We Earthlings like to live in gravity wells but also have a taste for the exotic and are a bit lazy. Why not make colonies that take advantage of local conditions?

Cloud cities over Venus, much less fun ones on Titan, an Atlantis under the ice on Europa, Domed potato farms on Mars etc.

It seems to me that making a series of earths would have dubious benefits and huge costs. Why change the spin of a planet when you can just genetically modify your crops etc.

So tell me why I am wrong. I will not fight you :)


7 Answers 7


There are reasons to Terraform and there are reasons to not Terraform.


For my answer, I take the term "Terraforming" as a generic term to create a passive closed-loop environmental system capable of sustaining human life. It's only external requirement is to supply the system with sufficient energy to drive the system. This includes massive Terraforming projects like making the surface of Mars habitable as well as creating passive closed-loop environments inside of asteroids or domed craters on various bodies (e.g. Mars and the Moon).


  • Closed-loop - The system only requires an energy input (e.g. sunlight) to keep it running.
  • Open-loop - The system requires constant replenishment of various components (oxygen, water, food, etc.).
  • Active - The system requires machinery and/or human intervention to keep it running (e.g. the Closed Ecological Life Support System (CELRSS)).
  • Passive - The system runs without machinery and/or human intervention (e.g. a biosphere).

Reasons to Terraform

  1. Maximize the space available for human settlement
  2. Minimize risks of environmental system failures
  3. For long-term habitability of the colony

The following sections discuss these 3 reasons but not in the same logical grouping.

Optimal environment

By definition, humans evolved to operate in a "shirtsleeve environment". Operating under conditions outside of this bound (too hot, too cold, too much gravity, not enough pressure, etc.), reduces the productivity of humans. In some cases, it also has adverse affect on human biology.

Terra-forming provides a technique to create a shirtsleeve environment in as large an area as possible. For planetary bodies, this ultimately would mean terraforming the entire body. Before that though, the colonists will Terra-form smaller sections (e.g. domed craters).

Domed Lunar Craters: Domed Lunar Craters

Environmental systems

As @o.m. pointed out. Humans require a pretty tightly controlled environment in which to live. You can either create these environmental conditions using an actively controlled environment or you can create a passively controlled environment that doesn't require human intervention.

Closed-loop active
Closed-loop active environment systems (e.g. something you'd put on a spacecraft) require constant supervision, maintenance, and other human intervention. If the responsible people get hit by a bus or the machines break and the people can't repair them - then everyone dies.

Open-loop active
Even closed-loop active systems are large and massive. For trips of less than 1-2 years duration it makes more sense to just bring the supplies and forget all of that machinery and chances for problems.

The problem is you throw everything away after it is used. Which means everyone dies if you don't get regularly resupplied.

Closed-loop passive (aka Terraformed)
For very long-duration missions, especially ones in which it's possible the crew will forget how to maintain the environmental system, then you will want a closed-loop passive system.

Passive environmental systems function with or without human intervention. For long-term colonies or generation ships, some form of closed-loop system will be a requirement and making it passive will be the goal.

Reasons against Terraforming

  1. Cost
  2. Project time
  3. Mass


How much might it cost to redirect comets to strike Mars and increase its volatiles inventory? Hundreds of billions or trillions of dollars? Even the "richest" entity capable of running a project like this on Earth (the US Government) would find it difficult to finance such a goal. Give the time horizon mentioned below, it would be exceedingly difficult for other organizations to justify the risk.

The current cost combine with the risk would not be worth the distant reward. Even when you ask questions like "how much might a fully terraformed Mars be worth" are actually not important when the payout goes to the great x50 grandchildren of the person making the investment.

Project Time

Terraforming even the most Earth-like planet (e.g. Mars) might take a thousand years or more. For even the most optimistic humans, this is well beyond our normal capacity to operate. We just don't think in terms of projects lasting that long.

Large corporations are arguably the best at long-term planning and execution and they aren't really capable of managing projects beyond a time-horizon of 10-20 years. By comparison, elected governments may plan for 10-20 year time frames by have problems with execution follow-through when elections replace the original planners.

After all the people reaping the rewards won't be born for more 50 generations. I suspect we won't see serious discussion of massive/costly Terraforming efforts until human life-expectancy gets to be close the length of time such an endeavor might take.


Passive environmental systems require much more "stuff" than the active ones. It won't make sense to use passive systems on anything intended to move around (e.g. most spacecraft) unless the mission requirements require it (e.g. a generation ship). Carrying the extra mass of the passive system would simply cost too much reaction mass to move.

