World ships: Feasibility and Rationale
WWorld ships: Feasibility and Rationale
Andreas Makoto Hein ∗ , Cameron Smith , Fr´ed´eric Marin , and Kai Staats Initiative for Interstellar Studies, 27-29 South Lambeth Road, London SW8 1SZ, United Kingdom Department of Anthropology, Portland State University Portland, OR, 97207, USA Universit´e de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, F-67000Strasbourg, France Arizona State University Interplanetary Initiative, Phoenix, Arizona, USA
Abstract
World ships are hypothetical, large, self-contained spacecraft for crewed interstellartravel, taking centuries to reach other stars.Due to their crewed nature, size, and long triptimes, the feasibility of world ships faces anadditional set of challenges compared to in-terstellar probes. Despite their emergence inthe 1980s, most of these topics remain unex-plored. This article revisits some of the keyfeasibility issues of world ships. First, defi-nitions of world ships from the literature arerevisited and the notion of world ship posi-tioned with respect to similar concepts suchas generation ships. Second, the key questionof population size is revisited in light of re-cent results from the literature. Third, socio-technical and economic feasibility issues areevaluated. Finally, world ships are comparedto potential alternative modes of crewed inter-stellar travel. Key roadblocks for world shipsare the considerable resources required, shift- ∗ Corresponding author. E-mail: [email protected] ing its economic feasibility beyond the year2300, and the development of a maintenancesystem capable of detecting, replacing, and re-pairing several components per second. Theemergence of alternative, less costly modes ofcrewed interstellar travel at an earlier point intime might render world ships obsolete.
World ships are hypothetical large, self-contained, self-sufficient crewed spacecraftfor interstellar travel. Large, artificial habi-tats appeared in the literature as early as 1929in Bernal’s ”The World, the Flesh and theDevil” [13]. However, the notion was exten-sively discussed for the first time in a spe-cial issue of the Journal of the British Inter-planetary Society (BIS) in 1984. Martin [56]characterizes a world ship as a large, lumber-ing vehicle, moving at a fraction of a per centof the speed of light and taking millennia tocomplete a journey between stars. Martin [56]presents a rationale for world ships, cost es-timates, and how scenarios for their construc-1 a r X i v : . [ phy s i c s . pop - ph ] M a y able 1: Crewed starship categories with respect to cruise velocity and population size. Population sizeCruise velocity [%c] < < > >
10 Sprinter Colony ship - <
10 Slow boat Colony ship World ship < An attempt to distinguish between differentconcepts for crewed interstellar travel was pro-vided in Hein et al. [38]. The distinction ismade based on two dimensions: cruise veloc-ity and population size. Crewed starships withpopulations below 1000 and a velocity higherthan 10% of the speed of light are called“sprinter”, slower starships with a similar crewsize “slow boat” and starships with a popu-lation size below 100,000 are called “colonyship”. World ships are defined as crewed star-ships with populations over 100,000 and a ve-locity below 10% of the speed of light. Asa result, we get the following three criteria,adapted from [38]: • Self-sufficiency: thousands of years3
Population size: > • Cruise velocity: < c , asthere is no physical or engineering reason whyworld ship velocities should be limited to be-low 0.01 c . As mentioned in the original paper,the most crucial parameter is trip time, whichwe would consider at least on the order of cen-turies.A more fundamental issue with the exist-ing definitions is that they do not explicitlyreflect on the meaning of ”world” in ”worldship.” A ”world” goes beyond self-sufficiency and a given population size. ”World” com-monly denominates Earth with all life and hu-man civilization. If this is what we meanby ”world” in ”world ship,” any spacecraftwith a closed habitat containing life and a hu-man civilization could be called ”world ship.”However, this interpretation of ”world” hasthe connotation of a habitat with an enormoussize. We can even imagine a habitat the sizeof a planet, along with the living conditionson a planet. We will later present such aplanet-sized world ship, based on the McK-endree Cylinder in Section 3. The etymologyof ”world” allows for an alternative interpre-tation, where ”world” indicates a material uni-verse or ontology. A ”world ship” would thenbe a ship which, for humans on-board, wouldrepresent ”all there is,” not only in a materialsense (what is inside the habitat, spacecraftsubsystems, etc.) but also in terms of what hu-mans would conceive as the ”reality” in whichthey live. Hence, departing from the existingdefinitions in the literature, interesting new in-terpretations of world ships are possible, goingback to the meaning of ”world.” World ships designs are usually dominated bya large habitat section and a comparativelysmall propulsion section. All other subsys-tems of crewed spacecraft are also present,however, their size is much smaller, com-pared to the habitat and propulsion subsystem.Only few engineering designs of world shipshave been presented in the literature. Mat-loff [58] presented a world ship based on anO’Neill “Model 1” colony [63] in 1976. Twocylindrical habitats are attached to the propul-sion system, which is placed between them.4igure 2: World ship based on the Deadalus fusion propulsion system and stacked StanfordTori [38]. Artistic impressions by Adrian Mann (left) and Maciej Rebisz (right).A Deadalus-type fusion propulsion system isused. Power is provided by fusion reactors.Deceleration is taking place via an electricsail. O’Neill himself proposed the use of anO’Neill colony with an antimatter propulsionsystem as a world ship [64]. Such world shipswould gradually move out of the Solar Sys-tem and embark on an interstellar trip. How-ever, except for the propellant mass, no detailsabout the design were given.More recently Hein et al. [38] have pre-sented a world ship design with stacked Stan-ford Tori for population sizes on the orderof to , shown in Fig. 2. Similar to[58], this world ship design is based on theDaedalus fusion propulsion system and a habi-tat design borrowed from an O’Neill colony, inthis case the Stanford Torus. Fig. 2 also showsthe dust shield put