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Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox

Robert A. Freitas Jr., Journal of the British Interplanetary Society 36 (November, 1983)

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Summary: The assertion that extraterrestrial intelligences (ETI) do not exist, based on the apparent contradictions inherent in the Fermi Paradox, rests upon an unproven and untenable presumption: That ETI are not now present in the Solar System. The current observational status of the Solar System is insufficient to support the assumption that ETI are not here.

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Xenology Research Institute, 8256 Scottsdale Drive, Sacramento, California 95828, USA.
Journal of the British Interplanetary Society 36(November, 1983):496-500.

Note: This web version is derived from an earlier draft of the paper and may possibly differ in some substantial aspects from the final published paper.

The assertion that extraterrestrial intelligences (ETI) do not exist, based on the apparent contradictions inherent in the Fermi Paradox, rests upon an unproven and untenable presumption: That ETI are not now present in the Solar System. The current observational status of the Solar System is insufficient to support the assumption that ETI are not here. Most advanced civilisations also would be either invisible or unrecognisable using current human observational methods, so millions of advanced societies may exist and still not be directly detectable by us. Thus the Fermi Paradox cannot logically be raised as an objection to the existence of ETI until these major observational deficiencies have been corrected.

 

1. INTRODUCTION TO THE ARGUMENT

The Fermi Paradox, attributed to a question from Enrico Fermi after a discussion of the possibility of extraterrestrial life during the 1940's, is traditionally formulated as follows: If there are intelligent beings elsewhere, then in time they must achieve the technology of nuclear power and space flight and would explore and colonise the Galaxy, as humanity has explored and colonised the Earth. Thus they should have been able to travel to Earth, but we see no evidence of such visitations, ergo they cannot exist. Fermi's question "Where are they?" implicitly construes the absence of extraterrestrials on Earth as positive evidence of their nonexistence elsewhere in the Universe. Dormant for many decades, recently the Fermi Paradox has re-emerged as a major modern challenge to the existence of extraterrestrial intelligences (ETI). For example, Dyson [1] asserts that the necessary technological advancement of advanced ETI implies that the Galaxy should appear far less "wild" if they exist. Hart [2] states "Fact A" that "there are no intelligent beings from outer space on Earth now," then suggests and disposes of four allegedly exhaustive explanations for this fact: (1) Space travel is infeasible, (2) ETI might choose not to explore or colonise, (3) ETI are coming but haven't arrived here yet, and (4) Earth has been visited in the past. Tipler [3-4] argues that advanced civilisations would quickly develop self-replicating machine technology, which at least one of them would employ to mine, organise and colonise the entire Galaxy in no more than 300 million years; our Solar System is still intact, hence ETI cannot exist. Past and recent [5-8] discussions of the Fermi Paradox make one critical assumption challenged in the present work: That the absence of extraterrestrials or their artifacts on Earth or in the Solar System is an undisputed fact, and, more generally, that advanced technology invariably leads to observable alteration of the large-scale physical environment. Cox [9] implicitly recognises this when he notes that Hart's "Fact A" is not an empirical fact but rather a theory, and Schwartzman [10] rejects the assumption to best reconcile Hart's arguments with those of the advocates of large N (the number of communicative civilisations in the Galaxy). But the extent of our ignorance of potential evidence of ETI in the Solar System is not generally appreciated. We have not yet looked very hard for extraterrestrials in the Solar System, or elsewhere. Until these observational deficiencies are corrected, the Fermi Paradox cannot logically be raised as a valid objection to the existence of ETI.

For purposes of the following discussion it is conceded that continuing reports of UFOs in our skies [11], however controversial [12-13], cannot be taken as convincing evidence of extraterrestrial visitation or technology. Hence the discussion is restricted to a brief review of the evidence that ETI are absent from the human observational Universe, excluding Earth.

 

2. NON-OBSERVABILITY OF ADVANCED CIVILISATIONS

2.1 Advanced Civilisations May Not be Visible

Why might ETI move out into the Galaxy, and not just stay at home? Possible motives might include physical resource shortages, the need for new habitats for expanding populations (Lebensraum), the drive for sociopolitical power or prestige, the thirst for adventure by certain segments of the population, mischief or criminal activity, genuine curiosity about the cosmos and the search for knowledge, religious proselytization of other sentient races, propagation of biological genetic material, and so forth. All of these mechanisms, in one guise or another, have proved very effective in driving mankind to colonise his entire planet. Similar drives are likely to motivate a highly successful, intelligent extraterrestrial species, but most may not produce astronomically visible effects.

