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I have a world (around 3000-3500ad) where human race is spread across various planetary systems - so interstellar travel it's possible.

The interstellar travel is possible via a Bussard Ramjet which uses the CNO Cycle to propel itself.

Each planetary system has a couple of planets/moons colonized and interplanetary travel is quite accessible.

Now, in my story, the idea is that human race colonized Mars and the Moon (all this before hitting 2500ad) and after that developed interstellar travel and went to Gliese 667 and colonized Gliese 667Cc.

But after thinking about it I thought "Why wouldn't the human race pospone interstellar colonization and instead terraform other planets/moons from the solar system (ie Venus, Mercury, Europa), and not go for interstellar colonization until all solar systems planets/moons are terraformed and habitable?"

And I couldn't really find a reason as to why.

So basically my questions are, what would be more likely to happen first? The colonization of the entire (as much as possible) solar system or interstellar colonization? Or maybe both? Which one would be easier? Why would human kind go for one or the another? One of the many reasons in my story is that human kind needs more space - immortality has been discovered so earth is lacking space.

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  • $\begingroup$ Hello Radu Rojas, and welcome to Worldbuilding and Stack Exchange. Interesting first question! However, it seems to be missing one key element: how well developed is terraforming in your world? That would seem to be a major reason why some planets might be possible to terraform and others might not. Also, can you tell us a little more about your method of interstellar travel? Particularly, does it have any downsides, such as the hypothesized gamma ray burst when a ship drops out of Alcubierre drive, that might make people unwilling to go FTL within a solar system? Please edit your question. $\endgroup$ – a CVn Jul 10 '16 at 11:32
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    $\begingroup$ Your "dc" dates are an unfamiliar system. Do you mean "AD" or "CE" dates? $\endgroup$ – John Dallman Jul 10 '16 at 11:50
  • $\begingroup$ @JohnDallman Yep, I meant AD, just changed it. $\endgroup$ – Radu Rojas Jul 10 '16 at 12:54
  • $\begingroup$ @MichaelKjörling Hi, thanks for the comment. I added the missing information. Though about the terraforming I can't say much because I have no idea how it /could/ work - as it's not really implemented in the story as of right now. My question was basically oriented towards a "what's most likely to happen in the future given what we know at the moment about this technologies (ie terraforming)". $\endgroup$ – Radu Rojas Jul 10 '16 at 13:17
  • $\begingroup$ Can people still be killed? $\endgroup$ – Ludi Jun 11 '17 at 14:56

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As I know very little about your world and your terraforming and travel methods here are some general ideas.

1) Terraforming is expensive — Terraforming in the solar system costs more than travelling to already habitable planets. Maybe interstellar travel costs next-to-nothing or terraforming requires a rare/expensive element.

2) Terraforming takes time — The planet needs space now. We can't afford the 1000 year wait for Venus to become habitable. The only way to gain space in time is to hop off to another planet in another system.

3) Some planets can't terraform — The moon and Mars are both rocky planets at a reasonable distance from the sun. Mercury and Venus are just to hot to work on. The equipment would melt. We can't terraform the gas giants and Pluto's too small to make it worth-our-while.

4) It's not allowed — Many areas are protected under conservation laws meaning no building can be done on them. Maybe something similar applies to our solar system.

If one of the above is true then I would suggest that humanity would go for interstellar colonisation first potentially with some terraforming going on slowly in the background. I would think humans would go for interplanetary ahead of terraforming even if they were roughly equally costly as humans often prefer to build something new rather than adapt/recycle something they already have.

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  • $\begingroup$ Thanks, that's quite helpful actually. And sorry for the missing details, I've just added the interstellar travel method usen in my story, maybe with that detail you can now articulate a little bit more on your answer. $\endgroup$ – Radu Rojas Jul 10 '16 at 13:22
  • $\begingroup$ There's also the possibility that terraforming would upset the orbits of planets, and the fact that there aren't enough materials in our solar system to terraform with $\endgroup$ – Xandar The Zenon Jul 11 '16 at 14:32
  • $\begingroup$ @XandarTheZenon How would terraforming upset planetary orbits? $\endgroup$ – kingledion Jan 12 '17 at 3:52
  • $\begingroup$ You can't terraform gas giants? Laughs in Supermundane Planet $\endgroup$ – TheDyingOfLight Dec 1 at 4:38
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What is given

What is given? Humans have a practical problem, they need living space.

