Journal of the British Interplanetary Society, Vol. 41 , pp.491-496, 1988. THE FEASIBILITY OF INTERGALACTIC COLONISATION AND ITS RELEVANCE TO SETI. MARTYN J. FOGG 44 Hogarth Court, Fountain Drive, Upper Norwood, London SE19 1UY. Since Fermi asked his famous question, "Where is everybody?" it has been increasingly realised that the feasibility of interstellar colonisation has deep implications for the search for extraterrestrial intelligence. In this paper, the feasibility of intergalactic colonisation is discussed. It is argued that a civilisation already technologically advanced enough to colonise its home galaxy would not find the task of settling a neighbouring galaxy to be insuperable. Most Geocentric arguments against SETl that involve interstellar colonisation require the probability of the independent origin of a colonising civilisation in a suitable star system to be~ 10-11 to account for an undisturbed Earth. Including intergalactic colonisation in the scenario requires the said probability to be ~10-22. Ultimately therefore, Geocentric arguments approach to Anthropic Principle arguments. It is contended that a Copernican hypothesis is more likely to be a true explanation for Fermi's Paradox than the "Anthropic" view that the Universe has to be the size it is to host one civilisation (us). 1. INTRODUCTION Many researchers have studied the feasibility of sending human populations on voyages to the stars (e.g. Martin[1], Matloff & Mallove[2], Jones[3], see also JBIS bibliography on interstellar travel and communication) and others have looked at the dynamics of mass interstellar migration (Jones[4,5], Newman& Sagan[6], Bainbridge[7], Jones & Finney[8] and Fogg[9]). Another strategy, that of exploring the Galaxy using "von Neumann machines (henceforth vNMs, von Neumann[10]), intelligent computers capable of self repair and replication, has been advocated by Valdes & Freitas [11] and Tipler [12,13]. What such work suggests is that, although the problems involved in interstellar exploration and colonisation are formidable, they would not be insuperable. There is little doubt that a sufficiently determined and technologically advanced species could 6 settle the entire Milky Way Galaxy (henceforth MWG), within a period estimated between 10 9 10 years. The realisation that within a few centuries the human race might itself be capable of taking its first step to eventual colonisation of the MWG has had profound implications for the Search for ExtraterrestriaI Intelligence (SETI). Fermi's question, "Where are they?", now looms large in any discussion concerning ETl. Scientists who have taken up a "Geocentric” position claim that since there is no evidence for extraterrestrial visitors to the Earth, either now or in the past, then they do not and have never existed (Hart [14], Tipler [12,13]). The geocentric argument is strong, but by no means immune to serious criticism (Freitas[15]). In particular, doubt has recently been cast on the claim that the existence of millions of extraterrestrial civilisations is incompatible with an undisturbed Solar System (Fogg[9]). Fogg has shown that it is possible to “universalize” the "Zoo Hypothesis” of Ball [16] by looking at the development and interaction of ETI in the MWG from a "historical" viewpoint. Assuming the Galaxy was fully colonised by ancient civilisations before the Solar System was formed, then there are powerful reasons why the Earth may have been left undisturbed. This modified Zoo Hypothesis has been termed the “lnterdict Hypothesis”. To appreciate some of the probabilities involved in SETI it is sufficient here to write a much simplified Drake equation: N = N*.fcomb (1) Where N is the number of civilisations now existing, N* is the number of stars in the MWG, and fcomb is the combined probability of the existence of a planet orbiting a given star on which a long lived civilisation can arise. It turns out that Geocentric arguments concerning ETI are actually more profound than they at first appear. This is because they are normally framed within the MWG context. For instance Hart [14] states in his abstract, "We observe that no intelligent beings from outer space are now present on Earth. lt is suggested that this fact can best be explained by the hypothesis that there are no other advanced civilisations in our Galaxy." Tipler [12] states in his introduction, “l