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The inner moons of Uranus, including Perdita

Perdita is the second outermost of the inner moons of Uranus. It is a prograde satellite, that orbits Uranus in 0.6 Earth days and has an estimated diameter of 30 km. Perdita is the outermost moon of the Portia group, nine satellites sharing close orbits. It doesn't orbit inside or close to a ring, however, there is a ring further away.

Spectral analysis revealed that Perdita is grey in color and its surface is composed of water ice. At a diameter of only 30 km, it should not be in hydrostatic equilibrium (it should not be spherical). However, as shown in cases of some small moons of Saturn, like Helene or Calypso, dust accretion from rings might create a smooth, almost spherical surface.

Advantages

Perdita should be the best place to build a space station to service the Uranus system. It is known that a larger cargo or passenger ship will require smaller costs then smaller ones per kg of cargo or per passenger. Also, flight windows are not so often. Such ships, which would be very massive, perhaps over 2 km long. One can imagine that landing a large ship on a moon with high gravity will be very hard, if not impossible. Also, such a ship will carry all goods and all people from and to all moons of Uranus at once.

Because of these reasons, it is cheaper to build a large space station on a small moon like Perdita. Other smaller ships will carry both passengers and cargo between the moons of Uranus.

Why Perdita?

There are three categories of moons in the Uranian system. The inner moons are small, usually orbiting close to or within the rings. The large moons, most important for settlers, are in the middle. The outer moons orbit at higher distances and are most probably captured asteroids.

The chosen moon for a station must be close enough to the large moons. It must not have a retrograde or highly elliptical orbit and it must not orbit in a too tilted plain compared with the large moons. All outer satellites are either too far away, retrograde or (in case of Margaret, the only prograde moon), have a too tilted orbit.

On the other hand, the inner moons sometimes orbit inside a ring (like Mab, which is further then Perdita) or are ring shepherds (therefore, too close to a ring). Radiation is not a hazard, since Uranus does not have dense radiation belts, like Jupiter. Another problem with the inner moons, is that they orbit sometimes too close to the planet, which can be difficult and will result in much fuel loss. Since Perdita is the second outer moon of the inner satellites after Mab, it is a good destination.

The chosen moon for a base must be large enough to support all the needed infrastructure. It must have a weak gravity, which will prevent untied containers from flowing into space. Also, a strong gravity will create problems for landing ships. Perdita has enough surface for infrastructure and a small gravity. Portia is larger and could also be used as a base. But, because Portia is closer to Uranus then Perdita, docking there will require additional fuel to break from gravity. The other moons of the Portia group are smaller and have a too weak gravity. Still, Unfortunatelly, Portia orbits within or very close to a ring.

Requirements

At the orbit of Uranus, the Solar Constant is small. Therefore, temperatures are low (about -170 C). There is enough light for the human eye to see, but this light is far not enough to generate all needed electricity. Also, it is possible that some dust from the Mab ring can reach the surface of Perdita.

The Uranian moons don't offer the same conditions as the moons of Jupiter and Saturn. Oberon and Titania have enough gravity to hold an atmosphere for some time, so they can be terraformed. The other moons are too small and might not be able to support an atmosphere for long enough. The Population Limit is also very small. Still, Paraterraforming is possible for nearly all moons. Overall, this means that there will be some passenger and cargo traffic, but not on the scale which is expected for Jovian and Saturnian systems. So, Perdita base should be smaller.

The Base

The Perdita base can be built in a similar way with Himalia and Helene bases that will serve Jupiter and Saturn. However, there are a few particularities. The base will have three separate sectors: support/maintenance, spaceport and storage facility.

Support - Maintenance

The support - maintenance sector is the first to be built. It must provide energy, offer a place to live for workers and passengers, offer medical services and repair ships.

Solar power can be used, but because Uranus is too far away from the Sun, there will never be enough power generated. Solar power can still be used by low energy consuming electronics. NASA is planning a space mission to Uranus that will be solar, so it is possible to use solar energy. However, significant amounts of energy will be used for heating. The base will require a nuclear generator, which will produce the needed heat and electricity. It is good also to have a backup generator.

Because Perdita orbits not too far from a ring, it is possible that its surface is covered with a layer of smooth dust. If this is the case, much of the base should be built underground. This will provide additional thermal insulation and will reduce heating costs. If this technology is used, light from the surface can be harvested with mirrors and lens and sent through glass fibers to the inside.

Uranus is far from the inner planets. Ships that will travel here must resist for longer periods of time in the cold interplanetary environment. A repair facility is mandatory. Before departure, each interplanetary ship must be checked for problems and must be repaired if needed.

An additional maintenance team must repair all systems of the station.

