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This question already has an answer here:

To live on another planet, we have deal with its environment. Let us assume that terraforming is not a near-time, realistic option, so we have to deal with none-or-thin atmosphere (pressure), with missing oxygen, with too hot or too cold temperature, with low gravity, and even with too long day/night cycles.

To colonize Mars, for example, we would need pressurized (and heated) domes, underground cities, or something like those. To leave those enclosures, we need suits. The low gravity we will not be able to do anything about, so we will have to adapt. On rock/ice balls without any atmosphere (like our or some Jovian moons), the effort will even be higher.

On the other hand, we could instead built space stations. Cylindrical designs with rotation would give us even a possibility to get Earth-like "gravitation". While Mars' thin atmosphere will give some protection against radiation and while Mars' temperatures are not so low/ high as in space, rock/ice planets without atmosphere seem not to have any advantages compared to a space station, except material structure.

So assuming that we can get the needed material into space (for example, by mining asteroids), what colony would be the easiest to begin with? Space stations? Rock/Ice planet without atmosphere (e.g. our moon or Jovian moons)? Or something like Mars?

Edit:

From the comments I take that I have to define what I mean with "easiest".

So assume for this question that we have to establish human colonies for some reasons and we have plenty of time. Transportation may not be a serious issue if the colony can sustain itself after it is established. We will not have artificial gravity nor will we be able to terraform a planet in the time scale.

What I really want to explore is if it makes any sense to colonize other planets, or (with the same technology that would be needed to colonize planets) it would make more sense to build space stations instead. My theory is that if we have the technology to colonize even the promising mars, it would be more feasable with the same technology to build space stations to give new homes to humanity.

Edit 2:

I already accepted an answer. But I also want to explain why I think it's not a duplicate. The other question asks about a feasible order in which we colonize our solar system. While there is many information there that helps me in my question, the difference is that I am not interested in a step by step plan what is easiest first and then next, but what would be the easiest if we have all the technology for every of the choices and start from the scratch.

So, think we are able to transport materials into orbit quite easily. We are able to do space flight to the other planets. We are able to mine asteroids, built mass drivers etc. Maybe even we have someday artificial gravity. So let us say we made the evolution that is described in the "order" question. Now we discover a stable wormhole that gives us easy access to another solar system, similiar to or own with similiar planets (except an earth).

In this situation and with all the technology at hand able to make a free choice, would it make any sense (other then scientifically) to build colonies on a mars equivalent, or on some moon equivalent or on some other (except earth) equivalent to some body of our system? Or would it make more sense to say "We can build giant space station colonies with paradise like environments as much as we want, and it's much harder to achieve something nearly similiar on any body, so we explore the other system for science reasons but we don't colonize it (except perhaps by building space stations also there)."

Perhaps this in the end explains what I was looking for. The accepted answer seems to indicate my assumption that with any given technology it is easier to go with space stations.

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marked as duplicate by JDługosz, Snow, Hohmannfan, kingledion, Azuaron Jan 10 '17 at 14:28

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.

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    $\begingroup$ Easiest as in "most feasible economically" or as in "shortest to build using current / near future tech, assuming money does not matter"? $\endgroup$ – Mołot Jan 9 '17 at 14:11
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    $\begingroup$ Relevant: worldbuilding.stackexchange.com/q/2507/627. $\endgroup$ – HDE 226868 Jan 9 '17 at 14:29
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    $\begingroup$ @SRM maybe you are right, but I'd really like to see clarification from OP, because it wasn't clear to me. OK, maybe it's my lack of English skills... But never hurts to ask? $\endgroup$ – Mołot Jan 9 '17 at 14:47
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    $\begingroup$ @Molot That's exactly why I asked OP to clarify. :-) $\endgroup$ – SRM - Reinstate Monica Jan 9 '17 at 15:21
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    $\begingroup$ @JDługosz Thank you. I tried to make my intent more clear in a second edit. But you seem to understand exactly, what I mean. :-) $\endgroup$ – Hothie Jan 11 '17 at 10:41
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Stations are better for sure.

Gravity well

In both cases, to be able to inhabit another planet or to build a space habitat humans have to leave the gravity well of their planet Earth. The challenge is the same in both cases in terms of transporting humans and their initial supplies.

