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I have a city in space that will have a local population and a lot of travel in and out (center of commerce). Should I place it out in open space or near a planet?

Being near a planet obviously involves gravitational forces that affect both the station and travel; my gut feeling is that that's a positive thing but I'm not strong on the relevant science. It also seems like placement near a suitable planet would provide resources the inhabitants might need -- food, water, raw materials.

If it should be situated near a planet, what is the best positioning? Should it be in line with a pole, in the direct line that runs from star to planet, something else? How far out should it be, with respect to the planet's gravitational influence? (I assume it has to at least be as far out as a Lagrange point or it'd fall to the planet, but they're sometimes depicted as seeming much farther out.)

Does the placement decision interact with the shape of the station, or are these orthogonal decisions? How does the mass of the station affect things? What other factors do I need to take into account?

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    $\begingroup$ In line with the pole wouldn't be stable, it would need to orbit. $\endgroup$ – Tim B Feb 12 '15 at 17:02
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    $\begingroup$ How self sufficient is your city (does it need food/water/material imports?), is defense a concern, and where does it plan on getting energy from (solar/ nuclear/ other?) $\endgroup$ – Twelfth Feb 12 '15 at 19:53
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    $\begingroup$ The method of travel determines the optimal placement of travel hubs. Ships require coastal harbors. Airplanes require flat open runways. How do your spacefarers get around? What are the limits and capabilities? $\endgroup$ – Foo Bar Feb 13 '15 at 2:49
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    $\begingroup$ Related meta question: meta.worldbuilding.stackexchange.com/q/1779/28 $\endgroup$ – Monica Cellio Feb 13 '15 at 3:01
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    $\begingroup$ As you're talking about Lagrange points allow me to state some basic things: Size/Weight most presumably will NOT have any influence on where you place your city around a planet, sure you need a certain distance to escape the drag of the air, but otherwise no influence. Safe Orbit is achieved by speed, not by distance, the faster you are away from an object the less speed you require. Lagrange points are points between 2 or more gravity wells which allow an object to stay in a stable position relative to these gravitational sources. $\endgroup$ – dot_Sp0T Feb 13 '15 at 12:59
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Near a planet.

A planet's orbital path will provide a clean bit of space, a star for energy, a way for people to reference the location, and other minor benefits. Put your city in very high orbit around a planet. In the plane of the solar system and in the direction of planetary rotation will work well (may as well be going the same direction as most of the mass, smaller relative velocity). You'll get minor gravity well and all the benefits.

Clear Space

Planets, by definition, clear their orbital paths of debris. If you don't want to be bombarded constantly with large solar system debris, a planet's orbital path is a good place to start.

Energy

Being in a star system will provide energy for your city. Most species develop in star systems [citation needed], so a star system is somewhere they can feel at home. You get free diurnal cycling if you rotate your city. Besides, what's life without a sunrise?

Reference

If you have a city in space then presumably the navigational equipment of visitors is good enough to determine how to get to any coordinates by looking at the stars and a watch. A human won't be pointing their ship at the second star to the right and cruising at warp two until dawn, the most complicated navigation they will have to do is to your web page to look up the coordinates of the city. The computer will do the rest. So, being in a star system near a planet doesn't matter one bit for interstellar navigation, but being able to say "Oh, this place has the best Korblian Ale, it's around Howsian-4" gives the place a natural language reference. If your city is called New Portland, it will be hard to find unless you've got a planet and star system (State and Country) to reference it by. Space is a big place, and Portland is an awesome place, obviously there will be a lot of New Portlands out there. Oh, the New Portland around the fourth planet of Howsian? Yeah, I know where that is.

Other Benefits

If the planet you're orbiting is an uninhabitable gas giant, then great! No one will ever live there! Guess where that radioactive-toilet-water is going? If it's habitable/near-habitable then it can be used as a rendezvous in an emergency or the city grows into a space port (a high class one, where droids are allowed).

It's something to look at. Space cities should be beautiful, put it above the rings of a Saturn like planet you will make a killing on the real estate.

You can build your city in a captured asteroid. Free solid ground, cosmic ray protection, minor gravity, and micrometeor defence? Yes, please. Minor gravity alone is enough there, make it a health spa, come here and you'll literally weight 10% of your original weight!

So, yes, around a planet.

