This is something that is starting to bug me, I'm creating a universe in which space travel is cheap and comfortable and space habitats can be built roomy enough that no-one actually needs to live on a planet anymore. If we can live in space, with gravity, open spaces, fresh air, and grow sufficient food off-planet too is there any reason to actually live on the "nasty heavy places" that are planets?

Answers should be applicable both for near matches to Earth for habitability and also for worlds that require rather more effort to colonise. Good answers will focus on the relative advantages of planets when compared to smaller but equally comfortable habitats, and bear in mind the effect of gravity wells on relative commercial costs.

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    Can you clarify what the scale of your civilization is? Both energy scale (K-type) and information scale (Sagan's alphabet scale) -- en.wikipedia.org/wiki/Kardashev_scale -- as it can change the nature of correct answers significantly. A K4-Z civilization is a very different beast than a K1-J civilization with regards to gravity wells and the like. – Yakk Aug 12 at 19:55
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    Planets are far more collapse resistant, if society collapses rain doesn't stop falling and air doesn't slowly turn toxic. – John Aug 12 at 20:35
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    Those are almost inconceivably big "if"s. – chepner Aug 12 at 21:31
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    @Benubird Only several orders of magnitude more information than we have now, quantity and quality are not necessarily synonymous. They could potentially harvest multiple universes worth of power if they needed it but they don't need it, K2 means 10,000,000,000 worlds worth of energy, or put another way total energy conversion of 20 Mega Tonnes of matter per second; given they live on less than a couple of hundred worlds they don't even really use all of the power of a true K2 civilisation. – Ash Aug 13 at 17:16
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    @John As opposed to our current planet, where if society DOESN'T collapse the air slowly turns toxic ;) – Whelkaholism Aug 14 at 15:57

21 Answers 21

up vote 31 down vote accepted

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I'm surprised nobody else mentioned that already. A space station is an artificial habitat within a hazardous environment. This artificial habitat needs to be maintained and run to avoid a breakdown.

A habitable planet is a natural habitable environment which regulates it self. Of course there are natural events like storms and eruptions, but we should know how to deal with them, avoid them.

All in all it's probably safe to say that a space station will fail much more easily than a planet.

  • This should be an important argument. It is very hard to obtain and maintain a huge infrastructure in space. Planets offer the base, you do not have to construct the surface and atmosphere. But the implication is that you would need to colonize planets that can be easily adapted and where you can completely modify the environment to suit you. – Andrei Ioan Danaila Aug 13 at 7:31
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    Individuals may value higher safety on a planet. The popupation as a whole is probably better off losing relatively small sub-populations more often then a large bunch at once every 65 million years. - Also a space station may even be moved out of the way of a large impactor, a planet - not so much. – Alexander Kosubek Aug 13 at 7:54
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    When you live on a planet, all the rock is underneath you. When you live in a space habitat, all the rock is above you. This is a significant advantage. And when dealing with the kind of civilisation that needs space habitats, habitable planets are not self-regulating. You may note that the one example of a habitable planet we have is failing to self-regulate a mere seven billion humans, let alone the hundreds of billions assumed by a K2 civilisation. – Yurgen Aug 13 at 17:50
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    A planet is hardly "self-regulating" into always providing nice enjoyable conditions or even a necessarily habitable one, especially if you needed to use some terraforming to make it suitable for colonizing. One only needs to notice the worrying number of mass-extinction events in our own planet's history to realize that, and all those lost species were on a planet they were inherently adapted to. – pluckedkiwi Aug 13 at 19:22
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    "A habitable planet is a natural habitable environment which regulates it self" Found the global climate change denier :D j/k – Shane Aug 14 at 3:48

If nothing else we would need planets for raw materials and therefore have at the very least mining colonies or penal camps.

You can't produce metals, plastics and all the rest from nothing.

Also you can't have a closed ecosystem without it eventually deteriorating, there is always a loss however fractional. Evaporation, energy, food and everything else needs input at the bottom, it's not just a matter of recycling. Even with our whole planet we're experiencing scarcity of some resources.

