Key word in the title is naturally. Ignore nearby GRBs, supernovas, black holes, super powerful aliens, and anything outside of the star system that would cause problems to the civilization. Whatever needs to be their downfall needs to be contained in the star system.

Suppose we colonize Mars, some Jovian moons, and Mercury.

Further suppose we have fusion and travel between planets with fusion ships, and have fusion reactors as a result, because I don't know we could harness it in space but can't make reactors on Earth.

Then suppose we blew the surface of Earth away with nukes, and dragged an asteroid down on it just to be sure things got bad.

Is this civilization ending if we have millions of people on the other planets, and we exclude the above?

Water is abundant so that is not a problem. Planetary travels with fusion is a thing, the fusion fuel source should be abundant, which means energy is abundant. Growing crops with fusion doesn't seem out of the question. A civilization wouldn't say "let's never bring soil with us and forget about growing food and fully rely on Earth for everything, or try to create soil with all of our energy, etc". Minerals and metals can be found in asteroids, or mined from the husk of Earth.

I can't imagine a way of having such a system where Earth gets toasted and ends up falling apart, unless I go into politics and start wars.

I am interested in if there's a way to build these worlds such that there is a natural consequence of destroying most of the surface of Earth where it is actually a problem and not an inconvenience that fusion technology solves.

Is a civilization with fusion immune to all these problems and the only cards to play are political wars if one wants problems?

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    $\begingroup$ The possibility of this doesn't seem to be some fact intrinsic to your world but the result of the events that you choose to have transpire within it. If you want them to die out naturally they will and if you don't want them to they won't. $\endgroup$
    – sphennings
    Apr 22, 2023 at 5:01
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    $\begingroup$ @sphennings is correct, but let me word it another way. Tell us where you want to go and we're really good at explaining how you can get there. Don't tell us where you want to go... ask questions like, "is this possible?" and suddenly we're stuck navigating vote-to-close reasons that boil down to "well... it depends on your story." Worse, asking us to prognosticate what might be true in a distant future is asking a lot. Take the story out of the question. Tell us where you want your story to go. Then watch us have fun telling you how you can get there. $\endgroup$
    – JBH
    Apr 22, 2023 at 5:04
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    $\begingroup$ @sphennings Why are you telling me "you pick the events so pick stuff to make it happen" when I'm asking for what a reasonable event is that makes coherent sense? We're talking past each other. If you aren't saying that, then your message is very unclear to me. $\endgroup$
    – Water
    Apr 22, 2023 at 5:24
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    $\begingroup$ @JBH Species X has fusion rockets/power plants and has spread to other planets. Homeworld Y for X is made uninhabitable. Habitations on nearby planets "not Y" need to somehow slowly fatally end by method(s) M given the loss of Y. Constrain M to not be any of catastrophic space events, or war between other planets, and M must be reasonable from a hard sci-fi perspective. Now you know where my story wants to go, and since you said "Then watch us have fun telling you how you can get there", then I hope this means you can fill in M. Does that help? $\endgroup$
    – Water
    Apr 22, 2023 at 5:31
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    $\begingroup$ @Water life dies when there's not enough energy (whether fusion, fission or directly from the local star as photosynthesis, fossil fuels and photovoltaic cells). That's the bottom line. $\endgroup$
    – RonJohn
    Apr 22, 2023 at 19:57

12 Answers 12


Short answer: The colonies cannot maintain fusion reactors on their own

Fusion reactor are very complex and apparently your colonies rely very much on them. Fuel is not a problem but from time to time, they require spare parts or break down completely. The production of those parts and new reactors requires massive industrial complexes and a lot of highly trained experts (look at the production of microchips for comparison). The necessary infrastructure, personnel and knowledge only existed on earth and was lost during the war. Your colonies will struggle for a while, maybe trying to reestablish the industry and keep the reactors running, but ultimately the last reactors fail and so do the colonies.

