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Space Elevators: Low Cost Ticket to GEO?
Crocuta writes "The current issue of Science
News features a cover story
that discusses the current developments in space elevator technology. NASA has been
working on such devices for many years, but private companies such as Highlift Systems are now jumping on the space
elevator bandwagon, no doubt seeing the huge potential profit in a low cost per pound
delivery system. PhysicsWeb has a somewhat
older, but much more technical article
on the formation and structure of the carbon nanotubes that form the basis of the proposed
tether cables. With a development like this, we could shoot entire boy bands into space and make
the world a better place."
Think of the space elevator structure as a 100,000-km-long highway that will require ongoing maintenance and repair," says Smitherman.
How unrealistic can an analogy be? If a crack forms in some remote stretch of interstate, there's no danger of the rest of the interstate system suddenly ripping away and falling into space. Repairs would have to happen instantaneously without ever breaking an almost unimaginable ribbon tension. And this wouldn't be a very rare occurrence, either, as the ribbon would present a surface area of five to eleven million square meters on each side (5 to 11.5 cm wide, 10^8 meters long). And remember that it's on the equator, which every piece of orbiting debris crosses twice during each orbit.
And the only mentioned solution for lightning strikes (one of which could be fatal to the ribbon) seems almost totally unworkable, and doesn't take into account that a 100,000-kilometer-high conductive tower would generate its own lightning. Remember the ill-fated (but educational) Space Tether Experiment? And the tether was only a mile long. A space elevator's ribbon would intersect a huge chord of Earth's magnetic field, including both Van Allen Belts. Seems to me that, even if the ribbon didn't immediately blow like a giant flash-bulb filament, you still couldn't get within a hundred yards of the base due to the continuous electrical discharge.
Don't get me wrong--I've dreamed about space elevators since I was a kid reading about Clarke's hyperfilaments, but the more I think about it, the more unworkable it seems.
Nothing can be protected 100% completely from attack by terrorists (or anyone for that matter). There is always a risk that if someone really wants to see something destroyed, they can do it.
That said, however, putting a ribbon to space out in the middle of the ocean, away from any shipping lanes, international flight paths, or human activity at all is a good start at protection. It's HARD to get to a location that far removed from everything without anyone noticing (especially if that location is under constant watch and guard.
Additionally, this operation, while not devoid of human workers, won't have so many people laboring at the anchor-station or on the cable to make a terrorist attack really that fruitful. There just isn't that much casualty potential (although the capital losses could be considerable).
But capital is just money. And the neat thing about money is if you spend it on projects which create wealth, you're not really losing it. If the cable can operate for a few years, it will have paid for itself, anyway, and very likely several additional cables will be built to expand capacity. These cables will most likely expand radially from earth all around the equator, under the control of diverse groups of people. We already know that humans want to get out into space and explore it, even at considerable expense. The proposed budget for the cable is not chump change, but nor is it unreasonable when compared to other space projects. America alone has spent considerably more on the Space Shuttle program over the past 25 years, and for that money, we'd be able to lift up as much material (measured by tonnage) in 2-3 years as we have in all the Shuttle missions combined. So the real risk of huge financial loss is if a terrorist destroys the cable in that initial timeframe. Additionally, since most of the cost is in the research, design and development, rather than the construction and deployment, another cable could be built if the first one is destroyed (admittedly, if the first one is destroyed very quickly, there will be a huge political barrier to overcome before a second cable could be deployed).
Also, since the thing is so cheap to operate, many more nations, companies, and individuals will be able to afford to undertake space-based projects.
The thing is, if the whole world is given access to space, There won't be that much motivation to destroy the means to that access. If one country or company jealously hordes the cable and doesn't lease out usage to everyone else, that country or company will:
1. Risk considerable reprisal, both in the form of economic sanctions by the rest of the world, possible military threats, and very likely terrorist threats
2: Miss out on a fantastic opportunity to enhance the economy of the entire planet, and line its own pockets considerably in the process.
Therefore, it will be in the interest of whoever builds such a machine to let the rest of the world use it as well, including the deployment of components for the construction of additional cables.
One thing I never see mentioned by all these proponents of nanotubes as a structural material is that extrapolating the strength of nano-scale covalent bonds to macroscopic dimensions is overly optimistic. "Calculations suggest... based on flexibility... 100x as strong as steel" sure. There are all sorts of materials, if you remove all the defects on an atomic scale, that are super strong. But saying that it is inevitable that we can scale up something from 1 micrometer to 100,000 kilometers is a bit of a stretch. If you made the cable out of solid flawless diamond, it would be stronger than out of nanotubes, and we can already make bigger diamonds than we can make nanotubes. I think a space elevator would be great, but don't hold your breath. There are a lot of details to be worked out in the materials science area before it is really a possibility. But nanotubes do hold promise, just not as much as everyone here seems to think.
So yes, there are many challenges to overcome, but they all, fortunately, seem surmountable.
This is just impossible! :)
But seriously, I did read it. Well, really just the section about nanotubes, and if the rest of the paper is equally fallacious, I think that would serve as pretty conclusive evidence of the imposibility of the space elevator. Using a combination of an overestimate of the strength of nanotubes with an underestimate of their density, the author uses a strength/mass ratio that is twice as large as the UPPER bound on the strength of nanotubes (which is the ideal strength). In practice the ideal tensile strength is typically many times higher than the yield strength. In case you're wondering, this is based on density functional calculations I performed myself--far better than the crude estimates refered to in the paper. And yes, I did just check his source. It's a review paper that refers to an extrapolation of a strength based on a strain from a tight-binding molecular dynamics calculation which the authors recommend taking with a grain of salt.
On the experimental side, noone has yet (to my knowledge) produced a composite based on nanotubes which is actually particularly strong. Even if these composites are developed (and probably eventually nanotube composites will surpas carbon fiber composites), they are guaranteed to pay a major hit in strength/mass due to the mass of the epoxy. Look for more like a factor of two over carbon fiber composites, rather than the factor of 50 or so advertised.
As mentioned in the paper, the mass of cabling needed is extremely sensitive to the strength/mass ratio. I don't know the relation (since I haven't looked up the Pearson paper), but he mentions that if you diminish the strength/mass ratio by a factor of 50 (using kevlar) from his fictitious nanotube ratio, the mass goes up by about a factor of 100,000. With an overestimate of the strength of nanotubes of at least a factor of two, probably much more, it seems highly unlikely that the cost of the elevator (already estimated to be rather high) will be within reason, and for all I know there may similar "rounding up" going on in the rest of the paper.
How could it be defended from someone who doesn't care whether he lives or dies as long as the target is destroyed?
How about removing the single point of failure?
What if the cable split into a few hundred strands, and was anchored in such a way that it covered a good 1KM radius on the ground, with lots of room between the strands? Perhaps a fully-loaded 747 could take out a 747-wide swath of the cable ends, but it couldn't hit enough of them to threaten the overall integrity of the elevator.
Basically, it's just an engineering problem. A single mass of cable would be pretty difficult to destroy already, and strategies like I've just described could make it even more difficult.
-jcr
The only title of honor that a tyrant can grant is "Enemy of the State."