Himalia, as seen by Cassini
Himalia is the innermost outer moon of Jupiter. It is an irregular, prograde satellite. We know that its diameter is about 150 km, which suggests it has an irregular shape. Density is estimated at 2.6 g per cubic centimeter. It orbits at 9.8 to 13.1 million km from Jupiter and needs 251 Earth days to orbit around the giant planet [1].
Because it is cheaper to send a single large ship to Himalia then to send four ships to all Galilean moons, it would be more feasible to build a base on Himalia, to handle cargo and passenger traffic. Large ships land more harder then small ones, so a cargo freighter can travel between an Earth orbital base and a Himalian base, while small ships will do the landing on Earth and the Galilean moons. So, for economical reasons, it is less expensive to build a base here. Compared to other Jovian small moons, Himalia seems the best place for this.
Advantages
Himalia is the second closest outer moon to the Galilean moons. The closest is Themisto, but that moon has some major disadvantages (see below).
Another advantage is that Himalia is the 6th largest moon of Jupiter (at least by mass), after the Galilean moons and Almathea. So, it has some gravity. An astronaut could jump on the surface of Himalia without the risk of getting lost into space. Still, the low gravity allows spacecrafts to land and departure without a significant fuel consumption.
Given its relatively high density, we can speculate that Himalia has a more rocky structure, containing less water and volatiles. There is also a chance that even metals can be found in its rocks. The high density suggests that Himalia is not fragile (so it will not break apart easy) and is not a captured runaway object from the Kuiper Belt.
Because it orbits at a high distance, Himalia is not significantly affected by Jupiter's magnetosphere. In fact, for a long part of its orbit, the moon is bombarded by the Solar wind, which is far weaker around Jupiter then at Earth's orbit. Therefore, a base is much safer.
Its orbit is prograde and it is the closest outer moon after Themisto. Retrograde moons are not good for bases, because spaceships traveling from or to the Galilean moons will need more fuel to change their movement. The most outer moons don't have very safe orbits and actions from the human colonies (for example a gas leak) might push them away into a heliocentric orbit.
Why not Themisto
Themisto is in fact the closest of all the outer moons of Jupiter. However, Themisto is far smaller and has a very low gravity. Compared to Himalia, if an astronomer jumps on the surface of Themisto, he will end-up as a satellite of the moon or will escape into Jupiter orbit. Many objects from the base could be lost in orbit and become space debris, only because of very small forces on their surface (for example, strong vibrations). Another major problem is that Themisto has a very tilted orbital path, taking it almost over Jupiter's poles. For a space ferry connecting Themisto base with the Galilean moons, this would result in more fuel consumption then if the base was built on Himalia instead.
Himalia is part of the Himalia Moon Family (5 moons with very close orbits). A theory suggests that all 5 were once part of a larger moon that broke apart after an impact. If so, then there is a high chance there are also debris left after the impact. However, the collisions might have occurred a long time ago and much of the dust and other small particles might have been blown away by solar wind and gravitational interference from Jupiter and other moons. Given the vast area of space where Himalia orbits, these particles should be dispersed on a large area and their density should be small. It is known that small moons lose matter after meteoric impacts and create rings (it is known the huge Phoebe ring created by Saturn's moon Phoebe, also Jupiter's rings are created by the inner moons). However, many spaceships have passed through these faint rings and had no problem. Particles from the ring orbit move around the planet with a similar speed and on a similar path with the moon they came from. So, their impact on a base located on the surface of the moon will be small. Themisto, on the other hand, orbits alone and there is a far smaller chance for debris to exist close to its orbit.
Colonization
Given its small gravity, Himalia is far too small to support an atmosphere. So, terraforming is impossible.
The Base
The base on Himalia should be able to handle cargo and passenger traffic. It must have a large number of platforms for ships. Given its low gravity, it should be good if the platforms are equipped with devices able to lock a spaceship on their surface. For passenger traffic, each ship must be parked close to a passage to the base building (most probably an underground tunnel, heated and full with air).