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    @Ash you're not going to get any oil, oxygen, most metals, soil or a host of other things from asteroids and comets. Just the important industrial minerals are processed from huge amounts of ore, you may get small amounts in comets and asteroids, but not enough to be worth processing. – Kilisi Aug 12 at 13:02
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    Asteroids can be up to 40% pure aluminium oxide, and while it would be wasteful as hell, just like it is on Earth, you could raise beef cattle in an O'Neil Cylinder using asteroid and comet sourced carbon dioxide and other elements as feed stock for carbohydrates. – Ash Aug 12 at 13:32
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    @Ash: It's not wasteful to raise beef cattle (unless you're doing the feedlot thing). You let them out on the range to graze, they take care of themselves, in the fall you round them up. – jamesqf Aug 12 at 16:36
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    @jamesqf Compared to what that land and water use could otherwise grow it is a gross waste of caloric potential, that is if you're farming naturally well watered arable crop land, if you're talking roaming herds on marginal brushlands sure it's a worthwhile use of the land but why would you make artificial marginal growing land on purpose? – Ash Aug 12 at 17:07
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    @Kilisi "you're not going to get any oil, oxygen, most metals, soil or a host of other things from asteroids and comets." I'm not sure how you came to such a conclusion, at least regarding oxygen and metals. We already know of plenty ice comets and asteroids throughout our own solar system, with enough water in them to satisfy all of your hydration and oxygenation needs. Likewise with metals, those are created in stars and would be plentiful in asteroids. There are known asteroids with more platinum and gold than has ever been mined on Earth. Now, oil and gems, those would be hard to find. – AugustDay Aug 13 at 18:57

Because they're beautiful. I don't know what exactly your habitats offer in terms of technology, but nothing short of full VR would convince me personally to live in a space station where nature is confined to parks and you cannot stand on a mountaintop and watch over miles of untouched taiga.

This is an important factor for human well-being, don't underestimate what people would pay for it. If the planet is earth-like in the first place, it wouldn't even be expensive to colonize. Land could in fact be extremely cheap: it's not needed for agriculture anymore and all the "urban" people are in space habitats.

Of course, nature isn't just for aesthetics alone, it also enables activities like hunting and hiking that just need lots of space and can't really be reproduced properly on a habitat (unless, again, you have VR).

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    @jamesqf VR will eventually do away with the whole "goggles and controllers" thing and transmit signals directly to and from the brain. – Starpilot Aug 12 at 17:32
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    @Starpilot: Even if it's feasible, transmitting signals to the brain doesn't do the job. It has to transmit forces to the muscles &c. Otherwise, it's just an improved TV. – jamesqf Aug 12 at 18:17
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    Even if it looks and feels like the real thing, it's not going to be enough, because people will know it's not real, and some will want the real thing, period. – Joe Aug 12 at 19:22
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    First off, Space structure can be massive. Orbitals form the Culture have 20 to 120 times the surface area of Earth. Nature would not need to be confined to parks. If people are willing to pay obscene amounts to go hiking and hunting, it still may be cheaper to have entire low population space colonies rather than settle/terraform a world. Moreover, even if going to the planet is cheaper, having planetary resorts might be a better option than actually settling a planet. Which is what this question is about. – Shane Aug 13 at 12:47
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    @jamesqf I guess we are talking about different things. When the OP said that they are thinking about space habitats that can be built roomy enough that no-one actually needs to live on a planet anymore, I assumed he meant space habitats roomy enough that no-one actually needs to live on a planet anymore. If they instead meant space habitats roomy enough to not need a planet except for all the things you would still need a planet for because it isn't roomy enough, then I guess I see where this answer is coming from. – Shane Aug 14 at 3:17

Yes - It's the Sky.

Although space habitats such as O'neil Cylinders can provide you with anything you desire in terms of atmospheric pressure, composition, and density, and also plenty of 'living space', only planets can provide you with an infinite sky.

This is your view in a Bernal Sphere: enter image description here Here is your view in an O'Neil Cylinder: enter image description here

When you look up, or out, don't tell me you wouldn't surely miss this: enter image description here

Long story short: people at the moment pay good money to be at an ocean's edge, looking up at the sky or across the ocean. Perhaps it's primal instinct, perhaps it's just because others want the same, but in the end you can't argue the endless sky isn't still desirable.

