The “magic number” of people needed to create a viable population for multi-generational space travel has been calculated by researchers. It is about the size of a small village – 160. But with some social engineering it might even be possible to halve this to 80.

Anthropologist John Moore from University of Florida tackled the problem as part of a combined effort with space scientists to determine how in future humans might successfully undertake century-long journeys out into space.

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In the past, attention has been focused on cryogenics, sperm banks and military-style modes of operation, says Moore, but “the ‘right stuff’ for a journey into space is the family – a million-year-old institution designed to assist reproduction.”

Moore has previously studied small migrating populations of early humans and has developed simulation software – called Ethnopop – for analysing the viability of small groups.


Marriage partners

For a space trip of 200 years, perhaps eight to 10 generations, his calculations suggest a minimum number of 160 people are needed to maintain a stable population.

This would produce around 10 potential marriage partners per person, he says, and if this seems a small number, “think about how many people you dated before you got married”.

Room would be at a premium on any spacecraft and reducing the number of people initially required might be desirable. Moore suggests two strategies. The first is to begin with young childless couples, echoing the practice of Polynesian seafaring colonists.

The second is to ask the space crew to postpone reproduction to later in woman’s fertile period, perhaps age 35 to 40, creating longer time gaps between the generations. This measure results in a stable population of just 80 but the consequences of the increased medical risks of late childbirth have not yet been considered.

A potential concern is that small populations can suffer a damaging reduction in genetic diversity due to inbreeding, says Dennis O’Rourke from the University of Utah. He considered the same 10-generation, 200-year journey as Moore and looked at both genetic drift and inbreeding.

“The decrease in genetic variation is actually quite small and less than found in some successful small populations on Earth,” he says. “It would not be a significant factor as long as the space travellers come home or interact with other humans at the end of the 200 year period.”

Gene screening

O’Rourke believes that a more serious concern would be the presence of potentially damaging genotypes in the initial space pioneers. Genetic screening might well be needed, he says: “Any harmful recessive characteristics might lead to increased healthcare loads which would deplete scarce resources.”

A final concern raised at the American Association for the Advancement of Science’s annual meeting in Boston was the possibility of infighting. Small communities isolated for long periods at research stations in Antarctic and even families travelling on long car journeys, provide examples of how small conflicts can quickly escalate.

But Moore points out: “Some small island communities on Earth have lived in peace and harmony for thousands of years because they have developed ways of solving conflicts. These are not taken to Antarctica.”