shall argue... that the probability of the evolution of creatures with the -10 technological capability of interstellar communication... is less than 10 , and thus we are the only intelligent species now existing in this Galaxy." i.e. for terrestrial civilisations to be the -10 only one in existence in the MWG fcomb < 10 /fc., where fc is the fraction of colonising civilisations. However, it is clear that this is not what Hart and Tipler actually believe. Hart [17] has published calculations in which he “generously” assigns the probability of the origin of life -30 on a given suitable planet to be ~10 . Tipler [12] has used an interpretation of the Anthropic 20 Principle to claim that the universe must contain 10 stars in order to produce a single intelligent species and terminates his Geocentric tour de force (Tipler [13]) with the confident sentence "...we shall have to accept the fact that we are alone in the universe.” In other -20 -20 -9 words, what Hart and Tipler truly require is for fcomb.fc < 10 . Since 10 << 10 the feasibility of intergalactic colonisation must be examined. 2. THE FEASIBILITY OF INTERGALACTIC TRAVEL ls it possible that, in the remote past, an extraterrestrial civilisation could have constructed vessels capable of crossing the vast intergalactic gulf? Could such ships have remained functional during the journey? Could they have brought life with them, and might they even have been capable of reconstructing a replica of the parent civilisation? The best way to speculate over these questions is to try and foresee whether in our own future we might be capable of colonising a nearby galaxy, given a considerable increase in technological capability. References relevant to intergalactic colonisation are scant and can be subdivided according to the principal information system carried as part of the galaxyship's payload. 2.1 A Human Population. 2.1.1 lntergalactic Worldships. "Worldships" the size of small nation states have been designed to take human populations on a multi-generation long journey to other stars (Martin [1]). However, because of the great length of time involved in an intergalactic voyage the problem of creating a closed ecosystem capable of supporting a viable population is far greater. The only self-contained ecosystem we have knowledge of that remains habitable over a time scale of millions of years is the Earth's biosphere. Life flourishes under conditions maintained by atmospheric, chemical, geological and biological feedback loops, driven ultimately by sunlight and the internal heat of the Earth. A galaxyship carrying a fully and continuously functioning ecosystem might thus have to be the size of a small planet and capable of carrying a powerful long term energy source. Burruss and Colwell [18] in a recent article have described a colossal galaxyship that they claim would be capable of transporting a population of fifty billion on a 5 - 10 Myr journey to the Andromeda galaxy (M31). Their design is in a sense too “heroic” to be taken seriously. 14 Not least amongst its many difficulties is ~5x10 kg of anti-matter is needed comprising ~50 percent of the ships mass. This is an absolutely enormous quantity – an amount that a million of the space based anti-matter production factories outlined by Forward [19] would take over a billion years to produce. Moreover, the bulk of this antimatter has to be held in perfect isolation for the first 50,000 years as the ship accelerates to cruise velocity. One single containment failure that were to occur over this time period would probably cause a selfpropagating explosion that would consume the whole ship. Another problem is that there is no provision for spin generated artificial gravity in the ship's design. The inhabitants would have to exist under conditions of near zero G for millions of years. Since this is comparable with evolutionary time scales, it would be likely that, should the Burruss/Colwell vessel ever reach M31, the crew would no longer be human. For these reasons, among many others, it is probably safe to discard Burruss and Colwell's model for a qalaxyship. 