  • Primal instinct could justify it indeed. Reminds me of an article whose theory was that human beings loved forests and trees because a more primitive part of our brains might have remembered / still be hard-coded for life in the trees (a resilience of our primates past). – wil Aug 12 at 20:33
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    The pictures are an extremely misleading comparison. Wide angle birds eye view designed to show the overall concept for the space colonies versus a ground level normal view for the planet. If you looked at a more reasonable comparison picture, you'd see a view that rivaled -- if not surpassed -- the beautiful sunset shown here. s3files.core77.com/blog/images/lead_n_spotlight/… (still not a perfect picture because it is not ground level. – Shane Aug 13 at 4:53
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    And if you are going to look at something like Halos or Orbitals, their views will absolutely blow anything planet based away. – Shane Aug 13 at 4:58
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    Burn the lands and boil the sea; You can't take the sky from me. – Sobrique Aug 13 at 9:58
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    I might be an odd pony out there, but since I grew up in a city, I would not pay money to go out of my way to see nature, when the city with its concrete forest is beautiful to me. – MaudPieTheRocktorate Aug 13 at 12:37

Too many people in space.

hillbilly relaxation ranch https://www.vrbo.com/874262

Space is very popular. Lots of people live there. The food is great. The schools are great. There are hundreds of interesting experiences all the time. It is like living in the big city: rich, diverse, amazing and stimulating.

And it can be too much. Some people get overwhelmed by all of that, and especially the proximity of so many other people. They want to be with one or two other people, and no-one else for miles. You cannot really do that in space with any quality of life. But you can in Missouri.

  • Comments are not for extended discussion; this conversation has been moved to chat. – L.Dutch Aug 15 at 5:26

Gravity - genuine, spacetime-based gravity.

There is one thing that planets have, by their very nature: genuine gravity, created by a massive body warping spacetime. Using known science, we can fake gravity decently enough by spinning our (lower-mass) space habitats, but it's still not quite the real thing.

Why would we need curved spacetime, rather than a rotating frame of reference? And why would we value it enough to pay the launch costs associated with a gravity well? That, I'm not sure of. Especially if we can build the habitats wide enough (and spin them slowly enough) that rotation-induced dizziness isn't an issue.

But, if you find some reason why genuine gravity is valuable, then this could very well be your answer.

  • "gravity" may not be due to a rotating frame of reference, but it is still due to a an accelerated one, and thus a fictional force just the same. – sp2danny Aug 13 at 14:29
  • @sp2danny I seem to recall some mention somewhere of rotating space stations being slightly different than planetary gravity. A more pronounced gradient would probably be part of that (due to smaller overall radius), though I forget whether anything else would matter (e.g., objects not co-rotating with the atmosphere? I'm not sure how obvious that would be) – Ethan Kaminski Aug 13 at 15:19
  • "if you find some reason why genuine gravity is valuable, then this could very well be your answer." And that can very well be a medical reason. We have a pretty good idea what the effect of microgravity on people (and animals, and plants, and crystals) is but you could easily make a plausible explanation as to why the effects of (fluctuations in) the artificial gravity are not beneficial in the long-term. In particular gravity may be much smaller than 1g (to reduce the amount of energy needed to generate it, or to avoid spinning sickness) leading to some of the known problems of microgravity. – CompuChip Aug 13 at 15:39

There isn't much reason, other than tradition and/or novelty.

If you can build a planet-sized amount of living area more easily than terraforming a planet, then why bother with the planet? Especially given that on a planet, you'd have to deal with a steep gravity well in order to get back to the rest of human civilization.

This isn't to say that zero humans live on planets, of course. The Earth itself is still thoroughly festooned with life, including humans, and will be until the expanding Sun renders it uninhabitable. And occasionally you get a group of colonists who set up shop on a planet just for the fun of it. But most of humanity? They're up in space, where land is cheap and delta-v costs are low.

Relatively easy creation of livingspace.

A spaceship has a lot of requirements to be liveable, like being airtight, not wasting space, requiring energy input to constantly filter the air, specific design for artificial gravity, protection against radiation etc. A planet even one in the process of terraforming, if it can be terraformed at all, would meet those requirements much more easily than a space-ship. Having a livingspace like a house that can be build from brick and mortar rather than airtight structure with radiation-shielding could be very convenient.

Planets can also offer stimulation in the views, vegetation, animals and challenges like the weather (if appliceable) that many humans could enjoy. While the creation of space elevators could be relatively costly, the experience, culture and livingstyle could be very welcome.

