Planetary ring

A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region. The most spectacular planetary rings known are those around Saturn, but the other three gas giants of the solar system (Jupiter, Uranus and Neptune) possess ring systems of their own.

Recent reports have suggested that the Saturnian moon Rhea may have its own tenuous ring system, which would make it the only moon known to possess a ring system.

Rings are made of tiny objects, from small dust grains to objects of a few km in diameter. They are known to exist around planets, around asteroids and around stars. Some stellar rings are made of hot plasma.

Classification
There are many types of rings.

Planetary rings
In the Solar System, rings are known to exist around giant planets like Jupiter, Saturn, Uranus, Neptune. Also, some asteroids have rings: Charilko and Chiron.

The star 1SWASP J140747.93-394542.6 is known to have a planet with a huge ring system.

There are two types of rings: below or above the Roche Limit. The Roche limit is the line where a moon orbiting a planet (or a planet orbiting a star) will disintegrate. Closer to the planet, the moon will tear apart, will break and form a disk of debris. Therefore, rings that are closer to the planet then the Roche limit, will never collide into a larger body. Dust and asteroids inside the ring will try to collide, but they will be pulled away by the planet's gravity.

Further then the Roche limit, particles inside the rings will try to collide. So, another force is needed to hold them in their place. This force can be a gravitational resonance from passing moons. In other cases (like Saturn's outermost Phoebe ring), particles are too small to have a significant gravity and are too far away one from the other, in order to collide together.

Stellar rings
The star Pleione from the Pleiades rotates so fast, that at its equator, matter almost reaches the escape velocity. The star is surrounded by a disk of plasma, with a specific purple glow.

By composition
The rings of Jupiter and Saturn's Phoebe ring are made of small, rarefied particles. This kind of rings are created when there is a small moon, with weak gravity. Meteorite collisions disrupt matter from the moon surface and throw it into the rings.

The classic rings of Saturn are more compact and made of particles varying in size from dust to a mountain.

Reason for colonization
Why should someone create a colony inside a ring system?

Tourism
One good destination for the rings of Saturn is tourism. An orbital station can be placed inside one of the gaps. From there, tourists will have rides through the ring system. Guided tours will take them on a cruise above the ring plane, with short entries inside them. Adventures will try to drive small, rented speed ships, through the rings.

Driving between the asteroids inside the rings will be a dangerous and extreme sport. Some people will try the more rarefied E ring, while others will go to the dangerous B ring.

We can imagine a group of at least 20 resort stations through Saturn's rings, in different gaps, creating a paradise for adventurers. Other people, not willing for adrenaline, will only stay and watch as the rings slowly move, as they safely stay inside the gap.

Pirates
Another possible destination for the rings is a pirates' hideout. Outlaws, scavengers and mercenaries can hide within the rings. The main disadvantage is that Saturn's rings are large, but very sharp, only a few meters across, so that space police can patrol them from 100 km above them. Nevertheless, our imagination can see in the future, perhaps in another star system, a giant ringed planet, hosting many outlaws in its rings.

The most known rings of Saturn have a width varying from 5 to 50 meters, with an average of 10 meters and a maximum of one km.

Technical challenges
Data from NASA's Cassini probe revealed that the rings are really sharp (less then 1 km). This means that particles inside the rings are close. They move with different speed. Collisions are common, even if probably not violent. Collisions are enough to slowly erode large particles and create more dust. Friction between dust grains will create electrostatic charged particles. So, we can expect that the rings are ionized. Ionized particles will try to settle on objects with a neutral or opposite electrical charge. This means that a spacecraft flying close to the rings will slowly be covered with dust. Saturn's rings are composed of water ice, so the dust will be like a layer of dirty snow, but nevertheless, it will affect the spacecraft.

Also, data from both Cassini and ground based observatories prove that, except for two larger divisions, the other gaps within the ring are not empty. They contain less matter. This means that a safe orbit cannot be obtained by flying through any gap.

