Interstellar Colonization

One thing I don't like about these models is that they tend to be based around "average" trip distances and speeds. However, the rate of expansion will be determined by the "pioneering" trip distances and speeds.

The sorts of interstellar propulsion I find plausible involve an incredible amount of initial investment and economic buildup, but then the marginal costs for additional colonization missions are small. This suggests that the third generation of colonization missions might as well be long range missions. The second generation of colonies will have saturated the nearby systems so the only direction to expand is into long range missions.

For example, suppose it takes 5000 years to build up from an initial colony into something that can send out missions of their own. In the meantime, the home system could be sending out colonization missions at a rate of one per decade. By the time the first generation of colonies is up for sending out colonization missions, the nearest 50 systems have already been colonized. The first generation then sends colony ships to fill out the nearest 2500 systems.

Assuming no one has yet bothered to try any long range colonization missions, the result is a compact ball of 2500 colonized systems, of which only a thin shell on the outside can expand with short range missions.

It seems to me plausible that at least some of the "core" systems will embark on long range missions. Maybe some of those long range missions will merely just barely outrun the expanding border. That's a rather short-sighted strategy. Other long range missions will daringly punch across the galaxy, starting up seeds which won't run into the "slowpoke" border for dozens of millenia.

The result is an overall frontier of expansion that is defined by sporadic long range "seed dots". They fill out eventually, but it's entirely plausible for the overall rate of expansion to be entirely defined by far reaching long range high speed missions from the home system or early generation systems.

Whether a planet of 10 million people - the equivalent of a single large urban region -- could realistically have a diversified enough economy to maintain and operate a fleet of starships seems a bit iffy, unless they are putting a massive effort into it, so massive that it may stunt their other prospects.

The most likely scenario for a world of 10 million people sending out a colony might be that they've decided their current home sux, and they're going to try their shot at another one.

Looking at the other end, how many people for a viable colony. I'd say 10,000 at the low end, with 100,000 seeming a lot more comfortable. That's the population of one semirural county. How many machine shops and such does it have, how much can they specialize for efficiency, and oh yeah, you need raw material, a mining sector and all that.

So maybe my figures could all be squeezed down by an order of magnitude, so that a colony of 10,000 is fairly viable, a colony of 100,000 can maintain a full industrial base, and one of a million people can keep its own starships in service. That helps for story settings, but you wouldn't generally expect worlds like that to be active colonizers.

Finally, and most central to time scale, how fast do colony populations grow, either from immigration or birth rate? I would call a million emigrants from Earth each year a benchmark figure for large scale colonization. That's several thousand people each day, one huge ship or several merely big ones, and it still takes a century of sustained effort to plant 100 colonies, each of a million people.

The other side of colonial growth is reproductive growth. Doubling the population each generation is about the historical sustained maximum. That corresponds to 10x per century, so Deseret World might go from 100,000 people to 10 million people in 200 years.

But even doubling per century is a pretty robust population growth rate. That's roughly 1.2x per generation. Unless you're growing 'em in vats, about half the women are having three or four kids, and one way or another the society encourages and accommodates itself to this.

It's no given that post industrial societies will generally have this population growth rate, though colony worlds may not follow the current trend in industrialized societies toward ZPG or even less.

Emigration from Earth to the existing colonies can continue after that, but at some point the rate will likely fall. Successful colonies will no longer want people dumped on them, unsuccessful colonies can't absorb them, so emigration falls to the level of people who can pay to go and want to go, or who the colonies are willing to pay for.

So. At some point around 2400, colonization has tapered off and emigration is tapering off. We can guess that there are at least a dozen or so full colony planets - if you can reach any you can probably reach about that many (and you need a good handful for a decent scenario).

The upward limit is about 100 or so true colony worlds, set - regardless of how many worlds are in reach of your FTL - by the postulated size of the colonization wave. A hundred million people, a hundred worlds - an average of about a million immigrants per colony, though the distribution may well be oligarchic by a power law, a handful of colonies getting a large share of total immigrants, growing to populations of up to a few tens of millions, while most have less than a million and kind of struggle along.

A few of these colonies already in 2400 have upwards of 10 million people and some potential to colonize themselves, but these were the immigration magnets, so they probably still feel short-handed if anything, not inclined to send lots of people off.

It will take 200 or 300 years for smaller colonies with rapid population growth rates to start pushing up into the 10 million population range, and might have the impulse and capability to colonize. But it might take closer to 500 years for a substantial number of the original colonies to have much motivation to colonize.

The early goers, though, will have filled in the next layer of easy pickings. Here is where your FTL really matters - whether you can light off freely into the vastness to hunt for a suitable planet, or are constrained by a colonization sphere that is starting to grow again.

But broadly speaking, it seems that secondary colonization couldn't be expected in any serious way until sometime well after 2500, and perhaps not in a big way till sometime around 2700-3000.

An interstellar domain can have no definite borders; stars are scattered too thinly, their types too intermingled. And there are too many of them. In very crude approximation, the Terrestrial Empire was a sphere of some 400 light-years diameter, centered on Sol, and contained an estimated four million stars. But of these less than half had even been visited. A bare 100,000 were directly concerned with the Imperium, a few multiples of that number might have some shadowy contact and owe a theoretical allegiance.

Consider a single planet; realize that it is a world, as big and varied and strange as this Terra ever was, with as many conflicting elements of race and language and culture among its natives; estimate how much government even one planet requires, and see how quickly a reign over many becomes impossibly huge.

Then consider, too, how small a percentage of stars are of any use to a given species (too hot, too cold, too turbulent, too many companions) and, of those, how few will have even one planet where that species is reasonably safe. The Empire becomes tenuous indeed.

And its inconceivable extent is still the merest speck in one outlying part of one spiral arm of one galaxy; among a hundred billion or more great suns, those known to any single world are the barest, tiniest handful.