Self-sustaining ecosystem and protection from long-range weapons such as particle beams.

As others have covered here, a self-sustaining ecosystem in an O'Neill Cylinder is difficult to produce, and requires constant maintenance. It may well be that living on planets is significantly less stressful for occupants, and may be preferred.

Another possibility, few really seem to have considered here is the wonderful ability kilometre after kilometre of atmosphere has to defend those living underneath them. It doesn't seem particularly unlikely that in our own future, and in that of a hard (or even firm) sci-fi universe weapons such as railguns and particle beams will be feasible technologies which can cheaply and effectively attack targets from distances measured in millions of kilometres, if not astronomical units. A good example of a cheap, long-range, one-shot killer would be something like a Casaba Howitzer concept, courtesy of Matterbeam. A single nuclear lance is, as that article describes, cheap to manufacture (especially for a spacefaring civilisation), devastatingly effective at long ranges, and unimpeded by even heavy spacecraft armour. Your fancy O'Neill Cylinder ain't looking so attractive a place to live when enemy combatants with cheap, mass-produced single-shot killers can slice it in twain, after all. But, as Matterbeam's blog dictates, the Casaba Howitzer can be stopped - atmospheres are incredibly effective against such weapons. This goes for pretty much any particle beam I can think of (that doesn't run on exotic physics like some kind of q-ball or strange quark gun) as well as any design of railgun that isn't inordinately expensive. The atmosphere simply absorbs all that energy.

Going on from this, if a paranoid government is involved, it may be that there's a benefit to keeping subjects confined to a planetary surface which can be covered not only by spy satellites, but also those capable of dropping "Rod From God" weapons from orbit, thus preventing some kind of major-scale uprising.

Another another reason might be using the entire planet as a heat sink. Some weapons, defensive or offensive, are simply not feasible to build in space (or sometimes, even on airless moons as there's little atmosphere to convect away heat buildup) but may be better built on planets, say using lakes or oceans to transfer heat away from a vast laser array, perhaps used as a weapon, perhaps used to accelerate lightsail craft to interstellar velocities. Alternatively, maybe vast computing needs also require considerable computational substrate and heat dispersal. Consider that the entire asteroid belt is estimated to be about 3-4% of the moon's mass - not bad, but if you're looking to build a galactic civilisation, maybe it is time to start thinking bigger. A gorgeous planet whose naturalist outer layers hide vast quantities of industry and computing power is certainly an attractive image in my mind, and one I'm currently toying with for a few short stories about future Earth.

Another possibility is as a status symbol - perhaps only the top 1% of the top 1% can afford to live on the relatively-rare habitable planets in your universe, being wealthy enough to also afford the cost have transferring themselves and any resources they desire up to or down from orbit. Alternatively, if structures such as Orbital Rings are common, it may be that the upper middle classes of your society live groundside, because they can afford to have goods shipped down to them, or get a flight up to planetary orbit for a line of profession that requires microgravity.

Lastly, maybe your setting's constructed physics places some limitation on such things. If FTL travel is restricted by gravity wells, and there's either no other way to interdict FTL objects, or it's terribly expensive, perhaps most non-military, non-corporate cities will use the mass of a planet or moon to protect themselves from vast armadas of DOOOOOOM.

Have fun :)

NIJNA EDIT: another possibility that occurred to me is that of truly enormous artificial habitats, such as Orbitals from Iain M. Banks' Culture series or a Ringworld from Larry Niven's, err... Ringworld series, might also be an option. Collossal enough that they feel like planets, generating gravity through spin over millions or tens of millions of kilometres, but also a vast testament to the hubris of man's works. Perhaps mixing and matching this with real planets might be an option, who knows?

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    So people on planets are inherently difficult to attack, while at the same time, people on planets are inherently easy to attack - something is a little off with that reasoning. – pluckedkiwi Aug 13 at 19:26
  • Some sorts of attack are easier on a planet than others. The main disadvantage is that attacks such as the Rods From God require either significant orbital infrastructure, or to be fired relatively slowly (to avoid burning up in the atmosphere) over significant distances, in which case basic Newtonian equations can allow planetary authorities to interdict them. Meanwhile, particle beams like the Casaba Howitzer are far more deadly than accelerated metal rods because they're fired at relativistic velocities, making them near-impossible to avoid. – Jack Aug 13 at 21:34
  • In short - no society capable of spaceflight is just going to allow the enemy to establish installations in orbit for RFG attacks, and probably have the capability to intercept longrange strikes. Meanwhile, the really terrifying weapons, like particle beams, are effectively useless against planets, but almost impossible to shield against on a space station. The only people who can attack a well-developed planetary-society with orbital bombardment (like Rods From God) are really the government of said planetary society. – Jack Aug 13 at 21:36

I'm going to run, largely, on the basis I established in this answer about space exploration.