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    $\begingroup$ This is probably the correct answer. On the colony worlds raw materials aren't a problem but if the majority of the space based infrastructure needed to maintain space flight and fusion tech etc is located around the home planet (where logically they would be) then those colonies are going to be almost completely reliant on the home world for big ticket, highly complex capital assets like space ships and fusion reactors. They can probably use advanced printing techniques to replace and repair small complex items but they won't have the industrial base for the bigger stuff, just maintenance. $\endgroup$
    – Mon
    Apr 22, 2023 at 5:16
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    $\begingroup$ P.S. The one exception to this is TIME. If the off world colonies have existed for some generations and economics make sense then over time at least some colonies are likely to grow to the point where they can sustain a space fairing culture. But we're likely to be talking a couple of hundred years a least to get to that point. $\endgroup$
    – Mon
    Apr 22, 2023 at 5:20
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    $\begingroup$ Agree (+1), but it's not just fusion reactors, it's almost everything. Looking at the other questions on this stack about minimum population to retain today's technological society, "millions" is not going to cut it, you need hundreds of millions at least. There are over a hundred sub-fields in medicine alone, and a few million people do not allow maintaining sufficient depth in all of them - not to mention chip manufacture etc etc. Sooner or later one of those specialties that gets triaged out due to lack of resources will be critical. $\endgroup$ Apr 22, 2023 at 14:02
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    $\begingroup$ Also, if you're an Earthling and you look at your colonies with a certain level of suspicion ("colony" is a loaded word after all), it makes sense to withhold something from them that is essential to their survival, to maintain your power over them. See in our real history how countries tried manufactured scarcity to tip the scales in their favour: Venice and mirrors, China and silk, etc. $\endgroup$
    – biziclop
    Apr 23, 2023 at 11:52
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    $\begingroup$ This is mentioned somewhere in Issac Asimov's "Foundation" trilogy. Protagonists in the story take a road trip, wandering among the remnants of a collapsed, interstellar civilzation. Inhabitants of one city that they visit are proud of their fusion power plant and, of the level of technology it affords them. One of the protagonists asks, "And, if a maguffin* tube cracks?" Their tour guide goes deathly pale, and then after a moment of awkward silence, changes the subject. [* I can't remember what actual name was used for the component the story.] $\endgroup$ Apr 24, 2023 at 14:15

Fear of separation and technological control

There is an easy problem to be seen with people being independent from Earth. If you feel different than the others, you can become opposing forces. To make sure colonies do not start to separate, you want to bind them in a way. Culturally is difficult, as they will near certainly develop something on their own due to the distance and different circumstances. So you want to do this technologically.

To bind people technologically, you need to have control over it. One thing is advanced manufacturing. If only Earth can create the required technology and ship it, you are dependent on Earth for repairs and enlarging your colony. Shipping it can be difficult however, so it is better to do it digitally.

Computers run the world, probably even more so in your proposed society. If you give a chip from our time to the scientists of 30 years ago, they probably won't be able to do anything meaningful with it. The advancements needed are too high to get there. If you then give out these wonders of technology for good prices, they will become dependent, and unlikely to progress on their own to such processors.

The technology given to the colonists is so advanced, it would take decades, even centuries to get there on your own. The technology is governed by computers, only allowing manufacturing if it doesn't harm Earth. They are maintained by the technology on Earth, sometimes needing some form of contact with Earth or subsidiaries.

When Earth is destroyed, the software can deteriorate or malfunction, causing the eventual failure of the colonies. It could be rapid, the computers instantly stopping if one if their main variables is gone, or slowly deteriorate as bit failure and the lack of updates slowly corrupt the system. The technology will fail before the colonies can adapt, causing all colonies to be destroyed in the process.

Keep in mind that technological control can be very mild. The colonies might expand to their hearts content, rarely triggering a technological blockade. Control doesn't mean dystopia.


Above is one method that can lead to the destruction of a space faring fusion energy civilisation. Earth trying to prevent war by technological control. When the Earth is gone this control fails, causing the colonies to fail over time.


Is this civilization ending if we have millions of people on the other planets

Unless the colonies have been very carefully designed to be self-sufficient, a few million people will not be enough to maintain the technological level needed. It is very likely that there are crucial technologies that are only manufactured on the Earth (by its 100-1000x larger population).

