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Space Elevator May Become Reality
mojotek writes: "The NASA Institute for Advanced Concepts has a study(15Mb pdf) about the feasibility of a "Space Elevator" comprised of a 22,000 mile long cable built out of carbon nanotubes. In theory, it would be able to carry loads of 20 tons to space without using a single rocket engine. Sounded way too sci-fi for my taste at first, but this article at TechTV actually helped fill in the holes."
I did the math and worked out that if you gibbed the cable (say 1m chunks), you'ld wind up with something like 25-30 thousand km (I don't remember the exact figure) of the cable crashing down on earth and the rest flying off into space. However, I didn't figure out if the cable would fall east or west (west would be better, but I think it's less likely). Either way, that's a little over 1/2 way around the world and while the only land mass likely to be hit is Africa, I don't imagine the impact with the water would be particularly fun (possible tsunami).
Bill - aka taniwha
--
Leave others their otherness. -- Aratak
The line might generate a lot of electrical potential if it didn't remain stationary relative to the earth's magnetic field... Also, wouldn't things like wind, static electricity, lightning and auroras cause problems with a 22,000 mile long cable?
"Leave the strategizing to those of us with planet-sized brains." -Tycho
Because if it fell down, it'd be about as destructive as a thermonuclear bomb (kinetic energy's a bitch). And NOBODY would want this in their back yard after 9/11.
On the moon, Mars, any other sparsely-populated/unpopulated body in the solar system? Sure. But not here.
You can't have the orbital part without a counterweight otherwise you have gravity pulling down on the vast majority of the cable and the whole thing falls out of the sky. So you need a mass at the end of the cable so angular momentum holds everything up. Last I heard you needed a lot of mass to do that -- like a trapped asteroid or something -- far more mass than we havet he technology to put into orbit.
The short answer is: Yes.
Physics works everywhere all the time. When you climb a flight of stairs or walk up a hill it slows the Earth's rotation - and it speeds back up as you walk back down.
No - seriously - just as an ice skater's rotation slows or speeds as they extend or contract their arms the same principles apply to all rotating bodies. Everytime we slingshot a space vehicle around the Earth we are effectively transfering some of the planet's energy to the vehicle and that energy has to come from somewhere.
But the amounts here are so small that the effect is not measurable or "effective" in the scale of anything we could notice. It's like the fact that anything with mass has a gravatational field - but you don't notice the effect of the gravity created by your pen.
=tkk
Bill Gates - Creationist?!?
There are some variations on the idea though,like this one, that are close to being possible with today's technology, and can even be provisionally costed. Basically the idea is to construct an elevated runway about 100km up, and use mass drivers to hurl stuff into orbit. At that altitude the saving from air resistance is huge and mass drivers become very efficient
At this stage, NASA speanding serious time thinking about space elevators is probably no more useful than daydreaming. Thinking about this kind of thing is probably more productiove though, becuase something might come of it in the medium term, and its almost as efficient as an evelator anyway - with the decided advantage of not being able to collapse and strangle the planet.
(Since I heard about this from a NASA researcher, maybe Im being a little harsh to accuse them of daydreaming)
Mars is a much better place to experiment with spacehooks like this. It's easier to build them there, they don't need to be as big, and there wouldn't be the same disasterous consequences if and when something goes wrong. Larry Niven's written a fair amount about it, see for example The Barsoom Project.
I hope I don't get modded down for this idea like I always do but here it goes anyways..
I've read several books which include the idea of a space elevator, and one of the key problems had to do with bringing that much cable to space, and the strength of the cable to stay together. The closer the cable gets to earth the harder the pull, the further out the "satellite" holding the cable in geo-synchronous orbit has to be. Instead of bringing the cable down to earth.. or putting it atop a very high tower, why not create a platform 50-80,000 feet up for planes to land on. This would save very large amounts of cable from being created, the satellite wouldn't have to be nearly as far out either to compensate for the gravitational pull from the cable below. Also, to compensate for the excess weight of the aircraft and payload while landing, the satellite holding the cable could move up and down to balance any weight added or removed to the cable.
Now, having a shortend cable would have added benefits too, in the event of a disaster, normally a cable attached to the earth would wrap around the planet several times causing an incredible amount of destruction. This could be minimized with my platform idea. Imagine something colliding with the cable causing immenant failure... why not create sections in the cable to automatically break off in the event of a disaster, this would minimize the amount of cable falling to earth, and the remaining cable would be either ejected into space, or depending on how an object hit, its possible the upper section could re-establish a geo-syncronous orbit after losing much of the cable.
Any pysicists out there able to agree/disagree with this? The tether would also most likely have to be conical in shape, thicker higher up, and thinner below to minimize the amount of carbon tubing used in the elevator.