But for how many? How fast population grows?

As for the first question, some believe that for Earth 1 billion ($10^{9}$) people is ok, and what is above is excess, there are bunch other believers. But solving the problem is to solve the problem, so export 0.1% of the population isn't solving it, 50% - yes maybe it's a significant change. As for today, 50% is 3 billion people. This way - how many? - is probably billion(s) of people.

How fast population grows, as for 2012 average grow was 1.1% per year. There where higher numbers in the past, 1.8%, 2.2% , World population. There are different beliefs about growth rate, how it will change in future, and why it is happening. But let us go with 1.2%. I will skip immortality as insignificant for that problem, as the problem will exist without immortality, and as a factor which consequences I can not predict. I think with immortality grow rate may be 0% easy (because mortality is one of driving factor for having children).
As note: grow rate isn't something fixed, as we may see, boiling frog works and people are happy to accept even negative grow under some circumstances, with immortals it will be 0% grow so using fixed value is more for illustrating purposes.

  • population, 11 billion, growth rate 1.2% per year, desired(optimal) Earth population 10 billion, and let say 10 billion per each terraformed planet.

Interstellar

There are some factors which are important and are pro- early interstellar colonization.

One star system is limited in resources, like energy, heavy elements, and because of exponential nature of population grow at some point grow will outcome ability to send people (sustain one human needs power roughly 10kW (24.7.365), sending it needs more even with almost free Bussard jet engine.) And at that point civilization almost stuck, until it looses some weight or finds some other solutions.

Let's say in each star system we will find average 10 bodies, which we are able to terraform, this way average population per star system will be 100 billion people.

So if we send at early stage 1 billion people in each star system, and left with 1 billion people in the solar system. For next 400 years problem is solved, as with 1.2% growth per year, it needs 200 to 10x multiply in population, and 400 years to 100x multiply. $\small 1.012^{200}=10.87$

But is that really a solution, or better to say longer to lasts solution, because expansion isn't the solution at exponential grow, because the exponent is damn fast, it beats everything we might suggest as a solution, at least for now.

Starting at 11 billion population, and have sent 1 billion to each of 10 stars nearby, after 400 years we will end with the need to send people to 10000 stars, 1 billion people for each star as we did it before - to buy another 400 years.(return to the state with 1 billion per system)

When does this strategy stop to work for the solar system? When it has to send people farther than 400 light year - they send 1 billion and they will arrive as 100 billion. So solar system have to send let say 100kk, just to give them some time after arrival to do something before they will have the need to send another pack of people themselves.

  • they multiply at travel, because otherwise, it makes no sense, they could stay at home, and not multiply there. They travel because they wish to multiply. (and for simplicity of the picture)

Good thing is, with that strict policy 100 billion per star system they will have plenty of energy to use it to solve that population problem, they are not stuck in the system, they have plenty of resources to just fly in another galaxy if they wish, or time and resources to find another solution. One of the problems is that each of them will begin to solve that problem in different time and they will have a different starting point and different optimal strategies.

But how fast solar system will have a need to send at 400+ ly?

Stellar density

  • The true stellar density near the Sun is estimated as 0.004 stars per cubic light year

So 400 ly distance is 1'000'000 stars in that volume, given 100 billion per each star, starting population 10 billion - this volume with radius 400 ly will be inhabited in 1400 years.

There can be better strategies in sending which propagates this time a bit. As example sending at the beginning right to the maximum distance (let's say 400 ly), and they will send also to maximum distance.

Sending at a greater speed to have time dilation effects as an example, this way we might send 1 billion people and they will arrive as 1 billion people. Hibernation trough relativistic effects.

Local system, not interstellar, not terraforming

There are some difficulties in sending people interstellar, and most important is: what will they find there? Green worlds or mostly rocks which they have to convert to habitable worlds. Most likely third bunch of planets not suitable for terraforming.

But let see what is our system maximum capability by supporting human life.

Sun Power is 3.828×1026 W

And if we assume 100kW of it per each person, and that is enough for food and lifestyle we(some of us) enjoy now it's enough for 3.828×1021 population.

For how long it might last with 1.2% growth per year, for 4166 years.

Problems overview, Critique planetary, and terraforming approaches.

