*** Spoiler Alert ***
If you haven’t read the book yet, best stop now.
Heaven’s River – The Topopolis
I’ve seen a few comments that Heaven’s River is not sufficiently well described in the book, so I’ve put together this post to describe it in more detail.
First, let’s start with an O’Neill cylinder, something most people are far more familiar with. Think Babylon 5. An O’Neill cylinder, at its most basic, is just a large drum, rotating around its axis to create centrifugal pseudo-gravity on the inside surface. There are different designs for introducing light into the cylinder, but the one used in Heaven’s River is a fusion-powered light source on a structure that runs down the center of the cylinder.
That’s great, but possibly a little exposed. One good meteor punch and all the air drains out of the cylinder. And like Johanssen said in The Martian, we need air to not die. So stick a non-rotating shell around the cylinder, made of rock or other friable material, to absorb meteor impacts. We make it non-rotating and separate from the cylinder so that it doesn’t place a load on the cylinder. In order to keep the outer shell and the inner cylinder from touching, we place magnetic bearings in the gap.
So far, so good. But with a single cylinder, you can enter and exit through the ends, preferably along the axis where there’s no pseudo-gravity. With a topopolis, not so much. Think of a topopolis as an O’Neill cylinder stretched to a ridiculous length. After all, for any given diameter, a length of ten miles isn’t any more of an engineering challenge that one mile. Or a thousand miles. Or, as in Heaven’s River, a billion miles.
The last thing we do is connect the two ends of the billion-mile-long O’Neill cylinder, so that now you have a loop. In the case of Heaven’s River, instead of going around the star once at a distance of three hundred million miles or so, the structure goes around the star three times at a distance of one hundred million miles or so. There was no engineering reason for that by the way, I just thought it was fun.
But now you have a structure with no convenient ends to enter and exit through. What do you do? Well, the only thing you can do—enter and exit from the side. Of course, that’s easier said than done, because the cylinder is spinning at eighteen hundred miles per hour. And it’s sitting inside a stationary shell. So you have to go through the stationary outer shell, somehow accelerate to a half mile per second while following the curve of the inner shell, then somehow get in through the inner shell. That’s where the Spin Transfer system comes in. You land in one of the nine space ports on the outer shell and get into one of the Spin Transfer system vehicles. The vehicle accelerates along a track on the inside of the stationary outer shell until it matches speed with the inner shell, then the vehicle clamps on to and docks with the inner shell. From there, it’s just an elevator ride to the inner surface of the rotating shell.
Heaven’s River itself has a fifty-six-mile radius, and was built in sections of five hundred and sixty miles each. Think of the sections as individual O’Neil cylinders that got glued together during construction.
Each section has a barrier at each end, which is made to look like mountains, in order to preserve the illusion. The barriers are actually designed to close off the end completely in case of some kind of catastrophic blowout.
The interior of Heaven’s River has four main rivers running through it, each going in alternating directions. But because the Quinlans are semi-aquatic, the rivers were designed to meander, to have tributaries, feeder streams, and so on. This increases the available river and shoreline, which is the ideal Quinlan habitat.
The interior terrain has topography, i.e. hills, valleys, and so on. But there is no reason for the builders to have carted in and dumped a bunch of rocks and dirt to make those hills. In fact, the extra weight would just increase the engineering requirements. So the topography is baked right into the interior shell. You see something similar in Ringworld, where the protagonists are able to see a negative of the ringworld topography when looking at it from the underside.
This design creates empty space under the hills and mountains, which is a perfect place to put infrastructure, including administration and maintenance centers.
A couple of other points…
Like a ringworld, a topopolis cannot have a stable orbit. Technically, it isn’t really even orbiting. So there will be facilities for adjusting the trajectory. I didn’t bring this up in Heaven’s River because it wasn’t relevant to the story, but I would visualize some kind of magnetic levitation system that plays off the star’s magnetic field.
A topopolis ‘bends’. Riker and Bill get into this a bit in the book, but the basic point is that the bend is so slight that no expansion joints are needed and there is really no issue of material fatigue. Most structures in our real lives bend far more than that on a regular basis.