First Earth...
What can we expect from Earth atmosphere ?
Concentration is very low at heights between 80 and 100 km. But an air capture device in orbit, moving at 8 km/s with a 1 m² capture surface, collects 8×10³ m³/s — that is 2.88×10⁷ m³/h.
Considering air density at 80 km altitude (1.4×10⁻⁵ kg/m³) and 100% capture efficiency, this results in about 400 kg/h of air.
With a 1000 m² capture surface, that means 400 t/h, or 9.6×10³ t/day — equivalent to the payload of 48 heavy launchers launched every day.
What about at 100 km altitude? Air density there is about 5.5×10⁻⁷ kg/m³.
That gives 16 kg/h for 1 m², 16 t/h for 1000 m², or 3.84×10² t/day.
One of the many challenges is to increase capture efficiency and reach the lowest possible point in the atmosphere.
The lower we go, the more air we can collect — but the higher the thermal and mechanical constraints on the structure.
To be competitive, we need to optimize both the capture surface and the number of capture wells.
What those figures show is that megastructures are made for a world turned to human expansion in space.
... then Mars and other planets