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Rewriting Life

This is how many people we’d have to send to Proxima Centauri to make sure someone actually arrives

Since it would take at least 6,300 years to reach the closest star to our sun, enough men and women to produce many genetically healthy generations would need to make the trip.

If humans are ever to colonize the galaxy, we will need to make the trip to a nearby star with a habitable planet. Last year, astronomers raised the possibility that our nearest neighbor, Proxima Centauri, has several potentially habitable exoplanets that could fit the bill.

Proxima Centauri is 4.2 light-years from Earth, a distance that would take about 6,300 years to travel using current technology. Such a trip would take many generations. Indeed, most of the humans involved would never see Earth or its exoplanet counterpart. These humans would need to reproduce with each other throughout the journey in a way that guarantees arrival of a healthy crew at Proxima Centauri.

And that raises an interesting question. What is the smallest crew that could maintain a genetically healthy population over that time frame?

Today, we get an answer thanks to the work of Frédéric Marin at the University of Strasbourg and Camille Beluffi at the research company Casc4de, both in France. They have calculated the likelihood of survival for various-sized missions and the breeding rules that will be required to achieve success.

First, some background. Space scientists and engineers have studied various ways of reaching nearby stars. The problem, of course, is the vast distances involved and the comparatively sedate speeds that human spacecraft can manage.

Apollo 11 travelled at around 40,000 kilometers per hour, a speed that would take it to Proxima Centauri in over 100,000 years. But spacecraft have since become faster. The Parker Solar Probe, to be launched this year, will travel at more than 700,000 kilometers per hour, about 0.067 percent the seed of light.

So Marin and Beluffi use this as the speed achievable with state-of-the-art space technology today. “At this speed, an interstellar journey would still take about 6,300 years to reach Proxima Centauri b,” they say.

Selecting a crew for such a multigenerational space journey would be no easy feat. Important parameters include the initial number of men and women in the crew, their age and life expectancy, infertility rates, the maximum capacity of the ship, and so on. It also requires rules about the age at which procreation is permitted, how closely related parents can be, how many children they can have, and so on.

Once these parameters are determined, they can be plugged into an algorithm called Heritage, which simulates a multigenerational mission. First, the algorithm creates a crew with the selected qualities. It then runs through the mission, allowing for natural and accidental deaths each year and checking to see which crew members are within the allowed procreational window.

Next, it randomly associates two crew members of different sexes and evaluates whether they can have a child based on infertility rates, pregnancy chances, and inbreeding limitations. If the pregnancy is deemed viable, the algorithm creates a new crew member and then repeats this loop until the crew either dies out or reaches Proxima Centauri after 6,300 years.

Each mission also includes a catastrophe of some kind—a plague, collision, or other accident—that reduces the crew by a third.

The algorithm then repeats each mission 100 times to determine the likelihood of this size of crew reaching its destination.

A key question is what degree of inbreeding can be allowed. Marin and Beluffi measure this using a scale in which breeding between identical twins registers as 100 percent; brother/sister, father/daughter, or mother/son is 25 percent; uncle/niece or aunt/nephew is 12.5 percent; and first cousins is 6.25 percent.

One option is to limit inbreeding to less than 5 percent, so partners have to be more distantly related than first cousins. Another option is to stipulate that partners cannot be related at all, so that inbreeding is 0. Marin and Beluffi use this second scenario in their simulation.

The algorithm then determines the likelihood of success over 100 missions for different initial crew sizes.

The results make for interesting reading. The Heritage algorithm predicts that an initial crew of 14 breeding pairs has zero chance of reaching Proxima Centauri. Such a small group does not have enough genetic diversity to survive.

Researchers have observed with animals that the genetic diversity of an initial population of 25 pairs can be sustained indefinitely with careful breeding. But when the Heritage algorithm uses this as the starting crew—25 men and 25 women—it predicts a 50 percent chance of dying out before reaching the destination. That’s largely because of random events that can influence such a mission.

The chances of success, according to Heritage, do not reach 100 percent until the initial crew has 98 settlers, or 49 breeding pairs. “We can then conclude that, under the parameters used for those simulations, a minimum crew of 98 settlers is needed for a 6,300-year multi-generational space journey towards Proxima Centauri b,” say Marin and Beluffi.

That’s interesting work that sets the stage for more detailed simulations. For example, fertility rates in deep space may turn out to be quite different from those on Earth. And the chances of a healthy child resulting from a successful pregnancy may also be much lower because of higher mutation rates due to radiation.

The chances of catastrophe because of accidents or plagues may turn out to be much smaller than the chances of catastrophe caused by social factors such as conflict. All this could be programmed into a more advanced version of Heritage.

Indeed, these issues have already been explored by science fiction writers. For example, in the book Seveneves, the author Neal Stephenson imagines a future in which humanity passes through a population bottleneck and all individuals are descended from seven women.

Given Marin and Beluffi’s work, Stephenson’s imagined future looks highly unlikely. But it is surely important to consider the scenario given the multiple threats that our civilization faces. 

Ref: arxiv.org/abs/1806.03856 : Computing the Minimal Crew For A Multi-Generational Space Journey Towards Proxima Centauri b

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