When terraforming will start, huge amounts of equipment will need to be lifted from Earth and sent into the outer space. Some processes require transport of huge amounts of water ice, frozen gasses or minerals from the Kuiper Belt and Main Asteroid Belt. For this, large ships need to be built, that are too large to be constructed on the ground.
Later, when terraforming processes will end, local economies will develop on all planets and moons. Trade Routes in the solar System will be based on existing flight windows, which in turn are dependent on planetary alignments, which don't occur too often. Flight costs per kg of cargo and per person are lower if we use larger ships then if we use small ships. Because of this, large space vehicles, with over 2 km long, will be used in many cases. Such big ships will find nearly impossible to land on the surface of a planet. This is why Space Stations will become very important.
The Earth space station will serve both the Earth and the Moon. The station will serve as an outpost for ships reaching planets, moons and asteroids in the Solar System. Cargo and passengers will be shipped between the station and both Earth and Moon with the help of smaller landing vehicles, designed to pass through an atmosphere.
From all celestial bodies in the Solar System, Earth has the highest Population Limit. For a very long time, the Earth will still be the dominant planet and the most powerful economy in the Solar System. Most of passengers and freight will travel from and towards the Earth.
Earth already has a space station, the ISS. However, this station is located too close to the planet and it is surrounded by flying debris, which makes docking of large ships very hard, if not impossible. Earth is well protected against the solar wind by its own magnetosphere, which in turn creates the Van Allen radiation belts. Nobody would like to build a space station inside the Van Allen belts. So, a station can be located below, between and above them. Below the inner belt, there is not enough space and there are many satellites and debris roaming. Outside, it is too far for a space station, which can get influenced by the gravity of the Moon. The best option would be to build the station at 2.5 Earth radii above Earth surface (22300 km from Earth's center). Sometimes, radiation belts are disturbed by solar flares and can reach in the gap, but it is a safe place for longer.
At Earth's orbit, Solar Constant has a value of 1.98. This can provide the base with enough energy with the help of solar panels. The needed energy will be solar. In addition, for the times when Earth or Moon eclipses the Sun, the needed energy can be stored in batteries.
Unlike Mercury Space Station and Venus Space Station, the Earth space station will not require for a shield to be pointed always towards the Sun. The base will also not require too many heating and cooling devices. Shielding will be needed because the space surrounding Earth is full of debris and solar and cosmic radiation, and because one wall of the station will be heated to 110 C, while the opposite wall will experience temperatures of -180 C.
In the beginning, the base will be used for many reasons. Plants and animals will be tested to see if they can survive in outer space. Many technologies will be tested here. For this, the base will require a special lab area. Later, this part of the base will become a museum.
In the Earth-Moon/Terra system, there will be many ships flying directly, but also many ships that will link earth with the earth station and the moon with the earth station. Flight windows for such ships will occur almost daily. Also, many people from Earth will travel to the station to see with their own eyes what space looks like. Later, when other planets will be colonized, people who cannot afford to move to another planet will still be able to pay for a flight to the Earth station.
Still, during flight windows to other planets, there will be massive fluctuations of people coming and going.
The Earth station must be able to handle a large number of passengers, as well as some tourists who will not go beyond. All these people need food and water, fresh air and something to entertain them. The station will have a large mall, many clubs, and hotels. In addition, for the richest, casinos and amusement parks will be readily available.
In the first years, when many new technologies will be tested, the Earth station will research enclosed environments and will try to produce its own food. Later, the station will still produce food, but at very high prices. Most of goods will be shipped from Earth or from the Moon. Still, the base will have water and air recycling systems. The base will import oxygen and food and will export carbon dioxide and human dejection.
The amount of passenger traffic will probably be comparable with the largest airports in the world. However, on a busy airport, there are hundreds of planes that take off and landing every day. On the Earth space station, the same amount of people will gather in a very short amount of time, getting aboard or leaving a single ship. This will create significant logistical problems. In order to handle all this, all personnel must be well trained. On Earth, if you lost a train or an airplane, you can take the next one. In space, if you lost the ship, the next one might be next year.
For cargo, things are different. Freight will be divided into four categories: containers, special containers, high volume items and fluids.
Like on Earth, most goods will be shipped in containers, which have an universal size and can be packed in large groups. Containers can be moved very easily, with robotic arms. The process can be done automatic. Containers will be transferred between local and interplanetary ships and sometimes placed in a large storage bay.