Many of those, who are not lazy to think about nonFTL future (and those who consider interstellar travel in generation ships - all of them) forget about a simple fact - to make generation ship or just long time traveling ship, the ship needs to be habitable in full sense of that meaning. Every detail we enjoy or consider as the reason to terraform a planet - have to be on that ship. Period.

It has to be this way, not only for the duration of travel but in the duration of terraforming a planet.

Terraforming is Art and Exam which may take 1000's of years to accomplish.

Obviously, I'm not sure about 1000's of years, and I may imagine something which may terraform planets in let say 50 years or maybe less - with perfect knowledge about what it doing, perfect knowledge about properties internal structure of planet, perfect prediction of important moments of how biosphere will affect that planet, and how that will affect biosphere. Not sure if that perfect knowledge is possible at all, because of nature of processes involved - waggly, unstable, changeable - on a fundamental level.

Nice thing about biological system is that it will adapt and find it's own equilibrium in most cases - but will we be happy about that equilibrium or will we complain like here Jellyfish Invading Japan. just a jellyfish, which can be eaten, not something we can't eat, and taste is fine, but no no, we do not like that.

That is kinda sarcasm, but a tiny difference will make significant a significant impact and create a system which we might not enjoy. All discussion I have seen about terraforming, in recent times in connection to Mars stuff - they forget a simple Fact - In a place where we can live, in the place can live everything that lives on Earth at the moment.

On a planet scale, to undo some error it may be impossible without destruction of the planet. I mean microbiological live mostly, especially one which lives deep in the soil, and which is important for the cycle of life and death. But with other nonmicrobial life, it also not so simple, at least at our current level of technologies, which sure is subject of change in the future, when we research and will gain more knowledge, but still, it might be not an easy task to control them.

Terraforming task is way much bigger than just blasting few nuclear bombs on poles to release ...

I consider disassemble of planets an ordinary task, it needs just force, tool, energy - and ok, done.
Terraforming, in a way we might enjoy the result, and accept as equivalent or better than Earth variant is opposite of that, it is almost no force, but pure Knowledge.

So talks about difficulties getting the atmosphere, water - it is just minuscule part of terraforming, the easiest part of it. As an example, growing a forest - it requires 100 of years, just for the first generation and as result it will be not a forest but a park - it needs more than that until it will begin to be a forest, especially in case if we do not prepare soil for it to be like in a forest, thus it might take thousands of years just for the thing to grow and create look and feel of a forest. I even do not talk about 1000years old trees and forest of them. Problems like that may be solved, but we even not close for that.

Why not go to other star systems as place to live

The problem with planets in other star system is one problem, I would say least important one. There is another more important problem, in nonFTL future - the speed of information traveling.

FTL is possible or not possible - not known at the moment. And to know it and probably build if it is possible - we need to do a lot of researches, lots of information have to be classified, data exchanges, a lot of work have to be done.

Placing part of our civilization in place with 400-year ping - do not help in that. Yes there are partial solutions for that information exchange problem - as an example nodes can be more independent in terms of information, each work on some direction, or randomly picking problems from possible problems, but it does not make everything better it makes it just less worse or same in computing power sense, but anyway important changes will distribute themselves slower and overall solving different problems like as an example FTL problem it will be slower. There are also other social identification problems, with groups - less likely to agree to work on common problems, and sense of what is the common problem will drift - work can be done, but that long ping and consequence do not make solving faster and easier.

And a planet oriented civilization(which expands and uses only planets) has 1400 years of its expansion, or less if there are not enough plants for them to use. If civilization relies on a just solar system and space habitat's and energy/matter delivery they have 4000+ years for their expansion with the same population growth rate. After that, both have to change the habits and try to maintain the 0-growth rate. Space habitat system might be established in other star systems too, but the speed of information exchange is a problem here.

How it might be done better.

  • do not terraform planets just for living - it's very inefficient use of available materials, losing scientific information about the universe, which may be important for our future survival.
  • Learn how to live in space, build space habitats - way much easier than terraforming planets, and needed anyway for long trips.
  • learn how to use resources in solar system on large scale
  • send tech seeds to other nearby systems, to research these star systems
  • grow seed to be able supply our system with energy and heavy materials we might need or are used here - the process of energy/material transfer can be combined, and be a very efficient one.
  • test that supply system on nearby systems, and begin to extend the system with 0.9c define collection points - make energy material transfer grid-system.
  • establish interstellar transportation system for those who would like to take nice pictures on vacation trips, send those people across the galaxy at 0.999999c.

