INTERSTELLAR PROBES:
A NEW APPROACH TO SETI

1. INTRODUCTION

SETI strategies also may be grouped according to the physical means of communication. So far as we know, all information must be carried on markers of energy or matter [1]. Thus interstellar communication can occur by exchanging data borne either on massless packets of energy (e.g., photons, neutrinos) or on massive packets of matters (e.g., artifacts, probes, ships). Most forms of contact with extraterrestrial civilizations fall within one of the four boxes in Table 1.
 
 

Ever since Cocconi and Morrison's seminal paper [2] on hydrogen-line signalling was published in 1959, classical SETI work has proceeded on the basis of a "beacon strategy." In this model [3-4], searchers assume that intelligent alien cultures are actively trying to make contact with other similar societies across the Galaxy using radio photons which are easy to generate and which suffer little absorption upon passage through the interstellar medium. ET signals may be broadcast omnidirectionally, or might be aimed at specific stars most likely to harbour sentient lifeforms resembling the transmitting race. Elaborate timing techniques [5-11] and other specific search stratagems [3-4, 6, 12-20] have been suggested. In each case the potential recipient is required to erect and maintain suitable detection apparatus, tuned to receive the alien messages on such frequencies and at such times as both races might agree are "preferred" [3-4, 6, 21-24]. To date, the great majority of actual SETI research has concentrated on beacon strategies. Probably about $10⁶ and 10⁵ observing hours have been expended worldwide on this approach.

The "eavesdropping strategy" also originated in 1959 with Dyson's discussion of the observable characteristics of advanced technological civilizations [25]. Dyson insisted that Malthusian pressures ultimately might drive an intelligent species to exploit all the mass and energy available in their entire solar system, resulting in a huge shell of artifacts orbiting the central star. No matter how efficiently the energy was used, eventually it would have to emerge as waste heat. Dyson recommended a search for these infrared emissions at about 10 microns. Besides evidence of large-scale astroengineering [6, 26-27], internal communication leakage radiation might be detectable across interstellar distances [3, 12, 28-29]. In fact, the electromagnetic signature of the Earth at radio frequencies has recently been analyzed from the viewpoint of alien observers attempting to discover intelligent technological civilization in our Solar System [30-31] To date, perhaps $10⁵ and 10³ hours of observation have been spent worldwide on extraterrestrial eavesdropping.

The remaining SETI strategies, each involving matter markers, were first suggested in the modern scientific literature by Bracewell as early as 1960 [29, 32-34]. Assuming sentient life is reasonably rare throughout the Galaxy, Bracewell believed that it might be more economical to send highly sophisticated messenger probes to other star systems in search of new members for the "Galactic Club." Both the "probe strategy" and the "artifact strategy" would involve looking in appropriate locations (moons, asteroids, Trojan Points) for evidence of alien devices right here in our own Solar System. Except for a few modest investigations of the curious but only quasi-SETI Long-Delayed Echo (LDE) phenomenon [35-46], essentially $0 and 0 observing hours have been invested in these approaches to SETI.

The imbalance of funding and effort appears to derive from the natural tech no-chauvinistic perspectives held by many radioastronomers doing research in this field. Since mankind now has the technical expertise to send out radio messages, the traditional argument goes, then must not ETs as well find radio the optimum medium for interstellar communication? Beacon searches frequently are justified on the grounds that such signals are all we are capable of looking for at this time. Fortunately, this simply is not the case.

The purpose of this paper is to consider the question of interstellar communication and contact from the extraterrestrial point of view. How might an advanced technical civilization go about making purposive contact with other technological cultures at similar or lesser grades of development? It will be shown that information transmission by photons (energy markers) or by probes (matter markers) are energetically and technologically indistinguishable alternatives for any society whose technical capabilities reach theoretical physical limits. When other factors are used as distinguishability criteria, probes fare much better than photons. The implications for a new, more balanced approach to SETI research are discussed and summarized in the last section.

