There are two competing models of space colonization; both are purely theoretical at this point, but they will not remain so for long.
One model focuses on sending a small number of people to another planet or moon for specialized, technology-mediated tasks, presumably with considerable support from Earth—creating, in effect, terrestrial versions of the International Space Station. This seems to be what the MarsOne project has in mind, at least for now: sending 24 people to Mars to live in a self-contained facility and receive what they need from Earth.
The other model focuses on creating sustainable, self-sufficient colonies that essentially operate as new, freestanding parts of the human story; this generally means a lot of extraterrestrial subsistence farming and other decidedly unglamorous work that has always been central to the independent survival of new human settlements in the past, and is likely to remain so for some time to come.
If you’re going to colonize the first way, it’s not really a colony in the traditional sense; it’s more of a scientific outpost. Real colonization implies multiple generations, long-term sustainability, and a new, emerging community that will develop its own cultural identity. It stands to reason that if you’re going to build one of those, you’re going to need more than 24 people to do it. How many, exactly? Portland State University anthropologist Cameron Smith has a number: 40,000. And his reasoning is pretty sound, at least from the standpoint of sustainable genetic diversity, economics, and so on:
“This number would maintain good health over five generations despite (a) increased inbreeding resulting from a relatively small human population, (b) depressed genetic diversity due to the founder effect, (c) demographic change through time and (d) expectation of at least one severe population catastrophe over the five-generation voyage … Almost no natural populations of vertebrates dip below around five to 7,000 individuals. There are genetic reasons for this. And when they do go below this, sometimes they survive, but many times they go into what’s called a demographic or extinction vortex.”
Smith points out another benefit as well: with 40,000 people, you can have some occupational diversity. Subsistence farming will still be a big deal, but there may be room for musicians, policymakers, writers, artists, fashion designers, physicists, sociologists, burlesque dancers, rodeo clowns, clergy, bankers—a wide range of human occupations working together to create a community worth living in. After all, what’s the point of creating an extraterrestrial colony that isn’t worth living in?