Some would be happy in habitats

Many people are happy with life as they are shown it - happy to extend the boundaries of their comfort, happy to concentrate on the details of contentment. Many people would happily live in habitats of one or other kind, enjoying the metropolitan cuisine and easy travel between sites in space, unwilling to descend the gravity well to a planet surface that is difficult and expensive to escape from.

Some would risk it all for planetary life

And yet. During the colonisation phase of the Americas, European migrants spent their life savings travelling in order to start a new life, one filled with hope and promise but also risk and hardship. Many did not have the resources to return once they arrived.

Why did they go? Some sought freedom from the strictures of their homeland, some sought opportunity to own their own land. These same motivations apply if you live on a habitat; habitats are not easily expanded, and unlikely to have large tracts of freely available space. If you're on a successful habitat, competition for space and resource will be high, and the pull of moving to a planet which has all the freedoms you lack will be irresistible to many.

Not the old, for whom the reduced gravity of the habitats made life easier, and who could enjoy zero-g swimming and other pursuits.

But for the young, eager to escape the dizzying complexities of hermetic community, the entrenched social hierarchies, the low-risk attitude which permeates every committee decision, it would be a chance to live their own lives.

  • I like this. It's for sure that a spaceborne colony will be far more regimented than anything on a planetary surface, it'd be the ultimate managed society. The prospect of living on a planetary surface where the very air you breathe isn't being tallied would be very attractive to the young – Ruadhan Aug 13 at 11:05

Sure you can have "all the comforts" in space, but that stuff is so darn expensive. Who can afford to pay for that for their whole lives? Planets have free gravity, free atmospheres, and the raw materials you need to build a settlement are already right there. It's certainly possible to live in a space habitat, but living on the surface somewhere is just so much easier and cost-effective.

Plus, a planet's surface gives you something to explore. No matter how hard you try, you can't make humans not explore something. That's the whole reason we got out into space to begin with. A planet's surface is just a giant adventure waiting to happen. Space habitats are predictable, controlled, and boring. Who would be content sitting in orbit, staring at that fun from afar instead of going down and living in the middle of it?

  • The question only said to suppose that space travel was cheap. Long-term habitation is a very different beast. – bta Aug 13 at 20:40
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    Travel over interstellar distances is long-term habitation. – Ash Aug 14 at 10:14
  • @Ash- That completely depends on how fast you're travelling. Also, living on a moving starship is a very different set of problems and needs than living long-term in something stationary like a space station. You should update your question to clarify exactly what sort of living environments you're dealing with. – bta Aug 14 at 16:27
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    Human curiosity has created a civilization, in which people sit in front of a screen all day, explore artificial universes in VR, use the time of their lives to play games in an artificially created virtual world because it's much more convenient and safe than to go outside and experience nature. Sure, some are different but don't underestimate the lazyness of the masses. – Otto Abnormalverbraucher Aug 15 at 9:29
  • On reflection just have the +1 for the second answer, at least the first one is food for thought. – Ash Sep 2 at 12:48

Space

A ship can only sustain so many people. Presumably, people don't want to be subject to extremely stringent rules about procreation. If these people were to never move off the ship, they'd have to be careful not to over-populate, otherwise they might end up in a situation where one life needs to be chosen over another. A planetary habitat is far easier to expand than a space ship, alleviating all these concerns.

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    A proper space habitat is bigger than some, quite small, countries. But I grant they do have limits but not ones you're going to come up against any time soon. – Ash Aug 12 at 12:19
  • "...and space habitats can be built roomy enough that no-one actually needs to live on a planet anymore..." The question presupposes that space isn't a problem. – T.J. Crowder Aug 12 at 16:21
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    We currently have the same problem on a larger scale on our planet, just that noone has come around to actually enforce strict procreation rules, yet. – Alexander Kosubek Aug 13 at 7:52

Natural instinct.