on top of the Stanford Torusfacing flight direction. The authors of [38]have subsequently further developed the de- sign, in order to reduce the overall mass of thespacecraft, which is dominated by the shield-ing mass for the habitat ( > . ∗ kg , and apopulation of ∗ , about 12 times the cur-rent human population on Earth [59]. A worldship of that size would be in principle feasible,given the resources in the Solar System [59]. Table 2 provides an overview of key param-eter values of world ship designs in the litera-ture. The population size and cruise velocity isof the same order of magnitude for all designs.However, there are orders of magnitude differ-ences for the dry mass and propellant mass.These differences are a result of different as-sumptions regarding the size of the habitat.The Bond and Martin world ships are repli-cating living conditions in sparsely settled ar-eas on Earth. The Stanford Torus rather repli-cates an urban or suburban area with a com-paratively high population density. Finally, theEnzman starship seems to rather replicate ahigh-density urban area. The population sizeis assumed to increase 10 times during the trip.For the habitat mass, equipment, and consum-ables, a mass between 150 t/person at the be-ginning of the journey and 15 t/person at theend is assumed. To conclude, existing world6igure 4: Size comparison of “Dry World” and “Wet world” type world ships (image credit:Adrian Mann).ship designs are based on a fusion propulsionsystem and a large habitat. The habitat sizeand mass depends on the underlying assump-tions about the environment in which the crewwould live. In the following, we decompose world shipfeasibility into biological, cultural, social,technical, and economic criteria. Biologicalfeasibility includes the genetic health of thepopulation during the trip and at the pointwhere they start a new settlement at the targetdestination. Hence, an important preconditionfor a world ship, we must assume that habitatsin which human populations can live out mul-tiple generations can be constructed. These will be informed by decades of life in otherbeyond-Earth settlements, such as Mars and/ or orbital communities, such as describedin [38]. Studies in closed-system ecologyare underway or have been demonstrated tosome extent with Biosphere-2 or BIOS-3. Weunderstand genetic health as states of beingadapted to a set of environmental factors wellenough to ensure successful self-replication.Cultural feasibility includes how knowledgeis transferred and preserved, including knowl-edge which is essential for living on the worldship and starting a settlement at the target des-tination. Social feasibility includes, but is notlimited to criteria that are related to the or-ganization of the society on-board of a worldship, such as its stability. Technical criteriaare related to the technologies used on a worldship, their maturity and performance. Eco-nomic criteria are related to the economic pre-conditions that allow for the development of7able 2: World ship designs from the literature with key values
Design Popula-tion size Dry mass[tons] Propellantmass[tons] Cruisevelocity[%c]Enzman world ship [20] · Torus world ship [38] · Dry world ship - Mark 2A [15] . · . · Dry world ship - Mark 2B [15] . · . · Wet world ship [15] . · . · The project of interstellar voyaging is ul-timately meant to preserve and spread hu-man life in space, an idea which is rootedin various cultural traditions, ranging fromthe ’Great Navigator’ in Polynesian culture to ’leaving thd cradle’ narrative by KonstantinTsiolkovsky. Therefore it builds out from thecentral concerns of the human body. For theexploratory period of short-term spaceflight,the concerns of the individual body were mea-sured in days and months, or up to a year.These are the scales of biology and flight phys-iology. As we move towards considerationof permanent space settlement and even inter-stellar voyaging to exoplanets, concern mustexpand to include issues of individual bod-ies arranged as families, families arranged ascommunities, communities as a population (ademe’), and populations as cultures. Theseare the special domain of demography, pop-ulation genetics and the scientific study of hu-manity, anthropology. In particular, anthropol-ogy studies human biocultural evolution as hu-manity adapts both by gene and (moreso in thelast 100,000 years) culture.Determination of a world ship populationdepends on the objective. Our objective isto allow a genetically- and culturally-healthypopulation to arrive at an exoplanet, wherethey may land and begin a new world for hu-manity. 14 stars are within 10 light yearsof Earth; propulsion engineering and otherissues related to the feasibility of reachingeach are explored elsewhere [49]. Alterna-8able 3: Overview of world ship feasibility criteria and their impact on key design parameters
Feasibility category Criteria Design considerationsBiological
Genetics population size, trip dura-tion
Cultural
Knowledge transmission population size, knowledgemanagement approach
Social
Societal structure habitat geometry, size, mod-ularity
Technical
Performance of technolo-gies Velocity, trip durationMaturity of technologies PrecursorsReliability of technologies Spare parts mass, mainte-nance system
Economic
Scope of economic activi-ties Scope of materialsWealth Affordable size, masstive destinations are introduced later in Sec-tion 6.1. Exoplanet discoveries are burgeon-ing, with a measured number of 3971 exoplan-ets discovered as of January the 23rd, 2019(see http://http://exoplanet.eu/). The currentparadigm is that there are “2 ± How many humans are required, as a found-ing population, to ensure that future genera-tions live in good multi-generational health?The question has been addressed mainly by(a) population geneticists, for theoretical in-terest, (b) conservation biologists, to help con-serve species at risk of extinction, (c) anthro-pologists, with an interest in human matingpatterns and ‘prehistoric dispersals, (d) space-settlement planners envisioning open popula-tions that may be expected to continue to bringin new members over time and (e), space-settlement planners envisioning closed popu-9able 4: Seven published and/or current world ship population estimates. Results of SIMOC,noted in the final row, are imminent and not yet published. D1 is the recommended interstellaremigrant founding population. D2, mentioned in the text, is not noted in this table.