Of course planets & moons will be likely targets for Terraforming. However, we might see Terraforming and development of passive systems for giant space habitats (e.g. O'Neill Cylinder habitats).

Interior of O'Neill Cylinder:
Interior of O'Neill Cylinder

  • $\begingroup$ Thank you for your answer. Terraforming is not the only method for setting up "closed-loop passive systems". Like I said in the post tailoring organisms to the environment we could produce life supports systems where the lower energy position is that it produces what we need for life. This would be cheaper and faster than trying to change the environment. $\endgroup$
    – King-Ink
    Commented Jan 25, 2016 at 0:14
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    $\begingroup$ True, but I think of Terraforming as the "ultimate" in closed-loop passive environmental controls. $\endgroup$
    – Jim2B
    Commented Jan 25, 2016 at 0:40
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    $\begingroup$ My way would be cheaper and involves floating cities. $\endgroup$
    – King-Ink
    Commented Jan 25, 2016 at 0:48
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    $\begingroup$ When I read the businesses where the best at long term planning I thought "what about governments?" Then I rethought my life. $\endgroup$ Commented Jan 25, 2016 at 1:01
  • 2
    $\begingroup$ Elected governments have trouble planning through election cycles. Other human governments certainly have trouble planning through one human lifetime, although that can be managed somewhat by dynastic succession and properly brainwashing your children. Maybe AI government can solve this. We just need someone to maintain the AI to close the loop.... $\endgroup$
    – kojiro
    Commented Jan 25, 2016 at 3:40

Safety and security.

Humans naturally desire to live in safer and more secure environments. The prehistoric ruins of Jericho show that humans have built fortifications around their living spaces (cities) over 10000 years ago. It is not unthinkable that much older settlements with defences exist, but have been lost by the ravages of time due to them being made of less sturdy materials (such as wood or loose rocks).

A spaceship or other self-contained environment is intrinsically unstable. While safeguards can always be built in, malfunction of that environment would result in catastrophic death to everyone inside that environment. For example, a floating cloud-city on Venus, if it leaked and crashed, it would spell certain doom for everyone inside it.

Such self-contained cities are also highly susceptible to sabotage, and considering the extreme warlike nature of humans, it is likely that enemies would make such self-contained cities a prime target, as they can provide the maximum damage to enemy lives and materiel with the smallest cost.

It makes much more sense to consider terraforming as an ultimate extension of humanity's need for security. A terraformed planet has a stable biosphere, and cannot have its biosphere easily destroyed (short of targeted asteroid strikes), and even in such cases, terraforming technology may be able to reverse much of the damage from the resulting nuclear winter.

Furthermore, a terraformed planet also greatly reduces the need for further maintenance once the terraforming is complete. While a Venusian cloud city requires constant input of lifting gases and possibly replacements to its corroded external compartments, a terraformed Venus would not require as much maintenance, and would also be far more stable.

Therefore, humans would generally not start with terraforming planets due to their large capital outlay, but as more and more people begin to settle on the planet, humanity's natural need for security would drive them to terraform it.

  • $\begingroup$ I have not been taking about spaceships at all. $\endgroup$
    – King-Ink
    Commented Jan 25, 2016 at 11:20
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    $\begingroup$ @King-Ink No, but March is, and it's a valid piece of an answer talking about how people colonize space. From what I've seen from you since you started using the site a few days ago, you come across as very antagonistic - from the way you comment or respond to your edit war on this question. Please see meta.stackexchange.com/help/be-nice $\endgroup$ Commented Jan 26, 2016 at 15:08

Stability, Safety and Security

Machines require power, machines break down, domes split or puncture, machines require parts and constant maintenance. Planets don't. I'm not going to doom my children to trying to survive against a bunch of aging machinery. I want them to have the stability of a planet. I want them to be able to play outside the way we did.

People are going on about cost, but what is cost. A home is a home, it has no value that could be measured by mere cost. We talk of the richest organisation on Earth, but what do they control compared to a whole world? The cost is nothing compared to the reward.


What's the lifespan of your machines? 100years? 1000?
What's the lifespan of a planet? 1,000,000years? 1,000,000,000?
For an order or two of magnitude on the costs? Really? False economy all the way. Stop pretending the bubble cities are anything other than temporary accommodation for the workers while they terraform.

Then there are the other costs

What is the cost of mining when you have to make the whole mine airtight or have every miner in a space suit?

What is the cost of popping over to visit a friend in the next city when it's 400C in the shade?