There are also many reasons why advanced ETI might not engage in colonisation, missionary, or exploitative activities. Unless interstellar cargo transport is feasible the most important resource is knowledge, implying noninvasive exploration rather than physical exploitation. Self-replicating systems would then be used only to build probes and libraries. Even if the material needs of a planetary system full of sentient beings requires the rapacious strip-mining of an entire galaxy, our Solar System would probably be spared if stellar systems with life are exceedingly rare.

Genes are environment-specific survival instructions, so transmission of pure genetic information seems pointless unless the target planet is terraformed. Interstellar ovum arks, generation ships, and automated bio-regeneration only generates more independent competitors for exactly the same limited galactic resources. Like the bottle-babies of Huxley's Brave New World, there might be little "parental" attachment or sense of community with cloned societies whose cultural interactions have information feedback loops with delays of 10⁴-10⁵ years, comparable to the timescale of speciation. In these cases each society would remain isolated from the others and never forge a pangalactic union, or might choose not to spread their clones throughout the Galaxy.

These kinds of arguments suggest that the mere possession of advanced technology and civilisation need not imply astronomically observable effects. Planetary (Type 1), stellar (Type II), and benign or diminutive galactic (Type III) civilisations are not precluded by existing data (see Section 2.3). Our Galaxy may have millions of such societies. Both ETI and very advanced technologies may exist - indeed, Type II cultures should possess any technical skill we can now imagine. But large-scale observational effects of the astroengineering activities of these organisations lie, for the most part, below the threshold of detectability of instruments employed by present-day mankind.

 

2.2 Cosmic Censorship of Rapacious Galactic Civilisations?

Such explanations for the absence of obvious astroengineering activity in the Galaxy, if ETI exist, cannot conclusively resolve the Fermi Paradox because they cannot apply to all ETI at all times after they achieve interstellar flight as required by Hart [2]. In theory, some civilisations might be unable to resist their rapacious sociological urges to colonise and to overpopulate, and might proceed to do both at a phenomenal rate. Others may be driven by motives too alien to contemplate, resulting in a "cancer of purposeless technological exploitation" [14]. Any one such species in theory would press to the physical limits of the Galaxy, creating an observably artificially-ordered, "tamed" Galaxy which, because of our failure to observe it, supposedly demonstrates the nonexistence of ETI.

This reasoning is unconvincing because it assumes that "one bad apple spoils the lot." Perhaps there can be no bad apples, because they self-destruct before damaging any others. The interstellar abyss may prove an excellent quarantine mechanism. We may see no spoilative Type III civilisations either because some unknown selective process [15-20] rules out their existence, or because they have never survived long enough to complete their galactiforming programme and leave any major observable effects.

Apparently Type III civilisations who make extensive and exploitative use of highly visible, very advanced technology are very rare or nonexistent. Why might this be? Extinction as a natural phenomenon is quite common on Earth, where biological life has a 99.9% species extinction rate. Social pessimists might argue that the almost instinctual voracity of humans, coupled with our technological capability for self-destruction, may eventually lead to the downfall of our planetary civilisation. It is logically acceptable to infer the existence of selective mechanism(s) which result in the "cosmic censorship" of civilisations who exploit glaringly obvious, rapacious technologies to the utmost.

The ultimate outcome of a mastery of fusion, starflight, machine replication and other advanced technologies is not necessarily an exploitative, colonial galactic civilisation, nor is the outcome necessarily astronomically observable. An information-intensive culture, for instance, would be very difficult to see. The present observational record can only support the much more restricted conclusion that no rapacious galactic civilisations are currently loose in the Galaxy.

 

2. 3 Limits on Observability of Advanced Civilisations

Visual observations of the Galaxy along the plane are limited to a few kiloparsecs; because of interstellar gas and dust obscuration. Even radio VLBI cannot resolve 1 AU Dyson-sphere-like structures beyond a range of a few kpc, and few celestial objects have been mapped to this extreme resolution. High technology activities by ETI such as terraforming and astrophagy might not be immediately obvious at interstellar distances, even to radio astronomers, and certainly a major fraction of the Galaxy could have been colonised without us being able to observe it. Advanced races could have made many starts on transforming the Galaxy but failed for various reasons, yet the observational consequences to humanity would be minimal.