Model Model type Spacefaringsimulations? D1 Current regard
ETHNOPOP Demographic Few <
300 likely lowSMITH Statistical Several > > > b ”), coauthored in2018 with particle physicist C. Beluffi, appliedan improved version of HERITAGE which in-cluded more complex mating and other repro-ductive rules, again evaluated at each comput-ing cycle for each simulated member of thepopulation [54]. In this simulation, popula-tions starting with D1 numbering 150 individ-uals (and a world ship capacity of 500) sur-vived not only centuries, but over six millen-nia, in good genetic and demographic health;an even smaller D1 figure (98 individuals) wasalso identified as viable on this timescale. Thiswas attributed to “adaptive social engineer-ing principles” that change the mating rulesen route, rather than applying a single rulethroughout the voyage. This is an entirely rea-sonable adjustment of HERITAGE and is en-coded in IF-THEN constructs such as the fol-lowing: “If the amount of people inside thevessel is lower than the [world ship’s predeter-mined capacity], the code allows for a smoothincrease of the population by allowing eachwoman to have an average of 3 children (witha standard deviation of 1). When the thresh-old is reached, HERITAGE impedes the cou-ples’ ability to procreate but allows womenthat were already pregnant to give birth even15f the total number of crew members becomesmarginally higher than the threshold.” [54]While the study proposed very low D1 fig-ures (compared to any other estimates) the au-thors did caution in the paper that “the impactof mutation, migration, selection and drift isnot included in HERITAGE ... [so] we empha-size that the minimum crew of 98 settlers wefound is a lower limit ...” and that further workmight well suggest a larger D1 figure. Themain advantage of HERITAGE is that it moreaccurately models real human mating behav-ior, which is not random and can thus, by con-sciously avoiding inbreeding, support smallerpopulations. In Marin’s third paper reportingresults of HERITAGE (“Numerical constraintson the size of generation ships from total en-ergy expenditure on board, annual food pro-duction and space farming techniques”), 500-person D1 populations were used as referencestudy [54]. The authors addressed the cru-cial question of how to feed the crew, sincedried food stocks are not a viable option dueto the deterioration of vitamins with time. Thebest option then relies on farming aboard thespaceship. Using an updated version of HER-ITAGE, Marin’s team were able to predict thesize of artificial land to be allocated in the ves-sel for agricultural purposes.Although no results have so far been pub-lished, the SIMOC (Scalable Model of anIsolated, Off-World Community) multi-agentsimulation is near completion. Orchestratedby Kai Staats as a project of the Arizona StateUniversity School of Earth and Space Explo-ration’s Interplanetary Initiative, SIMOC isdesigned to model and then analyze the re-sults of the physical characteristics of an off-Earth colony. In particular the habitat’s agri-cultural, life-support, recyclable and consum-able variables are modeled, as is the health of each colonist placed in the system (seehttps://simoc.space/): “to design a habitatthat sustains human life through a combina-tion of physio-chemical (machine) and bio-regenerative (plant) systems, and then scalesover time, with SIMOC Phase IV – V includ-ing options to grow the community with theaddition of inhabitants and infrastructure ...[based on] ... an agent-based model (ABM),a class of computational models for simulat-ing the actions and interactions of autonomousagents (both individual or collective entitiessuch as organizations or groups) with a viewto assessing their effects on the system as awhole.” [72].The project’s developmental phases are de-scribed below; at this writing the project isin Phase IV – V with expected activation andpublic release in the first quarter of 2019: • Phase I: Habitat modeling: low-Earth or-bit, on the Moon, in free space, or onMars. Attention was given to specific lo-cations, such as a valley, mountain top,or polar cap as each would inherit a par-ticular in situ resource utilization (ISRU)parameter; • Phase II: Physio-chemical modeling ofECLSS (Environmental Control and LifeSupport System) and bio-regenerativesystems; • Phase III: Agent modeling & integrationwith Phase I and II module; • Phase IV: Population modeling. Con-sumables tracking ; modeling which con-struction materials shipped from Earthversus were manufactured locally, viaISRU (In-Situ Resource Utilization);each expansion task is restricted by thecost of energy and time;16
Phase V: Modeling aging of the systemsand stochastic (entropic) breakdown suchas habitat gas leaks, solid waste proces-sor failures, or a space bolide strike re-sulting in catastrophic failure of a green-house and all crops therein.SIMOC is currently configured to model, asmentioned, physical rather than social dynam-ics, although the designers have expressed aninterest in the interactions and emergence ofsocial phenomena (see Fig. 8 for a summary ofthe potential SIMOC agents and interactions).Such social phenomena have been addressedin the field of multi-agent social simulations,capably defined as “... the intersection of threescientific fields, namely, agent-based comput-ing, the social sciences, and computer simula-tion ...” [21]. In the future, it will be very in-teresting to compare the results of SIMOC andHERITAGE. Current plans include compar-ison of SIMOC simulations with real-worldclosed habitat experimentation at the Univer-sity of Arizona, a form of “ground truthing” inwhich a mathematical model may be improvedby comparison with, and then better model-ing of, real-world systems. Social phenomenathat may emerge in SIMOC and other multi-agent simulations could be of great interest.At this writing we are aware of, but have notbeen able to review, W.S. Bainbridge’s 2019book, “Computer Simulations of Space Soci-eties” [10].
It is natural that a variety of populationsizes have been proposed for D1, the Earth-departing founding population, as researchers from different backgrounds have brought var-ious approaches to this question. We believethat some of the variation derives from differ-ent conceptions of human populations and hu-man behavior over time.Moore’s gravitation towards low figurescome from a long-term anthropological per-spective recognizing that hunting and gather-ing cultures have survived for many thousandsof years in low population densities, so thatjust few centuries should be relatively easyfor a D1 population less than several hun-dred. However, Moore’s figures appear some-what too low as he did not really account forthe fact that while humans may live togetherin breeding populations (demes) numbering inthe hundreds (the famous “Dunbar Number”of about 150 individuals is often quoted re-garding hunter-gatherer group size [26]), suchpopulations always have reproductive linkswith other groups. Also, his figures largelyreflect populations of hunting-and-gatheringfolk who move seasonally over large land-scapes, whereas in all conceivable world shipdesigns the subsistence mode would be agri-culture, which is characterized by residentialsedentism. However, the type of agriculturemight be diverse, including hydroponic andaeroponic farming, potentially extending tothe use of emerging