2.1.2 Hyper-Relativistic Travel. Another way to send human beings over intergalactic distances might be to greatly reduce the elapsed time experienced on board ship by taking advantage of the retativistic time dilation that occurs at velocities close to the speed of light (c). A ship that were to undergo a constant acceleration of 1 g to midpoint followed by a constant 1 g deceleration to the fringes of M31 would take 2 million years as experienced by an observer at rest, but only a little less than thirty years experienced by the crew. The problem of maintaining the habitability of the ship's environment over this time period would be relatively trivial. However, the energy required to propel the galaxyship would be enormous and could never be obtainable solely from fuel carried with the vessel (Purcell [20]). Bussard [21] has proposed that it might be possible to accelerate continuously to relativistic velocities by using a fusion powered ramjet, which gathers its fuel by scooping up the interstellar gas in its path. However, Martin [22] has shown that should it be possible to build a Bussard ramjet, there would exist structural and magnetic limitations that would impair its ability to maintain high accelerations at velocities close to c. Even if these problems could be overcome, the Bussard ramjet might have great difficulty operating in intergalactic space -6 where the gas density is lower by a factor of 10 . Froning [23, 24] has speculated that should it be possible to tap the quantum fluctuations of the vacuum, a “quantum ramjet” might be built - a spacecraft with unlimited power supply. This vessel would not be subject to the limitations of the Bussard ramjet and could indeed accelerate ever close to c. However, irrespective of the propulsion system used, hyper-relativistic intergalactic travel would be fundamentally limited. The velocity of the ship at midpoint would be so close to c that no interaction with intergalactic matter would be permitted. Dust grains would impact like cannon shells and hydrogen atoms would take on the characteristics of a lethal and penetrating form of cosmic radiation. Thus, leaving aside the feasibility of a "quantum drive", the use of time dilation to significantly reduce elapsed intergalactic voyage times would not be practical unless a way could be found of preventing impacts from oncoming particles. 2.1.3 Human Life Suspension. Might it be possible to overcome the great lengths of time involved in intergalactic travel by dispatching a human population in some kind of "frozen" or "suspended" state? Much research remains to be done in the field of human life suspension, and whether or not one believes that "suspended animation” will ever be possible depends more on one's faith in future technological advances than on current knowledge. The artificial induction of a form of hibernation, similar to what is seen in squirrels, in which the metabolism, and presumably ageing processes, are considerably reduced, shows some promise (Hands [25]). However, whilst this might be useful for interstellar travel, it would be worthless for intergalactic travel. What’s required is a complete suspension of all metabolic and degenerative processes. So far, no deep frozen human being has been revivified, but many more simple forms of life have, such as plant spores, bacteria, and zygotes of more advanced organisms. 2.2 Simple Life Units. It is likely that simple life units, such as bacteria and other single celled organisms, could remain viable for millions of years if maintained at temperatures close to absolute zero (Sneath [26]). lf protected within a spacecraft, it is likely that a viable population of bacteria could complete a long interstellar, and even an intergalactic, journey. Indeed, Crick& Orgel [27] have proposed the "Directed Panspermia" (Dp) hypothesis in which primitive life on the Earth originated by a deliberate act of seeding by ETl. They point out that should the ethical requirement of ETI be merely to spread life, throughout the stars rather than higher organisms and civilisation, then the dlspatch of bacteria would be ideal. Frozen bacteria could survive over far longer periods of time and could thrive in a far wider range of environments. Crick [28], in a book about DP, emphasised that whatever distance it is feasible for man to travel, bacteria could go further – as far as M31. However, DP as proposed by Crick and Orgell is unsatisfying because any complex biosphere on a seeded planet would take billions of years to develop from bacterial ancestry. In other words it is a very inefficient method for distributing diverse forms