2. ENERGY AND CIVILIZATION

Kardashev [48] has devised a particularly useful classification scheme which subsumes all conceivable extraterrestrial cultures, based on gross measures of total power utilization. A full Type I civilization has available to it all planetary energy resources (sunlight, oceanic deuterium, magmatic heat, and so forth), which many scientists agree probably cannot be released faster than about 10" joules/second without causing irreversible damage to planetary ecologies. Mankind, with present aggregate technological power output ~ 10¹¹ watts, is still but an emergent Type I society. A mature Type II culture, such as a "Dyson Sphere" of orbiting habitats, industrial artifacts [49], or "space colonies" [50-53], might have available to it the entire power output of its home sun. For typical F-K stars, this is perhaps 10²⁶ watts. Finally, a fully-developed Type III galactic civilization, an organization comprising billions of inhabited or colonized stellar systems, might command the power output of an entire galaxy, about 10³⁷ watts. It is against this backdrop of unimaginably powerful cultures that questions of interstellar communication techniques are most properly posed.

A few words for sceptics and technological pessimists: Table 2 represents a compilation of very real, tangible possibilities, regardless of one's personal view of their plausibility when measured against the limitations of present Earthly science. One need not believe that Type II or Type III civilizations actually now exist in order to concede that they may, or that their existence might be a necessary precondition for the achievement of large-scale astroengineering and interstellar colonization.

3. THEORETICAL ENERGETIC EFFICIENCY OF MARKER TRANSMISSIONS

On this basis Drake [57-58] has proposed the Principle of Economy, which holds that economy is practiced universally by all surviving classes of living systems. Thus technological civilizations throughout the Galaxy will normally choose those solutions to any technical problem which are least expensive. Of course it is conceivable that some extraterrestrial races may not be as subject to the same competitive rules of natural selection as are Earthly lifeforms [59]. Still other species might choose shorter-lived, more profligate lifestyles than ordinary thrift would dictate. But general living systems theorists agree that Drake's Principle should be applicable virtually at all systemic levels from cells to societies, since it appears to be a manifestation of the well-known principle of least effort [60-61].

Tentatively accepting the Principle of Economy as applicable to extraterrestrial cultures as well as our own, it follows that a technical communicative civilization will choose those means of communication which cost the least to do the job. The "job," in this context, is the transfer of information and complexity across the vastness of interstellar space.

The quantitative unit of information is the binary digit, or "bit,'. Assuming that all data must be carried on markers of matter-energy, the dimension of cost is in joules. Finally, data must be transmitted in some interval of time, measured in seconds. These quantities define a figure of merit which may be computed for any mode of communication. This figure, a measure of the energetic efficiency of information transfer per unit time interval is expressed in units of bits/joule- second

Consider first the possibility of signalling in beacon mode using photons. According to Shannon's classic theory [62-63], the theoretical maximum rate of information transmission (with an arbitrarily small frequency of errors) through a channel of bandwidth Dnis Dnlog₂(1+S/N) bits/ second, where S/N is the signal-to-noise power ratio. Such capacity is achieved only when all redundancy is removed from the signal, causing it to approach white noise across the band. While this may perhaps lessen its recognizability as an electromagnetic artifact for acquisition purposes, it increases its utility as a means of rapid interstellar data transfer once contact with another civilization has been firmly established. In any case, Shannon's limit places a maximum ceiling on the energetic efficiency of any photonic communication, whether an acquisition beacon or a message to regular correspondents.

What is this efficiency? The maximum bit rate Occurs when bandwidth equals carrier frequency, Dn= n. From basic quantum physics we recall that the energy per carrier photon is hn joules, where h is Planck's Constant. Hence the maximum theoretical photonic information transmission efficiency e_g is:
 

Now suppose that energy levels are used as information markers [64] in the data-bearing portion of the starprobe, and that these levels lie within some interval (0, E) where E is the maximum energy available within the material system. If energy levels can only be measured to an accuracy DE, then at most n = E/DE different levels may be distinguished. If no more than one energy level is occupied at any moment. then a maximum of H = log₂ (n+1) bits may be represented. If two markers with levels in (0, 1/2 E) each are used, maximum H = 2 log₂ (1/2n +1) bits. If n markers with levels in (0, DE) each are used, H = n log₂ (n/n+1)= n bits may be represented.