technologies such as arti-ficial meat [55]. Residential sedentism, world-wide and throughout prehistory, always leadsto higher populations, as we introduce below.Smith’s anthropological biases led himhighlight larger population figures because hu-man populations are always linked to oth-ers, normally in the thousands of individu-als, figures approaching the circa 7,500 popu-lation range for naturally-evolved populationsof naturally-occurring vertebrates. He is alsoconditioned by an emphasis on catastrophe:17or Moore, human populations have gener-ally survived quite well even small popula-tions in particular because they have often hadlarge landscapes and many resources avail-able; a local catastrophe could be averted bymoving to new resource territories, and if onegroup actually became extinct, humanity wasso widespread that others always continued.But for Smith, considering the perspective ofa closed population carrying all their resourceswith them, there is an expectation that eventu-ally some catastrophe will strike, and for thissingle, isolated population there is nowhereto go, no “geographical reserve”. To be sureto arrive in relatively safe populations (D2),Smith has gravitated towards particularly largedeparting populations (D1).For Marin’s approach with C. Beluffi, thereis an attempt to reduce D1 to an absoluteminimum as revealed in the paper title, withthe paper’s function stated as “to quantify theminimal initial crew necessary for a multi-generational space journey to reach ProximaCentauri b with genetically healthy settlers”.Here the focus is on propulsion at speedsachieved today with the Parker Solar Probe,resulting in the need to voyage for an esti-mated 6,300 years to Proxima Centauri b . Thephilosophy driving the search for the min-imum viable population here is that of the“scarcity paradigm” of crewed spaceflight. Inthis paradigm, we must identify the minimummass to transport to reduce cost. Marin’s com-paratively small figure of less than a hundredindividuals is identified as a viable D1 fig-ure under very strict adaptive mating rules thatmay change over the course of a journey back,then, to figures closer to the Moore thought-scape. From strictly mathematical, statistical and ge-netic perspectives we may say that Earth-departing D1 founder populations of humans,numbering in just the low hundreds of peo-ple, could theoretically survive for centuriesor even some millennia in health sufficient toserve as D2 (exoplanet founder) populationswhen mating is cleverly devised to avoid in-breeding. Smith mentioned this in the 2014paper, for instance, stating that “any popu-lation over 100 or so” would avoid some ofthe chief problems of small populations onsuch timescales. Marin demonstrated this withthe high-fidelity HERITAGE program that ca-pably simulates human social engineering tomanage population health.While the smallest figures may work bio-logically, they are rather precarious for somegenerations before the population has been al-lowed to grow. We therefore currently sug-gest figures with Earth-departing (D1) figureson the order of 1,000 persons. Because usefulmodeling is still underway, there is a practi-cal way to use such an estimate even at thisearly date. We propose that for habitat de-sign and modeling, 1,000-person modules (al-ternatively called villages) be designed, thatcould at a later date simply be multiplied aselements of a world ship cluster. This way,the Earth-departing population could be set toany figure one wants, for example 3 villagescomposing 3,000 people, or 10 for 10,000 orjust one for a departing population of 1,000.While this modularity does increase mass (ascompared to a single-vessel design using themost efficient enclosure of space by material)18nd thus the budget to be allocated for suchlarge missions, we feel the modularity is worththe trade-off. For instance, multiple, indepen-dent villages traveling in parallel, each witha population of circa 1,000, would reduce thepossibility of a catastrophe wiping out the en-tire population. The villages would travel to-gether on the same spacecraft but would besomewhat separated, with the possibility toallow travel from village to village. Trav-eling in parallel would allow people to visitother “towns” for pleasure, cultural exchangeand marriage and reproduction, but also to bequarantined (culturally and/or biologically) ifdesired. Such a concept of interacting habi-tats was previously proposed by Sherwood forspace colonies within our Solar System [67].The population on the order of 1,000 per vil-lage module is also viable culturally, as we ex-plore below. A more in-depth analysis of thistopic is provided in [72].
Before the interconnection of the modernworld, and before the radical changes of ur-banism that characterize modern and ancientcivilizations, early farming people worldwidelived in independent farming villages withmany features we think will be analogous tothose of interstellar voyagers. For example,Marin et al. [55] have used HERITAGE to alsomodel on-board food production, indicatingthat dried food stocks are not a viable optiondue to the deterioration of vitamins with timeand the tremendous quantities that wouldbe required for long-term storage. Having asustainable source of food is thus mandatoryfor such long journeys and the space needed for geoponics (or hydroponics/aeroponics)will strongly condition the architecture of thespacecraft. Among other results, Marin etal. found that for an heterogeneous crew of500 people living on an omnivorous, balanceddiet, 0.45 km of artificial land would sufficein order to grow all the necessary food using acombination of aeroponics (for fruits, vegeta-bles, starch, sugar, and oil) and conventionalfarming (for meat, fish, dairy, and honey).This translates into various spaceship lengthsand radii, depending on the level of artificialgravity we want to produce on-board.To learn from humanity’s long experienceof farming in independent farming villages wenote first that those populations were ratherself-supporting. While there was trade, it wasnot global, but among multiple villages ina relatively small region. This is much likeany world ship considered today; certainlytrade will be rather local, which in partshapes the economy. These villages were alsounfortified; while social friction did occur,so much time was spent in food productionand processing that it was not possible tomaintain standing military forces; such is alsoidentifiable in most world ship plans. Earlyfarming villages also had a rather domesticeconomy, where if you needed something,you generally made it yourself. Certainlythere were some specialists, but there was amore general self-sufficiency of fabrication.On reasonably-expectable world ships we feelsomething very similar will play out at leastin the lack of emphasis on, again, a largetrade in products. Rather many items willbe fabricated locally and on the scale of thehousehold or community rather than on thescales of a global market. Early independentfarming villages were also horticultural, ratherthan agricultural. That is, while they did farm,19igure 9: Reconstructions of Two Independent Neolithic Villages. Top: Demircihuyuk, Turkey(drawn by Cameron Smit); bottom: Chaco Canyon.the farming was again of a local character,serving communities or households, ratherthan for a market of millions or billions, andagain this