Exploring future worlds, terraforming, colonization and future trade will be very hard to do without an army of space stations, operating for various purposes, inside every solar system and probably orbiting every planet.
Today, cargo is shipped by sea worldwide. However, from the final port, cargo is carried by train or by large trucks to storage facilities. From there, smaller cars take the goods to each part of any town. In future, we can see giant interplanetary ships, over 1 km long, far too massive to land on a planet. They will transport much of the cargo from one planet to another, through space, but will be unable to land.
Another reason for building space stations is to offer an alternative for quarantine or emergency situations, for ships that cannot or are not allowed to land on a planet.
A third reason is that a space station can keep a closer eye on terraforming processes and on scientific research on the surface.
A fourth reason is that a military station will be vital for protecting a planet against meteorites and against foreign armies, if space wars are to become reality.
We are already using a space station in low orbit around Earth to facilitate space exploration, so it would by logical that we will build future stations around other planets.
There will be many kinds of stations:
Each station will be built at a certain moment and will be increased in size by time:
Before terraforming. At the beginning, scientists will send robotic and manned ships to a planet, to see if it can be terraformed. At this phase, they will build a small orbital station, where astronauts can return fast from the planet if anything goes bad. When humans landed on Moon, only the lander touched the surface, while the backup ship remained in orbit. Maybe, at the beginning, the orbital vehicle will be only there for some time, but as research efforts increase, there will be needed a permanent station.
During terraforming. When we decide to terraform a planet, a larger base will be needed. Let's take a single example, the most studied planet Mars. We will need to build a Terraforming Plant on the surface and to bring all ingredients needed (including extra water, greenhouse gases, additional chemical compounds where the amount of minerals is not good for Earth-like planets). The terraforming plant will be huge, covering maybe 100 square km. Some parts can be built with materials found there on the planet, but some parts cannot. We will need to ship at least one cubic km of pre-built material and equipment from Earth. The entire transport from Earth to Mars will require 3 different kind of ships and two stations. The first ships will take cargo from Earth to Earth's space station, then they will land back. Then, other ships, slower and larger, probably equipped with ion engines, will carry everything to the orbital station around Mars. A third model of ships will take cargo from Mars' station to the terraforming plant, then they will return to the station. Mars will also need water, that can be obtained from Kuiper Belt, by diverting planets. However, there will be other ingredients needed for life. What if a certain part of Mars needs cobalt, which is essential for life? We will have to take cobalt from somewhere else (for example a metallic asteroid) and send it to Mars. Then, we will send the cobalt in small amounts, under the form of guided meteorites, to the planet. An orbital station can manage all these processes much better then a surface station.
Early colonization era. This is the first phase, after terraforming. At this moment, colonies are small, but growing. The space station will have civilian functions. Passengers from Earth will come here first, have a health check and will be shipped for the planet, towards their new promised land. For giant planets, a space station can service all the moons. At this moment, passenger traffic will be more important then cargo shipping.
Later. As colonies are established and economies are developing, cargo will take over passenger traffic.
The following kinds of stations should develop and evolve:
Research stations. They will be made for scientific reasons (just as Earth's international space station is now). In future, we might see research stations on low orbits around gas giants. Many future freight and cargo stations can be research stations at first that will gradually grow in size.
Backup stations. They will appear as backup options for diverting ships in case of quarantine or emergency protocols. Also, they will have supplies, food, water, oxygen and medical equipment. If anything bad happens to the main station, they will be able to take over. Around Mars, such a station can be built on Deimos.
Freight & passenger stations. They will be the most important, the largest and the most long-lived of all stations. Having such a station allows the government on a planet to control all imports, exports and immigration. Contraband and illegal immigration can be handled more easily this way. These stations will grow very large, over 10 km in size. If the planet has a small moon, that moon should be used.
Small freight & passenger stations. Around asteroids, small planets and Kuiper Belt dwarf planets, there will be no need for large stations, but still, to handle the small traffic that will exist there, smaller stations will be required.
Military stations. They will be built and used when needed. They will be vital during space wars, to handle space pirates and to divert meteorites.
Interstellar stations. Interstellar travel will require the use of nuclear power on a large scale, the use of antimatter or other very dangerous substances. The explosion of a ship containing over one million tones of radioactive (or other potentially dangerous) materials is something that can threaten life on all planets, moons and colonies on a range of over one million km. Knowing the risks, it will be required that all interstellar ships do not come close enough to a terraformed planet or to a paraterraformed asteroid. The best location for an interstellar station would be at high distance, on a remote asteroid.
