History & Background (Dossier)
Downports are often more simply called airports or aerodromes among other terms.
Most sophont species do not evolve as flyers, they begin life as evolved walkers or swimmers. For such species, flight seems a near unattainable dream, something to observe and dream about, not a physical reality, soaring through the clouds and skies. But, technology eventually makes nearly all things happen.
Some enterprising sophont with the Traveller Gene eventually makes the first breakthrough to flight and voila, a surface bound species makes the transition to flight.
The earliest aircraft lead to vastly faster and more sophisticated craft. These first aircraft require increasingly more demanding resources and support and the first airports are born. Repairmen, technicians, fuelers, control towers, hangers.... All of those folks and infrastructure combine to enable flight.
Increasingly sophisticated aircraft lead to... the first spacecraft. Typically species first reach space in rockets but other technologies are certainly possible.
As the aircraft evolves into the first spacecraft, so does the airport evolve into the first spaceports. And as space becomes increasingly accessible, spaceports evolve into two distinct components: the Downport, a surface location, and the Highport, an orbital location.
Most futurologists and technologists believe that another paradigm change can be found on the technological horizon. Gravitics control technology will eventually allow starcraft that are indistinguishable from aircraft or spacecraft. These advanced craft will be able to enter and exit with a minimum of energy expenditure, very different from today's advanced TL–15 technologies.
The Ancients certainly had such craft!