How many astronauts to reach Proxima Centauri b?

One day, at least if science fiction novelists and dreamers are to be believed, humankind will set sail for planets outside the solar system. An extraordinary new chapter will begin in the story of our species, as we fly past the Kuiper Belt and prepare to take our place in the Drake Equation (while possibly solving the Fermi Paradox at the same time). Most likely, the first destination for a crewed exoplanet mission will be Proxima Centauri b, a planet identified in 2016 by the European Southern Observatory. There are several reasons for betting on this, among them the fact that the planet is only a bit bigger than Earth, and sits in the predicted Goldilocks Zone of its host star, Proxima Centauri.

The primary reason, however, is that it is by far the closest exoplanet in the sky. But the measurement is relative: Proxima Centauri b sits about 4.2 light years, or 40 trillion kilometres away. And that is going to make for one hell of a long journey.

How long? In a recent paper, Frederic Marin, of the University of Strasburg, and fellow French astronomer Camille Beluffi estimate 6300 years, based on the predicted 200-kilometre-per-second speed of NASA’s soon-to-be-launched Parker Solar Probe.

This clearly has some implications for crewing. Quite obviously, the astronauts who first climb aboard the Proxima Centauri Express are not going to be the same ones who clamber down the ladder at the other end.

Indeed, the folks who eventually arrive at the destination will be the descendants of a micro-community that has been living, breathing, eating, defecating, breeding, and telling stories in a high-speed tin can for longer than the time currently elapsed between the emergence of Sumerian culture and the invention of the Nintendo Switch.

And that, as any zookeeper will testify, brings a whole new set of problems that need to addressed before anyone goes anywhere, or thinks about anything. Any spacecraft is going to be constrained by mass – mega-ships such as the Star Wars Executor-class Star Dreadnought with room for 50,000 people would never get off the ground – so the size of the crew will need to be as small as possible.

But small here, too, is a relative measure. It is critical that the people on board the craft are of adequate number, appropriately gender-balanced, and sufficiently unrelated such that their great-great-whatever grandchildren do not all turn out to sport extra toes, crossed-eyes, and a penchant for playing the banjo.

In their paper, currently lodged on the preprint server Arxiv and thus awaiting peer-review, Marin and Beluffi attempt to calculate the essential minimum number of original crew, using an approach known as a Monte Carlo simulation.

The technique – widely employed by investment managers and economists – involves building models for possible results by using a range of values for every factor. The resulting numbers are then crunched many thousands of times, resulting in a spread showing the most and least probable outcomes.

The integrity of the model, of course, is dependent on the integrity of the data plugged into it, so Marin and Beluffi included as many real-world variables as possible. First of all (and very importantly), they assume inbreeding is totally forbidden and doesn’t happen. Then they throw in other likely outcomes that would affect the health and size of the crew, such as early deaths, illness, male and female infertility, restrictions in breeding to keep numbers under control, and menopause.

They even bunged in a hypothetical “catastrophic event”, occurring 2500 years into the journey, affecting one-third of the people onboard.

A first punt at using an initial crew comprising 25 breeding pairs was found to be far from satisfactory, failing in half the simulations run.

After doing more modelling, progressively increasing the number of initial breeding pairs, the researchers found that 49 – 98 people – was the minimum needed to have a 100% chance of a genetically healthy bunch arriving at the exoplanet.

(This is considerably more than the minimum number suggested by science fiction author Neal Stephenson in his 2015 novel Seveneves. The author envisaged the recovery of a functioning, technological human society 5000 years after its total destruction, based on the fertility and gene-editing genius of just seven women.)

Despite the impressive robustness of their modelling, Marin and Beluffi concede that there are a couple of crucial variables that need to be factored into future simulations before a mission to Proxima Centauri b can blast off with full confidence in a successful, eventual, arrival.

Over that many millennia, they say, matters of population such as genetic mutation and drift will have an influence. The perhaps deleterious effect of cosmic rays also needs to be taken into account. Sure, they note, it will be perfectly possible to fit the launch vessel out with a shield to protect the crew from such rays, but over six millennia it may well suffer a bit of, possibly lethal, wear and tear.