It may be that high, but the whole thing is NOT going to come down in one piece. If a terrorist hit it in the atmosphere and severed it (say, by ramming a 747 into it), then the vast, vast majority of it would simply leave the earth’s orbit (aircraft only fly about 7 miles high, and this thing is 60,000 miles high).
More likely, the forces involved would cause the thing to break into a whole bunch of small pieces. Depending on the height, they would A) fall straight down, B) Fall at an angle until they hit the atmosphere, then either burn up (if they were high enough to be going real fast), or rapidly slow down to terminal velocity and fall down. Btw, terminal velocity for something really long and thin is quite slow.
Worst case scenario is probably that a chunk maybe 50-100 miles long or something comes down over a populated area. How heavy is this thing per linear mile? Not that heavy, I’d imagine. Certainly at the thickest parts it would be pretty heavy, but not as heavy as an equivalent metal structure - the whole point to using carbon nanotubes is to get much more tensile strength per pound than steel.
So I think that part of it might be manageable. But that doesn’t account for the economic damage, which would be immense. Not just the cost of the tether, but the cost of not being able to launch all those payloads until another one is built. Just like the collapse of the World Trade Center wound up costing the economy more than 10 times the cost of actually building the towers.
Just the opposite. The ends are very thin, and the middle is extremely thick. This is because (e.g., at the end pointing towards the Earth) any given point needs to support the weight of everything lower than it in the gravity well, which dictates a minimal thickness, since (weight of everything below) = force = pressure * area, and the max allowable pressure is determined by the material’s strength. As you go higher up the tether, the force increases, therefore the area must increase. And when you do the math, it turns out to be an exponential increase. The only way to make it even plausible to build a space elevator is with magic materials (and I think even carbon nanotubes only make it “plausible” not “simple”).
As far as the stability question goes: I haven’t seen any studies on space elevators themselves, but I have seen some research on the control of large space structures. Unlike a simple bridge (which at most has some passive damping system), any space elevator (or similarly large space structure) would require active control of its motion.
ftg, powered flight was considered a joke in many engineering circles at one time. So what?
A ‘beanstalk’ would be constructed from geosynchronous orbit, the cable extruded both upwards and downwards. The upward mass of the cable would counter the downward mass – in other words, it ‘floats’ in a stable orbit.
1 hour before ‘touchdown’ the cable wouldn’t touch the ground. An hour later, it would. Clear?
I think ftg was referring to the dynamics of the tether. Before it touches down, you’ve got a loose end flapping around in the atmosphere. How do you grab it when it’s 10 miles up? By the time it gets to the ground, how far has the ground end drifted away from where you want it to be? And more to ftg’s point, does all that moving around due to wind send oscillations up and down the 100,000km length of the thing and turn it into a giant Tacoma Narrows?
They’re valid questions, but I’m reasonably certain the problems could be overcome. On the ends, you’d want to put some kind of set of rocket engines to maintain horizontal position, and along the length you’d want some set of oscillation dampers.
How are carbon nanotubes for strength in shear and other directions other than in tension?
What about resonant modes as this thing is being extruded? As it hits the atmosphere and is excited by atmospheric winds, and its length continually changes, what do you do when you hit a resonant mode that causes the amplitude of the swinging cable to diverge?
I wouldn’t go so far as to say that the tether is impossible, just that the 15 year timeline is nuts. You need to do a lot of research into hundreds of issues surrounding the thing. Plus, every single piece of equipment used to build it, from shuttle payloads to cable walking thread spinners to the cars that ride the tether and emergency systems for getting payloads off it would require basic research.
Many of these things can’t be researched on the ground. That means extremely expensive space-based experiments. Some of those experiments will fail, like the Shuttle’s tether experiment.
Materials will have to be tested under space conditions (extreme temperature changes, no atmosphere, bombardment by micrometerites and cosmic rays, etc.
If the U.S. got serious about building a tether, it might be able to do enough research in 10 years to start the actual engineering plans. The first construction satellite might fly 10 years after that. The thing might be open for business 15 years from that point, assuming no major snags are found, and assuming that it can be kept stable either passively or with active components.
In the meantime, other forms of access to orbit will get cheaper, and undercut markets for the tether. This in turn would threaten funding.
