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More generally, looking further into the future, habitats on the Moon would probably be just a first step. Suppose we do find a way to have millions living in space - I argue in my Moon First books that settlement in space has the potential to be hugely positive but it could also be hugely negative. It depends very much how it is done, and it may well turn out to be a good thing that we are likely to have comparatively few humans in space to start with.

Though I'm keen on humans in space, I'm no advocate for sending large numbers of us there as fast as possible (except as explorers and tourists). In any group of millions of people you may start to get some with strange and even destructive and violent ideologies, similarly to North Korea or ISIS. If a space colony develops such an ideology, they have space technology far advanced over ICBMs. We may get many peaceful, positive ideologies in space, but others might turn out to be as extreme as anything we have had on Earth so far, or maybe more so.

So, if the violence we get on Earth propagates into space, how can humans remain in space for long? Any habitats in space will be so fragile to violent actions, that even lobbing a rock at them at a few kilometers per second, an easy thing to do for any group of millions of people with space technology. With no air to breathe, there'd be no possibility of survivors hiding out in caves. If their environment control, hull integrity, or spacesuits are destroyed by the blast, how can they survive?

With hundreds of thousands, and millions of people in space - how can we restrict space colonization to the "good guys or gals" whoever we think those would be? So, let's not rush into that future. Slow and steady may win the race here.

What about colonizing other star systems?

a large city sized habitat can be closed system, producing all its own food, air and water, and low maintenance, once built, perhaps hundreds of thousands of dollars per inhabitant, then it may even be easier to live there than it is on Earth, at least once you have paid off the huge costs of building the colony in the first place. If the maintenance costs are high, costing millions of dollars per inhabitant per year, then I don't see how it can work at all. (See Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths)

In those books I also argue that with our lunar adventures, we will learn what humans can and can't do in space, and how to stay healthy there. We can also learn how to be self sufficient for months and then years at a time, without resupply from Earth. If we can do that on the Moon it will reduce costs hugely. Once we've done that, it will also be much more practical and safe to send humans not just to Mars but to the Venus clouds, Mercury, asteroids and further afield. Even Jupiter's Callisto, which orbits just outside its dangerously intense radiation belt, is less than two years journey away on a fast Hohmann transfer orbit from Earth (see Sending humans to Callisto or Ganymede (below) ) . Once we know how to keep humans healthy in space for years on end, then Callisto also should be within reach of Earth.

I raised the question in the introduction: if the violence, terrorism and extremism of all sorts that we get on Earth propagates into space, how can humans remain in space for long? Any habitats in space will be so fragile to violent actions, that even lobbing a rock at them at a few kilometers per second, an easy thing to do for any group of millions of people with space technology. With no air to breathe, there'd be no possibility of survivors hiding out in caves. If their environment control, hull integrity, or spacesuits are destroyed by the blast, how can they survive?

Longer term, if we are able to colonize other stars, the difference between positive and negative future outcomes may become even more stark. Perhaps in the next few centuries we will have the ability to visit other stars in person? It might seem great at first, to think of a "civilization" like ours spreading to fill the galaxy. But - is that so desirable long term? I would like to suggest here, that like the colonization of our solar system, it depends very much on how and why we do it.

If we expand because of rapid population growth, and we need to find somewhere for all our descendants to live, then that's the worst case of all. An exponential growth can’t keep going indefinitely. It will hit a crunch rather quickly. Even just doubling every century, if we did that for ten thousand years, would mean we’d need to create a sun’s mass of humans every century to keep going with the exponential growth.

Luckily we are already at “peak child” and it seems at least possible this will happen naturally, with the “middle of the road”projections showing the population leveling off by the end of the century.

If the future pans out like that, we should be okay on our planet, through to 2100, and if we can achieve a stable population by then, we'd be okay through into the indefinite future.

