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).