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This question already has an answer here:

Once a civilisation reaches the point where it can put reasonable quantities of manpower into space, it is generally assumed that it will start colonising other celestial bodies. However, as I understand it all the resources it needs will likely be available in moons, asteroids, comets, planetary rings, etc, all of which are way more easily accessible than those in the deep gravity well that is a planet.

So, why would a civilisation choose to colonise a planet instead of using those celestial bodies mentioned? Specifically, a colony that could be reasonably described as a town or city, not just remote research stations like we have at the poles.

Edit: This question is about why a civilisation would colonise any planet (regardless of whether they're in the home system or not) when moons, asteroids and other celestial bodies are much easier to access.

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marked as duplicate by L.Dutch - Reinstate Monica, Ash, Aify, Azuaron, Hohmannfan Sep 13 '17 at 20:55

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ worldbuilding.stackexchange.com/questions/56243/… seems like it is closer to duplicate. $\endgroup$ – user25818 Sep 13 '17 at 15:25
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    $\begingroup$ I'm inclined to disagree. One of those questions specifically targets planets outside the home system, and the other asks what order humans would exploit the celestial bodies of Sol. This is about why planets would even be considered as potential colony sites. $\endgroup$ – Kyyshak Sep 13 '17 at 15:29
  • $\begingroup$ I agree that it's not a duplicate, but the question I linked (which is linked from L.Dutch's link) has one answer suggesting that gravity wells will be mostly avoided, and the other does suggest limits to what's worth diving for. $\endgroup$ – user25818 Sep 13 '17 at 16:15
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    $\begingroup$ One obvious reason: people like living on planets. People evolved to live on Earth, and in the long term they will happier and healthier living in an Earth-like environment than in a space habitat where you can never go "outdoors" without a spacesuit. $\endgroup$ – Jeremy Friesner Sep 13 '17 at 17:49
  • $\begingroup$ First answer on linked question answers this question. $\endgroup$ – Azuaron Sep 13 '17 at 19:48
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  • Atmosphere: A planet with an atmosphere is considerably safer to a colony than a domed or underground base
  • More resources. A planet will almost certainly have more resources to draw on
  • gravity. Humans tend to lose bone mass if they are in low-G environments.
  • Growth. More room, more growth
  • Defensibility. You can dig in on a planet in ways you cannot on smaller bodies.
  • Food and water. Find renewable resources on a planet and you'll have an easier time than having to watch every last morsel of food and every last drop of water.
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  • $\begingroup$ Also you can easily overcome the "deep gravity well" by building a space elevator. $\endgroup$ – Till Sep 13 '17 at 13:56
  • $\begingroup$ Would the effort involved in moving to and from the surface be less than just collecting an equivalent amount of resources and building some sort of space habitat? If they're able to build a space elevator as Till suggested then surely something like a Stanford Torus (which will also cover most of your points) would be easier? $\endgroup$ – Kyyshak Sep 13 '17 at 14:22
  • $\begingroup$ @Till You and I may have different versions of easy $\endgroup$ – James Sep 13 '17 at 15:05
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    $\begingroup$ @MikeScott, once a planet may become too small for big enough civilization. Not known whether civilization even will grow after some point, however. $\endgroup$ – rus9384 Sep 13 '17 at 16:37
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    $\begingroup$ Elbow Room! $\endgroup$ – Spencer Sep 13 '17 at 19:21
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I think it all boils down to resources and energy use. In the short term (where we are currently) coming from a planet to space, it is much easier to bring resources from the planet to use in space, simply because there isn't sufficient infrastructure in space to allow resource production. In the medium term, low gravity well sources would provide cheaper (less energy) resources for use, but in the long term when you start exploiting resources at a very high level (a very large space population) those low gravity well material sources will run out and you have to go where most of the available resources in the solar system are and that pushes you to the planets.

For comparison the mass of the main asteroid belt is ~3×10^21 kilograms, with half of that mass actually being in the four minor planets. This sounds like a lot of material and it is, but it is only 4% of the mass of Earth's moon or 0.05% of the Earth mass. Even given that we generally can only access the atmosphere/ocean and the first several kilometers of the crust, this ~1% of the Earth's mass accounts for a much larger amount of available resources.

So at a certain point you need to go to the planets to get resources, and if you want to use a resource most effectively, you try to limit how much energy that use requires. For materials on planets that means you can most efficiently use the resource on the planet, without having to expend the substantial energy to boost into orbit. This would make colonizing the planet more advantageous than trying to use the materials only in space.

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A political realist would tell you that the question is not necessarily confined to "What is enough?" So long as there is more than one state, more than one company, more than one entity, the question will be, "who controls what?" and after that question "who controls the most and what can I (as an actor [state, company, or other type of entity]) do to control more?"

That has been the driving force behind geopolitics since at least the time of Alexander the Great--so it may extend to astropolitics as well (couldn't help myself). Superpowers are not content with having "enough" oil or any other important resource, they want to ensure that they control more than their opponents; states are rarely content with their economic or security agreements, they want to maximize their power; companies are not content with breaking-even, they want to maximize profits and increase revenues.

