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Mobile cities are a common sci-fi setting. They are a staple ingredient in novels like "Mortal engine quartet" and "The inverted world"

Plot set aside, what justifies mobile cities on land? Giant excavators are real machines which can be used as such. Still, examining them reveals some flaws:

  • large footprit (track print), i.e. it leaves in its wake a smothered natural habitat. This was frequently mentioned in Mortal engine quartet. The smothered lands the tracks left behind were named "the hunting grounds".

  • difficult to maintain. Replacing Large metallic chassis parts that corrode involves hard work. Indeed, One of the mobile cities in the novel was built haphazardly.

  • guzzle lots of fuel when moving

  • may still be dependent on land-based farmland, hence the need to plan trips ahead of time.

  • may need traffic control, similar to that of ships. This is where a caravan of smaller units, or even motorhomes ("private" houses) may prove more useful, as they may use existing roads and traffic control.

  • cannot handle all terrains.

Repurposed ships, aircraft carriers and cargo ships may solve most of those problems, but the question was narrowed down to land-faring societies: Would mobile cities be practical or justifiable enough to be so appealing? Do they have an advantage over classical nomadic societies?

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    $\begingroup$ If you’d like more backstory as to why the Mortal Engines Cities were considered a good idea there’s an entire pre-quartet trilogy called the Fever Crumb trilogy set at the rise of the traction era, and ‘the illustrated world of mortal engines’ that goes into a surprising amount of detail as to why everyone got so behind what is (on paper) a monumentally daft idea. It’s also a very pretty book. $\endgroup$ – Joe Bloggs Mar 7 at 16:29
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    $\begingroup$ Just as a different reference: In Absolution Gap by Alastair Reynolds, there is a religion on a moon of a gas giant that revolves around witnessing the gas giant blinking out of existence occasionally. The cathedrals move to stay under the planet, and the various cathedrals jockey for position as a status symbol of how faithful they are. $\endgroup$ – Stephan Mar 7 at 19:19
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If the planet is in the process of becoming tidally locked, there will be a period of time where the day-night terminator moves, but slowly enough to make noon and midnight still be deadly. Your city might need to move to follow dawn/dusk.

Note that this is an exceedingly unlikely scenario. On cosmic timescales, tidal locking is the blink of an eye. For example, scientists think the Moon formed 4.5 billion years ago, and took only 16 million years to become tidally locked. And the time frame in which the day-night terminator moves dangerously slowly is a minuscule fraction of that.

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  • $\begingroup$ If the world has some pesky oceans in the way, then hopefully the engineers designed some mighty pontoons along with the mighty tracks. $\endgroup$ – Giter Mar 7 at 17:39
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    $\begingroup$ Consider that human civilization came from cavemen to astronauts in about 15.000 years... So even if 16 milions year are few on an astronomic scale, for the society it is an enormous timespan (as even only 100.000 years would), so it is absolutely believable that society would undercome a big transformation to adapt to the situation $\endgroup$ – McTroopers Mar 7 at 17:54
  • $\begingroup$ Yeah it is. The problem is getting that society in the first place. It took ~3 billion years for complex life to form. What are the odds that the few millennia in which mobile cities make sense would coincide with the few centuries in which a society could build them? If people evolve just a few centuries too late, they reach the industrial age after the planet has tidally locked completely, and their cities will be stationary on the dawn/dusk terminator. If they evolve a few centuries earlier, they just colonize the solar system instead. $\endgroup$ – Ryan_L Mar 7 at 21:42
  • $\begingroup$ I think this idea was used in a Star Wars comic, that was moved to Legends. If I remember correctly, the city was built ontop a few hundrets AT-ATs, and commanded by Lando Calrissian. $\endgroup$ – DarthDonut Mar 8 at 8:08
  • $\begingroup$ I have seen cities like this proposed for Mercury. $\endgroup$ – Willk Mar 8 at 22:57
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I'm a bit of a fan of the Mortal engines series, and many of these questions are answered in in the prequel "Scriveners Moon" and supplementary book "The Traction Codex".

