I see. Yes, in the paper the scope was set to be model it on the space station: there's a habitable zone of the atmosphere about 50km up, and the mission parame

I see. Yes, in the paper the scope was set to be model it on the space station: there's a habitable zone of the atmosphere about 50km up, and the mission parameters were to base there for ~10 years with multiple human/robot missions to the surface and a rotating crew. Other approaches (such as a supersonic plane) weren't considered.