic (Lorentz force) interaction of such highly charged moving particles with the Galactic Magnetic fields, all "memory" of where and how the particles originated and the time of origin is lost, including the initial energy of the particle.
This kind of information carrier presents several disadvantages for interstellar communication. Due to the electric charge of electrons and protons and their interaction with the magnetic interstellar fields (~5x10 to the -6 power Gauss), a Lorentz force will act on both particles, but with opposite direction. The relevant relativistic equations are V/c = ((1-(M to the 2 power c to the 4 power))/E to the 2 power)1/2 and R = (MV/QB(1-V to the 2 power/c to the 2 power)1/2)sin alpha, where c is the speed of light, V is the particle velocity, M is the particle mass, Q is charge, B is the magnetic flux density, E is the particle energy, R is the curvature radius, and alpha is the angle between the galactic field and the direction of the particle beam.
Assuming the beam is aimed properly, it will curve around and just barely reach the target if R is one-half the distance thereto (Freitas, 1977). If we send a message via high energy protons across ten light years, E must be at least 1.4x10 to the 16 power eV if alpha equals ninety degrees. Of course this assumes a nice uniform B field all the way, a dubious proposition at best. In view of the galactic deflection problem, it is difficult to see how charged particle beams could represent a communication mode for advanced extraterrestrial civilizations.
The planet Earth is incessantly bombarded by matter whose information content is far from negligible. For example, the stream of extraterrestrial matter gives direct information about the abundance of the elements in the sites where the matter was produced, which mainly involves the history of the solar system. Meteorites are one such group of objects which can be handled and subjected to laboratory tests and analyzes. Meteoric dust brought to Earth by rain or snow can be studied in the same way. The difference here is that these particles or objects can be examined and re-examined in a leisurely way. They are not so transient as individual photons or fermions.
There have been some speculations that a simple biological system carrying a message and capable of self-replication in suitable environments may be one possible channel for interstellar communication (Yokoo and Oshima, 1979, and Nakamura, 1986). These kinds of ideas have several and severe objections. For example, the impossibility of predicting the environment of the target star in order to favor the self-replication of the molecular structure, the impossibility of avoiding the destruction of the content of the message by molecular mutations, and the impossibility for us to distinguish between a "natural" organism and a real biological interstellar message.
With the development of the Space Age, the way has been opened for a more direct engineered approach to many phases of extraterrestrial exploration using space probes. Such probes can transport sensing devices for direct measurements at distant locations and telemeter their findings back to Earth. The human being's capacity for observation and interpretation of diverse and unexpected phenomena make manned exploration the ultimate goal.
In the past thirty years, there have been several proposals of rapid space probes (v > 0.1 c) for interstellar travel purposes, including pulsed fusion and antimatter-powered rockets, laser-pushed light sails, and interstellar ramjets (Mallove and Matloff, 1989; Mauldin, 1992). The scale of the undertaking, from both a technological and economic perspective, is such that they are unlikely to be realized for at least a century (Crawford, 1990).
The most ignored factor in discussing interstellar flight is the kinetic energy that must be invested in the ship to make its tons of matter move at a substantial fraction of the speed of light (Oliver, 1981; Purcell, 1961; von Hoerner, 1962). This turns out to be the dominant energy requirement and is thus a useful lower bound on the total. If, in assessing the cost of interstellar travel, we find this lower bound too big, all the other costs will be negligible.
Bracewell (1960, 1975) and Freitas (1980a) have discussed the possible superiority of interstellar probes in missions of galactic exploration. Freitas (1980b) and Valdes and Freitas (1980) have raised the issue of self-organized machines in related context. The non-detectability of the proposed probes has been used as an argument to establish that we are the only technological civilization in the entire Milky Way galaxy (Tipler, 1980).
Under the assumption that an extraterrestrial probe will be interested in life in our solar system, a near-Earth search was carried out for these kinds of artificial objects by Freitas and Valdes (1980, 1983, 1985). Other proposals to search for extraterrestrial artifacts in our neighborhood were proposed by Papagiannis (1978, 1985).
At present the human race is limited in its experience of sentient life to the beings of a single planet in one solar system out of potentially billions in the galaxy. We must therefore be aware and open to the numerous ways in which advanced civilizations across the Milky Way and beyond may make themselves known to others in the galaxy, both directly and as the extraneous results of their technological activities. As we have just seen, there is a wide range of photon and particle phenomena potentially available for direct or indirect communication among technological civilizations.
Direct interstellar contact methods may range from radio and optical beacons or other transmissions to interactions with robot or crewed star vessels. Indirect (extraneous) means of contact may consist of similar methods, such as audiovisual broadcasts "leaking" from a planet into space, much as Earth has been doing for most of this century. We may also come upon the artificial "noise" from cultures conducting extensive activities in their solar systems. This activity may be anything from initial planetary exploration and colonization to the reconfiguring of entire star systems to suit the needs of their inhabitants.
Alien civilizations could be as varied as the means of interstellar communication. Humanity is just emerging into the galaxy, having barely begun to utilize the resources of our solar system and the incredible potentials of space travel. Other civilizations at a more advanced stage of technology may have turned their entire planetary system into an immense Dyson Sphere around their sun to capture every photon of solar energy. Even more advanced life forms may control a whole galaxy of star systems or groups of galaxies using technologies almost beyond our current comprehension.
It is quite conceivable that there are types of communication between the stars for which we are completely unaware of. Extra-terrestrial beings could be signaling us -- still using the fundamental forms of radiation we have summarized here -- with encoding and symbols which we can neither understand nor respond to. We could be witnessing activities and messages which we do not recognize as artificial due to our limited experience and knowledge.
Our ignorance of what dwells in the Universe should compel the human race to make even more extensive celestial explorations with as many techniques at our disposal as possible. If there are civilizations amongst the stars of the Milky Way, then it may be only a matter of time before we find them, if we have the patience and skill to search.
FIGURE 3: Nakamura (1986) examined the DNA structure of the SV40 virus. In (a) is shown a part of the genetic structure that the author considered to be a star map. In (b) is a representation of the map of the constellation Eridani.
FIGURE 4: Scheme for von Neumann's machines. The `universal constructor' A; when given a program In for constructing other machine N, A will read through that program, compute upon it, piece together components from its environment, and construct a copy of the machine N, as shown in the top left of the figure. In the bottom row is shown the self-reproducing automaton sequence, in which A is augmented by two new subsystems, B and C.
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