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Elon Musk has made very clear that his goal is a colony on Mars. That doesn't seem ideal to me at all. Certainly some people will be happy to live on Mars for the rest of their lives, but they will not experience full gravity or fresh air, having to live in habitats. Children born in such an environment might be at risk. We have no idea what a pregnancy in less than 1 G involves.

It seems to me that the long term sustainable expansion of humanity requires large rotating space stations to provide a comfortable environment equivalent to that in which we evolved. I realize that such stations would be an enormous engineering challenge (e.g., shielding from radiation, maintaining a biosphere). Some of these challenges would apply to Mars habitats as well.

So my question is: Is it harder to build such space stations than it is to build habitats on Mars? Are there other good reasons to choose Mars instead of space stations in some orbit?

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage ♦
    Commented Jul 27, 2020 at 14:10

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The biggest advantage of Mars is there are resources available on that planet. Run out of oxygen? Make your own! Same with water. Set up refining, and you can make your own metal. Large windows are more difficult in no atmosphere than Mars's thin atmosphere, which makes growing crops easier, at least according to The Case for Mars. I believe this is because of the reduced danger due to micrometeorites, Mars's atmosphere is thick enough that it will essentially negate these tiny flecks, but I'm not certain on this.

All that being said, this is a great debate, and is the single largest difference between Blue Origin's and SpaceX's views on solar system colonization. Time will tell which is the better option, but they both have their pros and cons.

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    $\begingroup$ I don't buy the "windows" argument - Mars has 0.006 bar, so windows still need 99.4% of the stability they would need to be used in a vacuum. $\endgroup$
    – asdfex
    Commented Jul 22, 2020 at 16:44
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    $\begingroup$ @asdfex It works a little better for greenhouses, because plants can handle pressures only a little above ambient, but yeah, that isn't a strong argument. The main benefits of the atmosphere are reduction of temperature extremes, radiation shielding, raw material, and greatly reduced propellant requirements for landing. $\endgroup$
    – Christopher James Huff
    Commented Jul 22, 2020 at 17:47
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    $\begingroup$ I borrowed that argument from Robert Zubrin. Now that I think about it, it's probably more about micrometeor protection, the grains of sand aren't as big of a concern when they go through even a thin atmosphere. Will change my answer appropriately. $\endgroup$
    – PearsonArtPhoto ♦
    Commented Jul 22, 2020 at 18:59
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    $\begingroup$ The implied economic argument: You can obtain materials by shipping them from Earth at a cost of thousands or, at best, hundreds of dollars per kilogram. Or you can take advantage of 10^23 kg of material that is already there. $\endgroup$
    – WaterMolecule
    Commented Jul 22, 2020 at 19:34
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    $\begingroup$ @leftaroundabout: while the processing facilities will be expensive, the per-unit cost won't be significantly higher than on Earth. Build once, use for a long, long time almost for free. A space station needs constant delivery of raw materials at huge cost. $\endgroup$
    – SF.
    Commented Jul 23, 2020 at 12:16
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The big problem is that space is empty. To build the space station, people need to haul every single gram there. Every single atom on a space station needs to be shipped there at cost--whether it's from an asteroid, planet, or elsewhere.

On Mars you have the ability to use local resources. You can easily dig and build using the resources at hand without extensive space travel to get the resources.

Another major concern are fail states. If the space station fails catastrophically, you lose everything: water, air, food, gravity, physical surroundings, etc. On Mars, even after a catastrophic failure, you still have gravity and a physical environment of things.

Finally, humans are a gravity-adapted species. For millions of years, our biology has tuned itself to work in gravity, and establishing gravity on a space station is just one step (and one less failure point) you don't need on a planet.

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    – called2voyage ♦
    Commented Jul 24, 2020 at 16:33
  • $\begingroup$ Also worth mentioning - rotating gravity has some very weird quirks that people would have to get used to. $\endgroup$
    – Tomáš Zato
    Commented Jul 25, 2020 at 20:36
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Radiation

I think this is actually the biggest concern, IMO. Astronauts aboard the ISS have a measurably increased risk of cancer due to their higher radiation exposure. Putting enough shielding on a space hab to reduce risk to earth surface levels would be unimaginably expensive. On the other hand, Mars has gigatons of surface rock which makes a pretty decent radiation shield if you have enough of it over your head. I think just 5-10 m of rock provides as much shielding as you get at the surface of the earth, and without the benefit of a magnetosphere or thick atmosphere.

