I get that a geosynchronous satellite can have a line down to earth that doesn’t move relative to us, however, sending anything up this line would require it to pull the rope thereby reducing this satellites orbit and bringing the whole ball of wax down to Earth.
Even with counterweights or whatever you are still ultimately pulling the satellite down.
Gravity itself acts on the rope in our atmosphere, the satellites orbit would degrade if we never even used it.
This is why the top has to be somewhere above geosynchronous height. The bottom part of the cable is trying to fall down, but the top part is trying to “fall” up, and balances it out (actually, more than balances: You’d want some margin of tension at the Earth anchor point).
As for the benefits of this thing, rockets are (barely) adequate for the sorts of things we use space for right now. But if we could bring down the cost of getting things into orbit by many orders of magnitude, it would open many more applications which aren’t even considered right now. You’d start to see things like asteroid mining, or zero-g manufacturing, or space tourism, or low-gravity geriatric homes, or many other possibilities science-fiction authors could tell you about.
Altitude is determined by the speed of the satellite (I’m simplifying here, don’t pick nits). Basically, the faster something is going, the higher it wants to orbit. The counterweight would be held by the elevator in a lower orbit than it “wants” to based on its speed. This causes tension to be formed in the cable, and so long as the weight of the cargo going up the elevator is less than this tension, the counterweight won’t be pulled down. Even if it were, it doesn’t matter, because orbit altitude is based on speed. Once the cargo got to the top, the counterweight would return to its original orbit. The only way the counterweight would come closer to earth is if its speed were reduced.
Fifty billion is a preposterously low estimate. (The $10 billion claimed by Brad Edwards is plainly dishonest.) I would guess that if we started now, a space elevator would cost, at an absolute minimum, one trillion dollars, and that’s if everything goes well.
Nobody’s even yet inented a practical way of making things go up the elevator.
According to this, we’ve increased the strength of this nano fiber 100 times over the last five years, and it needs to get about 180 times stronger than that, but it seems steady progress is being made in that direction.
The Japanese think they can do it for a trillion yen (£5 billion). I don’t believe that, but they are taking it seriously.
It’s the little matter of the counter-weight that people seem pretty optimistic about. Oh, just hook an asteroid to one end.
Uh, where are we planning on getting one?
Quite possibly, if we started now. That’s why we wait for other applications to drive the price of nanofiber down first, before we start on anything elevator-specific. That stuff has impressive enough properties that there will be demand for it, and there’s nothing that makes it inherently expensive.
That could be cool. Just think, North America vs. Europe in a giant game of tug-o-war.
Its most likely gonna burn up in the atmosphere, whether it falls straight down or goes flying sideways as it comes down.
Why not just manufacture a counter weight?
Start with a small cable with a small counter weight and send up more mass to be a counterweight. Then thicker cable then more mass. I don’t get the orbit dynamics either though.
If you’re pulling your way up the cable than you have to be pulling down on the cable and you can’t get blood out of a turnup. The energy has to come out of the counter weight.
Not quite IF I understand/remember this thing right.
It takes energy to climb the cable, but much? more is supplied by the cable system.
IIRC this “free” energy is supplied by the earths rotation being slowed down by an itsy bitsy bit.
Its late, I’m tired and this could all be BS though…
I’d say the carbon nanofiber will be the least of your expense concerns.
Explain to me how they plan to string it up fifty thousand miles. That task alone promises either disastrous or comical misadventure on a truly global scale; I can see the CNN footage now, “Yet Another Space Elevator Cable Floating In Pacific.” How’re they planning to rocket a space station that big into that high an orbit? How are they going to power the elevator - again, something nobody’s ever satisfactorily explained beyond “uh, like, we’ll invent a laser thing”? What amazingly strong edifice is going to hold the tether down?
A space elevator would not just be the biggest engineering project ever undertaken, it would be the biggest by at least two orders of magnitude. It’s a project of staggering complexity, construction effort, and orbital launch effort, and requires the invention of many things that do not yet exist. So as per the OP. it’s a pretty bad choice for a fiscal stimulus idea.
Oh, I never disputed that. It might be a good choice to stimulate the economy out of the Recession of 2063, but it definitely won’t be shovel-ready in time for this one right now.
Why are we still debating the practicality of building a ginormous space elevator? It doesn’t have to be huge. It has to be very long, yes. But it doesn’t have to be thick or wide. The first one can be 40,000 kilometers long, 5 cm wide, and 10 microns thick. The starting weight at geosynch could be less than what we’re currently planning to boost to translunar trajectory. If the first version can only lift a metric ton at a time, that’s still good enough to get servicable satellites into geosynchonous orbit, and then it can start lifting it’s successor.