Space is hostile to human life and even with the best technologies for creature comfort it remains a largely empty, deadly void.

Also, even the best space based habitats with plants and animals are still artificial.

Planets offer the only original and natural environment. Even though there are conditions threatening life and comfort, depending on the planet there is an abundance of zones most suitable for humans and resembling their origins.

Vast areas of nature trump cramped metal corridors and recycled air and water.

... unless your civilization builds planet size ships with their own ecosystem, including atmosphere and sun...

Why wouldn't you still have planetside colonies?

Planets are massive free living spaces covered in abundant resources (in some cases unique resources). For the most part they are self maintaining and even better zero energy (infrastructure wise). One colonized planet can house more humans and industry than millions of space stations.

Lots of industry is just easier planetside, especially industry that relies on combustion. Then you have materials that are only found on planets like large crystals, petroleum, or limestone (or a hundred other aqueous forming minerals) .

The other thing planets can do is support massive biodiversity, especially of massive living things. We are not going to build colonies with hundreds of cubic miles of artificial ocean to farm 400kg tuna but we might seed a terraformed planet with them. Trees are even less desirable on space stations, they need a lot of space but lumber is super useful. Shellfish, megafauna, trees, tree-fruit, gemstones, marble are all exclusive to planets or so much harder to produce in space that shipping cost is immaterial.

If energy is so abundant interstellar travel is commonplace then getting in and out of a gravity well is a triviality. Colonizing a planet is relatively easy compared to building enough space stations to support the same number of people, especially with comparable standards of living.

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    I question the presumption that terraforming a planet would be comparatively easy, as habitats seem more cost-effective - raw resources are abundant and environments can be custom built to suit the need. All materials can be easily reproduced (and you're not likely to be finding much marble naturally occurring elsewhere anyway), while megafauna and trees can be grown if desired (likely even easier as again habitats built to suit and precisely controlled) - on the other hand transforming other planets to be viable ecologies is an astounding planetary level endeavor. – pluckedkiwi Aug 14 at 16:21
  • The author states that many worlds are earth like, which will either have its own life (which will be hugely valuable in an of itself) and its products, or it will not and terraform would just be introducing earth life. habitats can be custom built but for many animals and plants they would be hugely expensive for comparatively little return. Tuna will need cubic miles of water at a minimum, and and trees will need a lot of space and soils maintained for a long time. Now consider how little return you will get from a habitat compared to how much free space there is in a planet. – John Aug 14 at 21:31

Earth like planets

Planets with a pre-existing earth-compatible biosphere surely would be nice, however they do not exist. The odds that life evolves on another place, in such a fashion that we can breath the air, eat the plant, etc. is small enough to be considered nonexistent. Any pre-existing biosphere would also have a major competitive advantage over earth life, due to already being perfectly adapted to the place, making such planets all but impossible to terraform.

Terraformable planets

To terraform an entire planet, to the degree that you could live there without any technology, is an major undertaking. Once done, it's a nice place to live on, but during the process, you are just as technology-dependent as in a habitat. On top of this, you have all the downsides of a planet (gravity well, weather, nighttime), combined with the downsides of a habitat (technology dependence), for, realistically, centuries.

Summary

We are going to look back at the predictions of terraforming, just as we are looking back of the predictions of flying cars and tube-mail. The planet-fetish that is evident in both sci-fi and even in some futurism, is a passing phase. If humanity sticks to planets, we need the entire galaxy to reach K2. We can do K2 without leaving sol. Planets are building material.

  • I wouldn't say that planets such as Earth's do not exist. Rather I would say that they are likely much to far away for us to reach in any realistic timeframe. – forest Aug 14 at 8:42

The resources argument is specious for raw materials: High temp materials -- sand (silicon, oxygen) Aluminum oxide, iron, nickle and heavy metals are present in the asteroids.

Comets are a mix of stuff but mostly water and methane and ammonia. Saturn's rings are mostly water ice. Jupiter's upper atmosphere is mostly hydrogen.

Energy is very cheap. Blow a bubble of plastic 1 micron thick 1 mile in diameter, plate the inside with a 10 atom thick layer of aluminum, Glue a hoop onto it, and you have two mirrors that put 20 million square feet of sunlight on a 100 foot diameter spot. Melt almost anything.