Also, I wouldn't want to move to Mars unless it had full-G facilities (i.e., the majority of the living space was inside a giant centrifuge). The centrifuge wouldn't need to spin that fast, since you only need to make up a fraction of earth gravity, but it would present some pretty goofy geometry, as "up" would be something like a 30 degrees-from-vertical vector towards the center.

Thermal Mass

The ISS faces a 300 C thermal difference between the sun side and the dark side. Just trying to manage that thermal gradient 24/7 must be a nightmare. On the other hand, Mars has a pretty regular surface temp, especially underground, for the same reason that Earth does: you've got gigatons of rocks to absorb and radiate heat. If you need to dump a lot of heat quickly, you can run emergency coolant lines through rock further away from your base, or you can even dump coolant above the surface and hope that it rains or snows back down for collection later, depending on what you are using. If you have a thermal emergency on a space hab, you can dump hot coolant outside, but then you have two problems.

Gravity

The nice thing about working in space is that you don't need to spend structural resources supporting your structure against gravity. The bad thing about working in space is if you lose something, you really lose it. Imagine a solar panel getting sheared off by an unlucky meteor strike. On a planet, if you have a disaster, at least your stuff doesn't float away. No need to manage the total delta-v of your critical systems.

Shielding

Not only is Martian rock good for shielding cosmic radiation, it's also good for shielding different parts of a habitat from each other. Imagine you run a nuclear reactor for baseline power. In a space hab, that's going to be a tricky business. You want it well inside your shielding layers, but you also want it well away from your living spaces. On Mars, you can just put it a few km away from your living quarters and run a power line to it. You can keep batteries and fuel cells in their own area as well, in case there is an overcharging disaster, etc. All that rock on the surface may seem useless, but it's very, very valuable, especially since you don't have to transport it there.

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    $\begingroup$ Even the atmosphere itself, as thin as it is, provides substantial radiation shielding. jpl.nasa.gov/spaceimages/details.php?id=PIA03480: "The range is generally from 10 rems(color-coded dark blue) to 20 rems (color coded dark red). Radiation exposure for astronauts on the International Space Station in Earth orbit is typically equivalent to an annualized rate of 20 to 40 rems." $\endgroup$
    – Christopher James Huff
    Commented Jul 23, 2020 at 3:12
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    $\begingroup$ In space, you still can build your reactor few km away from your living quarters and only have a power cable between the two points. The space is for free and the inverse square law can replace shielding to a good extent. But other points still apply. $\endgroup$
    – fraxinus
    Commented Jul 23, 2020 at 9:17
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    $\begingroup$ Full-G facilities on Mars? I don't think that's going to happen. Articifial gravity with acceptably low Coriolis forces is much easier to generate in space (using long tethers) than on a planet (where you need a full torus and complicated bearings). I think much more likely is that they will just design training equipment that allows humans to reasonably well keep their physical shape despite the low-G environment, which is after all not that low-G in Mars. $\endgroup$
    – leftaroundabout
    Commented Jul 23, 2020 at 10:25
  • $\begingroup$ @leftaroundabout Can you provide an example of a planetary Artificial-G-Facility (or at least the concept)? I have a hard time imagining it, my mind always returns to giant space-rings. $\endgroup$
    – JFBM
    Commented Jul 23, 2020 at 11:02
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    $\begingroup$ @jpaugh Excluding a centrifuge big enough to enclose a substantial part of the planet, the gravitation of Mars is on a vector orthogonal to the apparent gravity due to the rotation. You can't "cancel" any part of it by spinning faster. With the sloped floor, it just adds to the overall gravity experienced, which as stated allows the spin to be slower. $\endgroup$
    – Christopher James Huff
    Commented Jul 24, 2020 at 14:26
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Are there other good reasons to choose Mars instead of space stations in some orbit?