This way it is possible to grow influence sphere almost with speed of light, store energy for future use from many stars, do large scale projects, make efficient travel system(with almost 0 energy loss and at desired time dilation)

Do different researches with high energy demands, or if building FTL needs a lot of energy, then this way we might have needed energy. We all will have common goals and fast information exchange in a more compact group - and it might help to solve problems.

Storing energy of stars is a very simple, just by disassembling those stars as an example. Such process needs lot's of energy and there are lots of useful stuff besides H and He. Assembly them back to get energy back (not very efficient, but better than just loose lots of energy as it is now).

Most efficient and profitable way is to store energy in traveling people and ships(as kinetic energy) by building a global travel network, get that energy back at the arrival point, or extract it as needed in any system. More energy we will conserve at early stages more freedom-mobility we will have in the future. Each second of sun energy, conserved, will allow 3 more people to travel in the future with 10000-times dilation. Each year - 100kk.

At the moment galaxy is just wasting the energy which we might use.

Exponential grow

Exponential growth will catch up any strategy, even FTL strategy. Not a big deal trough - as may be seen on any creature living on Earth - humans included.

For how long it will last with 1.2% growth and expanding the sphere of influence at 0.9c speed, which collects all resources - energy, heavy materials - and send's them back to Earth(solar system). Without energy conservation for about 4000 years with the resources of the one-star system and additional 1600-1700 years with resources collected by the influence sphere.

It might take a longer time, with gradually slowing down the growth rate. But overall it's limited by the speed of light, and after 6000-7000 year might be something like 0.02% growth rate per year.

Expanding with nonFTL more than at 1000-2000 years distance makes almost no sense except travel and information gathering and establishing a galactic transport system. I mean, further expansion it makes sense for all reasons except solving the overpopulation problem. That's your great filter if you wish so.

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Coming late to this discussion.

One thing that has not been discussed is a far simpler way to accommodate a large amount of people.

Instead of trying to terraform other celestial bodies, we can do something similar with close to today's technology by just going underground.

Everything that humanity has accomplished has been done by exploiting just the top 500 or so meters from the surface. If everyone moved to an area let's say up to 1,000 meters below the surface, there would be room enough to house many more humans (let's say 50 billion or more people for round numbers).

You would need hydroponics (or similar) for the agriculture and Oxygen supplies. The main limiting factor would be sufficient clean water which would initially come from the surface. All the materials for supporting an acceptable lifestyle would come from materials excavated to make room for the habitats (AKA cities). With reasonable recycling, there should be enough for everyone to maintain at least a Western European lifestyle.

With hydroponics, you are not subject to droughts, bad insects or other adverse growing conditions. Every growing season would be the best it could possibly be.

The deeper you go under the surface, the more energy you can tap from geothermal. You should be able to generate 100KW/hr for each of the 50 billion people just through geothermal (that might be optimistic) plus "something else" that would be enough to host the people forever.

Below 1,000 meters, you would need to spend exponentially more on climate control (esp air conditioning) to keep the temps at a level that humans can exist and thrive. That might the limit of depth for now. There are also possible problems with air pressure with diseases similar to the "bends" that may also be limiting factors.

The surface could be left abandoned for the Earth to recover from humanity's abuses. I've read that if humans stopped emitting greenhouse gases, it would take up to 300 years for greenhouse gas levels to return to pre-industrial levels. Within 10,000 years, all evidence of humans ever being on the surface would be erased by natural forces.

Building underground would be many orders of magnitude cheaper than trying to move billions of people to another planet. In 2019, it costs something like $10 million USD to send someone into low Earth orbit. We know this because the Russians sell space tourist trips to the Space Station that cost about that.

And to get out of Earth orbit altogether, would be much more costly. Now multiply that number (whatever it turns out to be) by 1 billion people. This gets us a starting number of around $10,000 TRILLION USD. That's about 1,000 times the USA GDP for 2019. And that's price just to get the people away from Earth. Then you still have to land somewhere and build the habitats.

As to WHY people would want to migrate underground, that is an open question. Could be some eco-catastrophe or it could be something else. That's up to the story teller.

