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The design for a space elevator involves putting a long ribbon between the Earth’s surface and an anchor in geostationary orbit. If we had such a ribbon, how exactly would it be put in place? how could it be lowered from space to Earth?
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Could the ribbon be made with current technology? I know that currently nanotubes are only a few cm long. So are sheep hairs, but they can be spun together to make a thread any length you want. Can the same be done with nanotubes?
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Assuming it could be made at all, what would be the cost? How does that compare to the cost of a single rocket launch?
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The pictures I’ve seen show the car climbing all the way to the top of the ribbon. But is this necessary? Aren’t most current space missions in low Earth orbit? Why not have a space station about 200km up, with an anchor in geostationary orbit? The car never has to climb any further than the station.
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All the pictures I’ve seen show a car climbing the ribbon … but that’s not how elevators work, is it? In normal elevators the car is attached to a cable, and the cable moves, right? Why wouldn’t a space elevator be the same? Why not lift up the car with a block and tackle? Or maybe it could be like a cable car, which runs in a loop.
I suppose you could stop the elevator at that altitude, but anything that left the elevator would fall back down.
Being in orbit isn’t about height, it’s about how fast you’re going around the Earth. It’s like shooting a cannonball over the horizon; shoot it fast enough and it will go over the horizon, and the horizon beyond that, etc. Rockets go up to escape the atmosphere, but most of their energy is spent to go very, very fast downrange.
The exact details of your orbit (how fast you have to go, and how long each orbit takes) do depend on how high you are. At a certain height, each orbit takes 24 hours. Do it right and the Earth will appear to stand still underneath you; that’s geostationery orbit.
Something like the International Space Station takes about 90 minutes per orbit. (I did say it was going very, very fast.) The space elevator, stopped at that altitude, will be going a lot slower. Drop a cannonball out of it and it won’t go over the horizon.
As far as #5, it is currently infeasible to construct a structure that would span the distance required. If such materials could be made in the quantities needed, it would be quite expensive enough to construct a single structure upon which a traversing device or devices could travel. Therefore these “cars” would either be powering themselves or obtaining power from the structure. This is my understanding, at any rate.
- Lowering it from orbit is the way I’ve seen it done in science fiction books.
- Probably nanotubes or something similar but as yet undiscovered
- A lot more expensive than a single launch, but after it’s built it replaces all future launches.
- Low earth orbit is a compromise. It needs to be high enough to be above the atmosphere but going higher costs more due to the need for bigger rockets to generate more speed.
- If it worked like a standard elevator the winch at the top would have to lift the weight of 25,000 miles of cable as well as the weight of the car. Imagine how big a spool you’d need to wind all that cable.
A stairlift has its own power supply (battery) so that it can drive up and down with no external supply. The ‘car’ could work the same way.
At 200 km, the car and any payload won’t have enough velocity to remain in low earth orbit. Remember at any height the whole space elevator is “orbiting” with a 24 hour period. Things in low earth orbit are moving fast enough to have a 90-120 min orbital period. Drop your payload out of the elevator at that point and it’ll just fall to the earth.
There will be some small window below the station where a dropped object will be in a stable elliptical orbit with an apogee above 200 km. However, if you’ve got a working space elevator you shouldn’t put anything in orbit lower than geosynchronous since it’ll be a hazard to the elevator. The area swept out by the tether will intersect with every possible orbital plane!
Has anyone factored in the cost of constant maintenance and/or the cost of protection from sabotage? In fact, how would you protect something that long with a known location from sabotage?
Large perimeter exclusion zone where no one is allowed to travel (rigidly enforced), top of the line antimissile systems, blatantly draconian checks of travelers, luggage, and employees. Most airline protocols are ineffective because of the large volume of travelers; the space elevator is going to have far less in the way of travelers inherently.
You can’t make sabotage impossible, but you can restrict plausible successful attacks to blatant acts by nation states, who wouldn’t want to be blamed by the rest of earth for the damage caused.
I don’t see why you can’t have a “stop” 200 km up. It would actually have some gravity (.8 WAG).
By the way, of all the proposed technologies that aren’t Pure F’n Magic, this one is the least likely to be ever accomplished. A 25,000 mile structure is unbelievable. (actually 26, 199 plus the length of the counterweight).
Arthur C. Clarke’s 1979 novel The Fountains of Paradise is a must-read for anyone interested in space elevators. Good stuff.
Clarke came up with the concept of satellites in synchronous orbit, did he not, the Clarke Belt.
Not the first, but an early thinker on the subject, and he certainly did more than anyone else to popularize it: Geostationary orbit - Wikipedia
Except that it’s far, far cheaper to make the second cable than it is to make the first. All of the R&D costs would already be spent, and you could use the first cable to lift the second.
In fact, by raising and lowering the cars via cables, you could make the energy cost of running the thing arbitrarily low. Make your first cable static, with depots spaced along it at a regular interval. Put a second cable next to the first one, one interval shorter, with cars spaced along it at the same interval. Lift the second cable up one interval, and offload the upgoing cargo into the depots, and transfer downgoing cargo from the depots to the cars. Then lower it one step, and swap cargoes again, and repeat. The upper cars, which are trying to “fall up”, will mostly balance out the lower cars, trying to “fall down”. It’s effectively like a siphon.
The low cost of building additional elevators also helps to mitigate the sabotage problem. Once you’ve got one, build ten more, and put them in various locations around the world. Now, even if one comes down, you’ll still have the others, and can recover relatively quickly.