One big issue they missed is the fact that a carbon nanotube cable still isn't strong enough to support it's own weight without tapering the cable correctly, at the middle it has to be about 10 times thicker because the stresser are highest at geostationary orbit.
The deployment method they're using doesn't take account of the fact that you need the thickest part to always be at the middle - if you simply unroll it the way they suggest then the incorrect thickness profile will result in the cable exceeding it's breaking point and snapping.
What they need to do is unfurl a cable like this from geostationary orbit simultaneously up and down at the same time. The Mechanism to do this would have to be very delicate at unfurling the last kink or the cable will again snap.
The cool thing about this is if you figure out what kind of weight you want the cable to support then you can come up with an idea of the amount of energy stored in the tension. If the cable snapped at any point then the amount of energy released would be pretty phenomenal. From each end of the snap you'd generate a compression wave which would get stronger as it travelled along the cable, after a while of picking up energy it may turn into a shockwave and snap the cable again (essentially shattering the cable). If it doesn't then the wave will have energy equivalent to nuclear weapons when it reaches the endpoints and the waves transmit themselves into the supporting structure....
Are you familier with geosync orbit? The space shuttle is in low earth orbit and thus has to complete an orbit in approx. 90mins in order to "fall" around the earth. But if you get up to about 36,000k (I think thats right) your orbital time would be exactly 1 day. In other words you would be in free fall around the earth, but the ground below you would be rotating with you.
Look at the moon for example, it is so high that it takes ~30 earth days to orbit.
I hope you're not pretending to be evil while secretly being good. That would be dishonest.
If you'd like to see a surprisingly realistic sci-fi version of this, I suggest you take a look at Bubblegum Crisis 2040, an anime series that most geeks would really enjoy anyway, even if just for the interesting sci-fi ideas and the references to American sci-fi movies like Blade Runner and Alien.
It would totally depend on how far from the earth you were. If you are exactly on a geosyncronous orbit then you would definetly feel weightless no matter what. I'd assume that such a space elevator would be "anchored" on a geosyncronous orbit since otherwise it would drift and probably break the whole assembly.
If you are below the geosyncronous orbit you'd feel slight gravitational pull and above it you'd feel the effect of sentripetal force of the elevator keeping you attached to the earth - you'd actually be standing on the roof then.
Shuttles are normally orbiting the earth at a speed and height (mv^2/r=GmM/r^2) where earths pull is just enough to keep them on a steady circular course around earth - so they are technically free falling but never approaching earth. Geosyncronous orbit is just a special case where you're going at the same angular velocity as earth.
And it is exactly that, sci-fi. Sure, carbon nanotubes are incredibly strong. And they're also on the order of a few microns long. Now, this cable needs to be a few hundreds of thousands of meters long. You do the math.
The semiconductor industry figured out how to make large single crystals of ultra-pure silicon, then pattern the surface down to a ridiculously fine resolution. The fiberoptic folks figured out how to make glass so clear that a light pulse can go through many many miles of it and still be recognizable at the other end. Molecular biologists can "amplify" single molecules of DNA into macroscopic quantities.
I wouldn't be so quick to say that we will never be able to make carbon nanotubes that are long enough to be useful as structural materials.
Actually, there's strong evidence that the number of large dams constructed over the last few decades have changed the length of the earth's days. Not by a huge amount, but I think it has started to affect the introduction of leap seconds.
(The main reason the earth is slowing down, IIRC, is the tidal forces from the moon and sun. If the moon was gravitationally bound to the earth it would be falling, but since it's not it's slowly drifting away.)
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
I can take a good reason to build an elevator straight out of a book called 'science of the Discworld'. Basically, the argument goes that if you have an elevator into space, then you can reuse energy, whilst if you have a propulsion system then you cannot.
How does this work? Simple. After you have successfully sent so much stuff into orbit, your going to start to want to bring things back down, whether this be from mining other planets or simply getting the astronaughts back to their parents. Normally, we waste all of the energy on reentry because we don't use it for anything. With an elevator, the energy being exerted by gravity on the way down can be used to balance out the gravity being used to get other stuff up. Hence, you don't need as much energy overall to get stuff into orbit.
And as others have already stated, once out of the earth's gravity, you don't need that much energy to move around at all...
A sci-fi/sci-fact magazine in paperback form called Destinies had a story about this in their Aug-Sept 1979 edition. The story was called "How to Build a Beanstalk" by Charles Sheffield. He did some research into the material strength required, and to get the stalk to reach down to earth, or somewhere near it required a material with a tensile strength of 2 000 000 kg/cm^2, which was 10 times the current known tensile strength of known materials at the time.
"Beanstalks, originally called skyhooks, are an idea of the 1960's whose time may at last have come. They are used as important elements of at least two novels published in 1979, Authur Clarke's 'The Fountains of Paradise' and my own 'The Web Between Two Worlds' "
http://pcblues.com - Digits and Wood
Is carbon nanotube a good conductor of electricity ?