However, what about longer term, if we start to set up interstellar colonies? Even if this happens slowly, if it is an exponential growth process, then ugly problems soon rear their head again. Suppose we set up only ten colonies around nearby stars in the next thousand years, with one new colony on average each century. Let's suppose that each of those set up another ten in the next thousand years

(in practice some of them would set up new colonies even before the first thousand years is over, but to simplify the calculation, let's ignore those - the actual numbers would be a bit larger than for our calculation).

So now, after two thousand years, we have a hundred space colonies. So far, fine. Now each of those set up another ten colonies, and after three thousand years we have a thousand colonies, and so on. Still there seems no problem. Exponentials are like that. For a long time nothing seems to be happening much. But then it gets you.

After only twelve thousand years, suddenly we have a trillion colonies. Our galaxy only has 100 billion stars for them to colonize. But it's worse than that. Our galaxy is around 100,000 light years in diameter. Unless they have warp drive, they can't travel further than 12,000 light years (or 1,200 light years if they travel at a tenth of the speed of light). Clearly our galaxy would get crowded within a few thousand years at exponential growth, even at this slow rate of a ten fold growth per thousand years.

When you hit an exponential there isn't much you can do except to delay the effects slightly. Even tiny two gram colonists as small as the Etruscan shrews, are no help.

Even if we go all the way to a science fiction future of. massless colonists, total conversion of matter into energy, and warp drive, it only delays the inevitable by thousands of years. Even zero rest mass colonists need energy. If we have minimal energy "colonist photons" as vast feeble pulses of light with wavelengths equal to the diameter of the observable universe, still, none of that does more than to delay the inevitable. Soon, your exponential growth spurt has to stop. As usual, I'll indent the calculations so that they are easy to skip:

So, you will run out of matter to make new colonists well before 76,000 years even if the population increases only ten fold every thousand years, no matter how those colonists evolve and what technology they have, within the laws of physics as we understand them today. That's going to happen even if each colonist consists of only a single photon with the least amount of energy compatible with fitting that photon physically into the size of the observable universe.

If the exponential growth is very slow, say, a ten fold increase every million years, the calculation is the same. It's now 76 million years to the single photon colonist end point, even with warp drive and total conversion of matter to energy. Whatever the timescale is for a ten fold increase, just multiply that by 76 and that's the absolute limit of exponential growth within our observable universe.

What about populations that plateau or crash?

Typically in nature populations expand exponentially for a while, but they can't keep it up as they run out of resources. They may plateau.

Or you get boom and bust cycles, as happens with lemmings for instance. Every few years the numbers increase a thousand fold, then suddenly crash. Lemming populations don't crash through mass suicide - that's a myth.

If we expanded into our galaxy without any planning, we could expect something similar. Either our population plateaus or it booms and busts. You might think then - so what is the problem? It's obviously absurd to suppose any population increase continues exponentially endlessly.

Boom and bust - a nightmare future

In this future, from time to time parts of the galaxy might seem to be free of humans, or some part of it, for a while. But like lemmings, they'd never go extinct. A new wave appears and suddenly that region is full of them again, within a few thousand years. Or they are there all the time, but constantly at war and dying in trillions of starvation. The only respite from humans is when they are destroyed by some successor that is even more aggressive. We could go extinct but only as a result of something else taking over from us in these boom and bust cycles.

I think a boom and bust future like this would be the worst nightmare, not just for us but for any beings in this galaxy and eventually the entire observable universe

In this boom and bust future, if people learn to co-operate in one part of the universe - that’s only going to work in a small region, perhaps a few tens of light years in diameter. Beyond that, the chaos would just start up again or rather continue without check. If someone somewhere establishes a peaceful spot in the galaxy - they would never know when some horde of beings with strange ideas would suddenly appear having developed for a thousand years, at a distance of over a thousand light years away. Then they arrive at close to the speed of light using unfamiliar technology. They would be our distant cousins, or the creations of our cousins, but that wouldn’t help. And once started, how can this ever stop?