If memory serves (memory of papers read 3-6 years ago), the strength is still pretty good but the elastic modulus is pretty small. The things can bend very easily, but won’t break without a lot of work. I have a fuzzy memory of an experiment showing that the atomic structure of the tubes could change before breaking – basically, the tube ‘extrudes’ into a narrower tube in certain cases. But I really don’t remember where I saw this, so I can’t provide refs.
Ionized oxygen (basically O- ) is a big threat to the chemical stability of anything, especially anything organic, you leave in orbit for a long time.
I don’t know about other forms getting too cheap (this assumes such a project could get funding in the first place). If you consider just the cost of electricity equivalent to the energy needed to place something in orbit, the cost would be about $1 per kilogram (or maybe it was pound? I forget, but it’s ‘only’ a factor of 2) – this would be the cost of beaming that laser energy to the climbers. Even if you amortize the costs of building the tether, consider efficiences of lasers, and throw in some operating costs, the tether shouldn’t cost more than ~$50/kg. Current costs of launching stuff to space are on the order of $10,000 per kilogram. Future chemical rockets could maybe get down to $100/kilogram, but nothing on the horizon promises to even get below $1,000/kg. Heck, if other launch methods do come down in price, they might stimulate the markets to develop that would eventually demand something cheaper like a tether.
Well… I think it’s tough to put a dollar figure on amortization without having a better estimate for what this thing would cost. I have a hard time believing that a 60,000 km long tether made out of exotic materials and new engineering techniques could be built for less than the cost of the Gibralter bridge. That 15 billion figure is picked out of the air as far as I can tell, and doesn’t include major costs like financing, insurance, return on investment, time-value of money (i.e. you get to put up billions of dollara that won’t see a return for years, at which point the money would be worth much more), R&D, support equipment, maintenance, (how do you repair a flaw in the structure that occurs 150,000 ft up? 1000 miles up? Working in space would probably be even easier than trying to work in the thin upper atmosphere with 200 mph winds.)
Guys, guys; there’s an easy way to build this thing that nobody seems to have yet paid any consideration: temporarily remove the Earth’s atmosphere - Presto! no turbulence or friction problems when placing the tether. After a year or so (maybe less, but we don’t want to rush the job and cock it up) , we can put the atmosphere back and everyone can start climbing the thing.
My understanding is, you don’t. What ever ‘extrusion device’ is building the thing from geosync orbit just keeps extruding and the magic of orbital mechanics eventually orbits it down to ground level.
It wouldn’t be moving at all. It’s in geosynchronous orbit from start to finish. Which means its motionless relative to the ground.
A good question. But there would only be wind in the atmosphere, say the first 70km or so. That leaves 99,930km wind-free, which sounds like there’d be quite a lot of unmoving structure to help stabilize the rest.
If anyone’s interested, there’s quite a good, although too brief, write-up of ‘beanstalks’ (as well as other odd science-fictioney topics), in Charles Sheffield’s The Borderlands of Science, including material strength needed, taper factor of the structure, and possible construction techniques.
Personally, I think earth-based mass-drivers have a much brighter future in the near term than beanstalks, but that’s just my own hunch, FWIW.
Only if it’s held there, e.g., by something whose center of mass is in GEO.
Squeegee, go fly a kite. As an experiment, I mean. When flying a kite, you’re motionless relative to the ground, but your kite is not. Now imagine a tether with a cross-section that’s narrower than your kite, but much longer. It’ll get blown all over the place. And I didn’t mean to say you have to grab it when it’s 10 miles up, but you may want to if it’ll otherwise get blown into the next state.
Now we’re getting into the interesting stuff. A simple mass-driver wouldn’t be great for putting stuff (especially people, because they can only take 3 or so g’s of acceleration) into orbit because you give something all this velocity and then it loses a bunch of that velocity punching through the atmosphere. But have you heard of the various momentum-transfer schemes (Space Fountain, Launch Loop, etc.) that are based on mass drivers? You basically make a tube (preferably with a vacuum sealed in) and electromagnetically accelerate a stream of magnets inside the tube, then you electromagnetically push against those magnets further along to provide lift.