Hence the optimal use of a given amount of mass-energy E is achieved when n matter-markers with values in (0, DE) are used. Since Heisenberg's Uncertainty Principle defines a minimum energy measurement accuracy DE = h/Dt. where Dt is the uncertainty in the duration of the measurement, then in this case a message consisting of H = n = E/DE= EDt/h bits may be carried by the starprobe. Any self-contained material system of mass m is subject to an absolute limitation on the total mass-energy of all its information markers. This limit is the rest mass of the system mc^{2 where} c is the vacuum velocity of light, so H = mc²Dt/h bits may be represented by the entire material system. If these data are read out in a time interval Dt then the maximum rate of information transmission for starprobes is mc^2/h bits/second.

To find the energetic efficiency of matter-markers the transmission rate must be divided by total energy (rest energy plus kinetic energy) of the material system. From Special Relativity this energy is mc²/(1- V^2/c²)^1/2, where V is the probe's interstellar cruise velocity relative to the stationary frames of reference of the two communicating civilizations. Hence the maximum theoretical information transmission efficiency for matter, e_m, is:
 

Maximum photon efficiency e_g and velocity compensated maximum matter-marker efficiency e_m for data transmission are compared in Figure1.

Photon efficiency is represented by the vertical line at far fight, which shows energetic efficiency as a function of signal-to-noise ratio (primarily a technological choice). Note that the logarithm of S/N is strikingly nonlinear for S/N ³ 1. Even vast improvements in S/N using superior technology yield only miniscule increases in photonic energetic efficiency.

The velocity -compensated matter-marker efficiency curve ((e_mV/c) lies below the photonic efficiency curve for all S/N ³ 1. Even so, photons are only marginally superior to matter-markers in interstellar communication. If reasonable values for S/N and probe velocity are chosen for comparison, the difference in energetic efficiency is barely an order of magnitude or two. This small differential will likely be considered de minimis by highly advanced spacefaring Type II and Type III civilizations, who should have access to virtually perfect photon and matter-handling technologies restricted solely by theoretical limitations imposed by the fundamental laws of physics. Stellar and galactic technical cultures probably will view signals and starprobes as energetically indistinguishable alternatives for interstellar message-sending. Choice of communication mode thus should depend far less upon transmission efficiency than on other factors.

4. ECONOMIC AND POLITICAL FEASIBILITY

But will Type I societies transmit at all? Seeger [66] has proposed a powerful radio frequency beacon able to announce our presence to other sentient races in the Galaxy, which could be constructed on a crash program basis by 2000 AD using foreseeable human technology. This isotropic 10⁹ watt continuous transmitter would be placed in solar orbit to provide a. low Doppler drift rate and to minimize pollution of the local terrestrial electromagnetic environment. Is this a reasonable project for humanity?

At current prices of more than $10^-8/joule, the annual cost of the energy alone for Seeger's beacon installation would run about $10⁹. This is three orders of magnitude beyond present global annual expenditures on all SETI efforts and a full nine orders of magnitude higher than the total worth of energy released by mankind in beacon form, to date. (The Arecibo message on 16 November 1974 consisted of a beam of about $1 worth of energy directed toward the target globular cluster M13, 24,000 light-years away [67].) In order to afford a single, continuous 1 gigawatt beacon, it would appear that humanity must become from 10³-10⁹ times more affluent as a species. Since wealth is proportional to energy [47], mankind must expand its collective industrial -commercial base from its present power level of about 10¹³ watts worldwide up to at least 10¹⁶-10²² watts before interstellar beacons realistically can be considered economically or politically feasible. Of Course by then we'll be an emergent Type II civilization, and starprobe missions as well as beacon stations should easily be within reach.

Based on the only example of technological society we are aware of - ourselves - it seems that beacon transmitters are not economically or politically feasible for Type I planetary cultures. Only Type II and Type III organizations should find acceptable the tremendous expenditures of capital and material resources necessary to conduct a comprehensive programme of interstellar exploration [68-70]. Since starprobes and signals are energetically equivalent exercises for technically proficient extraterrestrial civilizations, either or both might be utilized in interstellar communication depending upon the particular purposes and needs of the societies seeking interaction.