This could be e.g. microchips or the equipment to make microchips. Even on Earth, the necessary technology is concentrated to a few companies, maintained as tight trade secret and new factories are difficult to set up even by established manufacturers. Imagine building a high technology chip factory, with no detailed knowledge how they work and without access to any newly manufactured components, while at the same time trying to keep life-support equipment running.


The NATURAL path of a space colony SHOULD include everything they need to be fully autonomous. The simple fact is that earth is distant, and shipping expensive. If we were to try and build a colony today, they would have to be able to survive without us- they would have to grow their own food, build their own spare parts, ect.

Now, if space colonies were founded later, after engine technology had advanced to the point that it would take days to travel from earth to the colony instead of months, then they COULD be dependant on earth.

Ultimately though, I think the only way an actual colony would be dependent on earth would be if earth FORCED them to be. Gundam Seed is an example of this- earth made it illegal for the colonies to grow their own food, forcing them to be dependent on earth to eat- probably to keep them selling space resources at low costs. So if earth had designed the system specifically to keep the colonies under the thumb, preventing them from owning certian high-tech manufacturing abilities, stopping them growing their own food, utterly preventing seeds from getting into space, then it could be done.


That One Secret Ingredient

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To keep the colonies functioning, there is some key material that must be imported from Earth every few hundred years.

Without the secret ingredient, the colonies fall victim to one of the billion-and-one mundane difficulties with maintaining a self-sufficient ecosystem in an environment that is otherwise fatal to human beings.

For example:

Deuterium: Used for nuclear power. Harvested from Earth sea water. Since there is no natural water on the colony worlds, Deuterium must be imported.

Zinc-Palliduim-Oxide: Unexciting but essential. Used in small amounts in transistors/vacuum tubes/ Neural gel packs for colony infrastucture. Fairly hard to manufacture, and impossible using only the readily available elements on the colony worlds.

Antigens: It turns out that without the constant arms race between humans and the ambient microorganisms present in the Earth air, our immune systems atropy, and we become allergic to peanuts and wheat and milk, and cotton, most metals, plastic, and the chemicals that keep the air breathable.


To answer your question you can draw a comparison with the colonies we have had during our history: the more autonomous and well established are the colonies in their settling environment, the less they depend on the relationship with the motherland.

A hostile environment can take a huge toll on recently established colonies, therefore in the early stages of settlement having frequent contacts and exchanges with the motherland to supply resources and people is vital in keeping the colony alive. Later on, once a sourcing network has been developed locally, the dependency weakens until it can be simply removed.

Therefore, depending on how mature and well settled are your colonies, they can be more or less dependent on contacts with Earth.



Here is my thoughts - Despite all the imported biomass that any colony would need - None of the colonies can sustain life naturally. The uniqueness of Earth is it's ability to create and sustain life - by nuking the surface, you seriously (and let's for the sake of argument say irrevocably) ruin it.

Over time, the Colonies species start to suffer genetic decline/drift, maybe from different gravity, maybe from different sunlight, maybe from just artificial atmosphere - who knows.

Stored samples can stave off the issue... for a while - but without the original on earth, the Colonies face the slow and daunting revelation that without Earth and it's ability to support Life naturally, they've screwed themselves.

  • $\begingroup$ Yes, came here to give this answer. The earth is the only source of naturally occurring flora and fauna humans depend on. If you've got heated gardens on Ganymede you're only one meteor impact, one pathogen, one mutated microbe away from a failed crop. Unless reactors and gardens are ubiquitous on the world you're on, yes everything can fail. $\endgroup$ Apr 24, 2023 at 16:40

Technological limits don't make much sense.

It's very expensive to haul stuff up to space, so it would make sense for them to be moderately self sufficient. It's even harder to haul stuff to another planetary system.

Colonies have a net negative death rate

Because colonies are unpleasant and painful places to live, they tend to have on net a low birth rate and a high death rate. Normally their population was topped up by regular recruitment from the home planet, but with the earth dead their populations mostly died out.

While some made efforts with artificial embryos and IVF to prolong the end, the depression over the loss of the homeland and the generally horrible life on colonies made these efforts ineffective.


Is it possible?

Of course it's possible. This should be never asked on this forum, because "possible" is almost always a matter of how good of a story teller you are.