Systems like SpaceX ITS can be used to solve the problem of exporting humans into space and it can do it for any purpose - a planet, a moon, a space station.

Building material

A planet may offer some advantages by offering building material for the price of energy of extraction of those materials(and transporting, and processing and ...). A planet as a source of such materials - is a big source of them, but not infinite or super good.
As an example, if we use a layer 1km thick on a planet like Earth for our constructions it will be 486'922'800 cubic km of materials - a number of materials which is equivalent to a dwarf planet with a diameter of 990 km.

But as we can see there would be problems in doing that on a planet if people live close to the surface(or on the surface).

On the Earth, a human can live under a rock and call it home - so we may say on Earth(and only there) a human do not need a lot of materials to build a home.

On a planet like Mars, material demands are higher than on Earth, everything (especially first few thousand years, for those who would like to terraform the planet, until they wait for the results of terraforming) have to be built as an enclosed system, the same way as a space habitat.

On a planet, materials are laying around...

That is kinda true, but when we take look closer at the problem, as for example Mars, things begins to be not so good, still probably in favor of the Mars, but how much in favor it depends on, and we can't say so about any planet, or even about Mars, just because we do not know yet for sure.

Mars have water on its pole, but the problem is where it have water it means not good solar insolation, the whole Mars have a problem with that, but some places are worse than others. Some suggest solar to be the source of energy for the Mars colony, and for reason of solar to be more efficient, strategically it makes sense to place the colony near the equator. But it may be more efficient to place the colony in the middle, energy station on the equator, make power line from the equator to the station, from the station to the pole etc(because of transportation problems, because of other factors which have to be optimized).

The point is - to be on the planet it does not mean all needed resources are under the feet. Planets are usually relatively big, Mars circumference is about half of the earth (21'280km). Not all resources colonists may need will be in one place and it does not necessary mean only materials, energy, but also seismic stable region as an example, or better landing place or better relief etc, the list goes on. So even a small colony may need relatively long routes of transportation.

Real world example, with good roads a tesla car uses about 500 Wh per km to pull about 1.5-ton trailer(and its weight is about 2.3-tons), it is about 2 times of its typical consumption. Useful payload there is the trailer, so we need to spend 500 Wh on towing 2 tons per 1 km on a relatively good road(Norway). It means 900J to transport 1 kg per 1 km on a road. (On mars it is hard to use fossil cars, for obvious reasons.)

The Moon escape velocity is 2.38 km/s, and it have no atmosphere, so we may have a mass driver launch system there, and to launch 1kg from the Moon with a mass driver we have to spend 2'832'200 J per kg, to Earth orbit or to an orbit around the Sun(!). The energy is equivalent to transport the 1kg over a distance 3146 km. And I would not say that such transportations will be an unreal case for a Mars colony, I think such distances may easy have a place there. (Saw suggestion to use ITS for transporting stuff at the begin of colony building from one place on the planet to another, makes sense actually)

But on a planet, they do not have to transport all their material

True. To build a space habitat one has to transport all needed materials into the orbit, where the construction will be.
But how much? O'Neill estimation for the first colony are

The nominal values for the first model colony are taken as: construction force, 2000 people; population, 10,000; total mass, 500,000 tons. When the design and cost analysis are done in detail for the entire enterprise, the need to fit a budget may force some reduction in size. The initial estimates have been aimed at holding the cost equal to that of one project we have already carried through: Apollo. The choice of 10,000 as a target population ensures that, even with some reduction, Model 1 will be large enough to obtain economies of scale and to serve as an effective industrial base for the construction of Model 2. A much reduced colonization project would be little more than a renamed space station, perhaps able to maintain itself but incapable of building the larger models that are necessary if the program is ultimately to support itself. It is an essential feature of the colonization project that Earth should no longer have to support it after the first two or three stages.

O'Neill, G. K: The Colonization of Space, Physics Today, vol. 27, no. 9, Sept. 1974, pp. 32-40.

50 tons per human for the construction and I would say it is pretty realistic. It is ISS mass proportion, but as volume grows proportionally to the cube of linear sizes we get more results for a bigger colony than we have with ISS.