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  • $\begingroup$ Welcome to Worldbuilding, Krandor2000, excellent answer. Good to see someone else contemplating a subterranean future. I've had similar ideas, but it a delight to find you've put numbers into what would be required. Going into the Earth rather than out to the stars, makes so much sense. $\endgroup$ – a4android Nov 29 at 21:38
  • $\begingroup$ Even after a large disaster like flood basalt Earth will still be more hospitable then any of the worlds in the solar system. Even the worst moments of the great dying it still had running water, oxygen, adequate gravity and magnetic field. If mankind has access to hydroponics, fusion power, deep mining and can ship some volatiles from the outer solar system, it can ride even the great dying. $\endgroup$ – Geronimo Dec 2 at 20:47
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If the only terraforming technology we have is what we have today or think is achievable in the near future, we would not colonize our solar system at all. Not even Mars or the Moon. They are simply too uninhabitable.

As it stands right now Mars will never have more than scientific expeditions. Much like on Antarctica right now.

If we could somewhat easily terraform any planet to be earth-like, we would probably fully colonise our solar system first, since any trip to Gliese 667 would take 24 years or longer, given your method of travel which does not permit FTL speeds.

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    $\begingroup$ Antarctica is practically trivial to colonize compared to even Mars, and Mars is, as planets come, quite similar to Earth in many ways. (I also suspect it's the one planet that has the best combination of colonizability, if that is a word, and actual knowledge of its environment. So with Mars, not only would we know what we're up against, it's also easier than with many alternative planets.) $\endgroup$ – a CVn Jul 10 '16 at 18:55
  • $\begingroup$ @MichaelKjörling I'm not sure what your point is. I used Antarctica as an example to illustrate the nature of any possible "colonization" that would take place on Mars. $\endgroup$ – Annonymus Jul 10 '16 at 18:58
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    $\begingroup$ My point is that we could probably make Antarctica quite hospitable with far less effort than would be required to do anything to make Mars hospitable or even habitable at all. $\endgroup$ – a CVn Jul 10 '16 at 19:00
  • $\begingroup$ Not quite, since the methods used are different, with current technology + radiation shielding and a few other things we could have scientific expeditions on on Mars, the same technology does not allow us to make Antarctica hospitable enough. If we can make Mars hospitable though, we most certainly could make Antarctica hospitable as well. However, that is besides the point, I was referring to current real-world Antarctica. I'll edit my answer to make that a bit clearer. $\endgroup$ – Annonymus Jul 10 '16 at 19:05
  • $\begingroup$ This. With the required time of journey, it's not so much interstellar colonization, it's colonization of empty space. You need to build fully self-sustaining ships that can keep going for many decades without any means of external help or replenishing of resources. Those ships could just as well stay in empty space as long as they like. $\endgroup$ – cmaster - reinstate monica Nov 29 at 23:18
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Venus

Venus surface pressure is not survivable. Massive sunscreen in orbit of Venus can cool it off, but to reduce the pressure is needed to take atmosphere out. Very complicated. Cooling the planet and put bacteria to make the atmosphere more suited to human life will help, but the pressure will still prevent people to live outside. It may be a big farm (provided plants can tolerate the pressure) and have people living in bunkers. The bunker-style colony does not allow for huge populations.

Cold Moons

The cold is a problem. Unless you invent some powersource that can heat up a planet, the colonies in the colder moons will be small. Bunker-style. There is an exception for Europa and Ganymede where the tidal force of Jupter heats up the moon interiors and lots of geotermal energy may be available. Huge power plants can be set on Io to take advantage of the geotermal energy and beam the energy to the closests moons. That is the exception. May support big habitats, but not even close to earth's population.

Titan, for example is cold, too cold to have anything but a small outpost. And even a small outpost will be challenging.

Terraformation Time

Terraformation can take centures. Mars has to be warmed to make a denser atmosphere and then put bacteria and plant to convert the atmosphere. Venus has to be cooled down, massive sunscreens built on orbit, then put bacteria and plant to convert the atmosphere. I say between 200 to 500 years process.

Actually terraforming may not even be feasible. The size of the mirrors needed to war mars and cool venus are huge. Possible the resources you civilization need to build the ramjets.

The Key is Size

In your universe the problem is population explosion. Mars, Earth's moon, Europa and Ganymede are small compared to the earth.