There are a few moonlets inside Saturn's rings. They consist of a central body with a diameter less then 30 km, but whose gravity affects matter from a wider area. The moonlets are unable to grow in size, since they are below the Roche limit, but they can create perturbations within the rings. Cassini usually cannot see the moonlets, but their effect on the rings. A large spacecraft with enough mass will act similar to a moonlet, getting covered with icy dust and becoming visible from above the ring.

If there is a gap between the rings, that should be the best place for a station. Gaps are created by a moon orbiting there or by tidal resonances from outer moons. In both cases, orbits are not stable, so that the station will always have to correct its position and orbit.

Depending on what are the rings made of, the colony can harvest their needed raw materials directly from the rings. In case of Saturn, it appears that the rings are made almost only of water ice. In case of Jupiter's diffuse rings, extracting materials from them is not feasible.

The major technical challenge is avoiding collisions. If the particles composing the rings are very small, they still can do some erosion on any spaceship or space suit. Each particle has a different speed. So, a piece of rock with the size of a bullet and with a speed of 0.5 km/s relative to you, will strike your space suit like a bullet, before you even have a chance to see it.

In case of Saturn's large, compact rings, the risks of a collision is obvious. There, it could be possible to build a small space colony inside some of the largest ring asteroids.

Another danger is static dust accretion. Dust can be electrically charged by friction. In the same way, even space suits or walls of a spaceship can be electrically charged. If the ship uses a ion engine, this can result in even higher electric charges. Charged dust can form a layer above the windows and can block your view and your cameras.

Inside the rings, there might be needed another kind of propulsion. With an abundance of matter all around you, you can simply grab dust and small rocks and throw them into the opposite direction from where you want to go. Saturn's rings are made of ice, but you don't need to liquefy the water and separate hydrogen and oxygen.

Update about Saturn's rings
Data from NASA's Cassini probe reveals that the particles inside the rings don't rotate fast. In fact, by the time they rotate around the planet, they rotate by half or a few times around their axis. Basically, this means that each particle rotates around its axis in a few Earth days. This is evidence that violent collisions between particles are rare, because any collision will result in a higher rotation speed. Also, the fact that the rings are so narrow (less then 1 km wide, sometimes only a few meters), suggests that collisions are not violent, because they would result in more changes in the orbital plane. It looks like particles in the rings are aligned in the way that they have the lowest kinetic energy. This is proven by the rings' low temperature. Collisions don't add too much heat to the system. If they did, they would have increased local temperature to a point where Cassini's infrared spectrometer could have detected it.

So, what do we have in our world that might look like the movement inside the rings? I imagine it to be something like the movement of small debris on the surface of a slow-flowing river. Closer to the shore, where currents are slower, you can see particles floating above the water and moving very slow. The further you get from the shore, the faster water will flow. Still, because nowhere water is flowing fast enough, you will never see violent collisions. Particles might touch one to another, they might stick together for a while, but then they will separate and move again with different speeds.

Nobody have seen the rings close-up. Cassini will come very close since December 2016, but still not enough to see the dust inside the main rings.

Let's suppose that there are two particles inside the C ring, noted A and B. Both move on circular orbits. Only that B moves 0.5 km further away from A. I calculated that they will orbit around Saturn nearly in the same timeframe, with a difference of only 12 seconds. Even more, the difference of speed between both particles will only be of 0.15 meters per hour. At this low speed, if you were inside the rings, you would say that they are not moving at all. Also, this low speed explains why small moonlets are able to disrupt local structure of the rings and make larger, visible structures.

Economy
Colonies inside the rings will have a dynamic life. The touristic stations will always need to adjust trajectories, to avoid losing trajectory from the gaps. Extra mass and volume are not needed. They will not have greenhouses to grow plants and all the infrastructure needed to recycle air and water. They will prefer to import all their food, air and all the goods. Tourism will provide all the money needed for the economy. No industrial activity will be feasible, because of the high collision risks.

Also, outlaws will not have the possibilities to grow plants on a large scale. A large base will be exposed to collisions and will become visible. They will need to bring all the goods they need. In this case, smuggling and contraband will power the economy. It is possible that, at some point, on a moon that lies within a ring gap, a trade center can take shape. It will be some sort of pirate capital, where slaves, drugs, prostitution, illegal technology and weapons will be for sale.