Touristic stations. Already, there are tourists visiting the International Space Station. A holiday on a station will always be interesting, both because of the views and for the services offered there.
Models For Solar System
The following is a model of space stations as I see they would be needed inside the Solar System. However, similar models should be highly effective around other stars:
1. Mercury - see Mercury Space Station - one large orbital station, to handle cargo and passenger traffic. The station will be in orbit at around 10000 km from the surface and will periodically need engine bursts to maintain orbit.
2. Venus - see Venus Space Station - one large orbital station, to handle cargo and passenger traffic. Orbits around Venus are much safer then around Mercury, so for a long amount of time the station can have a fixed orbit. A backup station would be needed, to handle traffic in case of emergency or quarantine.
3. Earth - see Earth Space Station - multiple stations. The current international space station will be too small for the colonization era and is too close to the planet. A second station, located about 20000 km from the surface, above the largest deposits of space debris, would be better. At least for the terraforming and colonization eras, this will be the largest station. A third, backup station, at higher elevation, would also be required. If all this is not enough, a fourth station would arise higher, at 30000 km above. Moon would require a small station, at least for terraforming operations, since Earth's stations are at some distance. However, major cargo and passenger ships will dock at Earth's station.
4. Mars - see Phobos - a large cargo and passenger station built on Phobos. Also, a backup station will be erected on Deimos. Mars has the advantage of two natural satellites, with stable orbits, so there will be no need for using engines to maintain orbit.
5. Main belt asteroids - see Ceres Space Station - small cargo and passenger bases, where needed. Majority of asteroids are small and their low gravity make landing not a problem for majority of spaceships. However, some large asteroids, like Ceres, Pallas or Vesta, will need orbital stations. The orbits around them are safe for millennia.
6. Jupiter - see Himalia - a large central station for cargo and passengers, most probably on moon Himalia. A backup station can be built on the moon Leda, which shares similar orbits. Small stations can exist in orbit around some of the Galilean moons. Callisto and Ganymede have safer orbits, around Io and Europa the orbits are not safe for long. In addition, a research station can be built on Almathea or another of the inner moons. Since Jupiter is close to the Main Asteroid Belt, a military station will be required to divert incoming asteroids or comets.
7. Saturn - see Helene - a large central station for cargo and passengers, most probably on the moon Helene. A backup station can be built on Polydeuces, which shares an almost identical orbit. Also, regional stations can exist in orbit around Titan, Iapetus, Rhea, Dione and even Tethys for a significant time, to handle local traffic. For Saturn, a research station can be placed between the rings and the planet. Touristic stations can exist inside the gaps of the rings or on the existing moons Daphnis or Pan.
8. Uranus - see Perdita - a large central station can exist on the moon Perdita, with a backup station on the moon Portia.
9. Neptune - see Halimede - an orbital station can exist further away from the planet. Neptune has prograde, retrograde and elliptical moons, which makes space traffic extremely difficult. A closer retrograde station could be used only by Triton, while a closer prograde moon will be used only by Proteus and the inner moons. An orbital station located in a Lagrangean point L4 or L5 relative to Triton will be efficient during terraforming processes and would prove useful as a local transit station. Orbits around Triton are not safe. A possible base can exist on the moon Halimede.
10. Kuiper Belt - see Styx - small stations would prove useful in many situations. The larger bodies of the Kuiper Belt are too far away for plant life to survive, but still can be a good attraction for settlers and industrial corporation. They will never be as inhabited as the inner planets or the moons orbiting gas giants. Around Pluto, we can use any of its four small moons for a base. Also, many other Kuiper Belt objects have moons.
Interstellar station - see 6 Hebe - should be placed somewhere safe. In one science fiction novel, an interstellar base was placed on the asteroid 6-Hebe, which is at a safe distance from Earth. As for current technology, interstellar travel is possible only with the use of nuclear power or antimatter. An interstellar ship carrying 100 passengers would probably require a 50 meters wide cube of plutonium. This is by far above the critical mass limit. If we use antimatter, the risks are far greater. Skeptics might argue that an interstellar base should be placed further away, probably on a centaur that orbits between Jupiter and Saturn. However, a terrorist can drive an interstellar ship, full with fuel, directly towards Earth or other inhabited planet. Knowing the danger, many would require that the interstellar station will be located in the Kuiper Belt and any interstellar ship would be banned to come closer then the orbit of Neptune. However, the use of atomic fuel or antimatter is extremely expensive and interstellar travel will be possible only when we will find cheaper ways to reach the stars.