Perhaps the following non-technical but social argument could be a good reason: Much like in the space-race to the Moon, exploring natural solar bodies with actual humans can have profound and inspiring effects on (both/all) colonies of conscious beings.

Having a spaceship, with humans/beings living in it, floating in some position in space is inspiring, and one could argue that it is another thing for humans to actually start living on the natural objects we can see in the sky.

Such actions might contribute to a change in human perspectives, and even though these changes might be (extremely) difficult to quantify (and predict), they could perhaps be the most important reason for large entities/corperations/goverments to make choices between expanding life on spacecraft and/or on natural solar bodies.

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    $\begingroup$ JFK put it nicely - "We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard, because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one which we intend to win..." $\endgroup$
    – Nuclear Hoagie
    Commented Jul 23, 2020 at 16:19
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    $\begingroup$ @NuclearWang "which we intend to win" That's probably the only reason he needed to have, given the historical context. $\endgroup$
    – jpaugh
    Commented Jul 23, 2020 at 22:26
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    $\begingroup$ I think this is a beautifull concept but a higly romantic and "idealistic" way of viewing history at best. The truth is that if there is no material reason to engage in such an endevour then material constraints will deter all efforts. The "overview effect" you are talking about would certainly be a byproduct of this endevour but the endevour wouldn't be sustained by it alone, unless the "overview effect" can yield a positive material impact on people's lifes to the point of been economically acceptable (which I think could be if in the long run but not in this needed first steps). $\endgroup$
    – Swike
    Commented Jul 24, 2020 at 12:34
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Other answers have already mentioned the availability of resources and the ability to endure disasters, which I agree are two key reasons.

I have not yet seen anyone mention what I consider the third key reason. Elon Musk does not just want to take us to Mars, he wants to make humanity a multiplanetary species. Sufficiently many people (a commonly guesstimated figure being a million) living on Mars could eventually be a completely self-sustaining civilisation. Perhaps a sufficiently large space station in orbit around Earth might also be self-sustaining; perhaps such a space station could even provide a refuge from climate change. Would it be safe from global thermonuclear war? What about an asteroid strike? Perhaps. But Elon's goal is to do better than "perhaps".

Settling Mars gives humanity more than one home, which a space station in Earth orbit does not, and arguably neither does the Moon for that matter. It "backs up the light of consciousness", which as far as we know may be unique in the universe - disaster could befall one planet's civilisation (and very likely will, unless we do something about climate change very soon), but the chances of two self-sustaining planetary civilisations meeting catastrophe are much lower.

(Not impossible. A nearby supernova or a wandering black hole or any number of other dangers could eradicate all life anywhere in this solar system. Eventually, for this reason, humanity may choose to expand beyond our solar system and colonise planets around other stars. We've got a lot of tricky problems to solve before that's even remotely plausible, so let's start with Mars, which is likely to be tricky enough).

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  • $\begingroup$ This is a nitpick, but based on our understanding of supernovae and their effects, a nearby supernova wouldn't wipe out all life in the solar system, or even all life on Earth: en.wikipedia.org/wiki/Near-Earth_supernova. $\endgroup$
    – Pitto
    Commented Jul 24, 2020 at 1:10
  • $\begingroup$ @Pitto thanks, that's an interesting link. $\endgroup$
    – BittermanAndy
    Commented Jul 24, 2020 at 1:14
  • $\begingroup$ @Pitto all life on an orbital habitat seems fairly likely though. $\endgroup$
    – Christopher James Huff
    Commented Jul 24, 2020 at 16:39
  • $\begingroup$ "which a space station in Earth orbit does not" - I think this is only true if you assume there's only going to be one space station; that it will be small; and that no further expansion of in-space manufacturing or mining will occur. It will take many decades, if not centuries, for Mars to be a true human backup (assuming we can gestate babies at 0.38g at all), by which time you could have hundreds of millions to billions of people living in spacious rotating habitats throughout the solar system at the same time - and unlike a planet, they can't all be killed by a sufficiently-sized asteroid. $\endgroup$
    – Snoopy
    Commented Jul 24, 2020 at 17:19
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    $\begingroup$ "by which time you could have hundreds of millions to billions of people living in spacious rotating habitats throughout the solar system"...a Mars colony could be self-sufficient and capable of supporting millions of people long before orbital habitats would be able to do so. The time and energy requirements of just reaching asteroids are sufficient to make this so, never mind all the zero-gravity mining, manufacturing, and fully-closed habitat technologies that would need to be developed and perfected first. $\endgroup$
    – Christopher James Huff
    Commented Jul 24, 2020 at 21:13
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It's motivational.

