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Optimal Worldship Populations

Although we tend to focus on propulsion as the major obstacle to reaching another star, the biological problems that go along with journeys lasting decades or even centuries are equally daunting. If we could devise methods that would get us to Alpha Centauri within a century, we’d still face the need to keep a crew alive within a sustainable closed ecosystem for that amount of time. If we’re talking humans in starships, then, we need a lot more data about how people interact in isolated communities, stay healthy, and manage critical self-sustaining systems.


Image: A habitat for humans over generations, a worldship poses critical questions about survivability and genetic diversity. Credit: Adrian Mann.

Centauri Dreams readers will recall Cameron Smith’s interest in these matters, as reflected in his article Biological Evolution in Interstellar Human Migration, published here last March. The author of Emigrating Beyond Earth: Human Adaptation and Space Colonization (Springer, 2012), Dr. Smith (Portland State University) looks at these issues over the course of generations. How large does a starship crew have to be in order to keep the population healthy? This article in Popular Mechanics gives a nice overview of Smith’s findings, which were published in Acta Astronautica and flesh out his earlier essay in these pages. The work was performed as a contribution to Icarus Interstellar and its Project Hyperion.

Working with William Gardner-O’Kearney, Smith constructed simulations to create scenarios for interstellar travel with the help of MATLAB, a widely used tool for numerical computation. One immediate result was to draw into question earlier calculations by John Moore (University of Florida), who had found that a 2000 year voyage aboard a generation ship would require an initial crew of no more than 150. In sharp contrast, Smith found that a minimum of 10,000 was necessary, while 40,000 would be a safer number still given the perils of the journey. Starting population size, which the duo calculated over 30 generations, is a crucial matter.

A key issue, as you would expect, is genetic diversity. Small groups like the Amish and Ashkenazi Jews suffer higher rates of diseases like cystic fibrosis and Tay Sachs largely because of intermarriage between relatives. I’ll send you to the article for the bulk of the researchers’ graphs, but I’ll show one below, illustrating what happens within groups of different sizes over time. A ship starting out with a crew of 150 loses 80 percent of its genetic diversity after thirty generations. Even 500 is too small a number, for it does not represent a wide enough swath of the human population. Somewhere between 10,000 and 40,000 is where we find a starting population that can maintain 100 percent of its original genetic variation.


Image: The decline in genetic diversity among smaller populations over time is evident here. Note the 150 line in red at the bottom of the chart, with the most robust, in purple and representing a starting crew of 40,000, shown at the very top. This number maintains 100 percent diversity. Credit: Cameron Smith/Gardner O’Kearney.

Just as we preserve a healthy gene pool with a larger population, we also safeguard against external risks, the kind of catastrophe that could snuff out the entire population of a small ship. This work makes the case that housing tens of thousands of colonists in a single generation ship would be a mistake. Far better, when launching our expedition, to use multiple ships, traveling perhaps close enough together for trade and other human interactions, but separated so that a single disaster wouldn’t mean the end of the entire venture. I’m invariably reminded of the expedition led by Sky Haussmann in Alastair Reynolds’ novel Chasm City (2001), a fleet of starships that confronts a human-caused calamity.

10,000 seems to be the minimum number for success. Says Smith: “With 10,000, you can set off with good amount of human genetic diversity, survive even a bad disease sweep, and arrive in numbers, perhaps, and diversity sufficient to make a good go at Humanity 2.0.” That’s a large crew, but history has shown us that there are always pioneers, adventurers, misfits and any number of other psychological types willing to give up everything they have known to chance their future in unknown lands. The guess here is that if a fleet of five generation ships needing crews of 2000 each is ever built, it will not lack for volunteers.

The paper is Smith, “Estimation of a genetically viable population for multigenerational interstellar voyaging: Review and data for project Hyperion,” Acta Astronautica, Vol. 97 (2014), pp. 16-29 (abstract).


Comments on this entry are closed.

  • John Frazer April 15, 2014, 4:12

    The Oort cloud cometary civilization (of small groups of maybe 500 in loose trade with others via what fast ships there are) deals with the sudden need to out-migrate due to catastrophies.
    6000 Au is a pretty good buffer for physical dangers, and nothing but total self-imposed isolation protects against harmful memes.
    I point out that Jones and Finney mentioned that out so far, the Sun is only a particularly bright star, and there are others brighter.

  • Randy Chung April 15, 2014, 16:26

    @John Frazer April 15, 2014 at 3:47
    Thanks for mentioning the book “Interstellar Migrations and the Human Experience”, it looks interesting! I ordered a paperback through Barnes and Noble for less than $13.

  • Cameron M. Smith May 9, 2014, 20:05

    Hi all, interesting posts. The Popular Mechanics article regarded the technical article I wrote for Acta Astronautica (https://www.academia.edu/5506161/Estimation_of_a_Genetically_Viable_Population_for_Multigenerational_Interstellar_Voyaging_Review_and_Data_for_Project_Hyperion). In that paper I made many assumptions, as one must for any model. These included the rate and lethality of expectable catastrophes, e.g. plagues, on such voyages, population growth rates and so on. I came to 10’s of thousands as genetically healthy populations of vertebrates almost never, in the natural world, fall below about 5,000. The oft-cited 150 people as a viable unit over generations is problematic for me in at least two ways. First, such tribal or band-sized populations were never entirely isolated; rather, they had extensive interbreeding links with other populations, and the human minimum viable population at 150 is a wide misunderstanding. So, I prefer to use about 10,000, which is closer to reality, genetically, for health in animals such as humans. Second, even if 150 were viable, I would not want to stake such a giant project on the bare minimum of what is possible, I would want to build in a safety margin, just as aircraft engineers build in a safety margin in their designs. So, my figure of 40k is roughly 2x the actual figure of 18k that I find ideal, for many reasons.

    I don’t entertain ideas like a single female with sperm bank or just sending egg and sperm banks, or entire ecosystems to colonize a plane by automation, or sending crews of explorers set up as ranked shipmates etc. I think it will be better to send humans in arrangements, e.g. social organizations like villages and towns, that are not new to the human experience, but have worked for the kinds of beings that we are for many thousands of years.

    One other note, on the moral implications; I think it is morally permissible to bring children up in interstellar voyage conditions just as it was permissible for ancient Polynesians to take pregnant women on ocean-exploring voyages, and birth children on distant, new islands. That was also ‘perilous’ etc. And keep in mind, these ships will be well-established for the safety of the children, certainly moreso, in fact, into the conditions to which most Earth children are born today.

    Thanks for your interest, even if you disagree with my positions.

    I have been meaning to write an item for Paul Gilster on cultural implications of such voyages, but have gotten sidetracked, and it will be some time before I can get to that.