The plateau future could be just as bad. It depends how the plateau happens. It might be that the population of the galaxy is steady as a whole, but underlying that is constant starvation, or warfare, and local booms and busts. It's not an appealing or enticing future, is it?

Slower growth, for instance cube or square law growth rates

Could we expand into our galaxy in a way that avoids this exponential growth disaster? Well yes, that's possible. One way is to simply fill the nearby region with space to a comfortable capacity and no more. The back story here would be that we expand in a star system until we reach a comfortable capacity similar to the 11 billion population prediction for future Earth (if restricted to a single moon or planet). Maybe with Stanford Torus style space colonies we can comfortably have ten or a hundred trillion or more colonists per star system. But at some point we reach the comfortable carrying limit for that star system. It's no problem for us however. Just as for Earth now, we reach peak child and our population within the solar system stops increasing. So the colonization is not driven by a population explosion.

In this slower than exponential growth scenario, then at this point the population is perfectly content to stay in that star system with a self sustaining population and habitats. It has no need to colonize. However, if there is another nearby star, then they will colonize it too, and indeed ten such stars or more every thousand years. But if all the nearby star systems are already colonized, they stop. They might even stop before that. Perhaps they like to colonize only one star in a hundred. If they see no way to add a new colony without "crowding the neighbourhood" according to their ideas, they stop, content. This gives other extra terrestrials and non intelligent species their own space to live in, and maybe this is something they value highly.

If we could have a civilization like that, then the population expands just depending on how far they have got so far (assuming no warp drive). For the first few centuries, all the colonies are surrounded by unpopulated stars, and so perhaps it expands exponentially, perhaps far faster than that ten fold increase every thousand years to start with. Here is a visualization of the trip to one of the nearby exoplanets in the habitable zone of its star, one the ones that seems most potentil to be habitable to date, at a distance of 41 light years, LHS 1140 b.

At a tenth of the speed of light, it would take 410 years to get there, either to live on the planet, or to live in its solar system and peacefully coexist with any residents the planet has already. With a population steadily filling our galaxy at a tenth of the speed of light, we could get there within a thousand years, easily. I don't think there is any need to factor in a time to "get established" at each colony before they start up a new one. At this stage surely they can make self sustaining habitats in the asteroid belts, or from moons, or other debris in the solar system. Indeed, they might be able to manage just fine in the distant Oort clouds using mini sun fusion devices. They might just hop from one Oort cloud to another.

If they continue to expand at a tenth of the speed of light, giving time to travel between the stars, and set up new colonies as they do so, then within a thousand years they fill a region 100 light years in radius from their starting point, in a way they find comfortable. After ten thousand years they fill a sphere radius a thousand light years, but by now they have filled the entire "thick disk" of the galaxy, (we are in the "thin disk" which has gas and dust as well as stars, only 400 light years in thickness, while the "thick disk" is devoid of dust and gas). So far it's been a cube law growth of population into 3D space, with the population proportional to the volume occupied. After that, they can only spread in two dimensions through the disk of the galaxy, apart from a few explorations to scattered stars in intergalactic space, and maybe nearby dwarf galaxies. So now they fill an expanding disk shaped region 1000 light years in thickness, so their growth rate has gone down to square law growth. This then continues until they fill the entire galaxy, less than a million years later. At that point they occupy a billion of the 100 billion stars in our galaxy, and see no need to colonize the remaining 99% of the star sytsems in our galaxy. Or they occupy every star system, but with sustainable moderately sized populations.

At this point, they see nowhere else to colonize except dwarf galaxies, and maybe they send off expeditions to other large galaxies too like the Andromeda galaxy, taking millions of years to get there. However, most of them won't be colonizing any more, not in this physical sense, but just happily content in their own star systems in a vast galactic empire. They have other frontiers to explore. After all even on Earth today many of us feel no need to colonize physically.