will be similar in world ships withtotal populations perhaps less than severaltens of thousands. Early farming villagesalso had populations in the 600 to 1,000range, similar to world ship estimates wesee above. Fig. 9 illustrates such villages atDemircihuyuk, Turkey, and Chaco Canyon,New Mexico. Table 5 presents summary population es-timates of early farming villages, worldwide(data derive from Smith 2019, in press.). Asmentioned earlier, the village populationswere managed in the low thousands, oftenaround 1,000. Villages were some kilometersfrom one another, such that while there wereinteractions with others, such that while therewere interactions with others, each villagewas self-sufficient. Self-sufficiency meanshere that a local production and consumption20able 5: Independent neolithic village population estimates. See text for discussion.Region Village Date (years ago) Population estimate1. SW Asia Jericho 10000 2251. SW Asia Netiv Hagdud 10000 1351. SW Asia Gilgal I 10000 901. SW Asia Dhra’ 10000 411. SW Asia Nahhal Oren 10000 181. SW Asia Ain Ghazal 8900 4051. SW Asia Tell Aswad 8900 3601. SW Asia Jericho 8900 2251. SW Asia Yiftahel 8900 1351. SW Asia Kfar Hahoresh 8900 451. SW Asia Catalhoyk 8600 60001. SW Asia Basta 8250 12601. SW Asia Ain Ghazal 8250 9001. SW Asia Wadi Shu’eib 8250 9001. SW Asia Beisamoun 8250 9001. SW Asia Es-Sifiya 8250 9001. SW Asia Ain Jamman 8250 6309. Europe Cyprus 6000 20009. Europe Serbian sites 6000 17403. East Asia Xinglongwa 7730 1003. East Asia Cishan 7700 1003. East Asia Zhaobaogou 7034 1009. Europe Germany 6000 1354. Africa Merimda Beni Salama 6000 16504. Africa Hierakonpolis 5500 17506. South America Real Alto 5250 1756. South America Loma Alta 4680 1752. South Asia Ban Non Wat 4000 7005. Mesoamerica Oaxaca sites 3300 3255. Mesoamerica Oaxaca sites 2900 19735. Mesoamerica Oaxaca sites 2770 17825. Mesoamerica Oaxaca sites 2600 18285. Mesoamerica Oaxaca 2600 10005. Mesoamerica Basin of Mexico sites 3050 685system exists. That such populations man-aged as relatively stable and self-sufficientunits for some millennia (in many cases for several thousand years before the adventof civilization) in arrangements that haveimportant similarities to how we imagine21able 6: *Table 5 continued.Region Village Date (years ago) Population estimate6. South America Titicaca basin sites 3250 6936. South America Titicaca basin sites 2900 17526. South America Titicaca basin sites 2500 35078. North America Snaketown 1000 3008. North America Galaz 1000 3008. North America Montezuma Valley 800 25008. North America Yellowjacket 800 20008. North America Zuni 800 16008. North America Sand Canyon 800 7258. North America Marana 800 7008. North America Paquime 600 47008. North America Sapawe 600 27708. North America Pueblo Grande 600 17508. North America Los Muertos 600 8007. Amazonia Rio Negro Sites 2300 12507. Amazonia Upper Rio Xingu Sites 1000 12507. Amazonia Central Brazil 1000 9648. North America Chaco Canyon 1300 6008. North America SW USA 1300 4008. North America Mesa Verde 1100 1008. North America Chaco Canyon MainVillage 1000 35008. North America Chaco Canyon hamlets 1000 2008. North America Moundville 1000 12008. North America Snodgrass 1000 3508. North America Lunsford-Pulcher 950 10008. North America Cahokia 950 1000 Average
Standard Devia-tion .6 Productive New Ways ToThink of Interstellar Voyaging
What aspects of culture and biology may weproductively address with the objective ofmaking the interstellar voyaging project mostlikely to succeed? Smith [72] investigatesthis question, concluding that we should fo-cus on humanitys adaptive tools, both biolog-ical and cultural. Culturally these include aset of human universals, domains of behaviorseen in all cultures that are often adjusted toaccommodate new conditions. For example,all human cultures have some conception of afamily, a cohabiting unit related often by kin-ship and cooperating often in resource acqui-sition. Adaptation of the size and structure ofthe family to the conditions is clear and manytimes predictable. For example, foraging cul-tures tend to have smaller families that cantravel nimbly, whereas farming cultures tendto have larger families for the many simultane-ous tasks of farming). In this case, the humanuniversal of family size and structure may beinvestigated for its adaptive range and poten-tial, and how it may be configured for inter-stellar voyaging conditions. Such an investi-gation is presented in [72] and is too extensiveto review here, but the point is that there existgood theoretical reasons to delineate the dis-cussion of culture aboard world ships alongthe lines of human universals. As a directconsequence, while each world ship might ex-hibit unique features of its population, theywill likely have common features which are aconsequence of human universals.We suggest a few anthropologically-guidedsuggestions that may help to shape more re-alistic world ship studies. First, we thinkwe should move away from the paradigm ofscarcity, and towards a paradigm of plenty. Certainly if setting off for a multiple-centuryor -millenium voyage, one would wish totravel with a large margin and surplus, not inarrangements that would be just mathemati-cally possible. Second, we would think aboutfamilies and communities rather than crews.Crews eventually go home and have a conceptof home being somewhere else; but on worldships, many will be born who will have no ex-perience of losing Earth or gaining the exo-planet, they will live out normal, small-townlives in the world ship. Third, we would sug-gest moving away from thinking of mating orreproduction rule as something of a problemfor the inhabitants. Indeed we think the peoplewho choose to voyage on these vessels will bethe folk who construct them in the first place,and they will naturally have rules about re-production to keep their population from ex-ceeding the world ship capacity, just as pop-ulations today have plenty of cultural regula-tions of various behaviors. Fourth, we wouldmove away from conception of the world shipas a vessel on a mission; again, it will be thehome of people who grow, live and die andit is hard to imagine that they will think ofthemselves on a ship or on a mission (exceptfor the earliest and latest generations aboard),rather people will be living normal lives. Fi-nally, we would attempt to de-exoticize the in-terstellar voyage. Fig. 11 presents some ex-pectable changes we may see in world shippopulation biology and culture over the cen-turies (or more) of an exoplanet voyage. Timemay be divided into departing, interstellar andarriving ages; the population may grow (if per-mitted); the language and biology will changesubtly. All of this will be carried out, however,on the individual timescales and experiencesof normal people living out daily life. It is thisanthropological perspective that continues to25nfluence our thinking about world ships.
World ship feasibility also depends on so-cial and technical factors. In the following,we will present various world ship destina-tions and what implications this would havefor a world ship mission and the settlementactivities for developing a new civilization.Subsequently, we present the population -trip duration trade-off, which helps determinewhich types of missions are feasible. Finallywe briefly present previous results regardingworld ship reliability.