Perhaps sheer scale in constructing observable artifacts is restricted by the Square-Cube law, gravitational tidal forces, available energy sources, and other fundamental physical limitations to astro-engineering projects which we may not yet fully appreciate. For example, large-scale artificial habitats may be more likely to consist of less-observable swarms of small O'Neill colonies rather than gigantic monolithic architectures, as this maximises the ratio of living area to mass. Also, potential but unknown linkage between the existence of ETI and such unexplained astronomical phenomena as the galactic "missing mass," quasars, and exploding galaxies like Cygnus A cannot be overlooked.

A well-ordered galaxy could imply an intelligence at work, but the absence of such order is insufficient evidence to rule out the existence of galactic ETI. The incomplete observational record at best can exclude only a certain limited class of extraterrestrial civilisation - the kind that employs rapacious, cancer-like, exploitative, highly-observable technology. Most other galactic-type civilisations are either invisible to or unrecognisable by current human observational methods, as are most if not all of expansionist interstellar cultures and Type I or Type II societies. Thus millions of extraterrestrial civilisations may exist and still not be directly observable by us.

 

3. THEY MAY BE IN THE SOLAR SYSTEM

Two general categories of evidence are commonly used to argue that ETI are not present in the Solar System. These are (a) lack of purposeful communications, and (b) lack of physical observables. However, we shall argue that null data in each of these two evidentiary classes may be due more to anthropocentric assumptions and the poor observational record than to the absence of ETI in the Solar System.

 

3.1 Lack of Purposeful Communications

The first evidentiary category is lack of purposeful communications: If ETI were here, they would have used their superior technology to contact us, or to get us to notice them. If they are here, where are they?

Ball [21] proposes that we may be part of an interstellar zoo or wilderness preserve, carefully isolated for our own good. Stephenson [22] suggests that if intelligence is rare in the Galaxy, it is more likely to be handled with greater circumspection and care. Kuiper and Morris [23] reiterate the old science fiction notions of culture shock and the idea that we might not be contacted until we reach some intellectual threshold to avoid "extinguishing the only resource on this planet that could be of any value to [ETI]." Papagiannis [24] suggests "confusion and indecision... they might be debating on whether to crush us or help us, postponing their decisions, waiting to see what we are going to do with ourselves."

The author has suggested [25] another more likely possibility - that there is no reason why They should not be silent. It is anthropocentric to assume, for example, that alien spacecraft entering the Solar System on a mission of reconnaissance or self-replication will feel the obligation to announce their presence to us or to request permission to proceed. Probes will probably just ignore us and go on about their business.

It is entirely conceivable that some ETI may not particularly care whether we find them or not, or may actually be interested in communicating with us yet be unwilling to initiate contact unconditionally. For example, they may refuse to speak to beings who occupy only one planet, fearing culture shock to a "One Earth" mentality unaccustomed to dealing with the environmental and cultural relativities of many worlds. The exercise of finding them would serve as an initiation, a kind of "entrance examination" to pass before opening contact. Platt [26] suggests that advanced civilisations may be "like the parents who do not talk to the baby until the baby wakes up."

Or, if life is not especially rare in the Universe, then, rather than carefully shepherding our development with extreme circumspection like an endangered species [21, 22], ETI may adopt a much more casual approach to contact. They may have tremendous confidence in their ability to manage potential contact events to successful resolution, based on numerous Previous similar encounters. In this case, humankind might rate neither ultraconservative wildlife management nor heedless astrophagic exploitation. The most likely response would be careful and unobtrusive observation, with no special effort to conceal the alien presence. Base site would be chosen for reasons of efficiency, maintainability, and low environmental risk.

Of course these arguments may be multiplied endlessly, illustrating the probative weakness of this particular category of null evidence. While we have no convincing evidence of purposeful communication between humanity and advanced ETI, neither do the null data constitute compelling evidence that ETI are not here because this conclusion is contingent upon unknown choices that may be available to such beings and our assumption that they would wish to communicate.

 

3. 2 Lack of Physical Observables

The only true compelling category of evidence is physical observables. For instance, we might observe their spacecraft, their self-replicating machinery, their habitats or strip mines, or a host of other physical manifestations which are the hallmark of technological activity. This evidence also depends on alien choice - ETI might conceal their technology for the same reasons they choose not to communicate. Some activities might be too obvious to disguise, such as planetary xenoforming or astrophagy by voracious swarms of replicating machines, but given a sufficiently advanced technology ETI should be able to conceal most of their mechanisms and activities to high perfection.