5. WHY PROBES ARE BETTER

First, there is the benefit of communications feedback. A probe which discovers a garrulous inhabited world may engage in a true conversation with the indigenes, an almost instantaneous interchange and interweaving of cultures. Interactive exchanges may require mere fractions of a second between questions and answers. On-site starprobes, perhaps in orbit around the host's sun or home planet, can carry on real-time educational and linguistic functions with a precision no remote signalling system could hope to match. As an added benefit, such intelligent devices could provide a noise-free channel of communication on any frequency of the contactee's own choosing (29). By comparison, the traditional SETI beacon acquisition scenarios appear little more than sterile data swaps requiring millenia per cycle rather than milliseconds. With interstellar photonic transmissions, delays are interminable and widely dispersed sentient species can never really converse. High probe. intelligence will permit such conversation, as if the sending race had made the journey "in person."

Second, probes have the advantage in acquisition efficiency. Beacons may radiate otherwise useful energy and information out into space for centuries, millenia, or even longer without getting any response or gaining any new information in return. This energy, since it was detected by no receiver, in essence was wasted and constitutes pure economic loss for the sending society. Such imprudence reflects an inordinate (and possibly selectively disadvantageous) degree of carelessness or generalized altruism on the part of the transmitting culture,

Starprobes, on the other hand, become independent agents as soon as they are launched [71]. If properly constructed, there should be no further need for energy expenditure by the transmitting society. Sophisticated messenger probes will be self-repairing, self -programming, perhaps even self-reproducing [72-74], and capable of refuelling or recharging at every port of call. They may be designed patiently to float in orbit for hundreds or even millions of years, awaiting the emergence of a communicative culture on suitable planets in the system; alternatively, they may be programmed to hop from star to star until they find communicative lifeforms, then enter into an exchange with them at no further cost to the original transmitting society. A subsidiary but nonetheless important benefit of starprobes is that they may serve as cosmic "safety deposit boxes" for the cultural heritage-and knowledge of the sending society. If the transmitting civilization is destroyed or the culture perishes for whatever reasons, the probes they sent to other worlds can still tell their story to any willing ears for perhaps geological time periods thereafter.

Third, starprobes hive the overwhelming advantage of military security for the transmitting race. Interstellar beacons are an invitation to disaster at the hands of unknown predatory alien civilizations.* In. any situation involving contact via signals, the sending society must give away the position of its home star system at great risk for mere speculative benefits. This terrible breach of military security [75-76] may be remedied by using probes instead of photons. If local technological activity is detected by an intelligent artifact orbiting some target star, the device may initiate contact with the indigenous technical species without ever having to disclose the identity or whereabouts of its creators. If it is deemed necessary for the starprobe to report its findings back to the transmitting society from time to time, this easily may be accomplished in a manner virtually impossible to trace or to decode (e.g., omnidirectional or "false trail" broadcasts into empty space, trapdoor function encoded messages, shifting relays through randomly dispersed repeater stations in uninhabited solar systems, and so forth.) In other words, probes help to safeguard the security of sending societies in any exchange between themselves and alien cultures.

6. A BALANCED APPROACH TO SETI

At the present state of knowledge, SETIists would do well to make as few assumptions about alien motivations and technologies as they can. As Stull [81] correctly points out, "all we can do is recognize the possibility that extraterrestrial technologies exist, and treat that possibility as an observational problem." Unfortunately, until very recently few researchers seriously considered the possibility that extraterrestrial starprobes might already be present in the Solar System. Here is a relatively simple "observational problem," right in our own backyard.

The author does not urge the immediate abandonment of all searches for extrasolar beacons in favour of interstellar probes. However, the gross disparity of funding of searches for extraterrestrial energy-marker transmissions over matter-marker transmissions (see Table 1) seems ill-advised in light of the arguments presented earlier in this paper. Probes are at least as plausible for interstellar contact and communication as photon signals. The most intelligent and equitable allocation of scarce SETI programme funding would seem to be roughly equal divisions of money and effort between energy-marker and matter-marker strategies.