World ship design is driven by trip time, asmentioned in the Section 1. Trip duration,however, is determined by the velocity of thespacecraft and the distance it travels. Distanceis determined by the destination to which theworld ship aims to travel.Since the World Ship Symposium in 2011,a range of new discoveries have been made,which may change significantly the rangeof destinations to which a world ship couldtravel.In Hein et al. [38], four types of habitatsare adopted from [28]: habitable planet, bio-compatible planet, easily terraformable planet,and using other resources for constructingfree-floating space colonies. [38] extend thelist by adding “moon” to “planet”, due to thepotential habitability of exomoons [41, 47].Furthermore, so-called rogue planets, whichare not bounded to a star and free floating havebeen confirmed via micro-lensing in 2011[24]. Rogue planets could be another type of destination for world ships. A summary ofthese destinations is given in the following: • Habitable planet / moon:
An environ-ment ”sufficiently similar to that of theEarth as to allow comfortable and freehuman habitation.” [28] • Bio-compatible planet / moon:
Possesses”the necessary physical parameters forlife to flourish on its surface.” [28] • Easily terraformable planet / moon:
Canbe converted into a bio-compatible orhabitable planet with moderate resourcesavailable to ”a starship or robot pre-cursor mission.” [28] • Rogue planet/comet:
Probably similarenvironment to outer Solar System plan-ets, moons, and minor bodies. • Free-floating space colonies:
Using otherresources for constructing free-floatingspace colonies.To our knowledge, rogue planets have notyet been treated as potential destinations forinterstellar spacecraft. Due to the limitationsof the observational technique of micro-lensing, Jupiter-sized rogue planets have beenconfirmed at the moment. Some of thesediscovered rogue planets might be brown orred dwarfs. One key criteria for colonizationis the existence of an in-situ energy source.Rogue planets seem to generate little to noheat and as they are free-floating, there is nostar in its vicinity to provide energy. Onepossible energy source could be fusion fuelsuch as Deuterium and Helium-3, as in gasgiants in our solar system [36]. Therefore,we can imagine several colonization modesfor a gas giant rogue planet. Either a free26igure 12: Crewed starship categories versus population size and trip duration.floating colony is constructed, possibly byconverting the world ship, or colonies couldbe established in the atmosphere of the rogueplanet, for example, via balloons [16]. Theatmosphere would be mined using techniquesdescribed in [16] and [36]. In case the rogueplanet is a rocky planet, surface or subsurfacecolonies could be constructed and Deuteriummined from water, which is hypothesizedto be available under certain conditions [1].However, rogue planets could also serve as anintermediate fueling stop for world ships. Thisoption would only be interesting if the rogueplanet could provide resources beyond fuelthat justify a deceleration and acceleration ofthe world ship.Nearby rogue planets are, for example WISE 0855-0714 at a distance of 7.27 lightyears [50]. However, it seems likely thatrogue planets at a closer distance will bediscovered in the future.We expect that colonies on or around rogueplanets have about the same characteristics asfree-floating colonies or on planetary/moonsurface colonies. The only potential differ-ence is the distance to a rogue planet, whichmight be much closer than the next star,rendering it easier to reach with a world ship.An updated table of potential colonizationdestinations from [38] can be found in Table7. In particular, we have updated the distancefrom Earth for most destinations in light of thelatest exoplanet discoveries. Six potentiallyhabitable exoplanets have been discovered27able 7: Potential destinations for world ships.
Habitableplanet/moon Bio-compatibleplanet/moon Easily ter-raformableplanet/moon Rogueplanet/comet SpacecoloniesInvestmentfor habit-ability
Small Establishecosystem Terraforming Colony con-struction Colony con-struction
Durationuntil habit-ability
Years Decade /centuries Centuries Decades Decades
Habitabilityduration
Millions ofyears Hundredthousands ofyears Hundredthousands ofyears Centuries –millennia Centuries –millennia
Availability
Rare Rare Rare High abun-dance High abun-dance
Distancefrom Earth(estimates) ≤ As demonstrated in Section 5, estimates for re-quired population sizes correlate with trip du-ration. The longer the trip duration, the higherthe required population size. In Fig. 12, weshow population size and trip duration for var-ious crewed interstellar spacecraft concepts inthe literature, using the population estimatesfrom [71], with the discussion presented inthis paper. The lower and upper estimates arerepresented as red squares for a trip durationof 210 years. The three red lines represent aninterpolation between population size valuesfor short-term missions (Mars mission with acrew of 3-6 and duration of 2-3 years) and theestimates from [71]. The area left of the redline is considered infeasible from a populationsize perspective. Hence, this chart can be usedto evaluate whether or not a world ship designis feasible from a trip duration - populationperspective. Furthermore, it allows for mak-ing trade-offs between trip duration, which islinked to velocity and energy, and populationsize, which is linked to spacecraft mass. Forexample, world ship designers may choose aslower but larger world ship with more peopleon board. Or they may choose a faster worldship with a smaller population. In any case,they would need to ensure that they are on theright side of the red line. For minimizing risk,they are likely to add a margin to the red lineto be on the safe side.Several world ship designs from the liter-ature are put into the chart, such as Matloff- 76 [58], Bond-84 [15], Hein-12 [38], and theEnzmann ship [20]. In case several valueswere given in the reference, such in the casefor Matloff-76, Hein-12, and the Enzmannship, they were also represented in the chart.In particular for the Enzmann ship, the popu-lation size does not stay constant but increases10 times during the trip, which leads to thedashed-line square with two population valuesfor one Enzmann ship concept and two trip du-rations. The chart shows that the upper esti-mates for population values from [71] wouldrender most of the world ship designs infeasi-ble, except for the Enzman world ship design.For making the infeasible designs feasible, ei-ther trip times would need to be decreased orpopulation size increased.As a side note, We have added RobertForward’s crewed laser sail starship from[29], which would fall under the category of“sprinter”.