It is good to have backup systems. An emergency nuclear generator will produce the energy needed, to prevent everything from freezing. Also, it is good to have a separate part of the base with its own support systems, where people could be evacuated just in case. An emergency ship can also be stationed, always ready to evacuate the base and reach a nearby moon.

Passengers & Ecosystem

Perdita orbits Uranus in 0.6 days. Therefore, flight windows to the other moons are frequent. Flight windows between Uranus and other planets are not so frequent, varying between roughly 90 days to Mercury to many years for Neptune. In this context, passengers and cargo will not stay for long on Perdita. People will stay for a maximum of a day, then will come long periods of time when the station will only be used for shipping goods between the moons of Uranus.

Still, there will be workers on the station that will stay there all time.

Perdita station must have resources of food, water and oxygen for bulk periods. Passengers will require at least a meal. Given the very low temperatures, everything can be kept frozen for a very long time. In addition, storage tanks are needed for carbon dioxide, used water and human dejections. These by-products can also be stored frozen.

It is questionable if Perdita base will require its own ecosystem or if it will prefer to import food and oxygen from other moons. Anyway, since the moon is made mostly of water ice, it is possible to produce drinking water and oxygen in situ. One thing is certain, that the base will require its own facilities to recycle water, to filter air and to store reserve food.

There will be apartments for station employees. Since passengers will not stay for long, they will not require their own apartments. still, a hotel should be useful. In addition, there will be restaurants.

A medical center is required to handle emergencies and to check the health of those who travel from regions with epidemic risks.

Between interplanetary flights, there will not be significant traffic. Still, because flight windows occur almost daily, there will be many people traveling between two moons with a stop on Perdita.

All buildings, on the surface or underground, need to have a good thermal insulation.

Commerce

The main activities will be transport and commerce.

Trade routes

The spaceport must be designed in such a way that it will handle all traffic. For Uranus, the Trade Routes, dictated by flight windows, are as follow:

Internal flight windows:

Perdita - Puck: 1.19 Earth days
Perdita - Miranda: 0.963 Earth days
Perdita - Ariel: 0.832 Earth days
Perdita - Umbriel: 0.743 Earth days
Perdita - Titania: 0.665 Earth days
Perdita - Oberon: 0.657 Earth days
Perdita - Sycorax: 0.633 Earth days. 

Basically, there will be daily flights to all moons, resulting in roughly 7 launches during an Earth day and 14 flight events (landing, liftoff) daily.

Flight windows to rocky planets:

Mercury - Uranus: 88 Earth days
Venus - Uranus: 227 Earth days
Earth - Uranus: 369 Earth days
Mars - Uranus: 702 Earth days

Flight windows to the other gas giants:

Ceres - Uranus: 4.88 Earth years 
Jupiter - Uranus: 13.8 Earth years
Saturn - Uranus: 61.8 Earth years 
Uranus - Neptune: 174 Earth years. 

When flight windows occur at more then 1000 days, a ship must fly even when planets are not aligned. So, if a ship is launched each time it has a flight window or every 1000 days when it doesn't have a good flight window, we get 8.73 launches during an Earth year or a launch at roughly 42 Earth days. Interplanetary flight events (arrivals and launches) will occur roughly every 21 Earth days.

These ideal flight windows are calculated using only orbit characteristics and do not account for gravity assists. Additional windows do exist.

As one can see, flight windows between Perdita and the other moons occur daily. So, a passenger will need to wait at maximum an Earth day to catch the next flight. Flight windows to the inner planets are not rare, but not too often. Between gas giants, flight windows are not frequent. There, spaceships will prefer to travel using a more frequent schedule, probably each 1000 Earth days, at the expense of fuel.

Interplanetary ships will land and wait for the next flight window to depart. While doing so, they will be moved to a special area.

Local ships will come and wait for maximum an Earth day for the flight window to depart. They will be more common, especially when an interplanetary ship should come or go.

Delta-v

The Delta-v shows the amount of energy needed for a ship to travel.

Delta-v requirements for Perdita station are shown below:

Mercury Space Station – Uranus Perdita 28.699 
Venus Space Station – Uranus Perdita 23.786 
Earth Space Station – Uranus Perdita 21.763 
Mars Phobos – Uranus Perdita 17.786 
Ceres Space Station – Uranus Perdita 13.323 
Jupiter Himalia – Uranus Perdita 11.323 
Saturn Helene – Uranus Perdita 11.335 
Uranus Perdita – Neptune Halimede 5.535 
Uranus Perdita – Pluto Styx 5.918 
Uranus Perdita – Eris low orbit 7.097 
Uranus Perdita – Sedna low orbit 7.245 
Uranus orbit (1000 km) – Uranus clouds 15.385 
Uranus orbit – Perdita 5.744 
Perdita – Puck 0.501 
Perdita – Miranda 2.115 
Perdita – Ariel 3.679 
Perdita – Umbriel 4.288 
Perdita – Titania 5.119 
Perdita – Oberon 5.334 
Perdita – Sycorax 4.266 

For comparison, an Earth surface - Moon surface flight will require a delta-v of 14.466 km/s.

