# City Design: Freefall

The Setting:

A Gas Torus around a star. It has enough atmosphere and elements for earth-like life, and is roughly analogous (plants, animals, etc). Small asteroids are present throughout torus, but nothing large enough to collect significant gravity. Life does grow on and use these asteroids for material. Water is present throughout, but is rare enough that generally the entire setting acts as an arid desert because there's no natural gravity-based water cycle. There's no night or day, but uneven heating plus orbital rotation does mean that the torus experiences things we'd call weather - wind and storms.

It's absolutely huge in scale, being a ring in the habitable zone around the entire star. Think thousands of earths in volume, with enough atmosphere to allow radiation protection and still have a fairly large habital area.

The People:

A sapient species has developed in this torus, and are native to a zero gravity situation. They can fly, have manipulating structures similar to our fingers and have roughly the same visual spectrum that we do. They're significantly larger than us, but not by too much because they do have to worry about moving their mass around even if they don't weigh anything. Something on the rough order as the same mass of a horse. Like us, they developed as cursorial hunters, and have recognizable social structures (if you think it matters, use western first world cultures as the baseline).

The Tech:

Pre-computer age - early 1900s America. They have domesticated animals for work, transport and food. If you want to put in some variation on tech, that's fine. Metals will be rarer than on Earth because they're more difficult to mine and smelt in free fall. Petroleum is available but very rare. Power is primarily generated through wind turbines and solar (no night, and they get some crazy wind).

The Problem:

What kind of cities would this species build? Specifically:

1. Would they have sky roads and buildings, or some other kind of organized structure?
2. How would they handle city logistics - water, power, trash, sewers?
3. How would they keep the city together and keep it from drifting apart?
4. Human architectural styles differ considerably based on culture, but are there any specific architectures that this species might develop as a result of their freefall environment?

The Notes:

As described I suspect this setting will require significant handwavium to keep from following apart. Answers should still be as science-based as possible, no outright magic.

• This is just impossible actually. – Anixx Apr 8 '15 at 18:38
• @Anixx: Planetary masses of handwavium? I'm just looking for "reasonable on the surface" here, I know the orbital mechanics and such would make the entire thing fall apart. – Dan Smolinske Apr 8 '15 at 18:50
• @Anixx I'm curious why. When Larry Niven wrote about a world like this in "The Integral Trees" and "The Smoke Ring", I thought he'd done his research. – BrettFromLA Apr 8 '15 at 18:50
• @Brett Juilly impossible to keep gas under pressure in empty space without gravity or constant (very powerful for Earth's pressure) source. – Anixx Apr 8 '15 at 18:58
• @BrettFromLA asteroids usually do not interact with each other (or interact via attraction) unlike the gas molecules at Earth's pressue which will interact frequently, chaotically and fill the whole available volume. – Anixx Apr 8 '15 at 22:36

In order to answer all the problems you pose, you must first understand the nature of your environment.

Everything in your proposed environment is in orbit around the primary. This means many of the fundamental notion of motion, forces, and propulsion to which we've become accustomed on Earth will not work the same way.

The Motion
Some principals of motion for things in orbit.

• If you release something (and it applies no other propulsion), you and it will intersect in exactly 1 orbit.
• Once two object's orbits intersect, they'll continue intersecting in every orbit until something exchanges momentum with one or the other of them.

So getting rid of garbage would seem as easy as extending a cable with the garbage on the end until tidal forces are sufficient to pull them away from the habitat. Unfortunately, you and that garbage will return to the same orbit at the same time every full orbit. Ultimately, you'll need to provide that garbage with some ability to change its orbit.

Thought experiment: try to get rid of garbage using tidal forces and a parachute

1. Create a long cable
2. Put the garbage bundle on the end of the cable
3. Drop the bundle either inward or outward (for our purposes the results are the same)
4. Play the cable out to its end
5. Tidal forces are pulling the bundle away from you (the amount is proportional to the $\frac {M_primary}{r^3}$ )
6. With this configuration, the bundle keeps returning to the same spot at the same time you do
7. If you included a parachute, the winds inward of your habitat will be faster due to its smaller orbit.
8. This will tend to add kinetic energy to the bundle.
9. This will tend to push the bundle into a wider orbit.
10. Which pushes it back into your habitat.

In this case, parachutes are bad for garbage but good for a man overboard situation.