World ship reliability is likely to be a majorfeasibility issue, due to the large number ofparts and the long mission duration [38]. As[32] remarks, the mechanical and electroniccomponents of a bioregenerative life support-ing system are much more likely to fail thanits biological components. Previously, [38]developed a reliability model for world ships.They demonstrate that reasonably high relia-bility values are only possible if componentsare either replaced by spare parts or replacedby repaired parts. The number of componentsthat need to be replaced ranges from three persecond for a 99.99% reliability value to oneevery 20 seconds for a 85% reliability value,as shown in Table 8.Detecting, replacing, and repairing compo-29ents at these rates seem to be infeasible forthe crew. [38] therefore conclude that an auto-mated system is needed. Furthermore, worldship components need to be easily accessi-ble and modular, in order to facilitate replace-ment. Nevertheless, given the complexity ofa world ship, the maintenance system likelyneeds to be very sophisticated and requires anadvanced artificial intelligence such as for theDaedalus probe [16] or probes described in[34].One way to address world ship reliabil-ity could be the substitution of mechanical,electronic, and software components by de-liberately engineered biological components,which exhibit self-healing capabilities [3].This might also work the other way around.Mechanical, electronic, and software compo-nents could exhibit self-healing capabilities[61]. Exploring the impact of such technolo-gies on reliability and habitat design would bean interesting topic for future work.
A civilization capable of building and launch-ing a world ship has a much larger economythan the current one. This also implies thatit has access to resources far beyond our cur-rent one, if we accept that economic activitiescannot be fully decorrelated from material re-sources and energy [9, 31, 30, 77]. There areTable 8: Component replacement rates forworld ship reliability values [38]
Reliability Replacement rate[1/s] t of material need tobe processed, assuming that on average only10% of the processed material ends up beingused in the world ship.The third argument is that of the globalgross domestic product (GDP). GDP is an in-dicator for the size of an economy in termsof the monetary value of all goods and ser-vices produced during a specific period. Mar-tin [56] estimates that at a growth rate of2%/year the required global GDP would be at-tained at some point between the year 2500 -3000. This estimate assumes that 1% of theglobal GDP is used for a world ship project.This range is consistent with similar analy-ses performed by [35] and [60]. For exam-ple, [35] assumes that a Daedalus-type fusionpropelled probe costs $ . [56] estimatesthat a world ship would cost about a factor100 more, which leads to a value of $ . In30igh GDP-growth scenarios, this value wouldbe reached before the year 2300 and between2500 and 3000 for medium GDP-growth sce-narios. Hein and Rudelle [39] estimate that aneconomy of such size would necessarily needto be to a large extent space-based. A sum-mary of these results is shown in Table 9.To summarize, building and launchinga world ship would require two economicconditions to be satisfied. First, a SolarSystem-wide economy with large-scale in-space manufacturing capabilities. Second,GDP growth rates of 2%/year or higher needto be sustained for the next 500 to 1000 years. Most existing publications on world ships fo-cus on world ships alone, without comparingthem to potential alternatives. Hein [37] boilsdown the interstellar colonization problem tofour fundamental functions. First, humans, inwhatever form, are transported from the solarsystem to the target destination, usually an-other star system. It is of course imaginablethat instead of a star system, the crew staysin interstellar space indefinitely or colonizes arogue planet.Transporting humans also entails support-ing objectives such as the transportation of anTable 9: Estimates for economic breakeven fora world ship construction and launch
Reference Year of breakevenMartin (1984) [56]
Hein (2011) [35]
Mode categories World ship Hibernation/ cryogenics Zygote /embryo DigitalDevelopmental state
Zygote X XEmbryo X XInfant XChild XAdult XElderly X
Metabolic state
Reduced XStopped X
Substrate
Biological X X XArtificial Xthey would settle, after being transported inone of these modes [51]. Finally, a more spec-ulative concept would be the transportation ofhumans on an artificial substrate in a digitalform, for example via brain emulation [40].We can speculate further and imagine that ar-tificial general intelligence may even mergewith or replace humans as the primary agentsof space exploration and settlement.How do world ships compare to these other forms of transporting humans between thestars? As an evaluation framework, we firstdefine some ideal conditions for interstellartravel in order to rank the proposed conceptswith respect to them.The ideal crewed interstellar transportationdevice would have the following characteris-tics: • No mass needs to be transported; • Instantaneous transportation of humans32able 11: Ranking of crewed interstellar spaceship concepts (1: best; 5: worst), adapted from[37]
Worldship Sleepership Seed ship Digitalemu-lationship DatatransferSpacecraft mass
Trip duration
Knowledge transfer
Development cost
Energy
Safety
Maturity • No cost for development • Needs no energy • • Technology available off-the shelf (matu-rity)These criteria are used for ranking the con-cepts from 1 to 5, where 1 is best and 5 isworst. As shown in Table 11, we select fiveconcepts for crewed interstellar travel, whichbroadly summarize existing concepts in the lit-erature such as in [37]. We assume that faster-than-light propulsion options are not feasible.However, if they are, such a spacecraft wouldlikely come out at the top of the ranking, atleast in terms of spacecraft mass, trip duration,knowledge transfer.Besides the world ship, the sleeper ship is aspacecraft on which humans are put into hi-bernation. It is currently unclear how far hi-bernation can be induced in humans and there are likely negative side effects. It is also con-sidered necessary to wake up the crew in cer-tain intervals [8, 65, 52, 7]. However, shouldhuman hibernation be feasible, it would poten-tially lead to a drastic reduction in habitat sizeand life support system mass, as only part ofthe population is awake at the same time [8].Seed ships [19] transport humans in a zygoteor embryonic state, thereby omitting the needfor a habitat and life support system during thetrip. Digital emulation ships are based on theidea that essential parts of a human, such as thebrain, can be transferred to an artificial sub-strate. In case only the brain is concerned, abrain on an artificial substrate is called brainemulation [66]. While it is unclear if this willlead to substantial mass savings compared tothe seed ship [34], the payload is likely to besmaller than that of the sleeper ship. Finally,data transfer is the process where the con-stituent data of humans are transferred to thetarget destination via electromagnetic waves.This concept is close to teleportation [37, 68].The results of the analysis are shown in Ta-ble 11, which is a modified version of the ta-ble in [37]. We can immediately see that the33orld ship is assigned the worst ranking ofall the concepts for four out of seven perfor-mance criteria, which is mainly due to its largemass, from which follows that a lot of energyis needed for propulsion. It also means thattrip times are comparatively long. This dis-advantage is partly balanced by the criteria ofmaturity, which is high compared to the otherconcepts. The technologies required for worldships are already available in a very embry-onic form of life support systems and closedecologies [32]. Also, it is known that isolatedhuman populations can survive over centuriesor millennia. Although this does not at alldemonstrate that world ships are feasible, it isat least possible to chart a pathway towardsworld ships, along with the identification ofmajor roadblocks and uncertainties. Accord-ing to the “theoretical technology” approachby [76], this indicates that world ships havea higher maturity than other concepts such asfaster-than-light travel, where we would beunable to construct such a roadmap due to thelack of knowledge of the underlying physicaleffects.In terms of knowledge transfer, it is rankedhigher than the seed ship, as on the latter,knowledge cannot be transferred via humans.Regarding safety, the world ship is rankedhigher than the sleeper ship, as there are lessintrinsic safety issues on a world ship. Forthe sleeper ship, it is still unclear whether ornot negative side effects of hibernation can beavoided [65].To conclude, world ships seem to performrather poorly compared to its potential alter-natives, except for its technological maturity.As we have addressed all feasibility categoriesfrom Section 4, we will provide an overviewof world ship feasibility in the following sec-tion.