For those 50 tons per human you get:

  • Earth-like gravity
  • radiation protection
  • Earth-like climate
  • Earth-like ecology (potentially) without the need to wait for the end of terraforming a planet, which may take some time, and have some bad planetary scale consequences.

  • Most important, we will get an environment over which we have full control, and that is not possible to achieve at our current level of technologies on the scale of a planet, even with partial-limited terraforming(some kind of big dome) it might be close to what we may have on the space station but not quite the same, not 100%(planet soil, tectonics, winds, dust etc).

  • We also get the ability to choose the orbit for the space habitat, where to build it - on an orbit around the moon, on an orbit around the Earth, Lagrange points, an orbit around the sun(closer to the sun, closer to asteroids). We can't do that with the planets. Different places/orbits have different advantages. Closer to Earth, less delay in everything (connectivity, supply, help, human resources, teleoperating, services(buy/sell)). Closer to asteroids - matter in shallow gravity wells. Earth-Sun L1 - plenty of energy 24/7/365, 1360W/m2.

ITS projected to be capable of delivering 450 tons payloads to Mars surface, with the price of 140'000\$ per ton - so even if we do not think to make the building easier by establishing a manufacturing base on the Moon, the project(O'Neill cylinder, 10'000 population, 500'000 tons construction) may cost 70 billion for material delivered by SpaceX ITS.

A planet is not the Earth

We tend to think that we know how to live on planets because we live on a planet right now. The fact is, a planet it not the Earth. Surviving each planet or moon in the solar system needs the same amount of technologies as to get there.

With space habitats, we can't say we did that, especially at such scale, and it is obvious for us we have to reinvent and adapt our technologies for the station, and that is true, but the same thing we have to do with any other planet, and the only planet where we do not need to do the adaptation of technologies is the Planet, the Earth.

Inhabit a planet or build a space habitat, problems are overlapping in 90% cases if not all 100%. And the difference is in result we get after our efforts.

With space habitats, we get highly scalable and tunable system, with a planet we get a very inertial system where is hard to implement things and almost impossible to undone the things, hard to predict the results, etc - all sorts of problems. The inertia of a planet as a system might be a good thing, for sure is so for Earth(because we do not have to change things here, we have to preserve them as it is), but for all other planets it is exact opposite, we have to change them almost in every aspect of their presence(if it possible at all) and only real thing they may offer for our efforts is gravitational mass.

The Mars with a 1 million colony for research of the planet, or even 10 million for the task(or 100 million or whatever number is needed) - not a problem, welcome and get us the science. A second new home for humanity - no, I do not buy that.

Microgravity and Energy

Microgravity is a big advantage of space, especially near Earth orbit(let's say L1), especially in terms of producing energy and converting it to useful work.

Near Earth, there is a constant flux of 1360 W per square meter of energy, 24/7/365 - no clouds, no wind, no weight, no so much dust, a little bit of meteorites, constant angle of sunlight(if our solar station makes 1 revolution per year), no birds to preserve.

The system like a real example of a solar plant which works with molten salt as a heat carrier. Its life cycle intended to be 30 years, and it gets at least 4 times less energy than it would get at near Earth orbit. It is more massive(and it means more energy was spent to build one of such) because it has to be robust to its own weight, winds and all forces which would be applied to it during those 30 years.

It could be replaced by just a top of the tower and energy producing machinery and with aluminum foils floating nearby the system guided by such ion microthrusters or another light weight tether-like, umbrella-like construction, which could significantly reduce the time it returns the energy spent on its construction, even with lifting materials from a gravity well.

Notes

Another reason why people think about planets, they think they will deliver colonists, and after that will be a business as usual, and we do not need rockets anymore. But why not. SpaceX works on reducing the price trough re-usability. But it is not the only way to reduce the price of rockets, cheap energy is also one of the ways to reduce the price of the production, automation of the productions is also one of the ways to reduce the production price. Energy in space is cheaper, resources may be cheaper if we deliver them from the moon, so why not to produce them in orbit. It is not something which is impossible to do (as note - micro-gravity actually will help with precise machining, less distortion of machines, no need to be bulky, less energy to produce such machines, no vibrations from ground) - and if we do their production, why not to continue to use them - build a mass-driver on Ceres and it will supply with water -> LOX-LH2 millions of those rockets - why should we discontinue their use.