The present analysis thus concerns only those ETI who, for whatever reason, choose not to use perfect means to hide all physical evidence of their technological activities. This is the most conservative assumption possible from the standpoint of the Fermi Paradox, as the more restrictive assumption that some or all activities are perfectly disguised leads to a trivial explanation for the apparent absence of ETI in the Solar System.

 

3.2.1 Physical Evidence in the Solar System

What sort of physical evidence might be available for us to find? Material artifacts of a commonplace nature including small tools, debris, garbage dumps, radioactive hot spots, and passive monoliths located on planetary surfaces [27] would be less than 1-10 metres in size. No body in the Solar System has yet been comprehensively mapped to this resolution except Earth, which is excluded from the present analysis, hence none of these commonplace items is observable by our present instrumentation.

If an exploitative Type II civilisation exists or had ever existed in our vicinity, then the Solar System would have been wholly converted to replicating machines mass, our planets sorted into their constituent elements for transshipment or industrial use and the Sun-stripped of its fuel - and of course we would not be here to discuss the outcome of these activities. Since humanity exists, rapacious nearby stellar civilisations are ruled out, much as exploitative galactic civilisations are provisionally excluded by the observational evidence as discussed earlier.

On the other hand, Kuiper and Morris [23] and Stephenson [22] argue that the only plausible interstellar mission which would be launched by a benign civilisation would be one of pure exploration, the pursuit of knowledge as a source of wealth. If this is true, then the most noticeable extraterrestrial artifact we might expect to find in the Solar System would be either a self-reproducing machine system which is building and launching interstellar probes bound for other star systems (or the aftermath of such activity) or just the probes themselves parked here in some convenient locale or orbit [28].

 

3.2.2 Observables of Interstellar Messenger Probes

Detection of probes would be especially challenging, as these could in theory be located almost anywhere. A typical alien probe might be 1-10 metres in size - this is large enough to house a microwave antenna to report back to the senders, and to survive micrometeorite impacts for millions of years, but light enough to fly across the interstellar gulf without consuming unreasonable amounts of energy [29].

A spherical Solar System boundary enclosing the orbit of Pluto consists of 260,000 AU³ of mostly empty interplanetary space and 10¹¹ km² of planetary and asteroidal surface area. To be able to say with any certainty that there is no alien presence in the Solar System, you have to have carefully combed most of this space for artifacts.

Currently the sky has been exhaustively surveyed to perhaps magnitude +14, the Palomar Schmidt Sky Survey extends to +21, and the best available magnitude limit for any telescope on Earth is about m_v = +24. This means that at best, current surveys from Earth might have detected an unmoving, mirror-shiny, optimally-oriented 10-metre object orbiting 0.01, 0.25, and 1 AU from Earth, respectively. If the artifact is smaller, moving, black, or canted at a different angle then it will be even harder to see.

So we can only scan the nearest 4 AU³ of space for probes, but we have at least 260,000 AU³ to search. Even if the Palomar 200-inch telescope was employed exclusively to search for alien artifacts it could reach at most one-millionth of the necessary volume. Orbital space, in other words, is at least 99.999% unexplored for 1-10 metre objects.

A more realistic assessment suggests that the visual detection threshold for alien probes for present-day humanity includes only 10^-5-10^-11 of the potential probe residence volume. This estimate assumes a random search pattern typical of past serendipitous observations which might have discovered alien artifacts if they were present [cf. 28, 29]. Radar and infrared measurements cannot substantially improve this current limit.

How about probes parked on planetary surfaces? Of the 0.1 trillion square kilometres of Solar System territory other than Earth, less than 50 million has been examined to 1-10 metre resolution. So 99.95% is still virgin territory as far as a search for extraterrestrial artifacts is concerned. If objects are buried somewhere or floating in a Jovian atmosphere, there is almost zero chance we could have found them up to now. Even huge 1-10 kilometre artificial alien habitats occupying the Asteroid Belt [24] would appear visually indistinguishable from asteroids to terrestrial observers, and the Belt population itself is poorly catalogued. So it is exceedingly unlikely that we would have spotted an extraterrestrial artifact anywhere in the Solar System unless it was desperately trying to get our attention. And why should it bother to do that?