Had JFK given us a more realistic goal than landing a man on the moon by the end of the decade, we would have failed. Such a goal might have been:

First, I believe that this nation should commit itself to achieving the goal, before this decade is out, of establishing a permanent outpost in space, in preparation for landing a man on our nearest neighbor, the moon.

Had it been successful, this would have paved the way for a permanent human presence in space, resulting a moon shot by 1975 or 1980. That initial moon shot would probably have had a larger payload, and might well have immediately established a crewed base.

And quite probably, the would have been quietly shelved by 1965. Putting a man on the moon (or Mars) is a simple, direct goal that anyone can comprehend, anyone can visualize. It's the perfect elevator pitch for a project.

As Apollo proved, if there is sufficient motivation, we can overcome the technical difficulties of the project. The problem lies in creating that motivation.

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Resources, as Pearson mentioned, are the key; specifically matter. An object in the inner Solar System is emphatically not a closed system, so the second law of thermodynamics does not apply: Any object here is embedded in the Sun's radiation, an inexhaustible source of low-entropy energy. This energy flux enables it to lower its entropy (by radiating even more of it away). But replacing complicated technological artifacts will require a technological infrastructure difficult to imagine even on a large space station.

Then there is leakage — the irretrievable dispersal of matter into space. It can be reduced but never completely avoided. Isolated space habitats are not sustainable over extended periods of time (as vague as this statement is). A space station would need long-term support from bodies like planets, moons or asteroids. (Read Stevenson's SevenEves for an in-depth treatment of this subject.) Any viable concept of a permanent space habitat involves a constant supply of matter like oxygen, hydrogen, carbon and other constituents of biological systems, as well as metals and other constituents of technical systems.

If we suppose that it comes from Earth we must suppose that there is some kind of space port on Earth, for refueling and maintenance, and the same is true for Mars or any other body. If we rely on a planetary base for supplies we can as well simply live there.

This leads me to the elephant in the room regarding plans to find replacements for an uninhabitable Earth. One of the most egregious examples of ignoring it is the premise of the movie Interstellar.

Even a severely devastated, climate-ravaged, irradiated and poisoned planet Earth will be an infinitely better planet for a base and will be infinitely better suited for, well, Terra-(re)forming than any other planet in the Solar System, let alone outside of it.

It is really hard to come up with disasters which make Earth worse suited for a human settlement than Mars. (This could be a question of its own.)

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  • $\begingroup$ I actually have a question about a potential way for a space station to make up for leakage: space.stackexchange.com/questions/45547/…. Though that question has yet to be answered, so I'm unsure about how practical that would be. $\endgroup$
    – Pitto
    Commented Jul 26, 2020 at 9:43
  • $\begingroup$ @Pitto I tried to contribute something to a discussion there, which led me to papers estimating the evaporation rates of different materials in vacuum. For some materials and temperatures it is substantial; in geological time scales it will probably become a problem. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jul 26, 2020 at 21:46
  • $\begingroup$ We can live on planets, yes, but that we do now is no reason to only live on planets, especially when the closest to Earth gravity we'll get is Venus, and we'd have to use aerostats to live there. So far as geologic time, I don't think that's a good argument against building colonies in free space. What does it matter if a habitat isn't habitable on a time frame of millions or tens of millions of years? That leaves plenty of time for our descendants to come up with technology we can't dream of. $\endgroup$
    – Snoopy
    Commented Jul 28, 2020 at 13:16
  • $\begingroup$ @Snoopy You may be right; I changed the wording to "extended periods of time". "Geological was a safe bet, but is probably not needed. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jul 28, 2020 at 15:09
  • $\begingroup$ Could you define an 'extended period of time'? If, say, we can practically live in a habitat for a thousand years or more, I don't see why we wouldn't pursue such an option. $\endgroup$
    – Snoopy
    Commented Jul 28, 2020 at 16:51
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I think the reasons are psychological - that occupying and building a life on a planet is more comprehensible and appeals better to people, by using the appearance of opportunities that are in a more familiar form. It is easier to popularise and market than promoting the building of space habitats. Any colony attempts will rely heavily on taxpayer funded programs paving the way, which require support of voters to sustain, so popularising the venture appears to be a prerequisite to having such programs and Mars beats Space Habitats in popular imagination.