Materials. Production of aluminium costs us 54MJ per kg, and launching aluminium ore from Luna costs 8% of the energy we should spend to make actual aluminium from it. So the cost of launching it might be not a major factor in the cost of making something from aluminium, but the cost of the energy where it is processed is the major factor. This molten salt tower, if such will work on mars (it probably will not) it would probably work for 50 years to make some profit, and that do not helps with reducing the cost of converting raw ores into useful materials and production.
Relative concentration of various elements on the lunar surface

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Although the topic is controversial, seemly the easiest off-world base to build and maintain for large amount of humans is a baloon base in Venus.

First, filling it with breathable air will make it float in Venus dense atmosphere.

Second, since it floats, you can make it fly around Venus every 24 hours, keeping our day/night biological expectations met.

Third, Venus gravity is about 0.9g, less than Earth, but not much less, thus probably not ruining the settlers health (settling in gravity-less places is expected to make Earth-born settlers weaker over time, and space-born settlers might end with some weird shape, maybe extremely tall, due to lack of gravity).

Fourth, the atmosphere in Venus, although dangerous (specially its stupid-high pressure and temperature near the ground), isn't THAT dangerous where the colony would float.

The only major challenge that showed up about this plan, is how to handle the sulfuric acid clouds, we would need materials that resist them very well, including for "space-suits", for maintenance colonists that need to step outside, and probably to handle ship docking.

As for resources, it is expected that the colony would use some kind of hook, to grab stuff from the ground in Venus, this is also a big unknown, we never build such thing before, and we don't even know how we would mine in Venus surface in first place, since the mining equipment would be forced to survive some extreme environment.

Thus the ultimate obstacle, might be just the transportation of the materials, since we have no idea how to mine in Venus surface, we will have to ship all the materials to build and maintain the colony from other places in the Solar System, it is an easy feat, but very expensive, like I noted earlier, Venus gravity is 0.9g, meaning space-flight inbound and outbound from Venus isn't much cheaper than on Earth.

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  • $\begingroup$ Grabbing stuff from ground in Venus would require some kind of mining (and preferably refining and manufacturing) facility on the surface. Which leads to surface colony (because even 99% automated facility requires human-only maintainance). Without this ground colony your floating city won't be any better than ISS nowadays. $\endgroup$ – Mr Scapegrace Jan 9 '17 at 14:47
  • $\begingroup$ @MrScapegrace what about a large bucket thing on a rope that can mine stuff, and have the manufacturing on the balloon? It is just a thought, but you do not have to do mining by hand. $\endgroup$ – Mark Gardner Jan 9 '17 at 15:00
  • $\begingroup$ And my point now is, if we are able to build such a floating ballon city on venus with such a grappling, mining whatever thing to get needed ressources, wouldn't it be easier with the same technology and money and ressources to build a rotating space station (self sustaining eco sphere) perhaps with ressources mined from asteroids? $\endgroup$ – Hothie Jan 9 '17 at 15:17
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    $\begingroup$ AFAIK up where your balloon would float, there would actually be very little sulfuric acid gas. It would actually be a pretty decently pleasant atmosphere compared to some other places in our solar system. $\endgroup$ – Adam Wykes Jan 9 '17 at 16:01
  • $\begingroup$ ABS plastic is pretty resistant to sulfuric acid (otherwise our car batteries would dissolve), I think polyethylene is pretty good too, so plastic shopping bag and milk bottle space suits for all... $\endgroup$ – Samwise Jan 10 '17 at 0:09
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The "easiest" space colony to build would be a free space colony much like the ones proposed by the L5 society back in the 1980's, although using much more modernized materials and assumptions.

enter image description here

Island 3 structure

I'm thinking the adjective "easiest" is being applied to the physical structure of the colony itself, as well as making that structure inhabitable for all people. Any colony on a planet or moon needs to be essentially custom built for that particular environment, and there will be a lot of things which your simply cannot change such as local gravity or the day/night cycle.

Free space colonies, on the other hand, can be "factory built" (one can imagine a "Willow Run" like megastructure cranking out habitat modules) since factors like gravity, day/night cycles, amount of insulation or radiator surfaces and so on can be constant. Depending on the distance from the Sun, you will have to adjust things like the external mirrors to provide enough light and solar energy, but even then platoons of external mirrors in co orbit can provide the additional light without changing the "factory standard" mirrors if you desire.