Mars surface is a bit bigger than the size of Africa. Moon, Europa and Ganymede surface is smaller than Asia. Venus surface is almost the same size than Earth.

In your universe, those planets can be in process of terraforming and still not being near enough to sustain the population you want.

Earth's Moon

Earth's moon is challenging outside mining and shipyards. Our moon probably will not support huge cities because the lack of water.

However I heard about a far-out ideia to make an atmosphere to the moon, by hauling ice from comets and asteroids. I do not know if the calculations was correct but someone in the usenet claimed that the moon can retain a thick atmosphere, thick enough to breath, by about a million years.

Conclusion

You are working with Bussard Ramjet, seems to me that you universe is conservative on energy. That means terraforming will be very hard. Venus and Mars need to move lots of resources to make the mirrors and then wait a long time to the bacteria and plants do their jobs. Earth's moon requires a lot of ice to be hauled from the outter solar system.

The Ramjet can put people in closest stars in a century. Terraforming takes a few centuries. And even then, does not go much further than doubling the habitable surface of our solar system.

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The last line in your question adds immortality to the mix. This opens a variety of psychological motivations to the mix. It could be that there are several layers of maturity waiting for us in the three and four digit ages, where we might come to see the world completely differently than we did when we were "young".

Children, Teenagers, Twenty-somethings and the other current age categories that we know about, each have distinct values and goals. Who knows what mental mandates might be waiting for us as the years go by.

It could be that after ten centuries living among nothing but humans, it is perfectly rational to buy a Bussard-Ramjet powered Corvette and race off to explore the stars.

Terraforming..? Leave that hard work for the young people! I have an entire universe to explore.

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I guess interstellar travel and terraforming wouldn't be invented overnight. It would rather be a long process. Before going on a risky and expansive adventure light years away technology would have to be tested before more locally on the Moon, Mars or Giant planets' moons for example.

Setting up scientific bases could be a first step then these bases could lead to industry (mining helium3 on the moon or drilling on asteroids for example) or tourism purposes.

Terraforming strictly speaking might not be most suitable options in our solar system but colonization could be started and tested with underground/dome cities...

In this spirit interstellar travel and colonization would only begin once there is a solid technological/economical base for it in the solar system.

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Neither

The general hard science consensus is that interstellar travel by human beings is impossible. (Also that Bussard ramjets do not actually work).

If we ever get off planet in a big way (still as large apes) then the most likely first point will be our moon, because it provides resources at the bottom of a much less deep gravity well and with no atmosphere in the way. Yes, no atmosphere is a huge advantage for manufacturing materials and putting them into Earth or Solar orbit.

The next stage will be O'Neill colonies.

Then Asteroid mining

Mars might eventually be possible to Terraform but that would be a long term project, and by that time humans would probably regard living at the bottom of a gravity well as hopelessly quaint.

But I also think that "as big apes" is an unlikely constraint. I suspect that we will either create AI's that will expand into the universe while (hopefully) leaving us this planet with its hopelessly hostile wet oxidizing atmosphere ... or we will become those AIs, through some sort of mental uploading or seeding. And AIs should be able to down- clock themselves and reach the stars.

Except as ever ... where are they? Fermi paradox, Von Neumann machine formulation.

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This question is plenty of scientific answers that may answer the technical issues. But I would like to address the philosophical question about why would humankind go for interstellar before finishing the conquer of Solar system.

Why are we spending more resources in knowing outer space than in our deep oceans? Because we are used to the things that are near and humans are always looking for the new. At the very moment we achieve the tech needed, we will try to go further, even if it is easier to stay.

Also, I dont have a clue about society in your world, but the Cold War made us go to the moon. Maybe one macro-country wanted to show more powerfull than another by putting a flag in Gliese 667Cc.