Of course, the cheapest searches should always be attempted first and the more expensive ones later (after the cheaper schemes have failed). Most the easy beacon strategy searches have already been carried out experimentally [4, 6, 82-94], whereas few probe search strategies have seriously been worked out even in theory [29, 44-46, 70, 95-106]. Table 3 includes 24 proposed SETI objectives, listed in order of increasing cost (author's best estimate), difficulty, and sensitivity or extraterrestrial detectability. Objectives appearing early in the list should be pursued in preference to later costlier items. (For monetary comparisons, the present Cross World Product is $1 x 10¹³ and the Planetary Net Worth is about $8 x 10¹⁴.)

In SETI programme planning, higher priority should be given in the near-term to those probe and artifact searches which can be carried out quickly and inexpensively, in preference to larger more expensive beacon searches which should be mounted in the decades ahead [107].

1. J. von Neumann, The Computer and the Brain, Yale University Press, New Haven, 1958.
2. Giuseppe Cocconi, Philip Morrison, "Searching for Interstellar Communications," Nature, 184, 844-846 (1959). Reprinted in A. G. W. Cameron (ed.), Interstellar Communication, W. A. Benjamin Inc., New York, 1963, pp. 160-164.
3. Bernard M. Oliver, John Billingham (eds.), Project Cyclops: A Design Study of a System for Detecting Extraterrestrial Intelligent Life, NASA CR-114445, Revised Edition (1973).
4. Philip Morrison, John Billingham, John Wolfe (eds.), The Search for Extraterrestrial Intelligence, NASA SP-419 (1977).
5. C. M. Tovmasyan (ed.), Vnezemnye tsivilizatsii (Extraterrestrial Civilizations), Proceedings of the First All-Union Conference on Extraterrestrial Civilizations and Interstellar Communication, Byurakan, 20-23 May 1964 NASA TT-F-438 (1967).
6. Carl Sagan (ed.), Communication with Extraterrestrial Intelligence (CETI), MIT Press, Cambridge, Mass., 1973.
7. Gordon W. Pace, James C. C. Walker, "Time Markers in Interstellar Communication," Nature, 254, 400-401 (1975).
8. T. B. Tang, "Supernovae as Time Markers in Interstellar Communication," JBIS, 29, 469-470 (1976
9. P. V. Makovetskii, "Nova Cygni 1975: A Synchrosignal for Extraterrestrial Civilizations?" Soviet Astronomy, 21, 251-253 1977).
10. William 1. McLaughlin, "On the Timing of an Interstellar Communication," Icarus, 32, 464-470 1977).
11. Cordon W. Pace, "The Use of Cary Stars As Time Markers in Interstellar Communication," JBIS, 32, 215-218 (1979).
12. I. S. Shklovskii, Carl Sagan, Intelligent Life in the Universe, Holden-Day, Inc., 1966.
13. R. P. Haviland, "On the Search for Extrasolar Intelligence," Spaceflight, 14, 217-219, 223 (1972).
14. James. C. C. Walker, "The Search for Signals from Extraterrestrial Civilizations," Nature, 241, 379-381 (1973).
15. Robert S. Dixon, "A Search Strategy for Finding Extraterrestrial Radio Beacons," Icarus, 20, 187-1 (1973).
16. Robert H. Cray, "Broadcast Strategies in the Galaxy: A Principal Message Source Locale," JBIS, 30, 341-343 (1977).
17. Mieczyslaw Subotowicz, 'CETI from an Earth Satellite Orbit," JBIS, 11, 109-110 1978
18. Ian Ridpath, "A Signalling Strategy for Interstellar Communication," JBIS, 31, 108-109 1978).
19. David J. Bihary, "The Music of the Spheres,,' Spaceflight, 20, iii (1978).
20. Robert Sheafter, "The Cosmic Blackbody Radiation: A Potential Guide to Interstellar Signal Beacons?" Spaceflight, 21, 155-1567
