In a universe in which faster-than-light travel is, to the best of our current understanding, impossible, journeys to other stars are likely to be measured in decades or centuries rather than days or weeks. A common reaction to that reality in science fiction is the proposed launch of generation starships: vessels in which those who arrive at their destination are the descendants of the original crew, born and raised on the ship itself. Perhaps inevitably, given the need for conflict and adventure in narratives, examples of generation ships in science fiction provide an interesting perspective on the many and varied ways in which such an endeavour can go wrong.

Perhaps the most obvious flaw with the premise of generation ships is one of intergenerational responsibility. The first generation crew may be dedicated to their ship’s mission and have made a conscious decision to pursue a goal which aligns with their interests and skills. However children born on the ship have a very different upbringing and no realistic choice other than to follow their parents’ vocation. Thus their own free will, and freedom of self-determination, has to be abrogated to support the needs of the mission.

While in previous centuries, submission to a career path dictated by parents or family circumstances was normal for a large fraction of the population, by the middle of the twentieth century that had changed. The formative era of modern science fiction, spanning the 1940s to the 1960s, was written against a dynamic social background of intergenerational conflict and radical changes in freedom of determination for young people. Thus issues regarding teamwork and deferral of reward feature high on a version of Maslow's hierarchy of needs as visualised by Cohen and Brody (1991) for generation ships (see image).

Perhaps unsurprisingly then, conflict of intention between generations is a fairly common cause for generation ship failures in science fiction.

An interesting example is “Lungfish” by John Brunner (1957), a novella which was collected in the recent volume Spaceworlds (2021), edited by Mike Ashley for the British Library Science Fiction Classics book series. Here a generational spaceship has a relatively short journey to consider, so that while the space-born second generation is adult before “Trip’s End”, the Earth-born first generation is still in command. In this narrative the different perspective of the two generations is writ large - while the space-born are willing to help their parents reach their destination, they have no interest in the colonisation they were expected to undertake. Indeed the idea of settling on a planet appalls them, and they share a consensus mentality and perspective that is alien to the older generation:

"These had been children like any other children: noisy, inquisitive, foolhardy, disobedient.... And yet they had grown up into these frighteningly self-reliant people who could run the ship better than the earthborn any time they put their minds to it, and still refused to take the initiative."

Both the speed of this cultural shift and the uniformity of it is perhaps exaggerated in this story - the implication here is that simply being born in space has somehow changed the way humans think, regardless of personality, shipboard culture or parental influence. Effectively a new species is being born. This seems unlikely. Even given the radical differences between generations which emerged in the twentieth century, human nature remained unchanged. Nonetheless, the sense of alienation between generations in the story is compelling and troubling.

Another, much more recent example of intergenerational conflict undermining a generation ship effort can be found in Neal Stephenson's novel SevenEves (2015). In this narrative, a world-ending catastrophe forces the rapid development of an autonomous swarm of linkable small craft - a Cloud Ark - which are populated by youthful representatives of the world’s nations. The intent is that, together with a central space station populated by more mature adults, these will provide a home for future generations which will enable humanity to survive until Earth is once again habitable. Unfortunately, the “Arkies” diverge in preferred strategy and viewpoint from the “GPop” or General Population of the station at least in part due to an echo-chamber effect caused by the use of social media to communicate between components of the Cloud Ark. This is encouraged by an unscrupulous politician, and the swarm splinters, with its crews ultimately opting to desert the central space station. By the time contact is reestablished there are few survivors.

While the journey here is fundamentally a circular one, designed to stay in the vicinity of post-apocalyptic Earth until it can be resettled, the intergenerational conflict is again starkly portrayed. In contrast to Brunner’s uniformity, Stephenson introduces a handful of extremely strong dysfunctional personalities and leans hard on the way social media can magnify the voice of a demagogue. He emphasises the divergence in communication methods common between young adults from the communication channels of older generations, using explicit analogues to Facebook, twitter and the blogosphere to explain how the young Arkie Community becomes disconnected from the motivations of the elder crew. There's also a similar contrast drawn between the approach of a very familiar-seeming young tech billionaire and those of the more established military and scientific community. As social commentary goes, this feels a little heavy handed, but it’s interesting how closely it mirrors the same fear of misunderstanding between generations articulated sixty years - three full generations - earlier.