So to answer your question, even if the actors, players, or competitors agree that the moons, comets, and astroids have more than enough of whatever resource for all to share, they will eventually move to planets for the same reasons that the Mongols, Conquistadors, and every other power in history has moved into new territory:

  1. Resources - not just to have sufficient quantities but to control resources and keep them out of the hands of opponents, or figure out a way to monopolize them somehow, "You can only buy lilac stardust in Dollars; not Euros!"
  2. Strategic locations (astrostrategic positions?) - unless space travel brings world peace, strategists, military and economic, will want to control certain trade routes and territories to force compellence or administer deterrence.
  3. Expansionism - Hey, more is always better--and this new stuff is interesting if only because it is new!
  4. Hubris - not only confined to Nero, Caligula, or Commodus.
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You don't put all your eggs in one basket

If you want to go on being a civilization for a long time. If you value your cultural addition to the universe. You will want to make sure your species and base culture will survive extinction level events.

Because once the big asteroid hits your planet there's no coming back unless you have a fairly decent breeding population somewhere else. Thus you may just as well forgo creating art, literature, technology and religion. It's all pointless endeavors anyway, just passing the time from here to oblivion.

So why planets

Well, people do like room to move around. Also, provided we have terraforming capability, a planet will be self sustainable. You don't have to figure out how to get around having almost no gravity at all. You have all the basic building materials you need for you colony. You have room enough to grow food. A planet colony will be much more resilient than some free floating tube or a hole on an asteroid. Those are nice for farming materials. But for a long term strategy you want a planet. (Preferably at least one in another solar system as well.)

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    $\begingroup$ The question isn't "Why colonise at all?". Rather, it is "Why colonise planets when asteroids/moons/etc are easier?" $\endgroup$ – Kyyshak Sep 13 '17 at 15:22
  • $\begingroup$ Fair enough, I've always been more of a "Why not planets?" kind of guy, but I've added my five cents about that. $\endgroup$ – Doomfrost Sep 13 '17 at 15:33
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Hmm ... may I suggest that, given a nice enough planet, it may take way less work to maintain a colony on a planet? A planet is big enough that mistakes you make (chemical spills, random explosions) can be smoothed away by the scale of the ecosystem, which -- bonus! -- you don't have to maintain. An orbital habitat, or any essentially self-contained ecosystem, has to be actively maintained. This takes time, expense, and effort.

There are also cultural factors. I can see having factions who prefer the hustle and bustle of space-hab life, as well as ... well ... "Space Amish" who want to settle deeper roots and not "live in a tin can". ;D

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Natural growth

Interesting things draw explorers, explorers draw technicians, technicians draw venders, venders draw merchants, merchants allow people.

We will first place bases (like our polar stations) various places probably starting with places many people have heard of like the Moon or Mars. And it will be found that the next science experiment requires more equipment and people to support that equipment. And then more people help make this other project work. Eventually it won't always make sense to ship people back, and babies will be born out there.

The city of Washington D.C. was built on a swamp and has little reason to exist except that the US government lives there. A city on the moon might be expected to exist for a similar reason, scientists need or want a number of things that support people could help with, and support people want a vast number of things some of which would be perhaps cheaper to make locally than ship from earth.

Logistics

Distance does matter, resupplying a Moon base is more convenient than shipping to the asteroids and much much faster. And shipping from the moon to any particular asteroid as needed may be a better support model than having each asteroid base have enough supplies for every contingency. And cities often grow at shipping hubs.

Solar power

It is better closer to the sun. And it turns out most of the mass close to the sun is down wells. The asteroid belt is more than 3 times as far from the sun as the moon is, so gets something like 10% the power from the same solar panels. That is a serious consideration in power intensive activities like growing food and refining metals.

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There is a very good argument for building space-based structures to live in, or settling on smaller bodies like moons or comets. As you note in the question, the resources available from these structures will be easier to obtain than those on a larger planet.

But the planets are there, and there will be people who want to settle on them. Ultimately, given enough time and technology, humanity will expand and settle on everything they can. It's not a case of doing one or the other.

Ultimately, it's about expanding to fill all available niches.

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It's the Law Colonizing smaller bodies could be banned as encouraging rapacious resource extraction over sustainable colonization. The greater gravity well and difficulty of extracting resources out of a planet's surface area means populations have to settle and develop more.

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Health

We are not completely certain about all the health effects zero-gravity has on the human body yet, but so far it is not looking good for the growth of healthy humans.

This is a major topic for nasa recently who put a man in space for a year who also had a twin on earth in the hopes of finding the most precise changes to his body. The results are not in yet, but the whole article is here.

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  • $\begingroup$ You don't need planets to get out of zero-gravity. In fact, finding a planet with acceptable gravity (in addition to all the other factors) might be much easier said than done... you're better off using a rotating habitat! $\endgroup$ – Adam D. Ruppe Sep 13 '17 at 21:34

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