The term "deep sky" refers to a part of the atmosphere that is often neglected by spacers yet is almost as important as low planetary orbit in the day-to-day operations of the space age. On Earth, the deep sky is a 90 kilometres thick layer extending from the lower stratosphere to the Karman line, the legal definition of the outer space border. On other worlds that may have different atmospheric layouts, the deep sky area is delimited by the envelope within which unassisted breathing is impossible (if applicable) but regular aerodynamic flight is possible. The deep sky, much like low planetary orbit and beyond, escapes the usual definitions of property and communal sovereignty on most worlds and is instead considered as a common ground, abiding by regulations similar to those that govern international waters on Earth. In a strange but ultimately understandable turn of events, on most worlds the deep sky is much lesser known than planetary orbit, owing to the incredible complexity of meteorology and climate on alien worlds.

Denizens of the deep sky straddle the limit between spacers and ground dwellers. In many ways, their environment is closer to space than the surface in terms of living conditions, with unbreathable air, limited aerodynamic flight at higher altitudes and high levels of radiation exposure on planets with weak magnetic fields. However, while a spacer is bound to spend long amounts of time away from a gravity well ("Once in the stars, forever married to the void" as per the Eloran saying), a deep sky dweller shares their life between the far blue sky and the surface. A very specific culture surrounds communes and cooperatives operating in the deep sky area, one that owes both to the "garage aerospace" ethos of the late interplanetary age and the test pilot ethics of the industrial era.

The main inhabitants of the deep sky are q-sats or pseudo-satellites.

A pseudo-satellite is exactly what it says on the tin: a vehicle that pretends to be a satellite while never bothering to be launched into space. Q-sats are often helium or vacuum airships that float at the upper edge of the stratosphere, using jet stream winds to travel around the planet or ascending to higher layers to remain immobile above their ground area. As they are solar-powered, q-sats may remain in flight for extended periods of time without servicing, sometimes months or years. Q-sats are very numerous in human space albeit it is somewhat hard to determine how many of them are in service, as they are less legally constrained than space satellite and as such only registered by local jurisdictions. They only require basic ground facilities and mobile tracking antennas to be operated, making them accessible to virtually any cooperative. They are often deployed on Venus-like planets as scientific outposts.

Pseudo-satellites are mainly used as communication relays and remote sensing devices. Though more exposed than regular satellites, their cost makes them very easy to replace. Their ability to loiter above a specific area without having to remain in a far geostationary orbit makes them ideal for civilian imaging as well as ad-hoc communications platforms on worlds with limited infrastructure. Remote sensing pseudo-satellites are very appreciated on worlds with significant high-altitude cloud cover such as Okean or Vyiranga where orbital platforms have trouble getting direct surface imaging in the visible and near-infrared spectrum.

Their close brethren are highflyers.

A highflyer is an unpowered aircraft that ascends to the upper atmosphere and then uses the local jet streams to remain in flight for extended periods of time several dozen kilometres above the surface. Highflyers are incredibly light, with their pilot or AI systems often representing the heaviest part of the aircraft. These gliders are made of carbon compounds weaved with an organic substance known as "pearl wood" often imported from the coral seas of Elora. Highflyers serve a different purpose than pseudo-satellites albeit they operate at similar altitudes. Most of them serve a scientific role, circling a planet to collect data on high altitude phenomena, observe storms from above, capture transient sylphs or blue jets on camera and sample lifeforms living at the edge of space. Highflyers are very common on high-pressure planets such as Okean, where the thick atmosphere enables them to keep bouncing between layers to keep their momentum.

A few cooperatives have been repurposing highflyers as mobile launch platforms for "all-in-one" nanosatellites carried aboard small integrated rockets attached to the belly of a stratospheric glider. Such nanosatellites are often used in tandem with highflyers in their scientific ventures.

On the faster, meaner side of things are Karman skimmers.

Karman skimmers take the shape of hypersonic, scramjet and aerospike-powered vehicles guzzling organic fuel and designed to straddle the thin border between high altitude aircraft and space shuttles. In the interstellar age, they occupy a rather strange niche as the only kind of aeroplane that can somewhat threaten spaceships. The idea behind Karman skimmers is to combine the advantages of space-based weaponry in terms of firepower with the laser diffraction and ground-based fire support offered by a planetary atmosphere. Such vehicles are meant to ascend to the edge of space where friction is minimal, fire their payload, then perform a steep dive to take cover from counter-battery fire in the troposphere.

The combat capability of Karman skimmers is doubtful, to say the least, and while there isn't a lot of usable examples, the Long War of Mars has shown that attacking FTL-capable spaceships with converted civilian spaceplanes is nothing short of suicidal. Modern Karman skimmers might be interesting in terms of payload optimization, however, given that they eliminate the need for an ascend module on long-range missiles which might give a much-needed edge to FTL torpedoes.

A more peaceful version of Karman skimmers - and as far as we know, more useful - are Skyhook subways.

Skyhooks are one of the most widely used means of surface to orbit transport on highly to moderately developed planets. They rely on cheap suborbital vehicles that carry a payload up to the end of the skyhook's cable to be reeled into space. Depending on local pressure and gravity conditions these vehicles may adopt either vertical or horizontal take-off/landing profiles. Though most of these "Skyhook subways" are drone vessels following exceedingly regular schedules, a few of them are piloted vessels. To sit in the cockpit of a skyhook subway is often reserved to ageing spacers with health issues preventing them from living and working in zero-g any longer. By volunteering as occasional skyhook subway pilots, they get to experience the thrill of grazing the edge of space for a short amount of time, gazing into the colours of the deep sky once again.

Finally, come the most elusive deep sky denizens, the surface to orbit airships.

However odd it might sound, the idea of carrying a payload all the way up to low planetary orbit with an airship is relatively doable, if one doesn't mind the lack of efficiency. The mission profile of a surface to orbit airship ascent encompasses the entirety of the deep sky. First, the payload is transferred to a stratospheric airship that ascends towards a pseudo-satellite station located in the higher stratosphere, where it connects with a space-capable vacuum airship that then ascends over the course of several days at supersonic speeds through the quasi-vacuum of the high planetary envelope. At the end of its journey, the vacuum airship effectively becomes a small starship capable of docking at a space station. It is then capable of going down towards the atmospheric station on its own.

Surface to orbit airships have undeniable aesthetic qualities, but their appeal doesn't stop at their elegance. Their ascent and descent profiles are the smoothest of all surface-to-orbit vehicles, only surpassed by a space elevator train. On planets devoid of such equipment, surface-to-orbit airships are ideal for payloads or passengers that are too critical to go through a regular ascent, even one carried out by a suborbital vehicle. 

Image credits, in order of appearance.

Airship by Jean Philippe Chassel, GNU license // Perlan glider by Airbus aerospace, all rights reserved // SR-71 and X-15 pilot, US public domain (USAF and NASA, respectively) // surface-to-orbit airship, JP aerospace, all rights reserved.