In Section 3, we have defined several worldship feasibility categories. In light of the re-sults presented in the subsequent sections, wecan now derive a few conclusions regardingworld ship feasibility.Table 12 shows the results for precondi-tions for world ship feasibility. It can be seenthat regarding biological feasibility, in partic-ular genetics, population sizes in the - range are required. It is currently unknownwhat population size would be required forknowledge transfer over multiple generations,assuring that critical knowledge for living on aworld ship and starting a settlement at the tar-get destination are not lost. Regarding the so-cial structure on a world ship, we have arguedfor an organization similar to early agriculturalsocieties, organized in villages. This wouldtranslate into potentially modular habitat de-signs, where each module would contain onthe order of people. Another argument formodular habitats is their redundancy in case ofa catastrophic event.Regarding the required technologies, oneresult from the population size - trip durationtrade-off is that the spacecraft velocity likelyneeds to be above c (trip durations on theorder of hundreds of years), in order to allowfor a sufficiently large margin from the line ofinfeasibility in Fig. 12. Furthermore, in or-der to mitigate the risk of world ship failures,technologies used on it would need to be testedwithin our Solar System for representative du-rations. Hein et al. [38] have presented severalstrategies for how the maturity of these tech-nologies could be increased, such as via theiruse in free-floating colonies within our SolarSystem. Reliability is another issue and devel-oping a maintenance system which is capable34f handling the detection, replacement, and re-pair of the large number of world ship compo-nents seems to be very challenging.Finally, from an economic point of view,a Solar System-wide economy with large-scale in-space manufacturing activities is re-quired, including the existence of their respec-tive supply chains. Regarding the requiredlevels of GDP, which can be considered as aproxy for wealth, the literature estimates that abreakeven would be reached between the years2300 and 3000, assuming current rates of GDPgrowth.Apart from these feasibility criteria whichpertain to the world ship itself, it is im-portant to consider potential alternatives, asthey might render it obsolete. We have seenin the Section 7 that world ships performpoorly when compared to alternative modesof crewed interstellar travel. Only in terms oftheir maturity are they competitive with the al-ternatives, as most of its technologies do ex-ist at a prototypical stage. However, assum-ing current rates of technological progress, itmight be rather unlikely that by the time worldships become feasible from an economic pointof view, at least one other mode of interstellartravel has not reached sufficient technologicalmaturity.We argue that the existence of a mainte-nance system that is able to assure world shipreliability goes beyond being a purely tech-nical problem. A society which will developa world ship will invest substantial resources.Reducing mission risk will be one of the keyconcerns of stakeholders. Demonstrating thatat least the technical subsystems of a worldship are sufficiently reliable will be crucial.To conclude, the main world ship feasibil-ity issues are rather economic and related tothe maintenance system. In particular, due to the large amount of resources needed forworld ship construction, the size of the econ-omy which can sustain such an activity needsto be several orders of magnitude larger thantoday’s. However, as it would take centuriesfor such an economy to come into existence, itis likely that alternative modes of crewed inter-stellar travel might already exist at that pointin time. From a technical point of view, themaintenance system on a world ship likely re-quires a sophisticated AI to fulfill its purpose,which is similar to the conclusion from theDaedalus report [16].However, even in a case where worldships have become obsolete, we can imaginethat free-floating space colonies equipped witha propulsion system roam our Solar System,similar to the vision of Gerard O’Neill [64].
10 Conclusions
This article dealt with the rationale andfeasibility of world ships, taking a varietyof feasibility categories into consideration.We determined preconditions for world shipfeasibility from a biological, cultural, social,technical, and economic perspective. We con-clude that due to the large amount of resourcesa world ship would require, its developmentis likely to start after the year 2300, assumingcurrent rates of economic growth. It is likelythat at that point, alternative modes of crewedinterstellar travel are already available, whichmight render world ships obsolete. However,world ships might still remain an interestingconcept for mobile deep space habitats withinour Solar System. For future work, areassuch as cultural and social aspects of worldship populations seem to be promising, asthey might shed light on societies in highly35able 12: Overview of preconditions for world ship feasibility
Feasibilitycategory Criteria PreconditionsBiological
Genetics Population size from - Cultural
Knowledge transmission Unknown
Social
Societal structure Modular habitat ( per section) Technical
Technological performance Velocities higher than > c requiredTechnological maturity Solar system precursors requiredTechnological reliability Order of 1-0.01 parts replaced per second, AI-basedmaintenance system Economic
Scope of economic activi-ties Solar System-wide economyWealth GDP breakeven in year 2300-3000
Alternatives
Emergence of other modesof crewed interstellar travel Likely to exist in year 2300 and beyondresource-constrained environments in general.
Acknowledgements
We would like tothank Michel Lamontagne and two anony-mous reviewers for their comments to a pre-vious version of this paper, which helped ussignificantly improve its quality.
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