That would work if they were peaceful and all think alike on matters that influence how they colonize the galaxy. They can peacefully colonize a galaxy like ours in a million years, and if their civilization is advanced, perhaps there is no need even for wars.

Perhaps some ETIs have already done this? If only we could find some way to guarantee such a reasonably peaceful future for ourselves and for our galaxy.

Could humans achieve slower than exponential growth like this?

Yes some of us could perhaps do this. Maybe even entire colonies might have this as their philosophy for expansion into the galaxy. But if this was a human future with beings like us, as we are now, then of the first thousand colonies, one of them is bound to be a more expansionist than the others. Maybe it develops the technology to spawn a hundred colonies every thousand years, and fosters a rapidly expanding population. If one of their colonies in turn is able to spawn a new colony every year, maybe through manufacturing clones of themselves, and with the help of self replicating machines, then they would be the ones that fill most star systems of all. If there is no intelligent oversight, the ones able to make the most colonies in the least amoung of time would win the race to fill the galaxy with their kind. It doesn't need to be through genetic evolution. Just through memes, ideas and culture.

So now, imagine us as we are now, as aggressive, as expansionist as we are now, but culturally evolved to be even more aggressive and expansionist, with strange future ideologies we can't even begin to imagine, developed by humans tens of light years from Earth. Our future distant cousins and descendants have the ability to modify their own genes, make clones, self replicating machines, cyborgs etc. They are also separated from us by the light speed barrier (assuming no warp drive). By, say, a thousand years from now, there may be colonies already a hundred light years away. After ten thousand years, some may be a thousand light years away.

By the time we've reached this stage, how could humans ever go extinct? Or rather - our cyborgs or genetically engineered super-humans, or "uplifted" animals, or self replicating machines, or whatever it is we have given rise to by then? Our civilization as a whole can't "self destruct" because if any parts of their civilization does this, well there will be others hundreds, or thousands or even 100,000 light years away that aren't affected. So wave after wave of their descendants, or cyborgs, or uplifted other creatures, or self replicating machines will wash across our galaxy from then on for all future time. With exponential growth highly favoured, then if we are unfortunate, this could become a boom and population crash future of short lives, of people (or their descendants or creations) dying in their countless trillions every year.

How can we make sure that such a future is reasonably peaceful? How can we stop it from turning into never ending waves of destruction in a future galaxy filled with remote cousins many times removed, with bizarre ideas and unfathomable technology approaching at close to light speed from thousands of light years away? This might be a significant and important future challenge that we have to find a way through. Perhaps all Extra Terrestrial Intelligences (ETIs) that develop space travel encounter these issues eventually.

Optimism - especially if we are not frst

Actually I'm optimistic there, especially if we are not the first extra terrestrial space capable species in our galaxy. Our predecessors have to have found a way through this, as otherwise the chaos in our galaxy from battling ETIs would be plain to view, and it is hard to see how it could end as it seems that they could never go extinct.

Once an aggressively expanding civilization has reached its nearest stars, then - it seems pretty much inevitable that it will fill the galaxy within a million years. And I mean totally fill it, in a population explosion. They'd have taken over Earth long ago, as they would need to use all the resources they could find to cope with their constant wars, exponentially increasing populations and resource crises. How could our Earth and solar system remain untouched?

Yet it is. We haven't found any extra terrestrial footprints or tracks on the Moon or Mars, and we can spot our rover tracks there easily.

There are no signs of mining operations, or any kind of exploitation, anywhere in our solar system. If any extra terrestrials have been this way in the past, their impact on our solar system has been minimal. Either that, or they are great at erasing all traces of their presence when they leave.

It's not just our galaxy either. There's no sign of distant galaxies being modified in large scale radical ways, as they would in a future in which intelligent creatures like us, with technology, fill a galaxy. .A search of 100,000 nearby galaxies has turned up no clear signs of galaxy spanning civilizations. They would expect to spot any widespread use of technology on a galactic scale.

