I think the problem with space elevators are not as much with the materials but with the logistics of deploying that. How do you get the cable up there? You can't just shoot up a rocket with a cable attached and expect the cable to stay upright: it would fall down unless you give it horizontal speed. But if you do that, you'd probably end up orbiting the Earth and tying it up with the cable.

The other option would be dropping the cable down, but you'd probably end up with a similar problem. I'm no physicist, but I'm pretty sure that dropping that amount of mass would shift the center of mass of the satellite-cable system and mess with the orbit.

Even if you manage to deploy the cable, you'd still have the problem of the counterweight. I've read that proposed solutions include a captured asteroid (we've just been able to land - or crash, depending on how you see it - in one), a space station/spaceport (that's definitely not cheap), an extended cable (which probably would require even more complex materials) or junk from the construction (still the same problem).

And we still haven't got to the point of security. How do you keep things (space junk, satellites, meteors) from hitting the cable? What would we do if several kilometers of ultra-strong cable fell down into the earth?

I think that space elevators are a nice fantasy, and just that. When we have mature enough materials and geopolitics, we'd probably be better off using them on other methods that seem to be far more viable.

> You can't just shoot up a rocket with a cable attached and expect the cable to stay upright...

Well, actually...

Package your cable up, launch it, move to geosynchronous orbit. Start deploying your cable. Once it gets long enough, say a few kilometres long, tides hold it rigidly pointing towards Earth. You solve the counterweight problem by also extending a cable outwards from your launch vehicle, so keeping the centre of gravity still. Eventually the bottom of the cable reaches ground level and you're done. On the plus side, you now also have a 36,000km long cable extending past geostationary orbit, ideal for interplanetary launches.

> What would we do if several kilometers of ultra-strong cable fell down into the earth?

Not much. If the point at which it's severed is low, the lower part will just fall down and you pick it up and weld it back on. If it's a bit higher, the lower part falls down and burns it. If it's a bit higher still, it goes into orbit.

People have been thinking about the engineering and geometry of space elevators for a long, long time...

The first issue would be touchy, I think. You'd have to find a way to deploy gradually the cable, or else each end would gain really high speeds that could break the cable when it stops deploying.

Regarding the second part... If the cable is severed, the lower part goes down and the upper part goews away as it does not have an anchor on Earth anymore (I think). Either way, you lose the cable. If it's a bit higher, maybe it doesn't burn up completely (for example, in Spain we've recently found space junk landing almost unharmed in a field [1]). And if it's several kilometres long (just a few hundreds) it could cause considerable damage.

1: http://elpais.com/elpais/2015/11/05/inenglish/1446724352_762...

>You solve the counterweight problem by also extending a cable outwards from your launch vehicle, so keeping the centre of gravity still

I don't follow. If you didn't do this, wouldn't your centre of gravity still remain still? Just start at geostationary and make sure you have enough mass to act as a counterweight already on-ship (doesn't have to be in the shape of a cable). The center of gravity will remain in geostationary, the cable will reach the ground, and the ship/counterweight will end up at just the right height to balance it.

No, it wouldn't. As you extend the not inconsiderable mass of the cable towards the earth, your centre of gravity will move inward with it. Assuming your ship massed nothing compared to the cable, then the centre of gravity will be at the midpoint of the cable.

For the space elevator to work, the centre of gravity must be precisely in geostationary orbit (otherwise the entire elevator will drift relative to the ground).

You could achieve this by having a massy ship, and then carefully moving the ship outwards as you unreel the cable. But it's easier to have two cables. It also solves some other engineering problems, such as reaction effects as you control the speed at which the cable unreels.

> As you extend the not inconsiderable mass of the cable towards the earth, your centre of gravity will move inward with it

What I'm saying is that without firing rockets, there's nothing you can do to change the trajectory of your centre of mass. When you "extend" the cable towards earth, presumably you are pushing on it in some way, and there will be an equal and opposite reaction. You don't have to "carefully move the ship outwards" - the ship (which is inherently "massy", as it has the rest of the cable as cargo) will move outwards by itself, exactly the right amount for the centre of gravity of the ship-cable system to remain in geostationary.

So I still don't see what two cables gets you apart from keeping the ship itself exactly at geostationary, which strikes me as unimportant.

FYI for all the readers here: Carbon nanotubes won't work, the phonon effects (lattice interactions) in that very large crystal usually add up to break them after a few cm. Think of it like like a very narrow and very long tube half filled with water. There are vibrations along the length of it, some big and some small in amplitude. When those random waves in the water tube interact, they can add up, sometimes so much so that they break the tube. I know that is now totally correct, but that's the gist.

http://www.ruf.rice.edu/~rau/phys600/p273.pdf

Many more here:

https://scholar.google.com/scholar?q=carbon+nanotubes+phonon...

Is it not possible to dampen phonons before they will break the thread? You probably don't need to have vary long monomolecular threads, you can have many shorter ones weaved in a thicker rope.

Good question. I am not an expert in this in any way. I'd think that you have to have dampners in the lattice of the crystal. The only way that works is to dope the lattice with either another 4 valence element, like silicon, or to dope the carbon itself with something. The only think I can think of is to modify the isotopes to be VERY heavy which is obviously therefore radioactive and would need to be replaced a lot. If you used silicon dope then you'd reduce the interatomic strength and therefore make the rope possibly too weak to be used.

I thought that the whole point of carbon nanotubes was that they are the only thing with the 'pull strength' (forgetting the term here) that could withstand the tension. If you weaved them, then you would rely on the Van Der Waal forces to keep the tubes together which is much weaker. Also you have to contend with the phonon interactions, anything tangential to that 'plane' has no effect on the strength, you have to be a part of the lattice to effect things.

Then again, I don't do research in this area.

I used to think space elevators are practically impossible because of interactions with lower flying space objects. Now I'm less sure; perhaps we can have a technology of active avoidance.

Other problems - like an extremely capable cable material, ways to climb up and down etc - are getting gradually solved over last years, so I'm cautiously more optimistic than before.

I don't really see the benefits of space elevators, to be honest. At an altitude of 100km the gravity is still virtually the same. And you still need a huge amount of fuel in order to put your rocket/plane into orbit. Unless you build an elevator with a height of 42'164km (geostationary orbit) - but that seems unrealistic to me.

> Unless you build an elevator with a height of 42'164km (geostationary orbit) - but that seems unrealistic to me.

Actually, much of the point is to build one that is significantly taller than that. The idea is that, if the center of mass sits at geostationary orbit, the elevator doesn't actually have to hold itself up. Centripetal forces do the heavy lifting. Try the wikipedia link posted by gp - the first thing you'll see is a very clear diagram.

> "I don't really see the benefits of space elevators, to be honest. At an altitude of 100km the gravity is still virtually the same."

You would have made the first 100km of the journey more fuel efficient, is that not a good enough reason?

I think it's just prohibitively expensive at the moment. 100km (I think it would have to be a lot more anyway) is not much in terms of space exploration. And even then you will always use the majority of your fuel for acceleration towards and deceleration at your destination (however fuel needs for deceleration might be much lower if you can do some sort of atmospheric braking).

There's a very real possibility that a conductive cable would generate tons of power which could be used by the climber plus surplus on the ground.

You have to build space elevators up to geostationary orbit (and beyond). That's the whole point; they don't work otherwise.