Hmm, that didn’t come out so coherent. How about this: Imagine aiming a hose with high-pressure water up at an angle. Now hold a plate in front of the stream, at an angle to the stream. The stream will be deflected, and the plate will feel a resultant force. Use a similar principle with mass-driver flung objects (generally magnetic small objects), and you can “hang” a platform up in orbit. These concepts do not require any magic materials – it could be done today. But it would be by far the largest engineering project ever undertaken. And you’d want to make damn sure your power supply couldn’t fail. Even more so than space elevators, these concepts would require a large demand for space access – either they’re up or they’re not. And if they’re up, they can put hundreds of thousands or millions of tons of stuff into orbit per year. It’s sort of like a railroad – you don’t build one for Lewis and Clark, but eventually it becomes worthwhile.
It’s a function of where the center of mass of the beanstalk is, which would be carefully arranged to be up at 22,500 miles, even though one end is down here in Des Moines.
Think of it this way: the Earth is 8000 miles thick, but the center of its mass is in the center (or close enough) of the sphere (okay, oblate spheroid). But this works for any shape, not just sphere. Take, say, a set of barbells 50,000 miles long; assuming you have same number of weights on each end, its center of mass is still in the center of the structure, and therefore it’s orbit depends on where that center, and not the ends, lies.
Hmmm, now, suppose the elevator is just a pair of weights connected not by a bar, but by a piece of fishing line; the centre of mass is going to be halfway along the fishing line, but this is pretty much irrelevant as the upper mass is in a geosynchronous orbit but wants to go much slower, the lower one is also in a geosynchronous orbit, but being much closer to the earth, wants to go a lot faster (conveniently ignoring atmospheric effects for a moment); the fishing line will break.
OK, so fishing line isn’t a good idea, but the point is that regardless of the theoretical effects on the centre of mass, we’re talking about a massive object which is going to experience immense internal stresses as a result.
As an ex-space station engineer, four important needs come to mind.
Cheap access to space would cut decades off the time to colonize Mars and/or the Moon.
The cheap access would allow us to build huge space stations, and replenish them at bargain rates. The whole viability and design of space stations would change.
The elevator could move loads at a very low acceleration, which means things like fragile people and equipment could reach space. Fragile things built in orbit could come down.
Insurance costs for “payloads” would come down an order of magnitude. This instantly reduces overall satellite cost by about 25%.
A possibility. But. If the elevator can stand up to hurricanes, it would take quite an act of terrorism. As far as air attack, the elevator could have a 20 mile no-fly zone, and anti-aircraft missiles. The space elevator would be under military protection, like existing launch complexes. No terrorist activity has happened there. Finally, we can’t let terrorists dictate about what would be one of the most significant steps in human history! So they blow it up. We build another one.
The elevator would break into three kinds of pieces: ones that fell in pieces to Earth, ones that started to fall, but were burned by the atmosphere, ones that flew off into space. The first and third are the worst, but the first could be mitigated by parachuting or destroying the falling pieces (as they do with out-of-control rockets).
Seems to me these are big challenges. The elevator would have many times the cross-section of a bridge like the Golden Gate. That has moorings the size of quite large buildings, several of them. Yow! Imagine moorings to control the biggest sail in history, in a hurricane!
As a point of clarification, you’re referring to the experiment where they were trying to generate electricity with the tether? Our elevator wouldn’t necessarily try to generate electricity…but then again, maybe it should.
Yeah, the guy they were interviewing did seem to have a vested interest, not in the social or engineering pragmatics of the elevator, but in getting a several million dollar grant. By the time the project failed or succeeded, he’d be retired and rich, either way.
Right. The elevator to geosyncy is kinda like Jules Verne deciding he had to shoot somebody to the moon all in one big bang. If super-strong miles-long strands existed there would be a lot of other possibilities. Yum. How about a Mars or Moon elevator? Elevators between the Lagrangian points?
Um, I think it couldn’t have a cross-section even as big as the Golden Gate, because if it did then they wouldn’t be able to launch it on the Shuttle (or anything else for that matter). Building a space elevator is only practical if the materials are so amazing that the earth-end (remember the ends are the narrowest points) only needs to be as thick as your arm or maybe your body (ie. wide enough to hold 50 tons or whatever it is that your climber + payload weighs). I’m not kidding – the bottom end could probably actually be much thinner! (My calculations suggest that, even at a material strength lower than you could use for a space elevator, the tether only needs to be about 1 cm - yes, less than half an inch - thick to carry 50 tons.) Because the required tether diameter increase exponentially, the tether stays pretty thin until you get close to the center of mass – it’s the last 1,000 km or so where the diameter starts going up like mad.