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  • $\begingroup$ I'd think it would be very hard to maintain coordinates for a free-floating city in deep space, constantly affected by the gravitational pull of hundreds of visiting ships, to a useful degree of accuracy. But everything else I agree with. $\endgroup$ – DaaaahWhoosh Feb 12 '15 at 20:09
  • $\begingroup$ @DaaaahWhoosh I disagree. It's not as if it's carefully placed there and then you say "NO ONE TOUCH IT". The city computers would certainly be looking at the stars and automatically adjusting drift with small attitude jets, thrusters, or precisely placed dance parties. $\endgroup$ – Samuel Feb 12 '15 at 20:15
  • $\begingroup$ In that case, I stand by my answer: it'd be easier (though perhaps not as fun) just to orbit a planet. $\endgroup$ – DaaaahWhoosh Feb 12 '15 at 20:16
  • $\begingroup$ @DaaaahWhoosh Though I agree orbiting is better, I disagree that the position maintenance is a good reason. You have to actively maintain an orbit too, for the same factors as deep space and more. Besides that, think about a city in decline, if the orbit/position mechanics breakdown it's significantly more of a problem to be falling into a planet than not being where people expect you to be :) $\endgroup$ – Samuel Feb 12 '15 at 20:23
  • $\begingroup$ All right, you have a point there. All I can say is that position maintenance would be cheaper with nearby energy sources, but you've already said that in your answer :) $\endgroup$ – DaaaahWhoosh Feb 12 '15 at 20:45
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This was originally a comment; so I apologize in advance for anything that got too stream-of-consciousness:

Think about what the original purpose of the city was - towns aren't often founded in the middle of nowhere, but tend to spring up around something and then grow into cities from there. So this space station/city will need an original purpose around which the economic growth adds population (which could have been planned for, or happened dynamically, depending on how you want the station/city to be put together).

Some examples:

  1. Gas-mining Jupiter - the city would probably be orbiting the planet for easy access. Core population would be working-class.

  2. Deep-space/nebula research - station would be away from planets/solar systems to reduce interference or inside the nebula for easy access.

  3. Pit-Stop - What was originally just a way-station grew until it was a city - this could be just about anywhere, but is more likely to be in inconvenient locations as far as resources and purpose. Most of the city's populace is likely to have a space-commute to another space-city/planet for work.

If you wanted to build near a planet you would probably want it in orbit so save energy - since it'll be easier to get pulled alongside the planet than if you tried to keep it stationary at one of the poles. See our own international space station.

This part is more of an educated guess - but the bigger the objects involved (planet and city station in this case), the further away it should be from the planet's center OR the faster it should go. See ISS vs Earth's Moon. Too close means you crash into the planet, too far and you lose too much gravitational pull to stay in orbit - just stay out of the way of other orbiting moons/debris (and don't let the citizens dump their garbage out the airlock or you'll come around the planet and smack right into it!)

As for raw resources - you can ship it from another planet but building above one that has the resources already there (or on a nearby moon) would make it easier to build. Once built I think you'll have a pretty heavy emphasis on recycling the resources they use (especially water) - even with regular shipments it would be an economic benefit since you wouldn't need to ship as much of it.

As far as shapes - anything with a clearly-defined relatively-consistent (accounting for the things/people inside) center-of-mass will make things a lot easier on you fictional designers when determining how to get this thing into a proper orbit. Away from a solar system this isn't really a concern - so beyond that, shape will be determined by a few other factors...

  1. Safety - What happens if there is a major breach? You need an easy way to seal off the affected section(s) - which means you may even build sections to have their own airlocks between one another - possibly even built physically separate from one another to minimize potential tearing across sections, although that can make determining a consistent center of mass more difficult. If above a rocky planet, it might contain emergency-landing-struts or something similar in case it gets force out of orbit in that direction.

  2. Purpose - A city-station with a lot of mining ships will need a lot of docking bays (or really big ones) for the mining traffic. A city-station that favors research will have a lot of researchy-looking things (antennae and satellites - big telescope domes and the occasional geodesic dome).

  3. Planned vs Dynamic Growth - A planned city would likely be modular and easy to expand. But one that springs up dynamically would tend to have a lot of growths - additions that come out randomly. An older growth-city would probably have the older sections remain ramshackle while the newer additions would fit together better as people realize what the city-to-be is becoming.

  4. Aesthetics/Architecture - this is going to depend on what the population likes. Lots or parks and statues and windows is going to look a lot different that the cramped spaces a military culture would probably favor.

The last major things to consider are: Communication Import/Export (Resources and Entertainment) Off-hours activities (Theater/Carnivals/Bars) But these are relatively minor or more dependent on culture/technology, so I'm not gonna touch them.

After that, you can do a lot with your space-cities to achieve the feel you're going for. How open is it? Are there a lot of places where you can see out into the great unknown or is your city more of a cramped borg-like space? What kind of people inhabit the city? Is its original purpose still active? (A city mining gas on Jupiter would have to shift if the gas stopped being used, for example - either collapsing as people left or adapting to a new purpose entirely).

Most of the above assumes the entire city is contained within a single structure - or structures that connect to one-another, as opposed to a "The Jetsons"-style city where everyone wears helmets and jetpacks.

