Larry Niven defines a macro-life civilisation as one that lives in interstellar spacecraft, independent of planetary resources and culture, full time and by implication keeps moving. This could mean small but self-sufficient generation ships or could entail something as vast as a ram-jetting Ringworld with a star for a fusion drive. Such a civilisation, once formed, would appear to have no resource reason to resettle on planets, (near asteroid belts yes; at the bottom of a planetary gravity well no), and in terms of long-term species survival macro-life seems, to me, the better bet.

The individual case answer will obviously vary based on cultural mores but I'm interested in the logical answer based on resources and long-term survival goals, to that end: Why would a successful macro-life civilisation give up their long held nomadic habits for a return to a planet-bound lifestyle?

Please note that for the purposes of this question it's all or nothing; either they colonise new planets or they stay interplanetary, travelling in habitats that don't need worlds and barely need star systems at all. Answers need to make a logical argument, based on long term survival, for planets being worth the effort of changing cultural norms that have existed for generations. The question makes the possibly unfounded assumption that there are suitable worlds at journey's end to be colonised at the option of these macro-life travelers.

Some contextual details that may have bearing on the question:

  • the macro-civilisation in question is composed of a small fleet, of ten to a dozen, very large (20+ kilometre long) slower-than-light generation ships, each of which has traveled roughly 25,000 light years when it reaches the colonisation target area.

  • the ships are capable of getting up to roughly 0.9C but accelerate at a maximum of only 0.2g.

  • any single ship is completely self sufficient of the technical equipment necessary to keep in resources and repair from material gathered from asteroidal and/or cometary debris.

  • any individual ship is capable of replicating itself over the course of a year of so given enough resources.

  • the trip has taken at least 4 very long generations for the ruling classes, several millennia at least.

  • the trip was initially undertaken for purely political reasons, the governors of the fleet left home rather than face the brewing revolt of their subjects and took large numbers of loyal subjects with them.

  • the governors have absolute power they can and will enforce whichever choice on the population as a whole, period.

  • $\begingroup$ Could you elaborate on the "purely political reasons"? Was it a choice or or were they forced off-planet? $\endgroup$
    – miltonaut
    Nov 19, 2018 at 15:18
  • $\begingroup$ @miltonaut Yes, both, I'll elaborate, slightly. $\endgroup$
    – Ash
    Nov 19, 2018 at 15:20
  • $\begingroup$ the "crew" are not human. - so what they are? How they think? any single ship is completely self sufficient of the technical equipment - but what about reaction mass and energy source? $\endgroup$
    – Mołot
    Nov 19, 2018 at 15:34
  • $\begingroup$ @Mołot What they are doesn't really bear on the question does it? How they think I thought was covered under "Answers need to make a logical argument, based on long term survival". Reaction mass and energy source(s) would be where "from material gathered from asteroidal and/or cometary debris" comes in. $\endgroup$
    – Ash
    Nov 19, 2018 at 15:47
  • $\begingroup$ @Ash asteroids and comets does not include any energy sources we know how to use. What they are dictates quite a lot, for example availability of other habitable worlds, minimal population needed for sustainable society without inbreeding, et cetera. And if we are supposed to assume human parameters, writing that they aren't human makes little sense. Last but not least, if you truly believe that what they are "doesn't really bear on the question", then what they aren't doesn't bear either and is just a distracting noise (in the signal to noise ratio sense of the word). $\endgroup$
    – Mołot
    Nov 19, 2018 at 15:53

8 Answers 8


Catastrophe beyond the ships' collective ability to repair. Not all the ships might be destroyed, but the remaining ships may not be able to support the influx of survivors from the damaged ships.

Piracy/Attack A variation of catastrophe--perhaps the spacefarers are forced to settle due to theft of or damage to their ships from outside attackers.

Destination Reached/Fulfillment of Prophecy Your spacefarers have reached a planet that they were either specifically headed for or unquestionably meets the criteria laid out in a prophecy.