Cargo transshipment. For cargo, there must be all facilities needed. However, cranes do not need to be as resistant as those found on Earth. The low gravity (0.006g) means that a human weighting 80 kg on Earth, on Himalia will be only 0.5 kg. However, the low gravity will not be strong enough to stop the inertia. An object that is moving up will need a lot more time to stop and return down. Cargo containers will need to be anchored by two different cranes and to the ground. Shipping open containers will not be an option, since their cargo will be lost thanks to the low inertia.
If we want to move cargo on the ground, we must think that the vehicles will also lose their way. Low gravity means low friction and that means a vehicle will accelerate or brake very hard. The best vehicles should be similar to trains in amusement parks, where the wheels are fixed both above and below the railway.
Overall, cargo transshipment will be more complicated then on Earth, but given the low gravity, buildings, cranes and rails will not need to be designed as robust as on Earth. A new, special design will be needed for the cargo docks, but still, the investment will be far lower then what we could expect on a planet with significant gravity. Also, if anything goes bad and a cargo container is lost, since Himalia has some gravity, it will fall back on the surface of the moon and not into outer space.
Fluids transshipment. Liquids and gasses can be moved more easy, through pipes. Himalia has no atmosphere, so during daytime temperature rises to round -100 C, but during night, it falls below -200. The pipe infrastructure should be built underground, where temperature is constant. Some fluids might freeze at Himalia's temperatures. It would be required to heat them up and also heat the pipes.
Liquids and gasses might need to be stored on the moon before another spaceship comes and takes them. So, large tanks will be needed. It should be better to build these tanks underground, where they will not be exposed to meteorites and solar radiation.
Waiting time. It is more cheaper to send a big spaceship towards Jupiter then to send 4 small ships to all the 4 large moons. Best launch windows for Trade Routes occur about every 90 Earth days for Mercury, 237 days for Venus, 398 days for Earth and 816 days for Mars. For the outer planets, the best alignments occur at 13 to 20 Earth years. Best launch windows from and towards the Galilean moons occur at maximum 20 Earth days. Perfect alignments occur with Almathea daily, with Io every two days, with Europa at roughly 4 days, with Ganymede every 8 days and at about every 17 days. Passengers can move from one ship to another in a few minutes, but cargo will require from hours to days. We can suspect that at least for the beginning cargo ships will travel towards the inner planets when the best alignment is achieved. Maybe towards Earth, there will be more ships. Transport to the other giant planets will be done probably every 1000 days, given the fact that the best alignments will occur too rare. Flights towards asteroids (for example Jupiter's Trojans) and the Kuiper Belt will be conducted occasionally.
At least for the beginning, passenger traffic will follow the same schedule. We might also see mixed passenger-cargo ships. People and freight will need to wait up to 20 days on Himalia, before and after the best launch windows. This is still little compared to the years needed for interplanetary flight, but still requires a lot of logistic challenges.
Cargo will be shipped to all directions (for example minerals from asteroids heading to Galilean moons, required for terraforming, technology from industrial corporations located on asteroids and outer moons, goods sent to other planets and minerals required for life exported to other planets, maybe also water sent to Moon, Mars, Venus and Mercury). However, people will come and go more often to and from Earth.
Passengers. The most important passenger route will connect Earth. We will see probably around 30 passengers on Jupiter - Venus or Jupiter - Saturn journeys, but thousands of settlers coming from Earth or going back to Earth. Also, when no correct alignment occurs, there will be no passenger transport for lengthy periods of time. So, on the Himalian base, there will be periods of time when nobody is there and periods of time when thousands will be crowded and waiting for departure. All of them need air, need clean water and food. Also, to compensate the low gravity, people will need to be placed in rotating chambers or will request other methods for artificial gravity. Another way is to force them to do physical exercises, but that also results in food, air and water consumption.
There are two ways to provide people with food, water and air. One way is to send the goods from a nearby terraformed moon (for example Ganymede), but that requires additional transport costs. Another way is to produce all the goods there, on Himalia, but that also requires the construction of a large closed ecosystem. Food and clean water can be stored with no problems, thanks to the low temperatures encountered on the moon.
On the other hand, people will also require spaces to spend the time, they cannot be kept in overcrowded rooms all the time. They might sleep 20 in a room, but they will want to practice some activities. A park inside a dome, some clubs and other places to hangout, a stock exchange center, a church and a mosque should be also built. Having 3000 people inside a closed environment also can cause medical problems. A hospital or at least a clinic would be needed. The fact that the place will be overcrowded for a limited time, followed by periods with little traffic and also periods of time without activity, means that many employees will have a part-time job.
