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How many people would humanity need to send on an interstellar voyager to ensure the survival of a colony beyond the solar system?

According to one expert the answer is a staggering 20,000 to 40,000 people.

A study suggests that, in order to ensure significant genetic diversity, such large numbers will be needed if any space colony is to survive a trip lasting 150 years.

Dr Smith from Portland State University says a future interstellar mission would only work if a large human colony was taken along for the trip on a large starship (illustration of a 'stanford torus', a proposed design for a space habitat made in the 1970s, pictured). The ideal number would be a population of 40,000

Dr Cameron Smith is an anthropologist and prehistorian at Portland State University in Oregon, and is also involved with Icarus Interstellar, an organisation dedicated to advancing interstellar starship research and development.

At Icarus he is working on Project Hyperion, a study of the various factors at play for a future deep space mission - including the number of humans to take.

And in a paper published Acta Astronautica he describes how taking less than 20,000 people on an interstellar voyage would be 'risky' over multiple generations.

'Designing interstellar starships for human migration to exoplanets requires establishing the starship population, which factors into many variables including closed-ecosystem design, architecture, mass and propulsion,' Dr Smith writes.

He continues: 'I find that previously proposed such populations, on the order of a few hundred individuals, are significantly too low to consider based on current understanding of vertebrate (including human) genetics and population dynamics.'

He says that having an 'Interstellar Migrant Population' of 40,000, 23,400 of which are 'reproductive males and females,' would be a good figure to ensure the health of a colony travelling for 150 years, or five generations.

Such a number of people is necessary to avoid interbreeding among the relatively small human population according to Dr Smith.

The study considered a hypothetical 150-year mission to a neighbouring star. Increasing spacecraft speeds by a factor of 1,000 would make a mission to a star such as Proxima Centuari (shown with date the distance was calculated) possible, which is 4.24 light years away

Pictured is an illustration of the IXS Enterprise, an interstellar ship envisaged by Nasa's Dr Harold White that could use warp technology to travel 'faster than light'. It could be an accurate representation of what the first mission beyond the solar system will look like

During a talk for Nasa's Future In-Space Operations, Dr Smith elaborated that no natural populations of vertebrates on Earth dip below five to 7,000, reports space.com.

'And when they do go below this, sometimes they survive, but many times they go into what's called a demographic or extinction vortex,' he explains.

The best way to ensure the survival of a colony on a long-duration journey to a distant alien planet would be to have a ship large enough to house a population of a town.

Another less preferred option, however, would be to send frozen sperm and eggs from Earth-bound people.

Dr Smith also explains how his study would be applicable for colonisation of worlds inside out solar system such as Mars.

For interstellar colonisation, however, he admits that it will require significant advances in technology before any such missions is possible.

But if spacecraft speeds can be increased by 1,000, then a mission to our closest neighbouring star Proxima Centauri lasting 140 years is feasible, says Dr Smith.