Many science fiction novels explore a much later stage of a generation ship’s journey, where the contrast between the experience of consecutive generations is perhaps less dramatic but the cumulative effect of decades of operational problems leads to a more general societal breakdown. These vary substantially from story to story, with potential problems ranging from disease of people or crops, environmental system imbalances, accidental mechanical or radiation damage, overpopulation and population collapse, to mutiny, division into completing clans by kinship or specialism, or other political unrest. A common premise is that disputes arise over whether or not to continue the journey, or that knowledge is lost due to the death of key individuals, or distortion by the process of repetition from one generation to the next.

A classic example (again collected by Mike Ashley in Spaceworlds) is “The Voyage that Lasted 600 Years” by Don Wilcox (1940), in which a single individual from the first generation, Grimstone, is placed in suspended animation and witnesses an entire process of social disintegration (much of it associated with overpopulation) through his intermittent awakenings. The starving population, grown to a far larger size than originally planned, are made to endure forced sterilisation, chaotic conflict and a growing ignorance until the sleeper found "the whole population of the Flashaway steeped in ignorance - immorality - superstition - savagery!". By the end of the story, Grimstone is forced to take control himself to force the ship back on course to its planned destination.

Image: Original illustrations from a magazine printing of "The Voyage that Lasted 600 Years", source: here

Highlighting the risk of disease to the confined population of a generation ship is a 1966 serial from television series Doctor Who - "The Ark" (written by Paul Erickson & Lesley Scott, directed by Michael Imison). Here the passengers of a migrating generation ship comprises a mixed population of humans and alien Monoids. This society appears to be stable, albeit with the Monoid minority suffering subjugation, until the Doctor and his companions introduce a cold virus which spreads rapidly, but is apparently as quickly contained. The time travel premise of Doctor Who [1] allows the crew to jump forward 700 years to see how the situation has deteriorated as the journey reaches its end, partly due to a resurgence of the virus and partly due to conflict between the different species on the Ark. The situation is further complicated by the realisation that the Ark’s target planet, Refusis, is already inhabited by a sentient indigenous species. While the aliens add an additional point of tension here, the impact of disease and the rebellion of a subjugated population could just have readily been explored between members of a human society.

A different form of social dysfunction can be seen in the excellent science fiction film Wall-E (2008, Disney/Pixar, dir. Andrew Stanton). Here, humanity has chosen to live on luxury liners for an interval of seven hundred years, while robots work to restore an Earth devastated by a culture of waste and buried under vast piles of discarded debris. The generations raised in luxury on the ships, including the starship Axiom. have become increasingly morbidly obese and dependent on robot assistance, until they are individually virtually helpless. This is strikingly demonstrated by the image (to the right) of the captains of Axiom who demonstrate the spiralling obesity problem in their succession. The passengers and crew also show little or no awareness of the potential or even desirability of returning to Earth. The goal of their journey has been forgotten amidst the luxury maintained by a self-repairing and robot-supported ship. It is not until a disruptive element (Wall-E himself) enters their world that the ship's captain seriously considers resettling the planet. The situation here is further complicated by an artificial intelligence in the main flight computer of Axiom which resists the ending of its role and the release of the ship-borne generations at the end of their journey.

A more extreme variation of this theme can be found in the 1981 BBC radio drama Earthsearch, written by James Follett. Here the second generation crew of the titular spaceship are killed by a meteor strike engineered by two megalomaniacal artificial intelligences known as Guardian Angels (Anciliary Guardians of Environment and Life), who then raise the four members of the third generation from infants. The crew are indoctrinated to believe the Angels are supernatural and infallible, and only gradually realise the truth. In the sequel, Earthsearch II (1982, Follett, BBC radio), a fourth generation is also threatened by corruption by the Angels. Thus we see two of the possible hazards of generation ships in one text: population collapse due to physical damage, and the risk of brainwashing and intergenerational differences when each generation’s education can be warped and varied from those of their parents in the enclosed society.

Interestingly, Earthsearch also provides an example of another consequence of societal breakdown on generation starships. The title ship’s crew discovered a sister vessel in which the descendants of the original crew have not only divided into multiple cultures, but have also forgotten that their world is a spacecraft at all.
(Image: cover of BBC Radio Collection audio recording release, showing the vast scale required for a generation starship. Artwork by Andrew Skilleter)

This is actually a remarkably common scenario in science fiction [2]. The distantly descended crews of generational ships in Robert Heinlein’s “Universe” (1941) and Milton Lesser’s “The Sense of Wonder” (1951) (both of which were adapted as radio plays by NBC’s anthology series X-Minus-One in 1955-56) have both come to believe that their ship comprises the entirety of the universe, with tales of the original journey or destination taken as mythology or holy writ (as was also the case in the Earthsearch example where a mythical Earth is equated with an afterlife).