I don’t think so, by virtue of the fact that it’s not cutting through the magnetic field. Since Earth’s magnetic field rotates with the Earth (it does, doesn’t it?), and since the tether ‘orbiting’ the Earth in a geosync orbit, it wouldn’t be moving relative to the magnetic field.
Partly Warmer: Yes, they were intentionally trying to induce a current in the tether by dragging it across the magnetic field lines of the Earth.
Cobalt: I don’t know the dynamics of the Earth’s magnetic field. But I should point out that the magnetic pole is about 11.5 degrees offset from the geographic pole, so if the field rotates with the Earth, anything rotating around the geographic center will still be cutting through the magnetic field lines in a sinusoidal fashion. So yes, this thing is going to generate a pretty substantial current. It may be manageable, and it might even be used as the power source to move stuff to orbit.
My point was simply that it’s going to take a lot of research to sort out issues like this before you can start drawing blueprints. And research that requires space launch is expensive and slow.
I forgot to mention before: you’ll only get an electric current if the object you’re dragging through the magnetic field is a conductor. Some nanotubes are, and some aren’t. I think making a tether which is not a conductor shouldn’t be too much of a problem. As far as using it as a power source, you wouldn’t want to do that, because the energy has to come from somewhere, and when you’re a tether in space, that energy happens to be your orbital energy. So if you keep taking energy out of the system, eventually you slow down and fall out of orbit. Which kind of defeats the purpose of putting the tether up.
I agree on both points. It’s that capital-intensive nature that bites you in the butt every time with space launch.
You can’t slow down the tether, because it’s attached to the Earth. What you would do is slow down the entire earth, but imperceptibly. It would be like the mother of all regenerative braking.
I don’t know enough about carbon nanotubes to make an educated comment about the ease of making them as insulators, but don’t forget that you also have the cars running up and down, you have ancillary equipment, cables, impurities gradually accumulating on the surface, etc.
That’s one reason why you’d need a TON of R&D before you can actually build one of these things, even if we knew how to make 60,000 km nanotubes today.
Oh, as for the strength of the bottom part - You DO need a monstrous anchor, because you need to put the entire tether under tremendous tension in order to maintain rigidity. If the center of mass of the tether were right at geosynch orbit, you could theoretically float the end 1" off the ground and not need an anchor at all. But the thing would be unstable. Most designs I’ve seen put gigantic masses at one end (like an entire space station or even an asteroid), and a huge anchor at the other. That means the thickest part will be much, much thicker, but it also means the thing will be under so much tension that other forces acting on it will be trivial.
The Earth can’t push against itself. IF the tether is moving relative to the Earth’s magnetic field, the action will be on the conducting material – ie. the tether. The current that is set up by the conductor’s motion through the magnetic field will cause a force that serves to reduce its speed through the magnetic field – ie. it will slow down relative to the magnetic field (same thing as decreasing its orbital energy) and fall to a lower orbit.
I’ve been thinking about your assertion some more, and I can’t visualize the effect you’re talking about. As far as I can figure it, even if the magnetic field axis is not aligned with the rotational axis, it’s still rotating along with the Earth, so the magnetic field at any point is stationary with respect to the point below it on the Earth (the magnetic field does not also rotate about its own axis). Can you explain in more detail why you think the fact that the magnetic field’s orientation matters?
Agreed. But just because your anchor is large doesn’t mean that the cross-section of the tether itself gets to be that large. (OTOH, I do agree that the wind forces on the tether will not be negligible.)
When flying a kite, you’re motionless relative to the ground, but your kite is not. Now imagine a tether with a cross-section that’s narrower than your kite, but much longer. It’ll get blown all over the place. And I didn’t mean to say you have to grab it when it’s 10 miles up, but you may want to if it’ll otherwise get blown into the next state.
A simple mass-driver wouldn’t be great for putting stuff (especially people, because they can only take 3 or so g’s of acceleration) into orbit because you give something all this velocity and then it loses a bunch of that velocity punching through the atmosphere. But have you heard of the various momentum-transfer schemes (Space Fountain, Launch Loop, etc.) that are based on mass drivers? You basically make a tube (preferably with a vacuum sealed in) and electromagnetically accelerate a stream of magnets inside the tube, then you electromagnetically push against those magnets further along to provide lift.