of life. This inefficiency might be overcome, given a higher leveI of technology that Crick and Orgel were prepared to consider. DNA specifies the complete genotype for a given organism. Should it be possible to dispatch higher organisms in the form of zygotes, then they might be artificially developed to multi-cellular maturity after a long space voyage. Human embryos have already been stored at low temperatures and then have been thawed and allowed to develop normally after implantation into the uterus. The day may not be far off when it might be possible (should ethical considerations change) to conduct the entire process of foetal development from embryo to baby under artificially controlled conditions. The cargo of an advanced DP ark could consist of a zygote bank containing an entire "potted ecosystem" and a von Neumann Machine. Upon arriving in a system the vNM could replicate, 4 terraform a planet, synthesise a biosphere and foster a civilisation on a time scale of 1O 5 10 yr (Fogg [29]). Should it be possible to construct such a DP ark to survive an intergalactic journey, then this is a way intergalactic colonisation might be achieved. 2-3 Raw Information. Tipler [12] has suggested that a better alternative to sending frozen cells on a long space voyage is to send a vNM with the blueprints for a complete biosphere within its memory. Upon arriving at its destination, and having located or constructed a suitable habitat, the vNM could hen create life forms by direct synthesis from inanimate material. Such an advanced vNM would have to be what is termed "universal", i.e. to have a capability of constructing almost anything not ruled out by the laws of physics, given the appropriate information. Whether machines with such "Godlike" powers will ever be built is uncertain. Suffice it to say that if they are achievable, then so is intergalactic colonisation. 3. AN INTERGALACTIC ARK. To arrive at an outline for a vessel that might seed another galaxy with life, the following constraints were deemed necessary: 1. The ship should have a cruise velocity of at least 0.5c . 2. Colonisation method follows the outline in Section 2.2. 3. More practical or feasible propulsion methods to antimatter or quantum drives should be used. 3.1 The Payload. A vNM and zygote bank would be the essential elements of the payload. Freitas [30] has designed a self reproducing interstellar probe (REPRO) with a vNM payload 6 of ~10 kg and with a reproductive time of ~1000 yr. The mass of the vNM payload of the intergalactic ark is assumed to be identical. (lt is possible that vNMs could be made orders of magnitude less massive with much faster doubling times (Drexler [31]), but here we shall confine speculation to the comparatively "primitive" bulk technology of REPRO.) Freitas noted that a Daedalus vehicle (Martin [32]) would be large enough to carry his vNM payload, but that since it would have to decelerate from cruise velocity of 0.1 c to rest in the destination system, an extra "zeroth" stage needed, 10 giving the mass of a complete REPRO to be ~10 kg. For an intergalactic cruise at 0.5 c, this configuration would be unworkable, as fusion fuels would require enormous mass ratios. Acceleration to this velocity is dealt with in Section 3.3; here we are dealing with the payload stage, which must have some manoeuvring capacity upon arriving at M31. If a second stage Daedalus propulsion system is used, for this purpose, 7 the payload stage would mass ~5.5x10 kg giving a ΔV ~0.04c. Including a mass of spare parts and facilities to be discarded after the intergalactic cruise, the total payload mass is 7 assumed to be ~10 kg. 3-2 Shielding. The vehicle must be protected from the erosive and heating effects of impacting material. Table 1 shows typical mass densities of gas and dust likely to be encountered by a galaxyship during its journey. TABLE 1 Martin [33] has studied the problem of bombardment by interstellar material as part of the Daedalus project. The general conclusion was that impacting dust would cause erosion, and interaction with gas would cause heating. Since the density of intergalactic dust is extremely low, the principal erosion risk occurs as the vehicle travels through the fringes of each galaxy. Therefore it is decided here to design the intergalactic vehicle to be able to travel through 2000 LY of galactic space. The following two equations from Martin [33] allow an estimate of the shielding thickness to be made. The flux of energy impacting the vehicle is: (2) where ρ = the density of dust (10 -23 kg/m ) and β = v/c = 0.5. 