 

3.2.3 Observables of Self-Replicating Systems

Observation of an operating replicating machine system would be only marginally easier. Likely sites are the Asteroid Belt [24] and the outer Jovian and Saturnian moons [30]. Recent technical studies [31] suggest individual replicating systems may be 100 metres in diameter or less, so a factory system for building probes should not exceed 0.1-1 km in size, again well beyond our ability to see it except on the Moon and portions of Mars. Ignition of fusion rockets to propel daughter probes out of the Solar System is detectable using amateur telescopes, but the observation window is very small and of very short duration. Self-reproducing probes should be able to replicate a whole generation in 1000 years [30] or less, and be quickly on their way, so only mining pits and small debris may remain at this late date.

The total mass of probes needed to explore even the entire Galaxy is astonishingly small. If each self-replicating probe, mass fully-fueled about 10¹⁰ kg [30], makes 10 replicas during each of 11 generations, enough to span the entire Galaxy [32], that is 10¹¹x 10¹⁰ kg = 10²¹ kg or about the mass of Ceres, the largest known asteroid. If the Solar System carried the burden of manufacturing all 10¹¹ probes to explore the entire Galaxy, how could we know if one Ceres-size asteroid had ever been removed from the Asteroid Belt?

And take the argument one step further. Assume that one million extraterrestrial civilisations each pillage the Solar System for materials to build and launch their own million independent probe networks, each covering every star in the Galaxy. The total requirement is still only 10⁶ x 10²¹ kg = 10²⁷ kg, about the mass of Jupiter. It is doubtful we could say for certain if even this much matter had been stolen away sometime in the remote past.

More likely, starfarers won't be greedy and will require each target star system to supply no more than one new generation of replicants. This is only 10¹¹ kg, enough to fill one 1-kilometre crater 40 metres deep or to make one 400-metre-wide asteroid. We'd never miss the mass.

Even more likely, ETI will erect their self-replicating probe factories in uninhabitable star systems to avoid disturbing us and just send nonreproducing exploratory probes here [28]. In this case, no local -mass would be missing and there'd be no surface debris either.

 

4. SUMMARY AND CONCLUSIONS

In this paper the critical assumption that ETI are not present in the Solar System, essential to the logic of the Fermi Paradox argument for the nonexistence of extraterrestrial intelligence, has been challenged. Observations suggest that the Galaxy may be devoid only of rapacious, exploitative civilisations, not of all ETI, at the present time, and that current knowledge of the Solar System is insufficient to support the presumption that ETI are not here. Hence the conclusion that we are alone, based on the Fermi Paradox, is without foundation.

The observational record is sufficiently incomplete that major galactic technologies may yet have gone unnoticed. The strongest conclusion justified by the data is: Due to some special selection effect, galactic civilisations who make extensive and exploitative use of large-scale, astronomically observable, very advanced technology are very rare or nonexistent. This does not rule out planetary- or stellar-scale technical civilisations, nor does it preclude benign, appropriate-technology" galactic civilisations. A well-ordered galaxy would imply the existence of ETI, but the absence of such order cannot prove the nonexistence of ETI.

Two categories of evidence might show that ETI are not present in the Solar System: The admitted lack of purposeful communications (uncompelling because it depends upon unknown choices available to ETI and our assumption that they would wish to communicate) and physical observables.

Astronomical searches to date are extremely unlikely to have observed physical evidence of ETI if it exists. The most easily observable extraterrestrial artifacts would be self-reproducing machine systems, the aftermath of their activity, or interstellar messenger probes. Orbiting probes could be located almost anywhere within a search volume of which only 10^-5-10^-11 has been reliably scanned by terrestrial telescopes, and few observations to date would have been capable of detecting operating replicating machine systems. Even building and launching probes to every star in the Galaxy generates an unobservably small local mass deficit, but most likely the probe factory and its mining pits would be located elsewhere so probes would remain the only observables.

Evidence from astronomy that could confirm or deny the presence of ETI in the Solar System is scanty or nonexistent. Cosmological data arguably also are incomplete, consequently cannot prove the nonexistence of all galactic-scale ETI. The Fermi Paradox cannot logically be raised as an objection to the existence of ETI until these major observational deficiencies have been corrected.

