Aerostats have been around for a long time, at least the concept has, the floating city. The floating city would eventually become a reality, but it would be centuries after the first space stations and colonies on other planets.
The Floating Cities
The first aerostats were dropped from orbit into the upper atmospheres of the gas giant planets. These unmanned stations drifted, gathering information and performing experiments, though the hydrogen, nitrogen, and other strange gasses. As technology developed, it became feasible to build larger and larger units. There eventually became a drop limit, as aerostats were built in orbit, hauled via ship to their destinations and literally dropped into the atmosphere of their host planets.
The applications of a floating city grow in importance as technology developed to support flying ships. Flying ships needed ports of call, and the first decades of flying ships were handicapped to the same facilities that tended maritime ships. Landlocked cities and states never built facilities to tend ships, because they didn't have them.
Aerostats are lighter than air habitats that quite simply float. This is a strong contrast to the spacecraft that deliver them, assemble them, or the flying ships that use them as aerial ports. The stations do not rely on anything other buoyancy to maintain altitude. As such, they are large, but vulnerable to damage, much like rigid airships. To this end, aerostats are typically non-combatant, non-participants in military operations, and are constrained to neutrality, scientific operations, and other non-strategic purposes.
Aerostats have limited mobility, and while they can move, most of their maneuvering is used to remain in a stationary location, avoiding inclement weather patterns and changing altitude. They do not have large engines or thrusters. They are lighter than air structures. (Aerostats are mostly found in the atmosphere of gas giants or Venus. Earth aerostats are small, and Mars has to thin an atmosphere to hold up anything of meaningful size. Aerostats are unarmed, and unarmored. The majority are used for scientific purposes, atmospheric extraction, terraforming, and other non-military purposes.
The Long and Troublesome Process
The aerostat is relatively simple, a lighter than air structure that floats, and is just a much larger and less mobile version of a rigid airship. The problem was that the concept of the aerostat was simply not feasible with contemporary technology and Earth. THe Earth has relatively high gravity, and a comparatively thin atmosphere. This meant that lighter than air ships had to be both light and large. When scaling up to a habitat size, such structures were beyond the pale of construction, and even if balloons miles wide were built, the logistics compared to the outcome were simply not worth the liability. The aerial fleets and militaries were content to use larger tender ships to maintain their fleets away from port, and some of the tender ships built be the Atlantic Federation and ACPS were quite literally flying towns built around docks.
Once operations began in space, and on other planets, the viability of the aerostat improved. The atmospheres of the gas giants are dense, and provide admirable properties for supporting large aerostat structures. Lacking a surface or large population, the aerostats themselves are not an eyesore or strategic in value. Venus also proved agreeable to aerostat construction, with it's dense atmosphere. With atmospheric pressures 30 times higher than Earth, Venusian aerostats could be dramatically smaller.
Aerostats are typically constructed in space, and are ferried to their destination planet by custom built tug ships. The ships then make powered descents into the atmosphere of the planet and retain in a hovering position while the aerostat is finished, and it's gas bladders are inflated and heated to provide lift. This is a dangerous operation, as entering the atmosphere with a large and heavy load is inherently dangerous, and if there is a power failure or engine failure on a tug, it could see the entire station and ship plummet. This is a hard crash and implosion on the surface of Venus, or a fall into the increasingly dense Jovian atmosphere until the ship and station implode under the massive pressure.
As purely atmospheric bases of operation, aerostats rely on their own carried aircraft. In an almost steampunkish turn of events, aerostats are frequently paired with rigid airships. These all rely on the same basic principles, and are high efficiency, especially as most cover their upper surfaces with solar panels, and use high efficiency electric rotors for propulsion. These craft can also go long distances, don't require extensive difficult maintenance, and if they lose power, they don't plummet to an unavoidable death. Helocraft, helicopters, and non-combustion based electro-jets come a distant second, as they have more components, and require more extensive maintenance and support. In an emergency situation, the rigid airships have the ability to operate as rescue vehicles, tethering up a damaged aerostat, or as long duration lifeboats, should a habitat actually fall.