As time will pass and new, faster transportation methods will appear, the flight windows will change. Ships will not wait for the best planetary alignment and we will probably see ships departing every 10 Earth days. Traffic will change, people and cargo will be seen all the time and in almost the same amount. They will not stay so many days here and employees will work all year round.
Energy. In past, all spaceships launched for Jupiter were nuclear powered. Juno is the first solar-powered. To power-up the base on Himalia, we will need huge solar panels, probably covering a large part of the moon surface. Solar panels will be exposed to radiation and impacts. Using a large nuclear plant is another option, that is also feasible. Residual heat from the plant can be used to heat the base, while solar panels generate only electricity. Given the fact that there will be many people and a lot of goods, providing the base with energy is vital. A backup generator must exist.
Insulation. The base on Himalia must be well insulated, to preserve heat. Outside, temperatures of -150 C are deadly for humans. Also, there is always the risk of impacts, which can result in air leaks. A good way to protect the people is to build underground. A good insulation is required, still.
Probably, only a limited number of buildings will be on the surface, the rest will be underground. This will ensure protection from impacts, solar storms and cosmic rays.
Telecommunications. The base will use many deep space and low range antennas, to communicate with other Jovian bases, with other planets and moons and with spaceships around it. Antennas will be placed in many locations. Since Himalia is not round, its rotation might be chaotic. We must always keep contact with everyone.
Trade Routes
Main article: Trade Routes.
From and to Jupiter, flight windows occur at the following frequency:
Mercury - Jupiter: 90 Earth days Venus - Jupiter: 237 Earth days Earth - Jupiter: 398 Earth days Mars - Jupiter: 816 Earth days Ceres - Jupiter: 7.53 Earth years Jupiter - Saturn: 19.9 Earth years Jupiter - Uranus: 13.8 Earth years Jupiter - Neptune: 12.8 Earth years.
Please note that all data is calculated based on alignment of the two planets and does not account for a gravity assist from another celestial body or complex deep space maneuvers. As one can see, flight windows are more frequent towards the inner, rocky planets. Orbital alignments between Jupiter and the other gas giants are rare. If no perfect alignment occurs, ships will still travel on longer and more expensive routes, probably once every 1000 Earth days. Flight windows between the moons of Jupiter are more frequent:
Almathea: about twice every Earth day Io - Himalia: 1.78 Earth days Europa - Himalia: 3.60 Earth days Ganymede - Himalia: 7.36 Earth days Callisto - Himalia: 17.9 Earth days
Suppose that an interplanetary ship will come each time we have alignment or each every 1000 Earth days when we don't have alignment. This will automatically mean that Himalia base will receive 8.38 ships every Earth year or a ship every 44 Earth days. Since these ships will also need to depart, we will see an arrival/launch every 22 Earth days.
Between the moons of Jupiter, we will also see many ships. In case of Almathea, there will probably be a ship every Earth day. Combined, there will be 1016 ships landing every Earth year, totaling for 2.78 landings every Earth day. That is roughly 5.6 landings and departures per Earth day.
If there will be a single ship for each flight window, one platform for interplanetary ships and two for local ships should be enough. However, if ships come and go in bulk, there will be more landing platforms needed.
Strategic Importance
Himalia has strategic importance. It is the best place for building an interplanetary base inside the Jovian System. The inner small moons (like Almathea) are exposed to Jupiter's strong magnetosphere. The strong radiation will endanger human life and equipment. On the other side, Himalia is far enough from Jupiter, but still not that far as other outer moons.
During terraforming processes on the Galilean moons, Himalia will serve as an important base, where cargo ships, carrying materials and equipment will stop. From there, small ships can take all the cargo to each moon. After terraforming, Himalia will probably be used as a large passenger and cargo base. It will be more easy to handle a single, large ship, to connect Earth and Jupiter, then to use four small ships, one for each Galilean moon.
If at some point Jupiter System becomes an independent state, Himalia will be its gateway to the Universe.