3 The mass loss rate per unit area is: (3) where ϵ = the fraction of the energy resulting in destructive damage (estimates range from 01, a reasonable value ϵ= 0.25); Hs is the latent heat of sublimation of the shield material 7 11 (taking graphite, Hs = 6x10 J/kg); and t is time in seconds (for 2,000 LY at 0.5c, t = 1.26x10 s). -7 2 Inserting these values in to equations (2) and (3) gives dm/Adt ~10 kg/m .s, with a total 2 mass loss per unit area ~12600 kg/m . Assuming the shield is made from solid graphite with a 3 density of 2,300 kg/m , then ~5.5 m of shield would be eroded. A safety factor of 4 requires the thickness to be increased to 22 m. A shield 80 m wide would be sufficient to cap the 8 vehicle, this gives a shield mass of 2.5x10 kg. Thus, the total intergalactic ark payload mass, 8 including payload stage and shielding ~2.6x10 kg. 3.3 Acceleration. Enormous quantities of reaction mass would be required for any vessel carrying its own nonanti-matter fuel to accelerate to and decelerate from 0.5c. Thus, a method of accelerating the ark to cruise velocity by means external to the spacecraft is desirable. A number of methods of beamed power propulsion have been suggested, such as pellet stream propulsion (Singer [34]), and photon pressure propulsion by laser beam (Forward [35]) or microwave beam (Forward [36], Jones [3]). Here we concentrate on the laser pushed method of launching the intergalactic ark. Forward [35] has outlined designs for laser pushed interstellar vessels of varying sizes. A beam of 1 μm wavelength laser-light is projected from a large array of solar pumped lasers, in orbit closer to the Sun and is focused by a 1000 km diameter Fresnel zone lens situated between the orbits of Saturn and Uranus. The beam is intercepted by a 16 nm thick aluminium Iightsail, which accelerates, towing its payload. Forward's largest lightsail was designed to be accelerated to 0.5c for a trip to ϵ Eridani and thus has performance similar to that required for an intergalactic mission. -5 The surface density for the lightsail and supporting structure was assumed to be 7.3x10 2 kg/m . The payload mass was assumed to be (sail + structure mass)/ e, adding a further -5 2 -4 2 2.7x10 kg/m , and giving an overall surface density of the entire ship ~10 kg/m . Thus for 8 8 the ark payload of 2.6x10 kg, the mass of sail and structure ~7.07x10 kg and the sail diameter ~3500 km. Erosion of the lightsail by interstellar dust grains, with a concomitant reduction in its performance, is a potential problem with the lightsail that Forward originally disregarded (Crawford [37]). However, so long as the lightsail is only required for the relatively brief acceleration phase then this hazard can be ignored. Forward calculated the thermal 2 acceleration limit of his lightsail to be 3.0 m/s . If the ark is accelerated at this maximum value, the laser power needed to provide the thrust is: (4) where M is the total vehicle mass, a is the acceleration and η is the reflectance of the sail (Forward’s value for η =0.82). 17 17 A power of 5.3x10 W is required initially, rising to 9.1x10 W at 0.5c to counteract the Doppler shifting of incoming laser light. (By comparison, this power is 12 times that needed to launch Forward's ϵ Eridani starship, but only half that required to launch the microwave pushed interstellar vessel of Jones [3]. The acceleration time to 0.5 c is 1.6 yr and the laser cut off point is 0.4 Ly. The bulk of the light sail and structure could then be discarded. 3.4 Deceleration. Dyson [38] has suggested that one way to avoid carrying large amounts of reaction mass to decelerate from high velocity is to make use of an “Alfven propulsion engine”. He proposes generating drag by electromagnetic coupling between the vehicle and interstellar plasma by extending a large sail of conducting wires 1μm in diameter spaced 10 m apart. Although such a system would work close to the Earth where the Alfven velocity is high, detailed analysis remains to be done to determine whether it might operate in interstellar space. However, should any difficulties be overcome, the stop time would be: (5) -21 where is the plasma density (10 