21. Robert Carl Sagan, The Cosmic Connection, Doubleday, New York, 1973.
22. Frank D. Drake, Carl Sagan, "Interstellar Radio Communication and the Frequency Selection Problem," Nature, 245, 257-258 1973).
23. V. V. Makovetskii, "Structure of Call Signals of Extraterrestrial Civilizations," Soviet Astronomy, 20, 123-125 (1976).
24. T. B. H. Kuiper, M. Morris, "Searching for Extraterrestrial Civilizations," Science, 196, 616-621 (197
25. Freeman Dyson, "Search for Artificial Stellar Sources of Infrared Radiation," Science, 131, 1667-1668 (1959). Reprinted in A. C. W. Cameron (ed.), Interstellar Communication, W. A. Benjamin, Inc., New York, 1963, pp. 111-114.
26. Freeman J. Dyson, 'The Search for Extraterrestrial Technology," in R. E. Marshak (ed.), Perspectives in Modern Physics, John Wiley & Sons, New York, 1966, pp. 641-655.
27. Adrian Berry, The Next Ten Thousand Years, E. P. Dutton & Co., New York, 1974.
28. Carl Sagan, "An Introduction to the Problem of Interstellar Communication," in Cyril Ponnamperuma, A. C. W. Cameron (eds.), Interstellar Communication: Scientific Perspectives, Houghton Mifflin Company, Boston, 1974, pp. 1-25.
29. Ronald N. Bracewell, The Galactic Club: Intelligent Life in Outer Space, W. H. Freeman & Company, San Francisco, 1975.
30. W. T. Sullivan, III, S. Brown, C. Wetherill, "Eavesdropping: The Radio Signature of the Earth," Science, 199, 377-388 (1978).
31. W. T. Sullivan, III, "The Radio Signature of Earth," Paper IV-4, Conference on Life in the Universe, NASA/Ames Research Center, Moffett Field, California, 19-20 June 1979.
32. Ronald N. Bracewell, "Communications from Superior Galactic Communities," Nature, 186, 670-671 (1960). Reprinted in A. C. W. Cameron (ed.), Interstellar Communication, W. A. Benjamin, Inc., New York, 1963, pp. 243-248.
33. Ronald N. Bracewell, "Life in the Galaxy," in S. T. Butler, H. Messel (eds.), A Journey Through Space and the Atom, Nuclear Research Foundation, Sydney, Australia, 1962. Reprinted in A. C. W. Cameron (ed.), Interstellar Communication, W. A. Benjamin Inc., New York, 1963, pp. 232-242.
34. Ronald N. Bracewell, 'Interstellar Probes," in Cyril Ponnamperuma, A. C. W. Cameron (eds.), Interstellar Communication: Scientific Perspectives, Houghton Mifflin Company, Boston, 1974, pp. 102-117.
35. Carl Stormer, "Short Wave Echoes and the Aurora Borealis," Nature, 122, 681, (1928).
36. Balthus van der Pol, "Short Wave Echoes and the Aurora Borealis," Nature, 122, 878-879 (1928).
37. J. H. Dellinger, "Observations on Long-Delay Radio Echoes," ST, 8, 42, 88 (1934).
38. K. C. Budden, C. C. Yates, "A Search for Radio Echoes of Long Delay," Journal of Atmospheric and Terrestrial Physics, 2, 272-281 1952).
39. V. W.. Crawford et al., "Possible Observations and Mechanism of Very Long Delayed Radio Echoes," Journal of Geophysical Research, Space Physics, 75, 7326-7332 (1970).
40. O. G. Villard, Jr., A. F. Fraser-Smith, R. T. Cassan, "LDE's, Hoaxes, and the Cosmic Repeater Hypothesis,,' ST, 55, 54-58 (1971)
41. Ronald N. Bracewell, "The Opening Message from an Extraterrestrial Probe," Astronautics and Aeronautics, 11, 58-60 (1973).
42. Duncan Lunan, Man and the Stars: Contact and Communication with Other Intelligence, Souvenir Press Ltd., London, 1974.
43. Duncan Lunan, "Space Probe from Epsilon Bootis?" Analog, 92, 66-84 (1974).
44. A. T. Lawton, S. J. Newton, "Long Delayed Echoes: The Search for a Solution," Spaceflight, 16, 181-187, 195 (1974).