Aphrodite 1 - Venus: one of the original aerostats, this station is dedicated to atmospheric research and high altitude experiments in planetology. It is a neutral outpost and is the nexus for information and groups involved with and interested in terraforming Venus.
Ishtar Prime - Venus: Ishtar Prime is a dual role aerostat, it is the logistical hub for the Ishtar province of Venus and has facilities to handle heavy air traffic, as well as being involved in building the Venusian Space Elevator. Once the tether between Ishtar Prime and the surface is completed, it will be ushttp://strolen.com/submitting/write_submission/8143ed to ferry terraforming equipment to the surface of the planet.
Vos - Jupiter: Operated entirely by sentient robots and smart machines, the Vos station exists exclusively for the extraction of heavy hydrogen isotopes from Jupiter's dense atmosphere. The existence of Vos, and it's regular shipments of tritium and deuterium do little to quell conspiracy theories.
Oroboros - Uranus: The Oroboros Station was built and dropped into the Uranian polar zone, and had two functions, space observation and viral weaponry. In case of emergency, the entire station could be scuttled, and everything aboard would be swallowed in methane clouds and crushed. Oroboros was associated with the Federation Scientific 66 space station, located orbit above it. Since the loss of 66, contact with Oroboros has also been lost, and planners assume that failsafes activated and destroyed the station.
Tethers and Space Elevators
Getting things from the surface of a planet into orbit and vice versa is one of the biggest logistical issues in the Cosmic Era. Re-entering atmosphere is one of the most dangerous things a ship will do on a routine basis, and getting from the surface into space requires a massive amount of energy. Mass drivers and flying ships have replaces rockets, but mass drivers are among the least gentle things in the solar system, and ships are not an economical way to move every sort of cargo. Aerostats function as nodes and control centers in the construction of space elevators. There are very few in the solar system, most are prototypes or proof of concept machines in ideal locations, such as the Whitestone Elevator on Earth's Moon. Until the technology evolves further, tethers are taking their place.
The operational plan is to use cost efficient craft to move goods from the surface of a planet to the aerostat, and the tether-vator (precursor to a true space elevator) to move things up and down the tether. This differs from the true elevator in that the tether-vator is generally only used for personnel, priority and courier operations, and other such small high priority things. The Aerostat will function as a hub, for multiple cars on the elevator to be handled and transitioned as needed. The second stage tether will connect the aerostat to a fixed position space station. This removes ships from having the be used for small things like moving small groups of passengers to and from space or the surface, and eventually remove ships entirely from moving finished goods, people, and materials from surface to orbit. Ships would remain in the comparative safety of space, and once constructed, the resources and energy required to move mass into space will drastically decrease.
This is a work in progress for the Cosmic Era, and rivals the Panama Canal in terms of issues faced by those doing the work and building the tethers and elevators.
Cumuloblanca - of all the gas processing facilities on Jupiter, the PCs had to walk into mine. As neutral locations, aerostats attract unique groups of people, especially the sort who want to be left alone, the neutral type, rather than the violent type. The PCs have been tasked with finding an eccentric doctor and his female assistant and bring them back to Earth for questioning. They can't take weapons, body armor, or even an armed shuttle to investigate the Casablanca of the Clouds.
The Flying House of Daggers - An aerostat is being used as a floating temple by a sect of monks. These monks eschew the trappings of Earth for the perfect serenity of Saturn's equator where they meditate on the planet's rings and expand their consciousness. The PCs are sent to investigate/hail from the aerostat due to the high number of monks who develop parapsychic powers after their pilgrimages to the station.
Terminus - The PCs have traveled to a remote aerostat, a lonely bastion above Neptune. They have been sent to investigate an anomaly and subsequent loss of communications. They arrive and discover the station has been the site of unholy experimentation and has been hit with a dimensional fatigue event and is now overrun by monsters, a pathogen, or the laws of physics aren't working as they should. They have to rush against the clock to fix their ship and escape before the aerostat self destructs.