What is the cost of food when you can't farm?

The last of these is the killer, you can exist in a bubble world but it can never be a true colony, it can never be a true home if you have to import your food or grow it in vats. Until it's terraformed it can never be a free world in its own right, it remains an experimental outpost.

How dare you condemn my children to living in a slowly decaying box when they should be dancing in the rain for the sake of short term costs.

“Finally, a planet is not a world. Planet? A ball of rock. World? A four-dimensional wonder. On a world there must be mysterious mountains. Let there be bottomless lakes peopled with antique monsters. Let there be strange footprints in high snowfields, green ruins in endless jungles, bells beneath the sea; echo valleys and cities of gold. This is the yeast in the planetary crust, without which the imagination of men will not rise.” - Strata, Terry Pratchett

p.s. Don't bother pretending you can change the mass/gravity of a planet. That's technology of another order.


On the long run, you want a self-sustaining ecosystem to live in. A place where people can live and work in shirtsleeves and survive if the computer network fails. And the best scale for that is an entire planet. Don't put all your eggs into one basket.

Edit in response to comments: You could gene-modify humans to live in various places, but that would limit the ability of humans to travel. The goal is a shirtsleeves environment for resident and visiting humans.

  • $\begingroup$ With GMO and other tech we could build self-sustaining ecosystems in lots of forms that take advantage of local conditions. Why make all your eggs the same. $\endgroup$
    – King-Ink
    Commented Jan 24, 2016 at 19:07
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    $\begingroup$ @King-Ink: because unprotected humans will die in seconds on any body other than Earth. That's an acceptable risk to relatively few people; if terraforming is practical, most people would prefer to not be one act of sabotage away from massive death. $\endgroup$
    – user243
    Commented Jan 24, 2016 at 19:19
  • $\begingroup$ less earth-like might be more stable $\endgroup$
    – King-Ink
    Commented Jan 24, 2016 at 20:06
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    $\begingroup$ @o.m. I think King-Ink wants you to be more detailed on why it makes more sense to terraform the planet. Why would the potential benefits outweigh the potential risks? $\endgroup$ Commented Jan 24, 2016 at 21:19
  • $\begingroup$ @King-Ink, less earthlike would mean humans from Earth can't visit as easily, and people adapted to the new environment can't visit the homeworld. $\endgroup$
    – o.m.
    Commented Jan 25, 2016 at 17:29


Nothing else, really.

When you try to colonize a uninhabitable place, there are 2 options:

  1. Convert the place to support your kind.
  2. Convert your kind to support the place.

There are two questions when choosing the best approach:

  • Technical : which one is more feasible?
  • Moral : can humans bio-engineered to live on Venus be still called "humans"? Can conquering of a foreign planet be called successful if the "conquerors" will never be able to set foot there?

The last one answers your question: our conscience tells us that option #1 feels right and #2 feels wrong. So it's just a "human thing" to do #1. (Nature does #2 and look how it ended.)

  • 1
    $\begingroup$ I think we could split the difference use GMO and metal tech to make nice and interesting places for people to live. $\endgroup$
    – King-Ink
    Commented Jan 25, 2016 at 12:35
  • $\begingroup$ "Nature does #2 and look how it ended." — It ended with creating humans. You might, of course, argue that this was a bad outcome. ;-) $\endgroup$
    – celtschk
    Commented Nov 27, 2016 at 12:06
  • $\begingroup$ @celtschk Exactly, it ended with creating a new species. We don't want that, we want to preserve the current ourselves. There is no need nor room to argue: a good outcome for the new species is a bad outcome for the old species. $\endgroup$
    – Agent_L
    Commented Nov 28, 2016 at 9:00

Have you ever red the Mars Trilogy by Kim Stanley Robinson? A lot of the questions over terraforming are addressed there. Most of these have already been raised in other questions, concerns like Security, Safety, Convenience, Stability, increase of space ETC, so I won't address these.

My point is that the Ethics of terraforming are simple if there is no native life. To parphrase a fantastic quote from (iirc) Blue Mars, Terraforming is not in any way an ethical concern. Mars doesn't care if it's lifeless red rock, or if it supports life inimical to humanity, or even if Mars is turned into a New Eden of life for the human race. Mars is a planet, not a person. It has no personality or consciousness that we are aware of.

I personally would argue that If we are to colonise planets, Terraforming makes the most sense, and is indeed ethically desirable.

We as a species are growing exponentially. We will run out of space on this planet, and as our species continues to expand, we will need more and more space. Bubble cities simply will not cut it space wise.