Is it plausible?

Not with hard science. Without FTL, the gulf of space is far too vast for us to set up colonies that rely up on spare parts from home. This has always been the case with overseas colonies. When colonizing The New World, there were numerous colonies that just failed. The colonists themselves didn't have the mental or physical resources required to build a sustainable life-support system, so they starved, died from disease or dysentery, were overwhelmed by predators or indigenous people, that kind of thing.

That stops being a possibility when a colony hits a self-sufficiency threshold. That threshold would be harder for space-faring colonies, but it would still exist. At that point, the colonies would mostly just rely upon the home planet for technological improvements, expertise in solving specific local issues, and maybe diplomatic connections.

Where are the weak links?

It won't be power. Even without fusion, we still have a reasonable power supply with solar collectors. Sunlight only becomes a problem beyond Jupiter, and materials aren't scarce. Yes, you'd need big mirrors to grow food without fusion outside of the asteroid belt.

You definitely won't have issues with food. There's no viable game plan that tries to feed millions by exporting food from Earth, so we'd undoubtably have space-based farms. The part that would be really weird would be the thriving industry of transporting waste products back to the farms for use as fertilizer, but any colony would NEED to grow their own food.

Similarly, we'd have air regeneration, medical systems, machine parts manufacturing, etc. Pulling things out of Earth's gravity well is expensive enough that there's a huge advantage to just making your own.

Intellectual Property Rights

Here's a fun weak link. Right now, many physical devices like tractors can't be fixed because the company that makes them encrypts all of the control systems. If everyone relied up on machines for which the only people allowed to fix them were dead, we'd have to re-invent everything. That could be a murderous burden.

This kind of rights could actually be something that people go to war over.

Proprietary material processes

At the moment, there are only around five companies on the planet who can make high-purity silicon. This ability is crucial to the creation of silicon wafers, which our entire electronics industry relies upon.

Let's say, in the opening salvo of the final war, one of the companies shot down the space-based silicon processing plants, taking their process knowledge with them. The global blow-up takes the rest of them down. After that, you're stuck re-using existing circuit boards until someone can redevelop that technology.

There are maybe a dozen linch-pin technologies that have a small number of production points. Rubber is another of those. Where are you going to get rubber after someone fireballs Asia? You don't.

The overall effect might mean that materials that we take for granted are suddenly just gone. It would be a slow collapse, but it would be inevitable, and provide plenty of opportunity for adventure.


There is a lot of talk here on self-sustainability. I'll try to couple two issues.


As already mentioned, it is quite hard for a high-tech colony to be self-sustainable. Once, because Earth would not want them this way (or else the colonies turn out to be competition in somewhat near future). This is a political issue, basically.

Once, because it's really damn hard to pull of, so most would not bother. Now, this is economy.

The issue is that we have with most resources is not that they are scarce. They are scarce at a given price, because somewhere else the is a mountain made entirely of iron ore; oil flows in fountains when you push a stick into the soil; a comet floats around, consisting of pure alcohol; deterium is 20 times more common in the ocean of that particular Jupiter moon; you get the point.

So, some resources are replaceble, but at a much higher price. The price might be so high, that no one thought of mining them at that colony. The price might be so high, no one thought of providing means to mine them (or means to produce means to mine them) to the colony. In the pre-disaster, perfect Sol-globalistic world it was not needed.

To add insult to injury, some resources are really scarce and are produced in a very few places. Those are the first ones you loose as a "lost colony". The spice is only produced at Arrakis, as we all know.

Technological level

The colony might not have the means to mine and process all kinds of resources it needs. This means that the moment it does not have a connection to the mother world, the colony can only rely on things it can do itself.

To put it short, the technological level drops.

Because they cannot produce the current means of production (or some iterations of it, they cannot produce the means to produce the current means of production, and so on), they are limited by the means they have and can produce. These would be more outdated.

Some tools are now unique artefacts. They work until they break. There is no way to produce a new such tool. In some cases the colonists know how, just cannot pull it off. In some cases they don't even have the understanding, not only the technology.

(Try building a modern processor – or any integrated circuit actually – with the top-notch technology from 1920s, for example.)