Perdita lies close to Uranus, deep in the Uranian gravity well. Because of this, any ship that attempts to reach Sun orbit will have to use more energy. Still, this energy requirement is not huge. Little energy can be saved by taking a gravity assist from Oberon or Titania.

Flights to the inner planets require a lot of energy, but delta-v can be reduced by taking gravity assists of Venus, Earth, Jupiter and Saturn, when this is possible. Also, flight routes to other gas giants and the Kuiper Belt require little energy, but the correct alignment is rare. Such flights, at the lowest delta-v budget, can take decades if not centuries. Ships will prefer to sacrifice more fuel in order to get their payloads in time.

Perdita is close to Uranus and not in the middle of the large moons. Because of this, internal traffic will not come too often to Perdita. Most local ships will travel from a moon to another. Not the same can be said about missions to the outer and the inner moons. Because Perdita is an inner moon, ships traveling to other inner moons (like Cordelia, Mab or Portia) will often dock at Perdita. Ships that travel to the outer, irregular moons, will most often travel to and from Perdita, because their delta-v requirements are smaller to travel to and from Perdita then to travel to and from another moon (for example Titania). Because of this, we can expect that Perdita will be significantly involved in commerce with the outer and inner moons, with missions towards Uranus itself and with interplanetary flights.

Spaceport

The spaceport will require at least two platforms for interplanetary ships and up to seven smaller platforms for local ships. Within the Uranian system, there could be passenger, cargo or mixed ships. However, for interplanetary trade, given the huge distances, mixed ships should be more common. This creates a complex problem, because the same terminal must handle both passengers and cargo.

People can be carried between the base and their ships with the help of small flying cars. Also, the use of underground tunnels is an option, but these tunnels need to be heated with significant energy costs.

Cargo containers are at present largely used on Earth. They tend to have similar sizes. It is expected that cargo containers will also be used in the space age. They will be lifted from each ship, transported to a storage area and loaded back with the help of special cranes. Because of the low gravity, each crane must handle its weight in a different way. Cranes don't need to be strong, since objects will not weight too much in low gravity. However, low gravity means high inertial movements. Containers will also need to be tied to the ground.

A special problem will be with containers that need to preserve a certain temperature inside. They need to be carried out fast and moved to a special storage area, where they will be connected to electricity. Frozen staff can be transported very easy.

Fluids are a significant problem because at the orbit of Uranus it is very cold. Only hydrogen, helium and a few gasses can be transported in liquid state and deposited as so. For most of the liquids, transport must be done in solid state.

Take into account the next scenario: Someone wants to send wine to Uranus. At high costs, the wine can be transported in glass bottles, kept into heated containers. A much cheaper alternative could be to handle the wine frozen in plastic recipients. however, without petroleum, there will not be enough plastic. So, a third alternative is to carry all the wine as large cubes of ice, which later can be melted and poured into bottles. In this scenario, all wine is shipped as large frozen cubes, while the bottles are shipped separately. Somewhere in the Uranian system, wine needs to be melted and poured into bottles.

Storage Facility

The base will require a special place to store ships and cargo. The presence of nearby Mab ring is not a problem on the short time. However, on a longer timescale, small impacts can be a problem.

Most ships waiting for a flight window can remain on surface. However, in some cases, at least certain parts of a ship must be token underground. In case of cargo, some materials (like mineral ores or metals) can be left on surface. in case of containers, it should be recommended to store them in a safe underground place.

A special storage place will be for containers which require a certain temperature. The best solution will be a heated underground chamber. It is important to remember that Perdita is made of water ice and maybe frozen gasses, which will melt or at least sublimate. The chamber must be insulated.

Telecommunication

Just like an airport, Perdita must have very good communication systems. There will be interconnected relay stations all over the moon. It is very important to communicate with each ship. In many situations, people might travel in cryogenic sleep inside interplanetary ships. There is a chance that even the pilots will be sleeping during the flight. To prevent a disaster, most of the ships will have automated pilot turned on. Perdita base must be able to interact with the automated pilots when needed.

Strategic Importance

Because nearly all external passenger and fright will come and go through Perdita, the station is highly important.

The station is a critical control point for illegal trade of drugs and weapons. Criminals will also try to enter or exit via Perdita base. Therefore, it is important to maintain a strict control.

Located close to the center of the Uranian system and with frequent flying windows to all other moons, Perdita is very important for strategic missions. If Uranus will be a dictatorship, a quick response military unit can be stationed here. If not, a rescue unit can be located instead.

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