45. George Sassoon, "A Correlation of Long-Delay Radio Echoes and the Moon's Orbit," Spaceflight, 16, 258-264 (1974).
46. A. T. Lawton, S. 1. Newton, "Long Delayed Echoes-The Trojan Ionosphere," JBIS, 27, 907-920 (1974).
47. Leslie A. White, The Science of Culture, Farrar, Straus, and Giroux, New York, 1969.
48. Nikolai S. Kardashev, "Transmission of Information by Extraterrestrial Civilizations," Soviet Astronomy-AJ, 8, 217-221 (1964).
49. Michael A. G. Michaud, "Spaceflight, Colonization, and Independence," JBIS, 30, 83-95, 203-212 323-331 (1977).
50. Gerard K. O'Neill, "The Colonization of Space," Physics Today, 27, 32-40 (September 1974).
51. Gerard K. O'Neill, The High Frontier: Human Colonies in Space, William Morrow and Company, Inc., New York, 1977.
52. T. A. Heppenheimer, Colonies in Space, Warner Books, Inc., New York, (1977).
53. Richard D. Johnson, Charles Holbrow, Space Settlements: A Design Study, NASA SP-413 (1977).
54. Erwin Schrödinger, What is Life?, The Macmillan Company, New York, 1947
55. Philip Morrison, "Conclusion: Entropy, Life, and Communication, in Cyril Ponnamperuma, A. G. W. Cameron (eds.), Interstellar Communication; Scientific Perspectives, Houghton Mifflin Company, Boston, 1974, pp. 168-186.
56. Enrico J. Coffey, "Life: Its Definition and Some Consequences," JBIS, 29, 633-640 (1976).
57. Frank D. Drake, "Intelligent Life in Other Parts of the Universe," in Hugh Odishaw (ed.), The Earth in Space, Basic Books, Inc., New York, 1967, Chapter 30.
58. Frank D. Drake, "Methods of Communication: Message Content, Search Strategy, Interstellar Travel," in Cyril Ponnamperuma, A. G. W. Cameron, Interstellar Communication. Scientific Perspectives, Houghton Mifflin Company, Boston, 1974, pp. 118-140.
59. P. H. A. Sneath, Planets and Life, Thames and Hudson, London, 1970.
60. C. K. Zipf, Human Behavior and the Principle of Least Effort, Addison-Wesley, Cambridge, Mass., 1949.
61. James G. Miller, Living Systems, McGraw-Hill Book Co., New York, 1978.
62. Claude E. Shannon, "A Mathematical Theory of Communication," Bell System Technical journal, 27, 379-423, 623-656 (1948).
63. Claude E. Shannon, "Probability of Error for Optimal Codes in a Gaussian Channel," Bell System Technical journal, 38, 611-656 (1959).
64. H. J. Bremermann, "Optimization Through Evolution and Recombination," in M. C. Yovits, C. T. Jacobi, C. D. Goldstein (eds.), Self-Organizing Systems, Spartan Books, Washington, 1962, pp. 93-106
65. Peter M. Molton, "On the Likelihood of a Human Interstellar Civilization," JBIS, 31, 203-208 (1978).
66. Charles L. Seeger, "Search Strategies," in Reference (4), pp. 75-91.
67. "The Arecibo Message of November 1974," Icarus, 26, 462-466 R715
68. Carl Sagan, "On the Detectivity of Advanced Galactic Civilizations, Icarus, 19, 350-352 (1973).
69. Bob Parkinson, "The Starship as an Exercise in Economics," JBIS, 27:692-696 (1974).
70. Anthony R. Martin (ed). Project Daedalus - The Final Report on the BIS Starship Study, JBIS Interstellar Studies Supplement (1978).
71. Nigel Calder, Spaceships of the Mind; The Viking Press, New York, 1978.
72. L. S. Penrose, "Self -Reproducing Machines," Scientific American, 200, 105-114 (June 1959).
73. W. Ross Ashby, "The Self -Reproducing System," in C. A. Muses (ed.), Aspects of the Theory of Artificial Intelligence, Plenum Press, New York, 1962.
74. John von Neumann, Theory of Self-Reproducing Automata, University of Illinois Press, Urbana, 1966.