I think neither Mars nor Space Habitats offer genuine and achievable prospects of true self-sufficiency with current technologies - not because Earth cannot make the technologies required but because such colonies cannot. They would have to be very large and comprehensively capable industrialised economies to make the full range of essential technologies themselves, without Earth input. In the absence of commercial opportunities via trade to sustain continuing financing to sustain ongoing supply and growth self sufficiency would have to be treated as a required initial condition, bypassing growth based on commercial opportunity but the absence of profitability may be an insurmountable barrier.

Space Stations based on Asteroid resources do have potential for commercial trade of physical commodities with the source of all essential technologies - Earth - and if successful asteroid resources may support continuing investment, occupation and further growth. Mars does not have any genuine prospect of engaging in physical trade to support it through it's time of dependence on imported equipment and resources and required period of growth to become the comprehensively capable industrialised economy that is essential for self sufficiency.

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    $\begingroup$ I'm not sure there's a reason why a colony on Mars could not make what it requires to be self-sufficient, given enough initial "seed-corn". For sure we don't know fully what minerals are available, so perhaps we'd find Mars doesn't have the right minerals to build Li-ion batteries, or something like that. That doesn't stop it being self-sufficient though, it just may limit what's available. If things need to be imported at high cost - well, you've just defined life for Fiji or Tonga or any small island country on Earth. $\endgroup$
    – Graham
    Commented Jul 24, 2020 at 9:55
  • $\begingroup$ Delta-v to Earth's vicinity from Mars is lower than from main belt asteroids, and it has an abundance of water and CO2 to be processed into propellant to drive such transport. Metallic asteroids are extremely poor in volatiles, and even carbonaceous ones can't compare to Mars...only Ceres comes close. Shipment to Mars is much easier since not only is the departure delta-v lower, landing vehicles can take advantage of the atmosphere for braking. If there's potential for trade based on asteroid materials, the same is true to a greater degree for Mars. $\endgroup$
    – Christopher James Huff
    Commented Jul 25, 2020 at 1:16
  • $\begingroup$ @Graham - I think you and others are seriously underestimating how diverse and complex and extensive the essential infrastructure and technology has to be for survival under Mars conditions, and how difficult to replicate those in isolation. No parallels exist with Earth's island nations - or with any prior colonisations, which were done using proven tech in widespread use. $\endgroup$
    – Ken Fabian
    Commented Jul 25, 2020 at 1:32
  • $\begingroup$ @ChristopherJamesHuff - Not convinced space habitats will be viable either but Mars sure isn't; delivering asteroid resources to Mars won't earn income, just add complexities and costs. There ain't no independent space economy. Why would any asteroid miners choose the asteroid belt when there are ones within the inner solar system or choose to service customers who have nothing - Martians - over customers on Earth, who have everything you need? No economic basis means no viable colonies. Mars has no economic basis. $\endgroup$
    – Ken Fabian
    Commented Jul 25, 2020 at 1:48
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    $\begingroup$ @ChristopherJamesHuff there are sufficient volatlies and other raw materials in the NEOs to sustain a population far greater than our own for many millennia, and there are many hundreds NEOs with delta-V requirements less than the Moon, and comparable to Mars. A colony in orbit near Earth will have the Moon, the NEOs, and Earth's industrial base to easily draw upon. A colony on Mars will mainly have itself. $\endgroup$
    – Snoopy
    Commented Jul 28, 2020 at 13:19
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Why will we go to Mars?

