Although this question is about a construction from Iain M Banks Culture Series, I’m asking about the actual properties if one was really built so hoping it can stay here in GQ.
An culture orbital is a smaller ring world, instead of encircling a sun it has a 3 million km diameter and is orbited at a tilt to the parent star so that the far side of the ring blocks the light from the sun for half the day. At this size rotation once in a 24 hour period gives you both 1 G of gravity and a standard earth length day night cycle.
See here for full details:
So lets say a culture General Systems Vehicle arrives in our system and announces to us that they are going to construct an orbital for 90+ percent of our population to live on so that Earth can be left as a nature park with only some tourism and recreation allowed on the planet itself, most of it to be left to revert to wilderness. According to Bank’s ideas, orbitals are usually constructed by gathering up all the mass from small bodies, asteroids, minor planetoids etc but leaving all the moons and planets intact.
My questions for GQ are:
Is there actually enough mass in all the small planetoids, comets , asteroids etc to construct a ring 3 Million kms in diameter, 1000 km wide and 10 km thick? Or if we have to dismantle one or more moons in order to make it, which moons would do?
Could all that mass be moved into the vicinity of earths orbit without messing up any of the other planets orbits?
Could an orbital be placed in the “goldilocks zone” close enough to our own orbit to have the same climate as earth without messing up earths orbit?
Forget the details of construction, for the purposes of this question the culture has effectively unlimited energy at their disposal and can build a fleet of self replicating drones to do all the work, I’m only interested in if there’s enough mass to actually make one, and what it would do to our existing solar system dynamics.
This structure would have a volume of around 8.5% of the Earth, or 4 times that of the Moon.
Now, is it made of normal matter, or some superdense Unobtainium stuff?
If it’s normal matter, then yes. We probably have 4 moons’ worth of assorted asteroids, comets, dwarf planets like say Eris, which could be ‘disappeared’ without people making too much of a fuss.
Ok great, so lets assume for the sake of argument that they are able to somehow form matter with the same average density as the earth into an ultra strong material to form the ring.
Can this much matter be placed into a stable orbit without messing anything else? Could it be placed at one of the earth sun lagrange points?
The asteroid belt has less than one lunar mass, so it would not be much use. I’ve seen estimates that the Kuiper Belt has about 3 Earth masses. That’s far more than is needed. So they could get enough matter from the Kuiper Belt and leave the larger bodies out there alone. Similarly, they wouldn’t need to take any of the larger asteroids, but probably would want to clean up the smaller stuff (especially the stuff that crosses any of the inner planets’ orbits) just to avoid the potential of a Tunguska or Chelyabinsk event in the pretty new orbital.
As far as stability, I wouldn’t expect such a small mass (compared to Earth, it’s small) in Earth’s orbit to have a significant effect on the rest of the solar system. They’d want to run a simulation on it first, though, just to be sure.
As for the orbit, L4/5 would be good or they could put it in a co-orbit with Earth.
Nitpick*, but I don’t think the bolded part is correct. It’s at a tilt so the far side doesn’t block its Sun. The half of the ring whose inner surface faces away from its Sun will naturally be in darkness.
Not just a nitpick, but a bonus free link fix too!
A bigger concern than the effect of the Orbital’s effect on the rest of the solar system would be the effect of the rest of the system on the Orbital. It’s so wide that you get a significant (severe?) tidal strain from the sun alone at the distance of Earth’s orbit. Or has this been accounted for?
There’s no reason to place it in the same orbit as Earth in the first place. The apparent gravity isn’t being generated by mass but by the rotation of the Orbital. You may have to play some games with the orbital velocities to get a nice 365 day year, though.
In the grand scheme of things, it’s not a whole lot of mass, so it wouldn’t mess anything else up, unless you put it right next to a tiny asteroid.
As in the novels, you’d want to keep an eye out for any small asteroids whose orbits might endanger the orbital. But that’s kind of true for Earth, too (though here, we don’t have many options if we do get hit and we are bombarded by meteorites constantly).
You could put it circling the Earth. You’d have the instability problem like the RingWorld, but I’ll assume that can be easily handled. Then everyone with even a small telescope could have a nice view of the Earth. You’d also be able to have satellites “orbit” the surface of the Orbital, by orbiting the Earth.