If it’s only 40,000km long, then you have to attach a gigantic counterweight to the end floating out there in space, and that makes deployment that much more difficult. Also, the materials to build the 40,000km ribbon… wait for it… don’t exist. And even if they did exist, the cable is a design nightmare. It’s tens of thousands of miles long and every single point along all that length is a point of critical failure. The entire mechanism is a single point of failure. And if it fails, it fails catastrophically. Best case scenario, the whole thing just floats off into space and is never seen or heard from again. Slightly less best case scenario, it burns up in the atmosphere before it gets the chance to kill anyone directly or give them cancer.
As for being thick and wide, if you try to build one out of any material actually known to man, it does have to be thick and wide, as well as long. That’s because your cable diameter grows exponentially as you approach the top, in order to support the cable hanging below. So unless you make it out of concentrated magic, it’s going to be both thick and long. So step one is to invent the strongest and lightest material ever, and then follow up by producing gigantic quantities of it, which you ship into space. Once you get it to space, you park it in an unstable orbit and just hope and pray that the stuff you built it out of is resistant to pretty much every form of radiation in the universe, as well as the thermal effects of cycling from daylight to darkness in a 24 hour cycle.
If you completely ignore all of the practical problems of building it, it just barely works in the idealized world of a single planet, with no atmosphere, and no other bodies in the universe. Truly, the space elevator concept is like storing your sharpened pencils balanced on their tips on your desk. They’ll never fall down because if you set them down just right, they will be in equilibrium and there is no power in the universe strong enough to upset that.
Arguing about building space elevators is a bit like arguing about building trans-Atlantic tunnels. Sure, it would be awesome to ship BMWs from Germany to the United States via automated underwater supertrains, but it’s not going to happen in your lifetime, or anyone else’s, because the project is just not feasible. Even if somebody with a Ph.D. made a poorly written report to a NASA subsidiary and has an equally poorly written book available for purchase on Amazon.
Tenebras, I think you’re way overstating the problems that need to be solved.
First of all, if space elevators were truly inherently unstable, somone would have produced a paper decades ago pointing this out (the way fans quickly calculated that Niven’s hypothetical Ringworld was unstable), and no one would even be discussing the concept today. Any tendency for the tether to either drift away or collapse to Earth is solved by the simple expediency of anchoring it to the ground and giving it a net tension away from Earth greater than any perturbation that would move it towards Earth.
Second, you’re still thinking that the tether must require millions or billions of tons of mass, if it’s to be made of “any material actually known to man”. We have a material known to man that is strong enough: carbon nanotubules. The only question is whether we can actually produce a real-world cable within an order of magnitude of the theoretical maximum strength that carbon nanotubules can possess. For strength ratios considered achievable, the “exponential” increase in thickness might be 50 to 1. So Space Elevator 1.0 would be a ribbon 10 microns thick at the ground and half a millimeter thick at the top. Weight to be initially boosted to geosynch orbit is on the order of hundreds of tons, not millions. We can’t do this today, we can’t do this next year, we probably can’t do it in five years. But it’s not wild-eyed fantasy to suppose we might be able to do it in twenty years.
Third, the single-point failure argument can be solved by simple redundancy, like any load-bearing application of cable or rope today. You have a 2-3X safety factor built in, and you use multiple strands so that the failure of any one doesn’t singlehandedly snap your cable. Protecting the cable against various sources of degradation is a serious concern, but again probably not an insurmountable one.
Anybody know how often that cable is going to be hit by orbiting debris?
Gut feeling tells me its is more than a year but not so long a time frame it won’t even be a worry either.
I obviously don’t think I’m overstating them. I actually thought I was being optimistic by admitting it would be possible if you could distill magic. 
A space elevator is, fundamentally, a geosynchronous satellite. It’s kind of a tricky geosynchronous satellite in that it has a feeler which reaches down to the surface of the Earth, but that doesn’t really change anything. That’s what makes the idea neat in the first place.
(If you want to make your life as simple as possible, you put it into a geostationary orbit, otherwise you have an even trickier engineering problem to solve, what with the bending and whatnot.)
Now, nobody needs to publish a paper to show that this orbit is unstable, because it’s already known. It’s not even particularly difficult to show, and neither is it a great mystery. Wikipedia knows it. Here’s a derivation. The reason the orbit is unstable is the same reason that any orbit is unstable, because small perturbations from other bodies such as the moon or the sun increase or decrease the speed of the satellite, which necessarily change its orbit. Normally this isn’t such a big deal, and you just put a small thruster on the satellite to push it back into place. When it runs out of fuel, you put up another satellite. If it drifts a tiny bit, no worries, it’s not like your communications satellite is a 50,000 mile string attached to the ground or anything.
As for attaching this thing to the ground, what exactly is the plan to do this? All of the space elevator people say “Oh, we’ll just attach it to the ground!” and pretend that’s the end of the story. Attach it where and with what? If you attach it out in the ocean you’re talking about a permanent sea base in the middle of the pacific, which I think is a much more likely project to be completed in our lifetimes. (slim chance being somewhat more hopeful than fat chance) If you attach it to the ground somehow, then you can’t very well move it around if you see an asteroid coming, now can you? More on this point later.