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  • $\begingroup$ hm, doubt we spend more money on space then on ocean. Fishing, oil, data transfer, sipping goods, tourism, global warming problem(ocean level observation), ecology, nuclear submarines - huge part of our activity is connected to ocean's. mexican gulf problem had more expenses that some of space programs. $\endgroup$ – MolbOrg Sep 5 '16 at 22:23
  • $\begingroup$ @MolbOrg I think that what MEPx meant was that there isn't that much money spent in solely exploring oceans (compared to space), and not things that are just connected to oceans in some way or the other. $\endgroup$ – Radu Rojas Sep 6 '16 at 12:16
  • $\begingroup$ @RaduRojas I bet lot of peoples wish it to cost less, so we could pay less, or get more. We pay much not because we wish to pay, and would reject possibility to pay less. This is only money we can get results for, at the moment, I mean practical results in form of knowledge, technologies, and our activities here on earth. Glad ocean costs not so much, I would miss all those deep see documentaries about funny creatures and online translation - at the moment we do not see practical use for them all. We are in space for practical results, and we get them. I mean A have false premise. $\endgroup$ – MolbOrg Sep 6 '16 at 13:49
  • $\begingroup$ @MolbOrg Of course, we spend a lot of money in the ocean, since boats are one of the most used ways of moving goods around the world and we eat millions of fishes. What I meant was really deep exploration, like going to the very bottom of every ocean trench (or as far as heat and preassure shields could go). The knowledge achieved by that would be as "practical" as the one that Voyager provides, or more. $\endgroup$ – MEPx Sep 6 '16 at 18:17
  • $\begingroup$ if that article is true - 18.7 billions for nasa and 4.5 billions to NOAA for 2012. I would call it is pretty compatible numbers, considering that ship rent might cost 10000\$/day, single launch 100M \$ and launch itself is not biggest expense. I do not get that 95% unexplored ocean argument, 99.999xx% of our solar system is not explored. Let say oceanexplorergov channel on YT, like their online stuff direct from see, very exited about science and public connection stuff and that direction. $\endgroup$ – MolbOrg Sep 6 '16 at 19:14
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Once teraforming is cheaper than habitat building, it makes sense to colonize planets. This way you can transport a bunch of settlers and have them colonize the planet on their own with resources from the planet (including Human Resources).

It would come down to whether there will be enough habitable land post-teraforming to make up for the cost of doing so.

Another consideration is agriculture, where depending on the starting elements within the planet it may be feasible to develop land for farming.

I would say teraforming is pretty much the same as building a habitat on or near asteroids. The main consideration though is the supportable population. It's worth the increased cost of return trips due to gravity/atmosphere if the planet will be able to sustain a large population for an extended period of time.

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There have been excellent answers on the technical aspects. Let me highlight:

Non technical reasons

If we ignore whether or not colonisation will ever become possible, which has been well discussed, it is conceivable, that humans initially don't want to leave their habitat earth until well after interstellar colonisation becomes possible. That is, they stay on earth and just explore space without colonising it. Then social or medical reasons force them to colonise and choose as remote a location as possible. Assuming people don't die naturally, but can still be killed, such social reasons might be:

External Adversaries

If they are not the only the only space faring race, conflicts may arise. It may be prudent not to put all the eggs in one basket. At present the capabilities for both space travel and terraforming are absent, but the bombs needed to render earth a very ugly place are makeable!

Internal Conflicts

If population grows drastically, groups that want to pursue radically different lifestyles might grow in size. In order to isolate themselves from "the others" they might opt to go more than just a "galactic stroll" away. By galactic stroll I mean what is easily within reach of those days.

Infections

If the reason for the first big migration is an infection, you want to go well beyond the current reach of "every day space travel".

Legal issues

These first settlers are not Earth's best and brightest, but rather a group of hideous criminals, who would do well to settle outside all patrols of the earthlings of that age!

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Given that there are lots of people, and only some of them will be colonising our solar-system, the second interstellar tech becomes available your analog of Elon Musk will make it happen asap. The colonisation of our solar-system will proceed without interruption while this happens, because of sunk-costs and long-laid plans.

So, colonisation of a region will start as soon as the capability is ready, and will progress until it is no longer commercially viable.

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Coming late to the discussion but...

None of the planets in the solar system seem to inhabitable even with massive investments. The Moon's dust is lethal - it will shred plastics and organic tissues because it is very small, jagged, and wasn't oxidized. Mars is covered in perchlorates. Jupiter's moon are inside a particle accelerator (Jupiter's magnetosphere) with deadly radiation. And I haven't mentioned the issue of wrong gravity for human life. If you can travel to another star system with a reasonable world, that is, reasonable gravity, running water improving the dust issue, no funny chemicals dissolved in the environment, it may be better to colonize this faraway solar system then deal with the nearby worlds.

The planets and moons in the solar system are last resort options. If you can build big rotating stations/ships or go to another system, they will be left underdeveloped.

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