If the answer is yes, then you have the world's tallest lightning rod. Sure hate to have billion of amps vapourizing the magic rope.
I studied this concept as part of a commercial space development group back when I was in college. It's quite compelling.
;)
/. - I may have messed up initially and buried this as a reply deeper down the treads.]
There're two significant challenges in implementation, though.
The fundamental flaw in the concept lies in conservation of rotational inertia. Think about a spinning ice skater - as she draws her arms in, she spins much faster. The opposite is also true - as a rotating mass extends from its center, its rate of rotation decreases.
The space elevator rotates at a constant geosynchronous rate, but as its payload is raised along that axis, the difference between its linear inertia at the surface of the earth and its linear inertia around the circumference at geosynch altitude (or any significant altitude along that axis) is absorbed by the elevator's structure.
Unless the payload applies some sort of thrust perpendicular to the axis of the elevator, that difference in inertia only works to pull the whole system back down to earth. Effectively, the amount of energy you'd have to put into the system to keep it up would equal the thrust expended to send the payload into orbit by conventional means.
Then there's the whole issue of vibrational harmonics. Accumulated shocks from winds, payloads, and even space dust would propagate up and down the string (any human structure of that incredible length would effectively be a string in tension) and create severe vibration problems. That'd take some *seriously* epic engineering to dampen.
NASA has done some experiments with tethered satellites which address the vibration issues (as well as accumulated electric charge from atmospheric drag), but they were intended more for spinning-wheel satellite applications than for space elevators.
It's a really cool idea that unfortunately is a something-for-nothing scheme. If there were some kind of cool electric thruster system which didn't rely on reaction mass, it'd be feasable, but then we're straying into Area-51 technology.
[This is my first post to
Questions for the astro peeps here...
What would the g-forces be like on the end of this thing going around so fast at that distance? Wouldn't it be like one of those machines they stress test pilots on?
The document describes it like swinging a ball around your head, but that means you've got an oscillating force. Would it be enough to make the Earth wobble a bit? Would that be comfy? Would we need two elevators, one in each hemisphere?
Something I haven't seen mentioned here (is the idea forgotten, or has it been proven to be flawed?) is the "construction ring" method.
Basically you launch your cable fabrication facility and create a *huge* loop of cable. Something long enough to encircle the earth at geostationary orbit. This loop is initially unstable and will require temporary station keeping engines. You don't care about north-south twists, but don't want in-out twists to grow to large. (Read any analysis of _Ringworld_ for details...)
You then turn the cable machines on their side and start laying cable towards/away from earth. The cables will follow local geopotential fields down and up, and eventually you'll have a starter cable touch down. This can be a temporary cable, designed to be discarded, that does nothing but throw mass up the cable to build the ballast and feed additional cable machines that are producing the production cables.
Eventually you have ring in geostationary orbit, plus numerous anchors along the equator. You supplement the ring at geostationary orbit with another ring a bit inside (or outside) of it so that it's always under tension.
Besides solving some construction issues, it eliminates many of the collapse modes. If the cable snaps, the upper portion is kept in place by the ring. Even if all cables are snapped, the ballast weights will keep the ring under tension and survivors can manage station keeping by dumping ballast. (Unfortunately, if all cables snap the rest of the system will have a different net orbital velocity and there could be a big jolt.) Since there are multiple anchors, there's little value to terrorists in destroying any single anchor.
I know that _3001_ mentioned a ring as an endstage after building the first beanstalk, but I thought I've seen papers suggesting they be used as a construction platform.
And the secondary benefits are huge. Let's say the ring is 250,000 km long, and there's a 500m wide band of solar cells attached to that ring. The solar constant is around 1370W/m^2, that's potentially 171 GW of pollution-free power than can be fed down superconducting cables - 540 trillion kWh/year. According to the USGS the US consumed about 9 billion kWh/year of power from all sources in 1998, so even if the ring has only 1% efficiency it would still provide every person in the world 300x more power than the average American consumed in 1998!
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
It _is_ a conductor... And it will be a 22,000 mile long generator that is powered by moving through the suns magnetic field. It should generate a lot of power. :)
I think that there are going to be a lot of issues with building a structure this big, and people will die and there will be disasters, but in the end everything will work out and riding a space elevator into space will be about as exciting as riding an elevator to the top of a tall building, or driving over a bridge.
Until space colonies are self supporting there will be a need for massive resupply from the ground to support even a few people and rockets make the shipping costs for these supplies prohibatively expensive.
The ability to ship people and supplies up in an elevator will make it economical for companies to start up their own space stations. It will make it fesible for small groups of wealthy people to start up their own space colonies. Space hotels will be able to make money. It will also make it cost effective to manufacture items in space and send them in the downward travelling containers.
-- Never make a general statement.