Interstellar genetics
A recent paper contemplates a very long voyage.
The stars are a long way away. Generations of astronomers have tried to come up with a metaphor or image that can capture just how lonely the Sun is, with equivocal success. Numbers alone don’t seem to have quite the impact they should. Distances are quoted in light-years, but to humans the speed of light is unimaginably fast. A recent calculation takes the speed of the projected Parker Solar probe, which is expected to set records for human-made objects, as about 200 kilometers per second. At that speed, the authors estimate that it would take about 6300 years to get to the nearest star, Proxima Centauri.
That sort of delay makes it hard to write gripping science fiction. The most popular way around the problem of distance is to postulate some sort of “warp drive” or “hyperspace,” so that spaceships can reach other stars in times reasonable by human standards. A second method for at least settling the Galaxy (if not exerting control through thrilling Space Navy battles) is to freeze your colonists so they don’t age during the voyage. Well, while people have been frozen, none has yet been revived, so that particular technology is still unproven. A third method, requiring the least technological development from our current state, is to make a starship big and self-sufficient so that it can conduct a multi-generation voyage: the settlers will be the many-times-great-grandchildren of the crew that left Earth. That’s the scenario our authors (referred to above) examine in a recent paper.
Leaving aside the truly daunting technological and social challenges of this kind of effort (6300 years is longer that human history to date!), our authors examine only the genetic aspects of the voyage. That is, how should the voyage be arranged so that the settlers arrive genetically healthy? Well, genetics is a science of probabilities. Each child is a throw of the genetic dice. So the authors ran many simulations of the 6300-year voyage using a computer, with random elements carefully built in. The results are not absolutely exact. However, they’re certainly firm enough for some conclusions.
The first is, that if a set of rules for having children (suggested by another author) is rigidly adhered to, the voyage will fail. Rules intended to avoid overpopulation, so that the ship’s resources remain adequate for its crew, lead to a population crash. No one is left of run the ship. Flexibility based on the current situation is necessary. If things like the number of children per couple can change, the chance of success is much better. However, something like 10% of the crew become dangerously inbred, related too closely to have children safely. If rules against close-relative couples are implemented, we begin to have a good chance of settling Proxima Centauri’s planet(s).
But we have to start with enough people (all assumed to be unrelated at the beginning). With 25 couples, the mission stands about a 50% chance of success genetically. With double that number, there’s well above a 99% success rate.
Well, recall from last week that people historically lived in, and chose their spouses from, their own village and the region nearby. 50 couples, 100 people, is roughly the size of a village; but those represent households, so we have to add children and old people. Our spaceship is rather larger than the historical village. And if we relax the rather strict computer rules on who you can have as a spouse, it would have to get larger yet. On the other hand, near the ship there are no outside villages for the more adventurous wooers (and if 10km is the average distance between couples’ birthplaces, that means some sought partners from farther away). So the historical and computer genetics are in rough agreement
We suggest, though, that the best way to settle the Galaxy may be to send constellations of village-sized ships. Most would find their spouses on their own spacecraft, but some would wander. Let there be love among the stars for the genetic health of the children!