If you have hydrogen fusion, then flitting about the system is easy. Earth to Pluto is 17 days at 1g. The moon is only 3 hours. So we're talking travel times comparable to the age of ocean liners and channel ferries. Shipping goods is really cheap if you aren't in a hurry. Minimum energy transfer orbits from jupiter moons to Mars, say, are a few km/s Do you care that it takes several years? For a tank of liquid hydrogen or methane, probably not. This year's fashion in belly button tee-shirts probably.

So, why go to a planet?

  • If it has an ecosystem, you have a huge selection of botanicals -- stuff premade instead of requiring many manufacturing steps. Not all things can be grown in captivity.

  • A place to holiday. Consider a future where most people live in space, and holiday on Earth or a terraformed planet.

  • A place of beauty.

  • A place of challenge. I have a tough time seeing the equivalent of the America's Cup on a habitat. Mountaineering without a space suit. Shooting rapids in northern Canada.

  • Gene pool. A place to verify that your genes haven't drifted too much. (See Gordon Dickson's short Call Him Lord) Also a source for things you didn't bring with you the first time. You may actually need a tape worm for something.

  • Storage. What place better than a planet to store a lot of spare air?

A good planet gives you a thick atmosphere and lots of stuff to burrow under for protection from the worst solar flares. solar flare

A bad planet has its own dangers which make it even less survivable than staying in space.

If you have a particularly benign, dependable sun then space habitats might be completely safe. Until some extremely rare event happens that tends to depopulate your many habitats.

On the other hand, with sufficient technology and material you might get a space habitat that was shielded well enough. You could collect a whole lot of material to use as shielding, and also create a giant magnetic field to deflect charged particles.

I can come up with at least three reasons--resource mining (that's a big one), fabrication, and niche entropy. (This is all ignoring just how much work would have to be done to make space that habitable, as it's included in your premise.)

I'm going to lump resources and fabrication together here, as I believe resources-alone was covered earlier. The reality is, there are many processes that do not occur outside of microgravity; which is precisely why we do experiments aboard the ISS. The only alternative is in free-fall, which is not economically viable. That said, an equal number of processes won't work in zero-G, inclusive of everything from distillation to lighting a candle flame. Many materials require gravity as an inherent part of their manufacturing process.

Second, I'm pulling the entropy card, but in the context of an environmental niche. It's likely everyone here has heard of the Heat Death theory for the end of the universe; but it happens on smaller scales, too. A life-supporting environment generally has to maintain an extended descent into chemical equilibrium, without actually getting there; if it does, everything goes inert (which, in the context of "people", means dies). As an example, each individual on the ISS goes through about 20 lbs of oxygen per day; generally created through electrolysis of H2O from the solar arrays. You can't recombine that with hydrogen without losing the very point of having the oxygen. These resources have to come from somewhere.

For a region the size of a planet, particularly one in a Goldilocks zone, that descent into heat death takes many orders of magnitude longer than it does for a space station. This means that even if living in space is affordable, it will still generally be more expensive than living on the ground.

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    Most designs of habitats are spun to simulate gravity - arguments against living in perpetual microgravity are therefore spurious. Habitats also allow being able to choose from a wide range of effective levels of force rather than whatever the planet happens to have. And yes, space habitats will have life support systems. This is obvious and generally assumed without needing to explicitly state that there will be life-support systems on a space habitat. And for materials, there are far more materials readily available throughout a system than merely what is on the nearby planetary crust. – pluckedkiwi Aug 13 at 19:35
  • @pluckedkiwi You're absolutely wrong about all of that. Spinning a habitat to simulate Earth gravity requires accelerating it at 9.8 m/s^2. You're providing the illusion of an environment, when that environment is not the lowest-energy state, so it will decay otherwise and the heavier the device, the more energy it takes to spin it. That means fuel. That extra fuel adds a lot to manufacturing costs. I explained above that life support is actually a problem. Additionally, the average pair of asteroids (in the belt) are 600 km apart. Please don't accuse me of being spurious without researching. – Michael Eric Oberlin Aug 13 at 22:59
  • @MichaelEricOberlin The only significant losses on a spinning object in space are tidal forces, and you'll note that objects set spinning billions of years ago are still doing so now. Spin gravity on a habitat is basically free, especially to a K2 civilisation. – Yurgen Aug 13 at 23:51
  • You also appear to be under the misconception that separation distance matters between asteroids in the belt. Since they're all in very similar orbits, the delta-V between them is much much lower than, say, the delta-V between the Earth and the Moon. Kicking minerals about the belt mainly only costs time, but rocks don't rot or get bored, so time isn't really significant either. – Yurgen Aug 13 at 23:55
  • I'm under no such misconception. Would you drive 600 km to get to your next dig site? How about if you were using a rocket and had to worry about deceleration costs too? Planets are loaded with resources. Astroids are maybe a hundred cubic kilometers in size. – Michael Eric Oberlin Aug 14 at 0:08