“The Ship is all! Praise the Ship!” is a rote response in the radio version of “The Sense of Wonder”, where the introduction of a strict religious and social orthodoxy has been implemented. Understanding has been lost in favour of strict obedience, and it takes a change in engine activity after 10,000 years of travel to inspire any questioning of that mantra. Indeed there is a suggestion that the belief that “the ship is all!” and the forgetting of the word “destination” was a deliberate act of social engineering on the part of its creators, presumably stressing orthodoxy and religious conditioning in an attempt to avoid the social unrest described in other examples. This theme is less prominent in the original text of the story, which suggests that knowledge was lost as a result of a revolt against an elite of elders. Either way, the crew's life, food and even reproduction are regulated entirely by buzzers, and their shipbound lives leave them unable to conceive of a world outside, let alone understand what to do when the ship finally lands.

The same themes are seen in the Star Trek episode “For the World is Hollow and I have Touched the Sky” (1968, written by Rik Vollaerts, directed by Tony Leader), where a generation craft takes the form of a hollow asteroid, 200 miles in diameter and over 10,000 years old. Unfortunately, this asteroid “Yonada” threatens to collide with a Federation world. It becomes clear that the ship houses the distant descendants of a people destroyed when their sun went supernova. Again, we see evidence for a culture that has lost awareness of the wider universe, for a limited knowledge regarding the ship’s function being preserved in the form of religion, and for a computer (the Oracle) exerting control over a population who have lost their understanding of its origin (although in this case the computer is not noticeably self-aware or self-serving). Interestingly, the episode leaves an important issue unresolved. Although Kirk and Spock manage to correct a flaw in the guidance system of Yonada, several characters comment that the ship is close to the end of its journey, with a strong implication that this is in fact Daran V, the Federation world with billions of inhabitants which was earlier considered to be at threat. So if the ship was in fact heading for a rendezvous with this world rather than a collision, why did its navigation need correcting? And what will happen when the ship reaches its destination and discovers a vast population and developed culture already present [3] rather than the empty new world “the Creators” had promised? These questions remain unanswered.

Whether or not it really is plausible for awareness of the outside world to be lost is an interesting question. Most starships in science fiction from the 1940s to the current day - including Yonada in the Star Trek episode - have been nuclear powered, presenting a radiation risk to crews. This, along with the ever-present danger of interstellar radiation exposure without the protection of an atmosphere, is of particular concern for generation ships. In a small, enclosed population, mutations to genetic material will be cumulative and potentially devastating [4]. As a result, burying the habitable areas deep within a starship, without straightforward access to the outer hull or viewpoints (or indeed deep in an asteroid) provides a logical approach to radiation shielding. It may also simplify thermal regulation, insulating the living quarters from both the cold of deep space and solar irradiation when passing through solar systems.

Nonetheless, a generation ship will require careful management of waste heat, mechanisms for illuminating crops and producing food, atmosphere processing (both oxygen production and carbon dioxide removal), recycling of material resources, lighting and innumerable other systems. All of these need to be functioning to a very high degree of precision and synchronisation. The same is, of course, true for hypothesised space stations, domed or underground colonies or any other artificial or enclosed environment [5]. For a generation ship these margins of precision would typically be tighter given the more limited resources and capacity available for environmental feedback, and the longer intervals between potential refreshment of resources from elsewhere. Here a hollow asteroid, sufficiently large to support a functioning biosphere, or a giant spaceship (such as that of the Darians in Space:1999 or Arthur C. Clarke’s artificial world Rama) would have an advantage.

Nonetheless, an error of 1 percent per year in the output of any one system would give a cumulative error of almost 50 percent after five centuries. Given that a change of Earth’s environmental feedback systems of just a few percent would render our civilisation unsustainable (and is in the process of doing so), any deviation of an environment from optimal could be disastrous. Thus any ship in which the crew becomes incapable of detailed technical maintenance must have both a highly automated system and an infallible or highly redundant self-repair process. This seems to be implicit in a number of science fiction stories, but not always convincingly so.

Certainly it is difficult to entirely believe stories such as “Universe” in which technical work is carried out by rote as a religious duty, without knowledge of the underlying principles. This suggests that every possible malfunction is specified in the instruction texts and also that a degree of detailed information is retained in technical areas which is in stark contrast to the most simple technical knowledge of all: that the characters inhabit a starship.