surface area of the Dyson sail. 3 4 kg/m ), Va is the Alfven speed (~l0 m/s), and A is the - To slow down from 0.5 c to rest over a distance of 1000 LY requires a deceleration of -1.2x10 2 m/s , over a period of 4000 years. The surface density of the craft must therefore be M/A = -6 2 1.26x10 kg/m , two orders of magnitude lower than the launch configuration, but a factor of 3 ~10 greater than that envisaged as the ultimate by Dyson. Taking the surface density of -10 2 Dyson's sail as 5x10 kg/m , then to achieve the desired overall surface density, a sail 5 diameter of 16,200 km is needed, with a mass of ~10 kg. This mass could be carried along with the payload or could be processed from 0.02 percent of the mass of the original acceleration sail. 3 3.5 Survival. A crucial question is whether the intergalactic ark could remain functional after a journey of 4 Myr. The temperature of the vessel would equal 3 K, the temperature of space, plus an additional increment from heating by interaction with local gas. Assuming an aspect ratio (the ratio between heated to radiating areas) of 0.1, the temperature of the vessel would be raised by ~8 K whilst passing through the intergalactic medium (see Martin [33]). Whilst travelling through the fringes of galactic space, friction with the much denser interstellar medium would heat the vessel by ~255K. Frozen biological material might need additional refrigeration at the beginning and the end of the mission, but during the intergalactic cruise, about 99.8 percent of the journey, the equilibrium temperature of the ark would be low enough to preserve the contents of the zygote bank almost indefinitely. A greater difficulty is likely to be the long term reliability of the ship’s systems and the vNM. Unfortunately nothing is known about potential reliability problems over such long periods. The provision of a nuclear power source capable of lasting the journey would be feasible as there are plenty of suitable isotopes with half lives well in excess of 4 Myr. Given such a power source, the ship would be capable of self testing and repair of components when they fail. A self-test and repair capability was found to be essential to ensure a high probability of success for the Daedalus mission (Grant [39]). The length of an intergalactic voyage would be 5 ~10 times greater than that envisaged for Daedalus, which suggests that ensuring the reliability of the intergalactic ark would be a problem five orders of magnitude more difficult to solve. However there are mitigating factors. Grant [39] (writing in 1978) noted that there had been an order of magnitude improvement of component reliability over the past thirty years, but to keep the Daedalus design within the scope of "foreseeable” technology it was decided to assume only one further order of magnitude improvement into the future. Should component reliability continue to improve geometrically at this same rate then the reliability required by an intergalactic mission might be achievable in a minimum of 200 years. To support the arguments of this paper however, such rapid improvement rates are not required. Galaxyships might only be dispatched after a civilisation has first settled its own galaxy - thus the technology available might be millions of years, rather than a couple of centurles ahead of our own. Grant [48] also points out that the self-repair scheme used by Daedalus (so called Repair-onRemoval) would not be the ultimate possible. A scheme where new items are remanufactured from materials recovered from worn out items incapable of repair would be the ultimate limit to keeping a machine in working order. Lastly, Grant [40] dismisses the importance of dormancy (the shutdown of systems) for reducing the component failure rate during an interstellar voyage. However, for an intergalactic voyage, where most of the ark's systems would be shut down for very long periods dormancy maybe a far more important factor in the survival equation. It would appear therefore that whilst vNMs and galaxyships built with the technology of the near future would be too unreliable to survive an intergalactic journey (Grant [41]), their survival for 4 Myr is nonetheless feasible in principle. lt is perhaps unwise to claim that Intergalactic travel will forever be so close to being impossible as to make no difference. This used to be claimed with confidence in relation to interstellar travel (PurcelI [20]) until the assumptions underlying the impossibility argument were exposed as over-restrictive. lf thousands of the intergalactic arks were to be dispatched, each with a large store of spare parts and raw materials, and utilising technology well ahead of our own, then some of them might well survive to complete the mission. 