We can be one of the wise ETs

My message here is one of optimism, not despair. Imagine if you gave the nineteenth century people present day technology. How long would the blue whales last, or the tropical jungles? How much chaos would they cause to their environment? What kinds of wars would they fight with modern weapons, including the capability for chemical and biological warfare? Theirs was a simpler time and there are many things nineteenth century humans took for granted, and regarded as acceptable behaviour, that would be unacceptable today. Their ideas and habits would also cause utter chaos if they were combined with modern technology.

We have learnt a lot already, and changed as a society, slow though the progress seems on a year to year basis. Our ideas and habitas are already radically different from those of the nineteenth century, and we have those in a global shared culture too, what's more. So where is this trend headed, a couple of centuries into our future?

Though we have stumbled a lot, we have made many good decisions, such as dealing with the problems of DDT and CFC's, human rights (a lot of progress though much still to do), preventing chemical and biological warfare (even in the almost all out conflicts of WWII neither side used the chemical weapons of WWI such as mustard gas, even though they stockpiled them and issued their civilians with gas masks). The Geneva protocol banning many forms of biological and chemical warfare came into force on 8th February 1928. I know there have been exceptions but most wars don’t use them. Imagine how different a world like ours would be, if it was inhabited by ETIs who were so aggressive and short sighted in their thinking, and so unable to negotiate that they couldn't agree to such a protocol at all? All their wars would use those kinds of weapons and probably worse ones too.

Actually, I don't think that nuclear weapons by themselves would be enough, to destroy creatures similar to us. To do that we'd have had to use Cobalt 60 bombs, with deliberate aim to make the surface of Earth radioactive and uninhabitable, and lots of them, and even then probably a few humans would have survived. Nobody was crazy enough to do that in our civilization. It's the same also with chemical and biological weapons. It is easy to target large numbers of people, but not so easy to kill everyone on Earth! (And who would want to attempt that?)

It's the same also for natural disasters and for the other risks we pose to ourselves. Some would impact severely on us, degrade our environment, make things more difficult for billions, kill billions. There are terrible things we could do to ourselves. But if you look at them carefully, I don't think any of them are extinction risks in the near future and quite probably never. We are lucky, that as a species we are resilient, omnivores, adaptable with minimal technology, able to live anywhere from the cold of the Arctic ot the dry heat of the Kalahari desert, or tropical rainforests. We are lucky also to live at a quiet phase on a planet in a quiet phase in its solar sytem in ia quiet suburb in our galaxy. For details see Not our "only precious window of opportunity" for space exploration (below) and on the resilience of human beings, and indeed our basic understanding of science, to mass destruction, see my Natural disasters - resilience of humans (below)

For an extra terrestrial to set back their civilization by more than a few decades, or to make themselves extinct, they have to be far more aggressive than we were. They would need to fanatically keep on using weapons of mass destruction of all sorts, when they can see that their home planet's population has been decimated and all hope is lost. Otherwise they'd keep knocking themselves back perhaps but restore their civilization within a few decades or centuries, a bit like the story of John Wyndhams "The Chrysalids". That would be just a blip on a geological timescale.

Other civilizations could go extinct just through bad luck. For instance if they arise on stars orbiting close to the central supermassive black hole at the center of our galaxy, or in dense areas of our galaxy prone to nearby supernovae - they might be destroyed by natural events. However, we are lucky to live in such a quiet part of our galaxy, and we are also very resilient.

At any rate I think our current prospect is quite hopeful, that we aren't headed for a future like that.

We’ve prevented starvation with the often forgotten Green Revolution between the 1930s and the 1960s, stopped the birds' eggshells thinning scenario of Silent Spring, the thinning of the ozone layer, stopped nearly all whale hunting, done lots of work to preserve species and environments etc, developed many international agreements that stop the worst of biological and chemical warfare and been able to prevent nuclear warfare for many decades. If you compare our present world with what it could have been without all those initiatives - I think it gives room for optimism for the future too. Also, I think we’ve made an excellent start on peaceful use of space with the Outer Space Treaty.