People can move in right away without any sort of adjustment from Earth's gravity or day night cycles, and getting raw materials or exporting finished products can be as easy as installing a mass driver down the central axis or a free flying one nearby, while asteroid mines and robot miners on various moons shoot "cans" of raw materials to the colonies.

The colony could be as simple as creating two huge "plastic" bags (some 22nd century super material) that fit one inside the other then filling the space between them with water to provide radiation shielding and thermal buffering, not to mention allowing light into the structure (about 5m of water would be needed for radiation shielding, according to Marshal Savage. As a bonus, you could actually grow algae in the water as part of your food and air recycling systems. If you are not keen on using water, then 5m of regolith distributed in the space between the bags will also do.

Factory built free space colonies will be by far the easiest way to colonize space.

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The problem with planets is gravity

A fundamental problem with a long-term colony on a planet or moon's surface is the unknown effects of long-term low gravity conditions on humans. Astronauts, who are generally in good physical condition, suffer a variety of negative effects from prolonged weightlessness. Attempting to live and entire life in such conditions could result in even worse health problems. We just don't know how much gravity is enough. Is Mars' 38% surface gravity enough? What about the ~15% on Luna, Titan, and the Jovian moons? We don't know.

The problem with space is radiation

The long term effects of radiation in space are also poorly understood. No one on earth is subjected long-term to the amount of radiation that someone outside of a magnetosphere will endure. Apollo astronauts did get short term exposure on their trips to the moon, but again, they were in good health and their exposure was brief on a scale of lifetimes. In particular, the effects of low level lifetime radiation exposure on human reproduction are completely unknown.

There are very few radiation safe zones. The Earth and Saturn's magnetospheres are safe zones, Jupiter is not since it produces its own, much higher, radiation levels. Venus' atmosphere is thick enough to provide protection, as is Titan's (Titan may or may not be within Saturn's magnetosphere at any given time, depending on orientation with the sun). Under the ice sheet on Europa or Ganymede would be safe, as would places far from the sun.

What is best?

The answer depends on the gravity and radiation problems, and if either one is limiting to long term human life. If neither gravity or radiation turns out to not be a big deal, then the best option would be the surface of Mars, which has lots of valuable space and minerals for growing food. If radiation is a concern, but gravity isn't, then the surfaces of well protected moons might be good, like Callisto, Europa or Titan. If gravity is a concern, but radiation isn't, then space habitats that simulate near 1g would be best. Finally, if both gravity and radiation are limiting, then the best options are space habitats within Earth's or Saturn's magnetosphere, or a cloud colony on Venus.

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  • $\begingroup$ "minerals for growing food." - they do not need them. They need microelements but not minerals. $\endgroup$ – MolbOrg Jan 9 '17 at 15:47
  • $\begingroup$ @MolbOrg Where do you get elements if not from minerals? Bacteria and weathering have a nice way of producing free nitrates and phosphates and such for free... $\endgroup$ – kingledion Jan 9 '17 at 16:08
  • $\begingroup$ First of all difference is in amounts of those elements. Second elements do not disappear or do not appear in to nothing - so human(animal) wastes will contain same elements plants need. Initially I thought you talking more about media where to grow the plants. As about matter from which those plants will consists - carbon, nitrogen, oxygen, hydrogen, Fe, Mg, K, Na, Ca, etc - planets are not exclusive source of those elements/building blocks. $\endgroup$ – MolbOrg Jan 9 '17 at 22:39
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On the surface is much easier because you have a supply of resources, whereas, if you colonize in space, you can only have the materials that you put in space (or maybe mine from an asteroid, but such asteroids are not found in likely locations to build a space station).

Also, planets with an atmosphere would be easier to colonize than planets without an atmosphere. There are plans to colonize Venus's upper atmosphere, which can be done because the air that we breathe provides lift on Venus (like Helium here on Earth) and Mars which, while it lacks that benefit due to its thin atmosphere, its atmosphere is enough to aid in slowing down from orbit. And both of these have the same benefits as colonies on atmosphere-less objects.

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