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REFERENCES

1. Dyson, F. J., The search for extraterrestrial technology, in R. E. Marshak (Ed.), Perspectives in Modern Physics, Interscience, pp. 641-655, 1966.
2. Hart, M. H., An explanation for the absence of extraterrestrials on Earth, Quart. J. Roy. Astr. Soc., 16,128-135 (1975).
3. Tipler, F. J., Extraterrestrial intelligent beings do riot exist, Quart. J. Roy. Astr. Soc., 21, 267-281 (1980).
4. Tipler, F. J., Additional remarks on extraterrestrial intelligence, Quart. J. Roy. Astr. Soc., 22, 279-292 (1982).
5. Martin, A. R. (Ed.), The Fermi Paradox: A forum for discussion, JBIS, 32, 4244 34 (1979).
6. Papagiannis, M. D. (Ed.), Strategies for the Search for Life in the Universe, D. Reidel, Boston, 1980.
7. Hart, M. H. and Zuckerman, B. (Eds.), Extraterrestrials: Where Are They? Pergamon Press, New York, 1982.
8. Martin, A. R. and Bond, A., Is mankind unique? JBIS, 36, 223-225 (1983).
9. Cox, L. J., An explanation for the absence of extraterrestrials on Earth, Quart. J. Roy. Astr. Soc., 17, 201-208 (1976).
10. Schwartzman, D. W., The absence of extraterrestrials on Earth and the prospects for CETI, Icarus, 32, 473-475 (1977).
11. Klass, P. J., UFOs Explained, Random House, New York, 1974.
12. Hynek, J. A., The UFO Experience: A Scientific Inquiry, H. Regnery, Chicago, Illinois, 1972.
13. Sturrock, P. A., Extraterrestrial intelligent life, Quart. J. Roy. Astr. Soc., 119, 521-523 (1978).
14. Dyson, F. J., Correspondence, Sci. Am., 210, 8-9, (1964).
15. Campbell, J. W. Jr., Unimaginable reasons, Analog, 65, 4-7, 175-178 (1960).
16. Hart, M. H., Habitable zones about main sequence stars, Icarus, 37, 351-357 (1979).
17. De. San, M. G., Hypothesis on the Origin of UFOs, Editecs, Bologna, Italy, 1979.
18. Stephenson, D. G., Extraterrestrial cultures within the Solar System? Quart. J. Roy. Astr. Soc., 20, 422-428 (1979).
19. Clarke, J. N., Extraterrestrial intelligence and galactic nuclear activity, Icarus, 46, 94-96 (1981).
20. Newman, W. I. and Sagan, C., Galactic civilizations: Population dynamics and interstellar diffusion, Icarus, 46, 293-327 (1981).
21. Ball, J. A., The zoo hypothesis, Icarus, 19, 347-349 (1973).
22. Stephenson, D. G., Models of interstellar exploration, Quart. J Roy. Astr. Soc., 23, 236-251 (1982).
23. Kuiper, T. B. H. and Morris, M., Searching for extraterrestrial civilisations, Science, 196, 616-621 (1977).
24. Papagiannis, M. D., Are we all alone, or could they be in the Asteroid Belt?, Quart. J. Roy. Astr. Soc., 19, 277-281 (1978).
25. Freitas, R. A. Jr., The case for interstellar probes. JBIS, 36, 490-495 (1983).
26. Platt, J. R., in C Sagan, (Ed.), Communication with Extraterrestrial Intelligence (CETI), MIT Press, Cambridge, Massachusetts, p. 151, 1973.
27. Foster, G. V., Non-human artifacts in the Solar System, Spaceflight, 14, 447-453 (1972).
28. Freitas, R. A. Jr., The search for extraterrestrial artifacts (SETA), JBIS, 36, 501-506 (1983).
29. Freitas, R. A. Jr., If they are here, where are they? Observational and search consideration, Icarus, 55, in press (1983).
30. Freitas, R. A. Jr., A self-reproducing interstellar probe, JBIS, 33, 251-264 (1980).
31. Freitas, R. A. Jr. and Gilbreath, Wm. P. (Eds.), Advanced Automation for Space Missions: Find Report, NASA CP-2255, 1982.
32. Valdes, F. and Freitas, R. A. Jr., Comparison of reproducing and nonreproducing starprobe strategies for galactic exploration, JBIS, 33, 402-406 (1980).

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