If the underlying physical principles are indeed understood by a ship’s residents, then it’s hard to imagine a scenario in which they don’t lead to the conclusion that the ship was a constructed artefact.

A final challenge frequently faced by generation ships in science fiction is that of relativity - in one form or another. An interesting instance of this can again be found in Earthsearch. Damaged by the meteor strike they themselves permitted to occur, the Angels lose their knowledge of relativistic time dilation - the fact that moving objects experience time passing more slowly than those at rest. The generation ship in this drama travels at near lightspeed. As a result, returning to Earth, they find that that a million years has passed rather than the 100 years or so of elapsed ship-time.

A similar time dilation issue affects the exploration crew in Heinlein’s Time for the Stars (1956), although here it is expected and planned for. While the novel follows a crew ageing through a single generation of ship-time, several generations pass on Earth before the crew returns. The manner of their return exemplifies the second problem relativity can introduce for generation ships: a phenomenon common enough in science fiction for the TVTropes website to dub it “lightspeed leapfrog”. Generation ships are a strategy to be adopted for sub-light travel. If, in the centuries their journeys may take, a form of faster-than-light travel can be invented then it is entirely possible for the ship to arrive at its destination to find a human settlement or crew waiting for them after reaching the destination in mere days or weeks of travel.

In Time for the Stars the many year, time-dilation-affected journey of the protagonist’s ship is ended when the crew are rescued by a faster-than-light vessel from Earth. However the effects of leapfrogging may well be less welcome - does any crew which sacrifices for generations want to find themselves superceeded and their virgin worlds already colonised? Classic early examples of this trope include Wilcox’s “The Voyage that Lasted 600 Years”, already mentioned above, and the 1944 A E van Vogt short story “Far Centaurus” in which an exploration ship with a small crew arrives at Alpha Centauri to find their target world already settled by later waves of colonisation, shaking their sanity as well as disrupting their plans.

Robert Sheckley’s “The Native Problem” (1956, again adapted as a radio play by X-minus-1) describes the experience of a lone pioneer, sent out at faster than light speeds from Earth to a world he dubs New Tahiti. When a generation ship populated by religious extremists arrives on the planet, they refuse to accept his story, assume he is a representative of a hidden native tribe and set him to hard physical labour with threats of violence. This is an interesting example of the way science fiction can explore both physical phenomena and social questions. In many ways this is a wryly humorous critique of the Pilgrim Fathers and their legacy in the assumptions made when western cultures approach indigenous peoples. Nonetheless, the flat refusal of the incoming generation ship’s residents to accept that they might have been overtaken is striking and provides an interesting insight into the potential clash of world-views between an isolated, ship-born population and the products of later emigration from Earth.

So Is a successful generation ship scientifically plausible? Arguably an entirely successful generation ship would not make for a good story and so perhaps its unsurprising that the examples in SF are overwhelmingly negative. As has already been mentioned, the tolerances for system imbalances compromising the ship’s habitability are very narrow, although larger vehicles and megastructures are less vulnerable. As many science fiction examples also make clear, the potential for social unrest or breakdown is very high. Since many of these stories are firmly rooted in human psychology - the tendency of people to be people, and crowd mentalities to develop - it’s difficult to see how this kind of unrest can be avoided. No matter how careful the selection of a first generation and plan for training those that follow, individual rebellion and shifts in goals and aspirations will occur.

Nonetheless, it is worth noting that despite the challenges they encounter, a number of science fiction generation starships are actually successful in their mission - whether or not this occurs in the manner predicted by their originators. The human survivors in Seveneves do indeed make a new human society which lasts thousands of years, despite political unrest and a population crisis. The passengers of the liner Axiom in Wall-E do indeed return to an Earth that is starting to regenerate its vegetation, successfully nurturing a paradise. After ten millennia of ignorance, the asteroid ship Yonada in Star Trek is indeed about to deliver its population, together with a book of instructions and a vast archive of knowledge, to a new world. And van Vogt’s colonists do indeed reach far Centaurus.

In truth, it is unlikely that humanity will ever overcome the very real challenges of maintaining entirely closed artificial environments, or entirely enclosed and captive populations. Generation ships are only ever likely to be seriously considered in the kind of dire emergencies described in SevenEves or "The World is Hollow and I have Touched the Sky", where the survival of a species depends on the attempt. Nonetheless, it is interesting to note that if such an eventuality did occur, science fiction has already provided a vast array of thought experiments, warning of hazards that may be avoided, and perhaps of a few for which avoidance is impossible.