Rebellion You say "the governors have absolute power they can and will enforce whichever choice on the population as a whole, period" but then why was there a rebellion on the home planet? Either the insurrectionists win and settle on a planet, or the governors put down the rebellion and settle the planet. This might also involve some aspects of Catastrophe. The cause of the rebellion really doesn't matter. Maybe they're just tired of living in tin cans and want to try something new.

  • 2
    $\begingroup$ Thanks the idea of the fleet being grounded by a disaster from outside is certainly a good one and something I really wasn't thinking about. $\endgroup$
    – Ash
    Nov 19, 2018 at 16:43
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    $\begingroup$ related to the third option is the evaluation by the ruling class, with presumably a keen knowledge & insight of the people, its development over its history, extending back into the pre-nomadic long history, that the population has degraded. in other words, adding a pragmatic imperative to a traditional one. $\endgroup$
    – theRiley
    Nov 19, 2018 at 18:03
  • $\begingroup$ This is, imo, the answer most responsive, so far, to the main question - what would impel the choice to settle, despite whatever disadvantages which may attend that choice, $\endgroup$
    – theRiley
    Nov 19, 2018 at 18:36
  • $\begingroup$ The real question is who did they bring and who did they leave behind, there was a rebellion at home after they left. $\endgroup$
    – Ash
    Nov 20, 2018 at 11:45

please note that for the purposes of this question it's all or nothing: either they colonise new planets or they stay interplanetary.

You have a problem there.

For starters, I don't think you could enforce it for a civilization. It cannot be enforced for even a small country. Throughout human history we've had people who move and people who stay, for more reasons than we can count.

Also notice that by choosing one lifestyle over the other, you make the civilization less tenacious against threats. Fleets will die when their systems fail and they are adrift in the f[expletive] void of space, whereas planets fail when a nova happens too close, or when they get bombarded by large enough asteroids or planet buster weapons.

That said, planets give you more mass than you can get in an asteroid belt (our own asteroid belt has less than 5% the mass of the Moon), and fpr non-rogue planets they can also provide you with a self-sustaining biosphere and a close-by star to syphon energy from.

  • $\begingroup$ That's why I initially noted that the beings are not human, but that caused some consternation, nevermind take it as read that the leadership cadre can and will enforce an either/or choice on the entire population. The rest of your argument is salient though, thank you. $\endgroup$
    – Ash
    Nov 19, 2018 at 16:12
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    $\begingroup$ @Ash I think part of Renan's point here is that it is difficult to enforce such a decision on every single person, even for dictators with lots of control. If you try, then "How could I guarantee that nobody could leave this situation forced on them by the ruling class?" and "How can I get the ruling class to all agree?" would be bigger problems for you to tackle than "To planet, or not to planet?" Though you could just say "sure, some might get away to a planet, but those losses are insignificant and will be ignored; we're focusing on the group as a whole" $\endgroup$
    – Loduwijk
    Nov 19, 2018 at 16:53
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    $\begingroup$ @Aaron Yes I understand that argument, but without going into endless gory detail "person" is not a truly applicable term in this context, if the rulers say "go here" the general population will. If it helps think of them as a hivemind, they're not but as a metaphor it'll do. A split in the ruling body might be possible though. $\endgroup$
    – Ash
    Nov 19, 2018 at 17:06
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    $\begingroup$ Indeed, there's a good argument that the asteroid-belt residents and the planetary colonists would provide each other a place for "misfits" to go. There would always be some people who grew up in one environment who think they'd prefer the other; letting the cultures trade them means they would exchange the disgruntled for someone who made the conscious choice to join their culture. The latter tend to be much more engaged than the former. $\endgroup$ Nov 19, 2018 at 18:01
  • $\begingroup$ If they where already escaping from a revolt, One ocurring inside of the ships isn't really hard to imagine. @Ash $\endgroup$
    – Tridam
    Nov 20, 2018 at 13:42

No matter how you look at it there is no logical solution.

One of the singular most crucial factors in a species survival is its ability to adapt to change thereby allowing it to spread.

The key here is spread. The larger the area a species thrives in the larger effort nature must expend to wipe it out. The larger the effort the lower the probability of occurrence.