What you need instead is a small propulsion unit to alter the trajectory of the garbage package (ideally $\frac {1}{2}$ orbit from the habitat but doing so shortly after release would be fine. Since the region is so big, the garbage need not have a powerful propulsive unit.

A good mnemonic to remember how the changing momentum changes your position relative to other nearby objects:

• Thrusting outwards moves you anti-spinward.
• Thrusting inwards moves you spinward.
• Thrusting spinward moves you outward.
• Thrusting anti-spinward moves you inward.

The Forces
To elaborate on the previous answer,

Spinward & antispinward
components will be subject to mild compressive and tensile forces. Pipe and wire connectors should be sufficient to hold members in place and provide for "pedestrian" & elevator traffic.

Inward & outward
These forces will be bigger than the other forces. The equation that governs this force is the tidal force calculation.

$$F = m\Delta rG\frac{M}{R^3}$$
F - Force on the body away from it's center of mass
m - Mass of the smaller body
$\Delta r$ - Distance between center of mass of the small body to its edge
G - Gravitational constant
M - Mass of the larger body (e.g. star)
R - Distance between center of mass of the two bodies

Components will be subject to mild tensile forces. Wire only connectors should be sufficient to hold members in place and provide for "pedestrian" & elevator traffic.

Northward & southward
Components will be subject to mild compressive forces. Pipe and wire connectors will be necessary (pipes for the compressive forces, wires for stability).

• All "cities" need to have each component physically connected or the pieces will drift away.
• Because the forces are small very small compressive members and thin cables, wires, tethers, ropes should be sufficient to keep outer pieces of the city in place.
• Connectors for components of the inner city may need to be substantially beefier in order to take their own forces as well as those of any components latched onto them.
• Air breathing propulsion is possible (jet engines, propellers, and wings). "Sky roads" in the city will be by way of the connectors - they could be as simple as hand holds for "pedestrians" or cable elevators or something else entirely.
• "Sky roads" beyond the city will require some sort of engine (you could go steam punk and make them all steam powered rockets or some such).
• I would think it'd be crazy to try to make a jump without knowing your destination, so the amount of $\Delta V$ required should be < 25 m/s.
• The city could harvest water by simply firing a harpoon with a tether into it. I imagine surface tension forces ought to wick the water along the tether
• The city's food would have to be cultivated in "farms" attached to the city. This would require much more work to "capture" and anchor to the city.

I would imagine the architecture would be open, 3 dimensional, and spacious. The place would have room to spare and then some. City expansion would depend much more upon the ability to capture and harvest resources floating around.

Think about this as you would a space station, you have to go and find everything that you need, maneuver it close enough to your city to utilize those resources, and then anchor it there. The original kernel of the city likely formed around especially valuable treasure trove of resources (dirt/mud ball, water ball, or tangle of native life and its "dirt"). This could still be around or totally used up by now.

It might experience "feast & famine" / "boom & bust" episodes as newly anchored resources leads to abundance. Then the famine sets in when they are used up and while you're waiting to capture the next resource. There might be periods where the city encounters lots of water but no soil or metals (or vice versa).

Steampunk like "airships" (only these don't require balloons to make them float) might ply the space scouting for more resources.

• "East takes you out. Out takes you west. West takes you in. ln takes you east. North and south bring you back." Also, really nice answer. – Dan Smolinske Apr 10 '15 at 14:46

I foresee hollow protoplanets!

Since this species develops in a free-fall environment within a space-based cloud of gas littered with resource-rich/-poor asteroids, the first major structures will be built on the asteroids for mining operations. Individuals rarely want to travel long distances to get to work (long is, of course, relative in space), so housing will be built on the same asteroid as the mining facility. As the facility grows, it will need access to more resources, so either the company will start another mining facility or haul other asteroids to an existing one.

Clumps of asteroids then become the foundation for larger settlements, which attract more attention, more business, and more inhabitants, which bring in more asteroids to support the growing population.

Independent facilities will produce a similar effect, though in this case "clumps" will be individual asteroids connected via trade routes. The collections will attract attention, which brings in business, inhabitants, and more facilities. To prevent unwanted drifting, asteroids would be tethered together to emphasize established trade and transport routes.

In either situation, the city is developing in three dimensions, expanding roughly evenly in all directions. This produces a spherical structure, with the oldest buildings at the center.

Tethers and nuts and bolts should be quite capable of holding things together; after all, they do quite well under constant gravitational stress from Earth.