According to that story line, the ancients (people near-future from us with orbital weapons etc.) nuked the world into oblivion, including managing to crack the earths crust, or in some other way destabilize the ground. This meant that there were lots of earthquakes, tidal waves, volcano's and semi-sentient weapons sweeping around the world. To deal with this, people became nomadic and wandered around in tribes. But because they were still reasonably advanced they could build vehicles. The vehicles started small, more akin to a bunch of camper-vans, but because there was combat over surviving resources, these advanced into "traction forts". From there it was an arms race of bigger and bigger until the monstrosities you come across in the main Mortal Engines quartet. Eventually the earth re-stabilized (with a few new mountain chains) but the traction cities now existed and didn't want to go away.

So, with this backstory:

  • They didn't care about the exological impact because the earth was already a wasteland from war
  • People can change car tires while in an operating vehicle. If you design your moving town right or have the right equipment, you can design it so parts can be replaced without stopping.
  • They had more efficient engines (if I recall correctly this was a plot-line in "Fever Crumb" or "A web of air" - I can't remember which)
  • The towns were big enough they had farms on their towns (and algae tanks on smaller towns)
  • You don't need traffic control if you're actively trying to hunt the other guy.
  • If you're big enough, you can handle most terrains. If you've got a footprint of a square kilometer and have enough weight, most ground is very quickly "flat". In the traction codex it is revealed that "the big tilt" that killed Toms parents happened while London was driving up and over a mountain range: the whole town was at an angle.

So in this case, what was their advantage over other nomadic tribes? You could scavenge for resources of a dying world better if you were in a mobile armored fortress....

Practically, building a mobile city is something we would struggle to do with modern tech, and is almost certainly a terrible idea. But that doesn't stop authors making it sound plausible.

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In the history of mankind, there have been a lot of nomadic societies, whose caravans could be considered a kind of mobile city. They used to settle for a time to exploit the resources of a place (basically for hunting or pastures, since this way of life isn't compatible with agricolture), then when resources were depleted or the climate became unsuitable, they moved to other places In the modern society, since all land (and relative resources) is owned by privates and/or part of a state, it is basically impossible to move freely to look for new resources to exploit (even because all worthy places are densely populated).

Since what you want is a city that can translate itself, not a group of carriages/tents, it is necessary to have a society with a suitable level of technology. So the best scenarios to allow for a mobile city would be
- colonization of a new planet
- aftermath of a catastrophic event (like Mortal Engines)
- a steampunk scenario, with a new run for colonies in underpopulated lands

A mobile city would be necessary if you have to exploit a fast-to-deplete resource (precious wood, places to scavenge in a post-apocalyptic situation), a highly seasonal resource or a mobile resource (migrating animals).
Of course, since on land it would be far easier to use temporary and easy-to-mount-and-dismount housing and trucks rather than translate a complete city on wheels, a mobile city would require some additional reasons to justify.
The best reason that come to my mind (paired with the first and most essential prerequisite, which is a reasonably flat land, like desert, prarie or tundra) would be a big external menace (carnivorous animals, predons, mutants, extreme atmospheric events...) that could attack the city with no notice: a mobile city, with walls and defenses always ready, would be less vulnerable during the shifts

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The planet Mercury is close to being tidally locked, but has a 3:2 resonance with the Sun. A city build on a railroad track that would keep the city safely inside day-night terminator safe zone would not only be practical, but necessary.

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  • $\begingroup$ Why would the city itself be neccessary? $\endgroup$ – Efialtes Mar 8 at 22:31
  • $\begingroup$ Well, if the Earth became uninhabitable for whatever reason we would have to build a colony somewhere. Outside of Mars and the Moon Mercury is one of the few options you would have left. $\endgroup$ – SciFiGuy Mar 8 at 22:35
  • $\begingroup$ That is true... $\endgroup$ – Efialtes Mar 8 at 22:57

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