3.6 A Programme For Intergalactic Colonisation The mission might be divided into the following stages: 1. Construction of laser subsystem and ark fleet. This would best be done in a star system close to the fringes of the MWG, in a relatively dust free region of space. Should the human race ever prove equal to the task of colonising the MWG and of reaching such a star system, the immense industrial effort involved in an intergalactic colonisation project should not lie beyond practical means. Disassembly of just one gas giant moon would provide enough mass for >40,000R EPRos (Freitas [30]). 2. Launch phase. Intergalactic arks are accelerated to 0.5c by laser pushed lightsail. One ark is dispatched every 2 years, per existing laser subsystem. 3. Post Launch phase. All but 0.02 per cent of lightsail and supporting structure discarded. Remaining mass stored behind erosion shield. Power and activity shut down to minimum levels. 4. Cruise phase. Long periods of "dormancy", punctuated by regular intervals of self test, repair and operation of sensors (4 Myr). 5. Pre-deceleration phase. Mapping of local fringe of M31, reconstitution of stored mass into Dyson deceleration sail. 6. Deceleration phase. Deceleration by Alfven propulsion engine from 0.5c to rest relative to the average velocities of local stars. Aquisition of a suitable target star system (4,000 years). 7. Arrival phase. Dyson sail discarded. Payload stage detaches from remains of shield. Arrival in, and exploration of, star system using fusion propulsion. 8. Growth phase. vNM replicates. Construction of an industrial base. 9. Restoration phase. 9a. Construction of space habitats, or terraforming of suitable planets. Ecosynthesis and fostering of a new human civilisation (~10,000 years). 9b. Launch of new DP arks to neighbouring star systems. 10. Realisation of final objective. Development of human civilisation within Andromeda Galaxy. 4. RELEVANCE TO SETI Since it appears that an advanced civilisation might be successful in an attempt to colonise other galaxies, it therefore follows that Fermi's question has relevance not only to visiting ETs -20 -11 from the MWG, but from other galaxies as well. In other words, if 10 << fcomb.fc << 10 then using the reasoning of Hart and Tipler, extragalactic ETs should still have colonised the Earth! Table2 shows the distance scales of large structures in the Universe. For the Earth to remain unvisited we require: comb c (6) where Vc is the intergalactic colonisation wavespeed, Tc is the time over which colonisation has been ongoing and R is the radius of the observable Universe. Taking Vc = 0.5 c, 9 -18 -11 Tc = 5x10 yr (Fogg [9]) and R ~20 MLY, then fcomb.fc ~ 5x10 . This value is still <<10 . The implication of intergalactic colonisation is that all Geocentric arguments against SETI ultimately approach to the kind of Anthropic Principle arguments considered by Tipler: The Universe is big enough to contain just one intelligent species, us. This requires the -20 acceptance of a probability as low as 10 for the origin of life and civilisation in a given star system. Two mutually exclusive consequences follow from this: 1. Terrestrial civilisation is an exceptionally unlikely fluke. 2. There is a flaw in the Geocentrists's arguments. This author favours consequence 2 - it is not right to set aside the Copernican cosmological principle when considering SETI. The position of human observers is not privileged. Thus, a Copernican hypothesis that allows for the simultaneous presence of both extraterrestrial civilisations and an undisturbed Solar System (such as Fogg [19]), and which would have a -20 probability of >> 10 of being broadly correct, might well provide a more likely explanation for the absence of ETs than the Geocentric proposition that we are alone in the Universe. 5. Acknowledgements My thanks to Richard Taylor for helpful comments and criticism. My thanks also for the assistance given to me by the librarians of the Royal Astronomical Society. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. A.R. Martin (ed.), “Special Issue on World Ships”, JBIS, 37(6) (1984). G.L. Matloff & E.F.Mallove, “The first interstellar colonisation mission", JBIS, 33, 84 (1980). E.M. Jones, "A manned interstellar vessel using microwave propulsion: a Dysonship,” JBIS, 38, 270-273 (1985). E.M.Jones, "Colonisation of the galaxy,” Icarus, 28, 421-422, (1976). E.M. Jones, "Discrete calculations of interstellar migration and settlement,” Icarus, 46, 328-336 (1981). W.l. Newman & C. Sagan, “Galactic civilisations; population dynamics and interstellar diffusion,” Icarus, 46, 293-327 (1981). W.S. Bainbr idge, "Computer simulation of cultural drift: limitations on interstellar colonisation,” JBlS,37,420-429 (1984). B.R. Finney & E.M. Jones (eds.) , “Interstellar migration and the Human Experience,” University of California Press (1985). M.J. Fogg, “Temporal aspects of the interaction among the first galactic civilisations: the Interdict Hypothesis," Icarus, 69, 370-384 (1987). J. von Neumann, The theory of self reproducing automata, ed. A.W. Burks, University of Illinois Press, Urbana (1966). F. Valdes & R.A. Freitas, "Comparison of reproducing and nonreproducing starprobe strategies for galactic exploration,” JBIS, 33, 402-408 (1980). F.J. Tipler, "Extraterrestrial intelligent beings do not exist,” Q.J.R.Astron.Soc., 21, 267-281 (1981). F.J. Tipler, "Additional remarks on extraterrestrial intelligence.” Q.J.R.Astron.Soc., 22, 279-292 (1981). M.H. Hart, "An explanation for the absence of extraterrestrials on Earth," Q.J.R.Astron.Soc.,16, 128-135 (1975). R.A. Freitas, "Extraterrestrial intelligence in the Solar System: resolving the Fermi Paradox,” JBIS, 36, 496-500 (1983). J.A. Ball, "The Zoo Hypothesis," Icarus, 19, 347-349 (1973). M.F. Han, "Atmospheric evolution, the Drake equation and DNA: sparse life in an infinite Universe," in Extraterrestrials: Where are they? Eds. M.H. Hart & B. Zuckerman, pp.154-165 Pergamon Press, Oxford (1982). R.P.Burruss & J. Colwell, "lntergalactic travel: The long voyage from home," The Futurist, 21(5), 29-33 (1987). R.L. Forward, "Antimatter propulsion,” JBIS, 35, 391-395 (1982). 20. E. Purcell, "Radioastronomy and communication through space," in Interstellar communication. ed. A.G.W. Cameron, pp. 121-143, W.A.Benjamin Inc. (1963). 21. R.W. Bussard, "Galactic matter and interstellar flight,” Astonaut. Acta, 6(4), 179-194 (1960). 22. A.R. Martin, “Some limitations of the interstellar ramjet," Spaceflight ,14(1), 21-25, (1972). 23. H.D. Froning, "Propulsion requirements for a quantum interstelllar ramjet,"JBIS, 33, 265-270 (1980). 24. H.D. Froning, "Use of vacuum energies for interstellar spaceflight," JBlS, 39, 410-415 (1986). 25. J. Hands, “Suspended animation for space flight," JBlS, 38, 139-142 (1985). 26. P.H.A. Sneath, "Longevity of micro-organisms," Nature, 195, 643-646 (1962). 27. F.H.C. Crick & L.E. Orgel, "Directed Panspermia ,” Icarus, 19, 341-346 (1973). 28. F.H.C.Crick, Life ltself, Macdonald & Co (Publisher) Ltd (1982). 29. M.J. Fogg, "The terraforming of Venus," JBIS, 40, 551-564 (1987). 30. R.A. Freitas, "A self-reproducing interstellar probe," JBIS, 33, 251-264 (1980). 31. K-E. Drexler, Engines of Creation, Anchor Press Doubleday, New York (1986). 32. A.R. Martin (ed.), "Project Daedalus,” JBIS supplement (1978). 33. A.R. Martin, "Project Daedalus: bombardment by interstellar Material and its effects on the vehicle." in "Project Daedalus," ed. A.F. Martin, JBIS Supplement, pp.116-121 (1978). 34. C.E. Singer, "Interstellar propulsion using a pellet stream for momentum transfer," JBIS, 33, 107-115 (1980). 35. R.L. Forward, "Round trip interstellar travel by laser pushed lightsails,” J. Spacecraft, 21, 187195 (1984). 36. R.L. Forward, "Starwisp: an ultra-light interstellar probe," J.Spacecraft , 22, 345-350, (1985). 37. I. Crawford, correspondence in JBIS, 39, 328 (1986). 38. F.J. Dyson, "Interstellar propulsion systems," in Extraterrestrials: where are they? eds. M.H. Hart & B. Zuckerman, pp.41-45, Pergamon Press, Oxford (1982). 39. T.J. Grant, "Project Daedalus: the need for on-board repair," in "Project Daedalus," ed, A.R. Martin, JBlS supplement, pp.172-177 (1978). 40. T.J. Grant, "ln-flight maintenance: Part 1- the policy choice," JBlS, 34, 381-391 (1981). 41. T.J.Grant, "Unreliable von Neumann machines," JBIS (forth-coming)