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Location location location

On Earth, cities developed along harbors, fords and defensible strong points (forts). These are their space equivalents:

  1. Home world start point star port. This Lagrangian point station is where the landlubers from the planet first get started off towards great new adventures. These will likely be the largest space cities, since for the near foreseeable future, the industrial and population capacity of the home world will be many orders of magnitude higher than all of the off-world put together. So these stations will likely act as a trading-house between the home world and the spaceheads, through which most of the planet-space trade will flow. Bonus points if it is a termination point for a planetary space elevator.

  2. Asteroid belt resupply. For all those miners out there, looking for a good time after 6 months supervising diggers and manufacturing plants on Ceres and other rocks. The population here will be transient, think more Wild West gold rush town than anything else.

  3. Saturn ramscoop and electrodynamo stations. All the system-outgoing ships must acquire reaction mass from a gas giant, or have sails pushed by laser beams powered by the giant's magnetosphere, so likely space stations will arise to supply the crew and the reaction mass, and maintain the giant lasers. Major spaceship construction centers will be here, far out from the sun's deep gravity well, or perhaps as far out as the Oort cloud. If large colony ships are being sent out, these areas will be intermediary staging grounds and could more permanently host those that decide not to go out-of-system after all.

  4. Military outposts. Early warning, radar sweeps, mass drivers, can be found here. With a small fleet, you might need pilots or drone overseers. Technicians for complex repair work, nano-factories for spare parts, etc. If distances are such that soldiers will only accept the posting if stationed there with families, could easily grow into full-scale settlement.

  5. Research installations. Dangerous but very valuable biological organisms that could wreak havoc on a planet if unchecked will likely be in deep space, with atomics for vaporization and escape shuttles. This could grow into a larger settlement if the output is particularly valuable or labor intensive to cultivate.

  6. Mercurian power stations. Close to the stellar power source, these gigantic solar panels send microwave power to other stations further out in the star's gravity well. Many energy intensive processes will take place here, so likely the supercomputers hosting the most advanced AIs with the largest power consumption will likely reside here. So this will be a major technological center...

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  • $\begingroup$ Forgive me if I don't get it, but in what way does this answer the question? He is talking about a city, defined as having a very large population. Not just a mere space station with a fifty man team. While some sort of merchants and adventurers would go to any station as a population (like in Roman Empire in Hadrians Wall forts) massive populations could not be cared for unless the facility was built specifically for that purpose. $\endgroup$ – JDSweetBeat Feb 12 '15 at 17:20
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    $\begingroup$ @DustinJackson, these are the logical agglomeration points that I can think of in space. On Earth, cities developed along harbors, fords and defensible strongpoints (forts). These are their space equivalents. $\endgroup$ – Serban Tanasa Feb 12 '15 at 17:52
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This is something that doesn't seem to be mentioned yet, and I'm not sure if it's a problem, but here's what I'm wondering:

If the city is alone out in deep space, how would you find it?

This is why I commented that it should be near a star: you can see stars from just about anywhere (within reason), often with the naked eye under clear conditions. This will make it incredibly easy for ships around the galaxy to get to your city, kind of like a free lighthouse that can be seen from anywhere in the entire ocean.

Once you're in the solar system of your city, you're again going to need a landmark to get you where you're going. At these distances, since I'm assuming you have FTL travel I can probably also assume you can find a space city with ease. However, if you instead look for a big ol' planet, maybe even a gas giant, no matter what technology you're using I'll bet it'll be faster and easier. Other answerers have shown the benefits of gas giants for spacefaring ships, I just think they're easy to find.

On the planetary level, I'd think most space navigators could either use their knowledge of the city's orbit in relation to the planet to get to the destination, or simply send out a docking request and wait for a reply with coordinates (since now we're within light-seconds of the city). Long story short, space is full of giant signposts, you might as well use them.

Oh, also, unless you're next to a nebula it'll probably look prettier to be next to a planet. You can charge more for apartments if they get a view, and the city'll look better in travel brochures.

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    $\begingroup$ That's true; I was needlessly conflating "not near a planet" and "in deep space", but "near a star but not a planet" is certainly an option. $\endgroup$ – Monica Cellio Feb 12 '15 at 18:38
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Lagrangian Points

A Lagrangian point with what? There are ten different Lagrangian points associated with the Earth, and four of them are stable. The most likely places to get early colonization are the Earth-Moon L4 and L5 points (trojans). Those are the closest stable orbits to the Earth. They may require some adjustment though, since they aren't stable against the sun.

If you want an orbit that doesn't require regular intervention, there are the Sun-planet trojan points. Obviously the Sun-Earth points are closest to us, but the Sun-Neptune points seem to be the most stable.

3753 Cruithne has what seems to be a stable system composed of two orbits. You might be able to build a city there.

Why near a planet?