75. Michael A. C. Michaud, "Interstellar Negotiation," Foreign Service Journal, 49, 10-14 (December 1972).
76. Robert A. Freitas Jr., "Metalaw and Interstellar Relations", Mercury, 6,15-17 (March/April 1977).
77. Richard O. Fimmel, William Swindell, Eric Burgess, Pioneer Odyssey-Encounter With a Giant, U.S.G.P.O., Washington, DE, 1974.
78. Entire issue, Science, 204, 1 June 1979.
79. Carl Sagan Linda Sagan, Frank D. Drake, "A Message From Earth," Science, 175, 881-884 (1972).
80. Carl Sagan, Murmurs of Earth: The Voyager Interstellar Record, Random House, Inc., New York, 1978.
81. Mark A. Stull, "On the Significance of the Apparent Absence of Extraterrestrials on Earth, " JBIS, 32, 221-222 (1979).
82. Frank D. Drake, "Project OZMA," Physics Today 14, 40-42, 44,-46 (April 1961).
83. S. Yloitskii et al. "Search for Monochromatic 927 MHz Radio Emission from Nearby Stars," Soviet Astronomy-AJ. 15, 508-510 (1971).
84. Gerrit L. Verschuur, "A Search for Narrow Band 21-cm Wavelength Signals from Ten Nearby Stars," Icarus, 19, 329.340 1973.
85. The CETI Program," Soviet Astronomy-AJ, 18, 669-675 (1975).
86. A. T. Lawton, "Conference Report: International CETI Review Meeting," JBIS, 28, 220-222 (1975).
87. "Satellite Searching Stars for Signals," JBIS, 28, 586 (1975).
88. Robert Sheaffer, "Project OZMA II," Spaceflight, 17, 421-423 11975).
89. 'Searches for Intelligence Beyond Earth Continue," Physics Today, 29, 18-19 (1976).
90. Robert S. Dixon; Dennis M. Cole, "A Modest All-Sky Search for Narrowband Radio Radiation Near the 21-cm Hydrogen Line," Icarus, 30, 267-273 (1977).
91. Boris Belitsky, "CETI in the Soviet Union", Spaceflight, 19, 193, 196-(1977)
92. Robert Sheaffer, "1977 SETI Progress Report," Spaceflight, 19, 307-310 (1977).
93. Bruce Murray, Samuel Gulkis, Robert E. Edelson, "Extraterrestrial Intelligence: An Observational Approach," Science, 199, 485-492 (1978).
94. Paul Horowitz, "A Search for Ultra-Narrowband Signals of Extraterrestrial Origin," Science, 201, 733-735 (1978).
95. Leslie C. Edie, Robert D. Kross, correspondence in Science, 136, 184 (1962) and 136, 913-914 (1962).
96. Roger A. MacCowan, Frederick 1. Ordway, III, Intelligence in the Universe, Prentice-Hall, Inc., New Jersey, 1966.
97. Ben Bova, "It's Right Over Your Nose," Analog, 81, 95-100 (June 1968).
98. C. V. Foster, "Non-Human Artifacts in the Solar System," Spaceflight, 14, 447-453 (1972).
99. C. W. Anderson, "Relic Interstellar Corner Reflector in the Solar System?" Mercury, 3, 2-3 (September/October 1974).
100. Edward S. Gillfillan, Migration to the Stars: Never Again Enough People, Robert B. Luce, Inc., Washington, D.C., 1975.
101. James R. Wetz, "The Human Analogy and the Evolution of Extraterrestrial Civilizations," JBIS, 29, 445-464 (1976).
102. Robert L. Forward, "A Program for Interstellar Exploration," Hughes Research Laboratories, Research Report 492A, Malibu, California, 1976. Reprinted in JBIS, 29, 610-632 (1976).
103. D. R. J. Viewing, C. Horswell, E. W. Palmer, "Detection of Starships," JBIS, 30, 99-104 (1977).
104. M. W. Saunders, "Databank for an Inhabited Extrasolar Planet: Purpose, Indication, and Installation," JBIS, 30, 349-358 (1977).
105. E. J. Betinis; "On ETI Alien Probe Flux Density," JBIS, 31, 2 217-221 (1978).
106. A. A. Jackson, IV, "Ultra-Relativistic Starships," JBIS, 32, 240 (1979).
107. A. T. Lawton, "Interstellar Communication: Antenna or Artifact?" JBIS, 27, 286-294 (1974).