Sustaining life

And now the next problem. The tech-level drops. But the natural environment of the colony cannot sustain Earth-based life as it. People (and all the Earth ecosystem on the colony) need life support. Life support is also technology. Did I mention, that tech-level drops?

So, at some point, the pre-catastrophe, advanced life support is beyond repair. Probably, some kind of a more primitive life support system can still be built. It would probably support less people or in a worse manner. This downgrade of a life support can happen multiple times.

I can actually image one of the latest space-faring trips of the colony to get some particular rare Earths or some particular carbon nano-tubes or some tritium from nearby asteroids – as long as they can. Because developing mining / local geology / nuclear industry takes too long and they cannot wait, so it's easy to trade one unrenewable resource (space travel) for another, also currently unrenewable, but much sorely needed resource. This is a prolongation of an agony, if they are not able to contact and trade with other colonies. If they can, space travel suddenly becomes much more important. (But tech level decrease bites here as hard as with life support.)

If the colony is caught in a downwards technological spiral (tailspin, actually) and cannot rely and re-base their tech to the currently available materials and resources and still manage to keep some kind of a life support running, then the colony dies out.

This is basically one of those "not with a bang, but with a whimper" stories.


There could easily be anything on the home world that the civilization can't go on without. You didn't specify that we're talking about a human civilization, and the lost home world is Earth. If it's not, you could construct just about any biological, physiological, ecolocical... need.

Think for example about a butterfly species whose caterpillars need a very specific plant to feed on. Or think about plants with complex flowers that can only be pollinated by a very specific species of bees. Or think about turtles that return to the very same beach they hatched on to lay eggs themselves.

The easiest way would be to interrupt procreation in some form. Let's say, for example, children need to eat the leafs of a specific plant at a specific age to mature into adults. This plant is highly adapted to its ecological niche on the home world and the ecosystem there. It can't be cultivated anywhere else, and the leafs are highly perishable. Once the home world is gone and therefore the plant is gone, it won't be long until no children can mature into adults any more, no new children will be born, and the civilization will die out.

A similar idea would be that the children need to spend a month in the radiation of the home world's star to mature. No other planet of no other star has the exact same physical properties, and the radiation can't be simulated artificiality.

Or young mothers need to give birth in the same place where they were born themselves. For hundreds of millennia, every child of this civilization has been born in one of three caves on the home world, and the social standing of the child is determined by which cave it was born in. This didn't change even after the civilization became space faring. But now the home world, including the birth caves, isn't anymore, maybe as the outcome of a horrible civil war.

If we're talking specifically about humans and Earth, physiological or ecological dependencies like that are harder to construct. But it's still possible to construct a social, religious, legal etc. equivalent.

Let's say for example young adults need to spend a week back on Earth as an initiation rite before they're allowed to marry, or to have kids. Maybe they need to visit the temple that has been built on the place where the first starship has launched from Earth.

Now that Earth is basically gone, they technically could forgo that that requirement. But there's this fanatic terrorist cult that brutally murders any child that has been born "unnaturally", without their parents having completed the proper rituals. Depending on how "effective" this cult is, the civilization can be gone very quickly, too.

By the way, it's still possible to go with biological needs for humans. Maybe you need people with special natural abilities to pilot the starships, and those abilities can only mature on Earth (because of the exact physical properties of Earth or of the Solar System, because of some ecological interconnections with hundreds of animal and plant species etc.).


There are catastrophes that could wipe out a solar-system-sized civilization.

One good one is the gamma ray burst. These are insanely powerful, for a few seconds they outshine whole galaxies. And we don't really know how wide the beam is; it is possible there could be one that's strong enough and has a wide enough beam to fry every settlement in a whole solar system. There may be a few orbital habitats shadowed safely behind planets, but can they survive the economic upheaval of losing 99% of the rest of the civilization?

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    $\begingroup$ I am looking for things that are not space-based catastrophes such as supernovas, GRBs, black holes, etc (see first line of my post to avoid such space-based events). I will edit my post to be extra clear. $\endgroup$
    – Water
    Apr 23, 2023 at 16:45

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