Survival

There will always be some who take the long-term view, and care that their species or their part thereof has a long-term future. All history suggests that civilisations always fall. The fall of a space-based civilisation in artificial habitats would be fatal. They are nodes in a mutual support network. It's harder to see how the fall of a planetary civilisation could be so fatal (though it's by no means impossible). Usually some survivors on a planet that's habitable, would start to build civilisation from scratch.

I won't try to make this case any further. Vernor Vinge makes it absolutely perfectly as Pham Nuwen's back-story in "A Deepness in the Sky", including how an advanced and peaceful civilisation might fall, even absent any malign intent at the start of the fall. Read it, if you haven't already.

Also, space

Slightly paradoxically, a space habitat will always be crowded. A planetary surface need not be, if the number of people living there is controlled.

and sky?

We don't yet know if there's an instinctive longing for real sky in human beings. There are some hints that the longing for grass under out feet is more than a cultural artifact, but of course there's no reason one cannot grow lawns in a decent space habitat. A really big one could even fake up the basics, with a blue cylinder / low G engineering section down the centre axis of the cylindrical habitat. You wouldn't see the ground opposite as "sky" or the ground curving up more than 30 degrees or so.

It's fairly late for me but I will try to write a passable answer and add details tomorrow.

Too hard to sustain

A colony entirely in space is hard. You cannot recycle everything. There are things which will be consumed (by humans, machinery, etc.) that cannot be reduced to its base ingredients easily.

Even on our Earth, which is a closed system (similar to your spaceship, albeit on a much larger scale) where trees solve our air problems and dozens of other natural processes do much of the regulation for us, we are still experiencing shortages. On a spaceship, where the majority of your energy will likely be going towards maintaining a livable atmosphere, these problems are only exacerbated.

On a planet, you can focus on secondary things to survival like building a society, in order to make sure that people don't start murdering other people. On a spaceship, you also have to worry about keeping people alive for longer than an hour.

Population

The population on the space station will grow unfettered, as there are no natural threats.

Assuming the people on your ship are mostly healthy and not in danger (e.g. there are probably few infectious diseases and the crime rate is low), the population will exponentially grow. Even if the birth rate is strictly enforced, this only slows the onset of the problem.

Also I just realized the first sentence rhymed

One way to counter this might be gladiatorial kill-offs where people are pitted against artificially introduced threats imported from another planet. Survivors can then be recruited for physically intensive labor, and the fights can also serve as entertainment for the desensitized space slave citizen. If you're willing to go full-on dystopian, the corpse can be served as food.

Atmosphere

Massive amounts of energy will be needed to keep a breathable atmosphere. Also, if something goes wrong, a lot of people die. You can potentially solve this with a very powerful energy source that will not be exhausted in the forseeable future, e.g. a Dyson sphere, but that brings up other problems.

The view

People really like looking at nature, as evidenced by the fact that people go great lengths to essentially look at the terrain. (To be fair, Earth is pretty.)

Why stay in space?

The question does not give much details on this, but there is no real reason to stay in space. Planets are there already, and there is no construction necessary. Even if the costs are negligible to build a mega-space station, planets are capable of sustaining much more people than a space station (depending on the size).

TL;DR: Space gladiators = dinner for everyone + population control

Population crisis

A space ship can only sustain a very limited population, and a strict population management is needed for survival on the ship. On a planet, there is a lot of space for a growing population (and a lot of stuff to build more generation space ships).

I remember reading one science fiction story where the crew of a generation ship is pushed out of its comfort zone by a programmed growth of population to force it to colonise the target planet.

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