The common architecture is going to be something much like what's on the ISS. There's no real down direction, so doors can be in any flat surface, possibly even in cones, corners, or hemispheres. In the case of asteroid clusters, paths of travel will be delineated by the structures themselves, where walls and solid connectors work together to create tunnels through the whole structure. In the case of asteroid collections, paths of travel will, in the macroscale, be defined by the inter-asteroid tethers, while local conditions will be like those for asteroid clusters.

The tethers and pipes used to connect structures and asteroids can double as infrastructure. Tethers can provide electricity and fiber optic cable. Pipes supply fresh water and remove waste.

(All of this ignores the viability question of your scenario, which I am in no way capable of answering.)

The setting is actually more predisposed for the creatures to develop a nomadic hunter/gatherer existence rather than an urbanized one.

Since things tend to stay where they are in a free fall environment, one of the issues confronting any sort of life form is getting away from wastes. Unless a large impulse is given to the waste, it will remain in the same orbit that you are in. This is true for everything from exhaled C02 (assuming an oxygen metabolism like terrestrial creatures) to urine and feces to crumbs and dead bodies; hardly what you want to have hanging around next to you.

The ecosystem will then have to consist of various life forms devoted to scavenging wastes, plants that feed off clumps of "processed" matter (possibly resembling venus fly traps or other predatory plants on Earth), and highly mobile herbivores followed by equally mobile carnivores. This will resemble the ecosystem of an ocean more than anything else.

Your intelligent creatures may resemble schools of squid, which can organize to hunt schools of "fish", and eventually develop intelligence and a form of nomadic civilization.

Structures, if needed at all, will resemble nets of various size to keep important items secure in free fall and accessible to the creatures when needed. Since they will probably have evolved some sort of organized "school" structure (placing the young adult males on the outside to guard the more vulnerable females and young, for example), more permanent structures may be developed to mimic this (birthing rooms in the centre, and rooms radiating outwards organized by age, sex and status). To prevent the accumulation of waste, it will resemble a series of "bird cages" rather than a building the way we recognize it.

Slightly different ecosystems will develop around asteroids, and creatures evolved around there will resemble the creatures living in a reef, predisposed to hide in caves, nooks and crannies either for protection or to hunt their prey ambush style. If intelligence evolves here, then expect a predisposition to cave dwelling.

Concept
Or for another perspective is based upon The Millennial Project: Colonizing the Galaxy in Eight Easy Steps - specifically the section on a space colony (I believe it is Step 4).

Consider the much of the terrestrial radiation shielding comes from its thick atmosphere. If you wished to have an extraterrestrial habitat with the same level of radiation shielding, then it would need about the same amount of radiation shielding.

You can create such radiation protection out of gases if you don't mind it being hundreds of miles thick. However, you could build it just 32 feet thick if you use water instead.

So imagine a giant hollow water drop. The water serves many essential purposes with radiation shielding being one of them. Interestingly, the water is transparent so it will transmit light for any gardens you wish to grow inside your water drop.

Your hollow water drop will require membranes (hopefully self-sealing) on both the outer and inner surfaces to keep the water where you want it.

Scaling
When I ran through the calculations to find a body whose self-gravitation would completely compensate for an interior membrane pressure of 1 atmosphere, I got something ridiculous like r = 32,000 km. This would be about the size and mass of Uranus. Clearly most of the space of such a large body would be unusable (either that or I made an error in my calculations).

So it'd be better if you instead scaled the thing so that the center was still habitable. Unfortunately, the integration requires me to do too much work so let's hope someone else will do it for us.

My guess is that it could be as large as 200 - 400 km in diameter.

• I'm expecting this to be more on the scale of a full-sized Ringworld since it's around the full star, with the volume of at least thousands, if not millions, of earths. – Dan Smolinske Apr 9 '15 at 19:54
• I figured I'd throw out a couple of other ideas. A (small) planet sized habitat would make an interesting setting. With the clear outer membrane, you'd still see its Sun and stars. Plus, the inside would be a microgravity environment. – Jim2B Apr 10 '15 at 0:04
• I don't see what this answernhas to do with the smoke ring habitat of the Question. – JDługosz Apr 10 '15 at 2:03
• Almost every aspect of living a smoke ring type habitat will be shared by this type of environment. The main exception is the size. – Jim2B Apr 10 '15 at 14:37