Sure there is: You want it to get about the same amount of sunlight as the Earth, to keep the temperature comfortable. That doesn’t necessarily mean that you’ll be in exactly the same orbit as the Earth, but it does mean that your orbit will at least come close to the Earth’s, which means that you’ll have significant interactions between the Earth and the orbital that you’ll need to account for.
But that’s something you can control in no small part through your atmospheric mix. The Goldilocks zone is fairly large as it is.
You could put it in orbit halfway between the orbits of Earth and Mars with appropriate climate engineering and Bob’s your uncle. No worries about the affects of proximity of pesky planets.
Frankly, I’ve never understood the sci-fi idea of the “gigantic” orbital station.
You could use the same mass to make a bunch of much smaller stations (10km diameter or something) where your materials are not under very much structural stress. This kind of redundancy would also be much safer to live in : if humanity is spread out among thousands of separate stations, there really isn’t much that could kill everyone.
You’d want to be farther from the star than the goldilocks zone, to reduce the radiation dose, and you’d beam power to the stations from solar power plants that are closer in.
This little tangent got me thinking about whether it’s even necessary to tilt the orbital’s plane relative to its… er… orbital plane (you know what I mean). I don’t think it is. Suppose the orbital is roughly at the same distance from the Sun as the Earth is. Wherever you are on the orbital, the far side is 3 million km away. That’s, what, ten times further than the moon is from us? And the orbital has a width of some 1000km vs the moon’s 4000km. So given the moon and sun have the same angular diameter for us, the orbital’s far side would have an angular diameter of about 1/40 of the sun. That would be imperceptible to the human visual system I’d have thought. So the orbital’s far side would not appreciably block the Sun out, but only a thin strip of it, easily lost in the glare of what remains.
The idea is to make a habitat big enough that its interior is comparable to a planetary ecosystem- weather, oceans, forests, etc. that support self-sustaining populations of plants and animals. Even the larger space colonies envisionable with contemporary materials (O’Neill cylinders, etc.) would be more like space farms than true ecosystems; heavily dependent on artificial regulation of the plants, animals, water and possibly even the air. Whereas a big enough megastructure essentially becomes a true artificial world- build it, seed the interior and it continues on from there indefinitely.
This is very likely true for us Earth humans, but it’s the Culture building the orbital, and the average Culture citizen is either bio-engineered to buggery, or a drone. Enough of them will be able to see the dark band that the tilt is included in the “DIY: My First Orbital” glyph files just for aesthetic reasons.
That’s a good point, it would only block about 3% of the Sun at Noon. The percentage would be higher at other times, but not too bad. It would double to about 6% at 8 am and 4 pm, and double again at about 7 am and 5pm. You’d be in full shade for over 12 hours, but when it’s around 6 O’clock, there’s a close portion illuminated from almost directly behind your portion, so that would be pretty bright, and you’d get a decent twilight.
FWIW, the far part of the ring would be brighter than the full Moon. If that was a problem, tilting the ring would help.
“Normal matter” does not have the tensile strength to construct something like this. Not by a long shot.
If you put radial supports (“spokes”), the requirement for tensile strength is dropped by many orders of magnitude (maybe tens of orders?) but even then, you’re talking about a 1.5-million kilometer cable that can support its own weight and more. It’s similar to a space elevator, except 30 times longer.
In the Culture’s case, you’d have to call it easilyobtainium as they could make it out of (virtually?) any raw material. How dense would easilyobtainium have to be for there to not be enough available mass in the solar system to make an orbital of the specified volume?
The mass of an orbital is dependent on the thickness of the shell, which is dependent on the strength of the unobtanium. Assuming that the unobtanium is arbitrarily strong, you only need to estimate the mass of the rocky layer on top of the unobtanium, a mass which turns out to be about the same as the Earth’s Moon. This relatively small mass would probably not disrupt the Solar System to any significant degree.
Note that Banks described the Orbitals as being held together by forcefields rather than unobtanium; I have no idea whether the infrastructure required to generate these fields would be very massive or not, but the lighter the better for a structure that tends to fly apart due to centrifugal force.