Look, here’s an incredibly optimistic paper (pdf) written by a guy who works for a Space Elevator Booster corporation. (Get it, booster?) Check out Figure 2. CNTs have a density of between 1.3 and 1.4 g/cm[sup]3[/sup], which puts us between the yellow and the green curves. (If we build it out of water, we can use the red curve.) According to wikipedia, in 2000, somebody built a nanotube that measured a tensile strength of 63 GPa. This seems to be a reference to this paper. (abstract only) You’ll note that the tubes ranged in tensile strength from 11 to 63 GPa, although 63 GPa is the figure which seems to be cited as the “tensile strength of CNTs”. You will also notice that this was not a test on a macroscopic scale. The entire testing apparatus fit into the imaging chamber of a scanning electron microscope. (That is, it 'tweren’t very big.)
Back to the point, referencing figure 2 of the incredible optimistic paper, we see that if we have a tensile strength of 50 GPa we’re talking about a mass of between 400 and 700 metric tons. (I’m estimating the green curve, obviously, but it looks to be a bit larger than 1.5 times the yellow curve, and both curves ought to be exponential.) That’s if the cable can uniformly withstand 50 GPa and we accept a factor of safety of 1. If the cable is weaker, the situation only gets worse, of course. (Remember, this is the graph which gives bare minimum masses supplied by the true believers who think the whole thing can be put into operation for only $1B.)
For comparison, the mass of the ISS in October 2008 was 227 metric tons. (wikipedia or the NASA fact sheet (pdf) the wikipedia number came from)
If you put two of them next to each other and one of them snaps, I’ll wager my dollars against your nickels that the broken one takes out the whole one. That’s rather the problem with a cable several tens of thousands of miles long under truly ridiculous tension.
Now, about moving it around to avoid collision, and other fantasies. In the first place, it’s an incredibly long ribbon trailing off the Earth. You have a snowball’s chance in hell of moving it in an equatorial direction if you’re very careful and your god smiles upon you, but you can’t move it longitudinally because, well, because it’s a big ribbon trailing off the surface of the Earth. Even if you don’t have it based on the equator, once you get a couple thousand miles up, you might as well be. Of course, trying to move it is just going to set the thing waggling, at which point it rips itself to pieces. Congratulations.
Speaking of which, suppose that we build this wild and crazy thrill ride in the sky. Every country on Earth has its own little sector of geostationary orbit mapped out for it. If any one of those countries puts a satellite in orbit whose orbit then decays (after, for instance, the satellite runs out of fuel) it’s going to go straight through your cable. For that matter, there are very few orbits (read none) which don’t cross the equator. So every single piece of detritus in orbit around the Earth is potentially going to hit your cable. Since nothing is in geostationary orbit for long (on account of that pesky moon of ours, for example), all of that stuff is going to be swirling around our precious space elevator.
I saw one of the people writing about this idea said that it would be safe from terrorist attacks because you could build it in the middle of the pacific, and then just not let anybody within a thousand miles of that patch of the Pacific. Pretty theory, but completely worthless when you consider that Iran just popped a satellite into orbit, and the Chinese and the Russians have been doing it for years. If they really wanted to piss us off, they’d just lob a fragmentation grenade into orbit and then watch the pretty light show as the elevator collapsed. Or, blow up a satellite in orbit and make a big ass (technical term) debris cloud.
Depends on what you mean by orbiting debris. Something big? Doesn’t happen that often, but it definitely occurs. Little stuff? All the time. NASA FAQ on orbital debris. Here’s another page talking about it. Geeky blog (astroprofs) NASA sent a satellite into low Earth orbit and then recovered it. In just under 6 years they recorded 20,000 impacts. NASA page on orbital debris.
Funny story, they sent the “let’s get smacked with debris” satellite up and intended for it to orbit for around a year, but then Challenger happened and they couldn’t get it back until almost 6 years had elapsed. Ok, not funny-ha-ha funny, but I still like it.
Well, to be fair, using the Shuttle to place mass into orbit aint optimum at all.
Though , no matter what current or near current rocket system you use to get all that mass up there is going to be pretty darn expensive.
Not that most of the rest of your post doesnt have good points…
I don’t think that “unstable” means what you think it means. Most orbits are not unstable, unless you’re talking about things like gravitational radiation which would take longer than the lifetime of the Universe to bring a satellite down. Unstable does not mean that there are perturbations on the orbit; there’s perturbations on everything. The question of stability is how a system responds to those perturbations. If a system is unstable, then the perturbations will grow exponentially; if it’s stable, then the perturbations will be damped out or reversed. Geosynchronous orbits are stable, which means that the effects of the perturbations will not be significantly larger than the perturbations themselves, which is to say small. A space elevator is even more stable than a standard geosynchronous orbit, since it’s further stabilized by being anchored to the Earth at the bottom.