For example a small volcano wiping out an island and whatever unique species it holds vs a meteor wiping out the dinosaurs.

There is no natural logic in reducing conquered surface area and access to resources.

There may be a diplomatic reason:

If they encountered a militarily superior species whom segregated them to a single world, maybe out of public opposition to genocide or for biological research. That might be a semi logical reason.

  • $\begingroup$ I feel like there's a good argument in here but I can't quite understand what you're trying to say in particular what do you mean by "conquered surface area"? $\endgroup$
    – Ash
    Nov 19, 2018 at 16:16
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    $\begingroup$ @Ash yeah that one is a bit butchered. I was trying to relate and quantify a species survivability to the 'distance' it populates. In many respects this is a fairly 2D relationship even for aquatic species who thrive in different aquatic layers or across different layers. For instance lets say the entire ecosystem needed to support 1 shark could exist on a football field, that same cubic volume could not support the shark if constrained to the square of the sharks length and unconstrained in height. Assume opaque walls. I think the root of this notion is related to energy distribution. $\endgroup$
    – anon
    Nov 19, 2018 at 16:42
  • $\begingroup$ Example: imagine if there was an ecosystem on a jupiter sized planet as dense as the Earths. It would take a much larger meteor to kill it than would be needed for the Earths. $\endgroup$
    – anon
    Nov 19, 2018 at 16:46

From what little we know about your life forms and their situation, I think this sounds like the best bet for them, especially since they seem dictatorial...

The choice is to stay in space. This offers so many advantages:

  1. Easier mobility; dealing with gravity wells is very expensive

  2. Enhanced probability of species continuity; it's easier to avoid extinction if you are mobile and especially spread out at least a little bit

  3. Resources in space can be detected from a distance and, if enough advance notice, intercepted with minor course corrections instead of digging through millions of tons of rock hoping that you find what you want

  4. Resources in space are sometimes more pure than what you find planet-side; hence the old "sword of star material" trope since some meteor iron was purer than natural earth-bound iron

... and the list goes on, you could probably come up with dozens of reasons of your own.

Now, here's the rub...

There are advantages to being planet-side too. They just do not outweigh the first couple points mentioned for staying space-side for your people. But the biggest benefit of being planet-side is that there are some things you just cannot get in space, or at least not nearly as much as (or efficiently/easy as) on a planet. However, your people can have the best of both worlds even while they stay in space...

Your group can decide to leave a very small contingent of people on a planet with the tools necessary to acquire the resources the primary group needs. Or they could use other animals or races to do the work for them. Either way, set them up to have what you need by some deadline. Come back to this area by that time, and take the resources then continue on.

This requires that the people either stay in one area long enough to wait for those planet-side to do their work, or requires the people to double-back to this spot at some point. So it does slow them down, but they still remain nomadic.

In fact, this sounds very much like nomadic cultures ruled by a chief whose word was law. They would move around, but they would have to stop at certain areas for a while out of necessity to gather materials, build, or perform other tasks. Some of these tasks would even be difficult, possibly expensive for them, such as cutting down large trees if they did not have metal blades. They may even leave people behind to continue on and gather other resources before returning to people working on their long-running tasks.

The main difference between your people and ancient nomads is that they are wandering space from celestial object to celestial object instead of wandering on the surface of a planet from resource area to resource area. An analogy with desert nomads would be even more apt since they too would be venturing through long stretches of "empty space".


Despite the question constraint, the answer is very obviously "both".

You have an entire society focused around and specialized in maintaining an interstellar society. It makes no tactical or logistical sense to try to convert the whole society and all their equipment into terraformers.

It makes a lot of sense for both the survival of the species and the survival of the society to allow those who want to land, to do so. This along with those that the society does not wish to remain in the society. So they seed all habitable planets in their wake.

We can comply with the criteria of the question if we consider each planet in isolation, though. You could make four simple rules:

1) At each mineable planet, reproduce at least one ship.

2) At each habitable planet, use at least one ship as a seed population.