Therefore, it depends on how your universe is developing. If you want something quite cyberpunky and dark, go with Bubble Cities, Floating Towns, Undersea Atlantian metropolis.

The slums of an atlantis would be fantastic for some cyberpunk - dingy light filtering through from the surface, corporations rationing your o2, dodgy recycled "rations"...

Alternatively, if you're going more utopian, go with a green garden planets your characters can frolic in, or make them part of the process :)

  • $\begingroup$ Thank you for answering this as a question about world-building. I am actually agnostic to whether terraforming is even possible. That aside I think non-terraform colonies would be more aesthetically interesting in fiction. Either in a cyber-punk or Gee-whiz future. $\endgroup$
    – King-Ink
    Commented Jan 25, 2016 at 12:33
  • $\begingroup$ I think it's eminently possible. Look at how well we've terraformed the world. Yes, it's not always gone well, but with what we've learnt as a species about ecology in the last 200 or so years, we're in a good place to manage and develop our own terraformed planets - just this time with lots more technology! $\endgroup$
    – Miller86
    Commented Jan 25, 2016 at 12:39
  • $\begingroup$ Additionally, one other point. Cost is a factor of time. Over a long enough time period, terraforming is not as expensive and complex as you may think. Start off by getting the atmospheric pressure right over a long period of decades or centuries, then add breathable air, water and life over a similar time period. $\endgroup$
    – Miller86
    Commented Jan 25, 2016 at 12:41
  • $\begingroup$ Could you clarify the statement/paragraph: "We as a species are growing exponentially"? When I read it I assumed you meant population, but population growth, in exponential terms, is slowing globally, has been for >50 years, and is tipped to plateau and decline within a few generations. (Maybe human population over time will look more like a tremulous parabola?) $\endgroup$
    – Kimball
    Commented Jan 27, 2016 at 7:22
  • $\begingroup$ Interesting, I was on the understanding that growth is still increasing. Where does this data come from? $\endgroup$
    – Miller86
    Commented Jan 27, 2016 at 9:13

While o.m. raises a valid point that you want your structure to be sustainable, terraforming wouldn't be worth it. There is the potential to upset the balance in the solar system, and it's overall way too much work. It would be much easier to just reinforce their space bases.

It would be a lot easier to take it in small parcels, a dome here, and asteroid there. Maybe your people have metal woven into their clothing, and live above giant magnets. Or create some kind of gravity generator. Or spin an asteroid.

  • $\begingroup$ I think sustainability is the primary goal and in a lot of cases terraforming might take us away from that goal. Cloud cities 50 km above venus might be more sustainable than trying to keep the planet away from its natural state. $\endgroup$
    – King-Ink
    Commented Jan 24, 2016 at 20:15
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    $\begingroup$ @Jon of All Trades By making Mars bigger, and better able to host an atmosphere, you could accidentally upset some the planet's orbits. Things like that. $\endgroup$ Commented Jan 25, 2016 at 3:57
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    $\begingroup$ @XandarTheZenon: Uh, no. Not remotely. First, I've never heard anyone suggest a terraforming project which materially impacts the mass of the target world: even a thousand comets would increase Mars' mass by less than one part in a million. Second, even if one somehow could increase Mars' mass to equal Earth, that wouldn't affect the other planets' orbits detectably. They orbit the sun, with over 300,000 Earth masses. $\endgroup$
    – user243
    Commented Jan 25, 2016 at 12:37
  • 1
    $\begingroup$ But Xandar, neither changing mass nor heating the core are things anyone has proposed for terraforming, and if it has, the context would seem to need to be such high technology that the whole question would be different, since presumably the technology to be able to do anything like that would imply who-knows-what other super-advances in technology that the speculation would be extremely open-ended. For example, most super-tech sci fi that has "artificial gravity" which we currently have no idea how to do, do not try to say they can change a planet's mass significantly, nor heat its core. $\endgroup$
    – Dronz
    Commented Jan 25, 2016 at 17:20
  • 1
    $\begingroup$ For a planet like Mars, by adding more heat and gasses (including evaporating atmospheric elements in more solid colder forms already on the planet). Mars has less gravity but not too little to have an atmosphere. Note Venus has less mass than Earth but 92x the surface atmospheric pressure. It's thought Mars once had more atmosphere than Earth does now, but lost much of it in asteroid strike(s). $\endgroup$
    – Dronz
    Commented Jan 25, 2016 at 22:51

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