It certainly is not ideal to build a colony on Mars. There are many threats to human life there - it is one of the deadliest environments a human could be in.

To Elon Musk, inspiration is the key to human progress and motivation - we will go to Mars for the same reason that humans do space exploration at all - it inspires us and we get to see what no man ever saw before - further we go towards infinity.

Effects on humans

There are theories that a pregnancy on mars (0.38 G) could be harmful to the child. Certainly, children of this kind would have serious problems when returning to earth - just like our astronauts when returning from the space station.

Certainly, artificial gravity on a space station would be another option - and will continue to be developed. However, there are major disadvantages:

  • The artificial gravity you mentioned also acts different on humans than earths gravity (even when their magnitude is equal)
  • A space station is way more vulnerable than a base on the ground (fuel explosions, thin walls and vacuum around).

Comparison: What is harder to build?

As you figured, both, a huge station (with artificial gravity) and a colony on Mars will be incredibly hard to build - it requires thousands of engineers all over the world to make such a project work.

Building a space station, even of the size of the ISS (51 meters long, 420 tons) is an enormous project. Now think of something we could fit a hundred or maybe a thousand people in - the challenge of building it is even greater.

For a colony, despite having to transport resources further, it is easier to assemble. You can just take a few astronauts with tools to build something habitable (of course not that easy but easier than using shuttles to assemble it).

Another crucial question is:

Which one of the two is harder to maintain?

And that is definitely the space station.

On Earth, we hardly have to do anything to stay alive - we just live. In a colony on mars, once we bring the tools there that let us grow plants (thus get fresh air) etc. , we are safe. People could life there for generations with hardly any extra effort compared to earth (except or some safety measures etc).

A station is harder to maintain. Let's consider the ISS:

  • The ISS has to be pushed back to orbit by a rocket every now and then, because else, it would fall into the ocean (less likely: land)
  • Without artificial gravity, humans have even worse problems when they stay in orbit for long... in a colony, you wouldn't have to deal with that (at least it wouldn't be as bad)

Concluding

I hope you (at least partly) understand, why we will go to colonize Mars instead of a huge space station ^^

I really liked all the other answers - make sure to upvote them too - thanks for this great thread I am glad to contribute ^^

PS: If I ever could, I would go to Mars right away ^^

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    $\begingroup$ For building a colony on Mars, it would make more sense to send in robots first, at least to do the heavy lifting. $\endgroup$
    – Pitto
    Commented Jul 29, 2020 at 1:42
  • $\begingroup$ @Pitto I certainly agree - however, in something goes wrong, a robot won't be able to rescue itself (like Curiosity in the dust storm that killed it ^^). Also, humans can lift about three times as much mass on mars ... I think it's best having machines /robots that are controlled / supervised by humans (we can't control them from earth). $\endgroup$
    – publicdomain
    Commented Jul 29, 2020 at 5:03
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To add to the other answers, another reason why Mars or another planet is being colonized is the idea to have a "back-up planet" in case the Earth gets hit by a global killer or something. Colonization in habitats is just the first step. The 2nd step will be living in domes and the 3rd will eventually be terraforming. So a terraformed Mars (or another planet) will have fresh air and the goal is to make it an Earth-like planet.

A space station (let's say the Stanford Torus) is like you say very challenging to build, and it would have artificial air and climate. The materials must come from some planet, probably Earth. It can't serve as a back-up location for humans in case the Earth became devastated, unless its very, very huge and reliable. If we're so progressed that we're able to build a torus where a billion human civilization can live forever we're also able to terraform another planet.

As with gravity, Mars' gravity (0.38g) isn't that low and actually pretty high compared with other solid planets, dwarf planets and moons in our system, only Venus and Earth have higher surface gravities. I think Mars' gravity is sufficient for much. While it would be better to colonize Venus with its more similar gravity, size and mass to Earth, one day Venus is expected to be swallowed by a red giant Sun. Therefore, forever probably only the outer planets can be colonized. And perhaps we will build floating habitats in the atmospheres of Saturn, Uranus and Neptune one day, all of which have Venus- and Earthlike gravities.

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