3) If the number of ships grows larger than a certain point, fork the society.

4) If it shrinks below a certain point, don't seed any more planets until you reach the necessary minimum.

This does lead us inexorably to Fermi's paradox, though: why has this not been done? (https://en.wikipedia.org/wiki/Fermi_paradox)


A desire for exotic foods, materials and experiences. Being on a generation ship, everything you eat and use is known to you. People can make new materials and foods but they won't be as complex as those found and developed through evolution. Your rulers are sick of eating the same types of food with several decades between new discoveries and hearing the same old stories and looking at the same fashion trends repeat themselves. They want something new.

So instead they start to setup colonies on planets so that they can harvest these goods for them. Food, plant materials and animals. Hunting experiences, unique landscapes and different perspectives. But the colonies can't send the goods to a constantly travelling generation ship. Instead they create a central planet where all the goods are sent, and your rulers eventually settle there so they can get the latest exotic goods delivered to them.

I would say any society which reaches reaches self sustainability will start to turn to the arts. Not just pictures and statues. Different tastes, feelings, sounds. Different ways of expressing and viewing things. Creating heart wrenching stories and plays. Creating huge emotional responses. Spreading out among the galaxy would allow them to collect both resources and experiences they would of never considered or thought about before.

  • $\begingroup$ Have you been reading the fall of the Eldar, that last paragraph sounds real familiar. $\endgroup$
    – Ash
    Nov 20, 2018 at 11:39
  • $\begingroup$ @Ash Nope, I have never heard of it. $\endgroup$
    – Shadowzee
    Nov 20, 2018 at 22:55
  • $\begingroup$ In Warhammer 40,000 the Eldar are basically space elves who's civilisation destroyed itself by getting bored and then chasing ever more powerful emotional outlets. I have a starter that I keep not getting very far with about what might happen to a civilisation that hits Clarke III levels of technology. $\endgroup$
    – Ash
    Nov 21, 2018 at 9:09

A few possibilities:

  1. Living in space is having a slow generational degrading effect on the species, they're only travelling at 0.2g of acceleration which may be less than ideal. The realisation that they're going down a path they don't like may be enough to make the civilisation change to a planet-bound lifestyle again.
  2. They have been forced to by an external influence, for example encountering Self-replicating Berserker probes which destroy any artificial structure found in space but leave planets alone. Such a foe would be far too dangerous to face in the long term, retreating to the bottom of a gravity well would be a safe strategy.
  3. The rise to power of a faction of their civilisation that believes they belong on planets and mandates No More Nomad Lifestyle. If that faction were powerful enough and spread across the whole fleet they could revert the entire civilisation to a planetary lifestyle and dismantle the fleet.
  4. Trapped - The fleet has run out of something critical in an inhabitable system and cannot resupply here because the resource is not present. Without the ability to travel further, the fleet's inhabitants opt to terraform and populate the planets rather than live in space.

1: Because it learned how to move planets.

2: Because it needs to begin a massive buildup of 'resources' for some industrial project and for whatever reason that can be done more efficiently (either with people on the ground, or at least out of the habitat-vessels)

3: Because it was forced to by peace treaty.

4: Because whilst it may have been stable, it may by way of that nature cause instability in other systems. As I understand it, any society that straps itself to a rock is vulnerable to any society that does not, MAD theory tells us that a feeling that one can act with (even posthumously) retributive strikes reduces fear of and the likelihood of a strike occurring in the first place(see 3.) Ergo, the knowledge that there is some space-faring civilisation out there capable of unrestricted xenocide is bound to cause a deal of potentially civilisation destroying paranoia. Maybe they feel guilty?

5: Because as a society it holds(or decided to hold) that life needs be catalogued/preserved/studied

6: Because something about the nature of the habitat vessels (the power source/motive force) causes instability in the fabric of reality.

Yeah, grasping =)

  • $\begingroup$ Option 6. is surprisingly insightful, not an instability that has bearing on this question but that is a thing. $\endgroup$
    – Ash
    Nov 20, 2018 at 11:27

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