Leonard David has an article in today's Space.com on the rapidly-approaching feasibility of a space elevator. Apparently, rapid advances in the manufacturing of buckey-tube-based materials of unprecedented tensile strength are making this a viable near-term technology, which in turn makes it possible to build a tower to the heavens.

The basic concept is that if you place a satellite in geostationary orbit (where most communications satellites reside) it will, by definition, remain at a single point over the earth's surface (at the equator). Drop a cable all the way down thousands of miles to that point, and tether it (just as suspension bridges often start as a single cable across a gorge). Now beef up the structure, and put the center of mass of the system beyond geostationary altitude, which puts it into tension.

Build elevators into the structure, and you have a means of getting into space for the costs of the energy alone (plus, of course the amortization and maintenance costs for the elevator). This is just a few dollars per pound, which is orders of magnitudes less than the current methods of using rockets.

That would make a space vacation possible for almost anyone who can now afford a trip to Hawaii. It would also make space a much more practical location for the storage of nuclear waste and the construction of solar power satellites that might eventually render nuclear plants unnecessary.

Unfortunately, as was brought home most dramatically last September 11, it would also make the most visible and monumental target possible for a terrorist.

The potential energy in such a structure would be unimaginable (though not incalculable). If it were somehow released from its equatorial mooring (in addition to the tremendous loss of capability and loss of life of whoever was on it), it might whipsaw around the local landscape like a python on meth, potentially causing tremendous damage on the ground before finally drifting out into space (where it would become a major navigational hazard for orbiting satellites, facilities, and even tourist hotels). It's possible (though unlikely) that it could even ultimately strike the Moon. It would make the events of last September look like a Sunday-school picnic.

This is, of course, not an argument against doing it. But it does add some additional requirements for its construction that might not have been considered prior to the WTC attack. For instance, the structure near the base should probably be capable of withstanding a small nuclear detonation, if possible. It should certainly be capable of withstanding a collision with any existing aircraft (including supersonic). Security in the area should be strict (at least as far as explosive devices go), with a large keep-out zone on the ground and in the air.

I might be using this as the basis for the Fox News column tomorrow, so I'd appreciate any other thoughts that people have on the subject.

[Update at 5PM]

OK, having given it a little more thought, it seems to me that the problem with the article was that it didn't mention any of the problems. It was gung ho about how the technology to do this is almost here, which means to me that we now have to give some serious thought to the real showstoppers.

I see two serious issues, either or both of which are likely to keep this from happening for a long time, and perhaps forever.

First, if a structure is towering from the equator to a third of the way to the Moon, no objects can safely orbit the earth at any altitude below that. No GPS, no remote sensing satellites, no space stations, nada. The only satellites that can safely orbit are the geostationary comsats. The reason for this is that all other orbits will eventually intersect the structure, resulting in a spectacular collision, unless they are managed carefully, and they can't be managed that carefully--such an accident is inevitable.

The second problem is the one that I mentioned above, and it's potentially much worse. If it breaks off in space, while the part above the break will go flying off into an elliptical orbit, or perhaps out into the solar system, the part below will come crashing down to earth. Much of it won't burn up, because it won't have much velocity.

So, as technically neat as skyhooks are, I have trouble seeing any political conditions under which such a risky project, requiring the total obsolescence of our existing orbital infrastructure, to fly. We are going to have to continue to work at creating new markets that can drive down cost of the launch rocket-based space transports, because I think we'll be stuck with them for a long time.

A couple of quick thoughts, mostly technical ones. How will you make this withstand the differential winds as it climbs through the atmosphere? The major limitation on skyscrapers is not the strnngth of material, but the increased strain caused by transverse forces (i.e. crosswinds). I am not sure that there is some intrinsic limitation here, but it seems that it needs to be thought through a bit more. I envision a tethered spaghetti string which flops around within some radius, pulling on the satellite. You could probably put some slack in the tether, though, to keep from pulling the satellite out of orbit. I am not sure how you would move something, but if the tether did have slack, you could induce waves in it, and use the travelling wave to move things up (now that's cool!!). Anyway, I am not enough of an engineer to know the answers to these questions, but I'm guessing you could give it a shot!

The portion in the atmosphere would be very strong and stiff. Or are you asking about during construction? The trickiest part is getting it the last few miles down through the atmosphere before it's tethered to the ground. I suspect that the way to do that would be with a high-altitude heliostat to control it as it was lowered.

Actually, I was asking about the operational device, but after reading the article, I think I can envision it better. I am leery of the physics of the actual "lift", as opposed to the cable. The laser propulsion system seems like the weakest link in the chain to me. Very cool futuristic stuff! Where's my hovercar??

Speaking as one who used to be responsible for both Environmental Impact Reviews and liability insurance requirements for space projects, I get a headache even imagining what those issues would be like with a space elevator!

Arthur C. Clarke's The Fountains of Paradise (mentioned in the article) lays out most of the issues in quite plausible detail. It's been a long time since I read it, but I believe he dealt with the satellite issue by having the skyhook actively controlled to flex out of the way of approaching satellites.

As much respect as I have for Clarke (and it is immense), I don't think that his solution was realistic. Again, it doesn't take much of a screwup to make for a major disaster.

The idea isn't totally worthless though, it's just that we want to build the pilot project within a complicated and messy environment.

Skyhook on the moon anyone?

Winds and such would be an obvious hazard, but what would the electro-magnetic challenges be like? Might this thing turn into some weird current conduit or something? Or how about the relativity factor? Atomic clocks in orbit run at different speeds than on the ground, because the lessening gravity affects the passage of time. Might that in some way bollix the thing's computers?

Wouldn't a better use, and one less likely to result in a catatastrophe if it failed, would be use them for an interplanetary transport system? Imagine one located at a place with a lot of room, like a Lagrange point. Rotate it up, then attach loads and fling them at your target-- Mars, Ceres, Europa, whatever. Have the same arrangement at the destination,which catches your loads, too. The advantage there is that incoming loads can offset the momentum loss/gain of outgoing. You can adjust the incoming and outgoing speeds by changing your release/catch point.

Now I haven't done the math, so I don't know if this is at all feasible. For example, at what speed would a 1000km cable be moving if the end were at 1g, and how does that compare to rockets? It may not be any good for people (too high g forces, or too long) but might be perfect for commodities, sort of a Union Pacific of the sky.

F = w^2 x r
sqrt (F/r) = w
sqrt (9.8 m sec^2/10^6 m) = 3.1 x 10^-3 radians/sec

x 500 km = 1,600 meters/sec

BTW, I think the regulatory environment (even in a completely privatized world) will never permit the construction of space elevators or "skyhooks" on Earth, but there may be possibilities elsewhere in the Solar System.