John Hawks takes a look at a fascinating article in Popular Mechanics that looks at what the minimum crew size would need to be for an interstellar generation ship, and relates it to what is known about genetics in anthropology.
Popular Mechanics asks, “How Many People Does It Take to Colonize Another Star System?”. The basic problem is that a multigenerational star voyage requires the trekkers to mate and reproduce many times while maintaining a limited population size. Too few people, and the colonists will rapidly lose genetic diversity by genetic drift.
The article starts by noting the work of anthropologist John Moore on the question. Moore concluded that the social structure necessary to prevent inbreeding was essentially that of clans or extended tribes of hunter-gatherers — strong kin avoidance rules to prevent inbreeding and a population size of 150-300 people.
A new paper by Cameron Smith focuses instead on the worst case scenarios, concluding that the “safe” population size would be much higher:
Personally, I have strong doubts that a generation ship is a feasible concept, even if the minimum crew size issue can be worked out. The sheer cost in energy of accelerating 10,000-40,000 people to an effective interstellar cruising speed, and then decelerating them back down to interplanetary speeds at their destination, is appalling. And that’s completely separate from maintaining a closed ecosystem for centuries.
Then there’s the ethics of consigning generations of people to a life they have no way to opt out of. As Hawks discusses, it would take an intense cult of some type to contemplate and carry out that kind of action, and those aren’t known for their scientific and technical acumen. Other than as a desperate ark of some kind, perhaps fleeing environmental collapse, it’s hard to imagine such a mission happening.
I think the possibility of such a mission goes up considerably once we’ve achieved extended life spans, where the crew have some chance of living to see the destination. The closed ecosystems are still an issue, but the other ones fade somewhat. The nice thing about this option is that the minimum crew size shrinks back into the hundreds since, theoretically, there would be no constraint on population growth at the destination. The energy requirements are still appalling, but far less so than for a crew of over 10,000.
As I’ve noted before, given those appalling energy requirements, my personal prediction for interstellar exploration is that it will be done by miniature robots, with humans only experiencing it by possibly transmitting their uploaded consciousness to those robots.