We've also had a measure of luck, that Apollo didn't return any microbes that were harmful to us or the environment of Earth. Though it was disappointing to find the Moon uninhabited and uninhabitable, it might have been a risk to us if it was. Perhaps some other ETIs become extinct at that point. If we have wars in space, we could easily create clouds of debris around our Earth of the exploded satellites, making space travel difficult for centuries. Perhaps an aggressive immature culture coupled with space technology could eventually make itself extinct through wars with its space colonies?

Although it’s frustrating that we don’t have warp drives or even the Star Trek “Impulse drive” they use to zip around from one planet to another in a solar ssytem, and don't yet have easy low cost ways to build habitats in space, I actually think it helps, that space is so hostile. Hopefully by the time we figure out how to live sustainably in space habitats, we will also have figured out how to do it peacefully, or reasonably so. With competition of course, but more like the Olympic Games than WWIII. Hopefully we can become more forward looking as we continue to colonize space. Perhaps the increased resources from space can help us to become more peaceful if we can handle it right.

If so, we might well eventually have a chance to explore even our entire galaxy peacefully, and without harmful consequences to ourselves and other intelligent species that may exist in our galaxy. And if we meet ETs, the ones that still retain space technology, then they also I think would be ones that have figured out how to explore the galaxy in a similarly peaceful way.

Surely amongst the many ETIs (if we do have neighbours) there will be those that are competitive, as with the Olympic games, innovative, eccentric, genius. But we need to find a way to embrace the good sides of those qualities, in ways that work in the universe we live in without being a nuisance to ourselves and the other species we share our galaxy and universe with.

Any expansionist interstellar ET must have minimal impact on the galaxy - or else - we are the first such, at least in our galaxy

Will we be an invasive weed, or a flower in the galaxy?

I think that’s rather how I see the future of us in the galaxy if we just expand into it without foresight. But far worse than a weed on Earth. We’ve unnaturally made ourselves almost impossible to go extinct already by our technology, and if we expand through the universe without evolving social breakthroughs of some sort, to catch up with our technological breakthroughs, I think we’ll become the ineradicable weed of the galaxy. But harmful to ourselves as much as to everyone else, and able to create even more dangerous replicators through our technology.

It's not just us. Our galaxy may well contain many non technological species, for instance intelligent fish-like or octopus-like creatures, living in the oceans of icy moons, or ocean planets, where they have no chance to develop control of fire. Or creatures that are just not very strong, and don't have good "hands" like us for manipulation, like parrots or crows. Even an elephant would have a lot of trouble building a fire and smelting metal. Ancient civilizations, perhaps advanced in mathematics, art, poetry, music, perhaps socially very advanced, yet without technology they would be especially vulnerable to a new technological species spreading out of control like an ineradicable weed through the galaxy. Out of all the intelligent creatures on Earth, I think only humans also (and the other great apes) had a decent chance of developing technology based on fire, even with intelligence. I can't see parrots, crows, dolphins, whales, octopuses, elephants, dogs, cats, developing technology like us, with tools, fire, metal working, however clever they became and however skilled at communication. So if that's a good basis for generalizing, then the non technological civilizations universe wide may well outnumber the technological ones many to one. So, even a billions of years old civilization could still be highly vulnerable to a few centuries old civilization of technological ETs such as ourselves,

I think any sensible ET will look at that possible future for themselves and the galaxy, and find a way to become a flower of the galaxy instead of a weed that will eventually choke all the species in the galaxy, including themselves. If they can’t see a way to a future like that, then if they have any sense, they just stay at home until they can. And if they haven’t the sense to do that, I think, perhaps, that they either make themselves extinct, or they keep destroying their own spaceflight capabilities, and get nowhere, until they develop some sense.

Let’s be one of the civilizations in our galaxy and universe that flowers like a beautiful flower.
- Amazonian giant water lily.