Special containers require being connected to a source of electricity, in order to have a specific temperature or to be handled with care. For example, many items shipped to the giant planets will be delivered at low temperatures. It is easy in outer space to create low temperatures, by avoiding light from the Sun or reflected light from the Earth to reach in.
High volume items, which don't fit into a container, might include equipment, vehicles and mineral ores. They cannot be shipped with standard robotic arms. Such items will be shipped with larger arms or with local flying cars. In low gravity, it is very easy to move objects around. In case of mineral ores, there is a risk that fragments can escape into space and become space debris. To avoid this, it will be a good idea to ship all mineral ores inside large bags or crates.
Fluids can be shipped with the help of pipes and stored for some time in pressurised tanks.
In addition, the base will need to have a checking and repairing facility. Fuel of all types should be stored here too. Maintenance ships will bring food, oxygen and fuel from the Earth or the Moon when needed. Also, these ships will take back carbon dioxide and human dejections. The local ecosystems will not be able to produce all the food and air required. 2
For Earth, flight windows occur at the following rate:
Mercury - Earth: 115 Earth days Venus - Earth: 584 Earth days (1 year and 219 days) Earth - Mars: 777 Earth days (2 years and 47 days) Earth - Ceres: 466 Earth days Earth - Jupiter: 398 Earth days Earth - Saturn: 377 Earth days Earth - Uranus: 369 Earth days Earth - Neptune: 367 Earth days.
However, all flight windows listed here don't account for gravity assists and deep space maneuvers. Instead, they only take into consideration planetary alignments. Additional flight windows do exist, but they might require longer time or more fuel. For example, each 12 Earth years, Jupiter is in the right place for a gravity assist towards the Kuiper Belt.
Based on these numbers, we can estimate that there will be 8.92 launches during an Earth year, all done with large interplanetary ships. That acquires for a ship every 41 Earth days or one space event (departure, landing) every 20 Earth days.
In addition, there will be ships connecting the Earth base with the surrounding asteroids, to many Main Belt asteroids and to the Kuiper Belt. Flight windows towards the surrounding asteroids will occur at less then 1000 Earth days. Flight windows towards the main asteroid belt occur at 1 to 2 Earth years. Flight windows towards the Kuiper belt occur at less then 360 Earth days. Ships that fly to asteroids will carry less people and more industrial products. We can only guess how many they will be.
Some asteroids and Kuiper Belt objects will not have too many passengers and too much cargo to be shipped. Instead, those places will only be linked to one or a few planets. At least in the beginning, that planet will be Earth.
It is possible that flights towards asteroids and Kuiper Belt objects will outnumber flights towards the planets, because these little places will not have many other available destinations.
We must also take into account the many ships that will travel between the space station and both Earth and Moon or other stations around the Earth. These local ships will by far outnumber interplanetary spaceships.
The delta-v shows how much energy a ship will need to go from one place to another.
The following chart shows delta-v budget to reach destinations around the Earth:
Mercury Space Station – Earth station 19.309 Venus Space Station – Earth station 8.609 Earth station – Mars Phobos 8.585 Earth station – Ceres Space Station 13.281 Earth station – Jupiter Himalia 18.216 Earth station – Saturn Helene 22.447 Earth station – Uranus Perdita 21.763 Earth station – Neptune Halimede 18.033 Earth station – Pluto Styx 17.565 Earth station – Eris low orbit 17.357 Earth station – Sedna low orbit 15.858
Earth station – Earth orbit (100 km) 3.468 Earth station – Earth surface 11.425 Earth station – Moon orbit 2.979 Earth station – Moon surface 4.703
For comparison, an Earth surface - Moon surface flight will require a delta-v of 14.466.
It is interesting that flying to Mercury consumes more energy then flying to Pluto. For Saturn and Uranus, since their stations are close to the planets, delta-v is higher. For distant destinations, gravity assists can decrease the delta-v budget. Missions to the outer planets can take advantages of Jupiter or Saturn flybys.
It doesn't consume much energy to move around the Earth. Landing on the Earth requires less energy because aerobraking can be used. Launching a ship from Earth to the station requires with 1.5 more energy because of the aerobraking effect. In fact, only the gas giants require more energy for launch-to-orbit then Earth. Because of this huge energy requirement, it is possible that, in the space age, smaller planets and moons will develop their economies much faster.
The Earth station will be the backbone of Solar System colonization. Its strategic importance is out of the question. Only later, when the Earth will cease to be the most powerful economy, the Earth station will no longer have its importance as a major strategic station.