Why not near a planet? Cities tend to arise in places where it's natural to have traffic. On the Earth, this is often at the intersections of waterways. Pittsburgh is at the intersection of the Allegheny and the Monongahela and the Allegheny with the Ohio. Cleveland is at the intersection of the Cuyahoga with Lake Erie. New York City is at the intersection of the Hudson River with the Atlantic Ocean.

In our solar system, the obvious destination is the Earth. It seems likely that space cities would arise near to Earth first. Start with the Earth-Moon trojans. Then the Sun-Earth trojans. Perhaps 3753 Cruithne to provide easy access to the Earth, Venus, Mercury, and Mars. Also as a jumping off point for further out in the solar system.

The Sun-Neptune trojans may serve as natural stopping places for interstellar travel. If that ever became common, perhaps one or both would become a large city. Power might be a problem though. Not nearly as much sunlight there as closer to the sun. Of course, you can make the panels of arbitrary size.

The planets are already in the most logical places for stable orbits. The next most logical places are the Sun-planet trojans. It's unclear if any of the planets are interesting enough to attract space cities of their own. Perhaps if we terraform Mars or Venus.

You could locate a space city somewhere like the asteroid belt. There may well be a stable orbit there. But would you? We could build a floating city in the middle of the Atlantic Ocean, but we haven't done so. These are certainly possible places for cities, but they seem unlikely.

Why in space?

Why put people in space rather than on the nice, cheap ground? Presumably because you want to do something in space. It's expensive to lift people from the ground to space.

I'm also not convinced that space will be more expensive than the ground in the long term. Once we get self-replicating robot miners powered by solar energy, space construction is effectively free.

Shape

As I said last time, I think that the most likely shape for something we'd call a city would be a cylinder. I still think that.

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I think it depends on what 'near' is. If we can travel to Jupiter in a couple days, then almost anywhere in between will be reasonably close. If it takes 6 months to travel to Mars then being in some kind of orbit around the Earth, moon or a populated planet would be a much better idea. it would be possible to orbit the Earth even outside the moons orbit.

A very large city orbiting say just outside the geostationary orbit would be able to cause effects in the tides. The further out it is the less it will interfere with tidal systems. Orbiting the moon should have very little affect, unless it's large enough to make it wobble in it's orbit.

The shape of the city would likely be in a torus or tin can structure in order to easily provide some kind of gravity to those residing inside (unless you discover how to generate gravity/antigravity fields).

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  • $\begingroup$ Wouldn't being in (or very near) geostationary orbit not play a large effect on the tides? I mean, for example, if the moon was in geostationary orbit there wouldn't be tides at all, just a consistent high spot. $\endgroup$ – Samuel Feb 12 '15 at 20:43
  • $\begingroup$ @Samuel, that is probably true, but that seems pretty close to me to have the city. $\endgroup$ – bowlturner Feb 12 '15 at 20:48
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The distance from planets would basically depend on its purpose.

What I can imagine, however, is that a future civilization might see things completely inversely: for us space is tough terrain, and it takes too much energy and resources to frequent places out there very often. In the future, it may be the other way around and people might consider descents to planets as unusual, maybe even unnecessary efforts.

It is completely possible that the far future will see a civilization that has managed to upload itself to computer networks ;) We're not too far from that, are we? In that case all we care about is energy availability. Places look different if you're a bunch of electrons. A city in space in cyberspace has fewer constraints.

As an aside, I immediately remembered the movie 'Elysium'.

In the movie, an artificial city-state had been built for wealthy super humans, away from all the pollution on Earth. A class metaphor. It appeared to be not too far from Earth (maybe roughly Earth-ISS-distance).

But that's more a digression than an answer...

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Converting my comments into an answer: where the city is should flow from why the city is. Real-world cities exist where transportation and geography intersect. Seaports are places to transfer between ships and road vehicles. Airports transfer between planes and road. Also, waypoints form at oases of habitability along important but difficult travel routes, such villages on the silk road.

In a space civilization, there are two good breakpoints: ground vs local space, and local space vs interplanetary/interstellar.

  1. If your ships are FTL-capable but cannot land, space cities should be in planetary orbit. There you use short-range methods to transit the gravity well. The outer end of a space elevator would be excellent.
  2. If your ships are sprightly (e.g. zero-point anti-grav) but do not have self-contained FTL, stations should control jump points. Make up some law of nature that requires them to be off-planet and limited in number (e.g. natural alderson points and/or expensive star gates). The city grows there.
  3. If your ships can lift off, escape planetary gravity, go to warp independently, and come back down to another planet, then spaceports would be ground-based. A space city would require some other reason to exist. Next to an uninhabitable planetoid rich in rare elements? A deep space safe haven amidst light-years of otherwise-impassable dangers?
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