I've flapped my gums now and again about small space habitats like Stanford toruses, Bernal spheres, O'Neill Cylinders, and asteroid terrariums. From Orion's Arm (of course), I have learned of space habitats that dwarf even these. Basic habs are kilometers across and support thousands to millions. These "mid-sized" habs are thousands of kilometers across and support hundreds of millions to billions. Perhaps the proper labels are not small, mid-sized, and large, but colossal (O'Neill cylinders, etc.), really colossal (the topic of discussion here), and stupidly colossal (ringworlds, dyson spheres, etc.).
So you people are probably thinking Jakob, what are you banging your keyboard about now? Meet the Bishop Ring:
- Numbers. It's not out of the realm of possibility to imagine hundreds, even thousands, of small habs drifting around a star system. These guys? It's difficult to imagine an economic justification for building more than a few in any given star system.
- Materials. They're too big for asteroid mining, unless you'd like to break up a terrestrial planet or hoover up every bit of free-floating rock for billions of kilometers around. We need basic starlifting as a prerequisite for gaining the materials, otherwise it'll be a hopelessly expensive white elephant.
- Inside culture. Space is limited in smaller habs. You don't have to go very far to find an edge. You can see where it ends. Here though? We are talking about artificial constructs the size of worlds, and culturally they would have more in common with planets than space stations. It doesn't have the look and feel of a space station, it has the look and feel of a planet (albeit with odd geometry):
Why would we want to build one? A real estate developer hoping to muscle in on a crowded star system by whipping up an artificial planet? Colonists hoping to claim an empty system, but wanting a planet to put down roots? Simple showing off, like many of the skyscrapers in the Middle East today?
And when can we hope to build one, bearing in mind the need for demand as well as technology? I would lean towards the fourth millennium, when starlifting is the norm and inner systems are likely to be getting crowded. In my timeline, the first ones are built during the Reconstruction and early Kingdom Era to replace planetary surfaces destroyed by the War of Gods and Men as well as showcase the Kingdom's wealth and technological progress.
PS: I count supramundane planets in the same spiritual category, even if they aren't technically space habitats. But I already spoke of them.