Space Elevator Kickstarter

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Re: Space Elevator Kickstarter

Postby AvatarIII » Tue Aug 28, 2012 10:13 am UTC

SlyReaper wrote:
AvatarIII wrote:
gmalivuk wrote:
Wnderer wrote:I was thinking about the Lagrange points, but maybe the cable would naturally be pulled through the Lagrange points. But then there are two of those between the earth and moon.
No, only L1 is between the two, and it marks the point where something goes from being pulled toward the Moon to being pulled toward the Earth.


so the centre of gravity of the cable would have to be in L1?

iirc, one of the hardest things about going to the moon is landing with no air resistance, a space elevator would certainly help on that front.

The center of gravity would have to be above the L1 point, otherwise anything climbing the cable would pull the whole thing down.


Unless the mass of the cable was sufficiently large compared to the elevator car, then any movement caused by the climber would be minimal, there would probably need to be jets of some sort to keep the cable stable anyway.
besides, the further beyond the L1 point that the centre of gravity is, the more tension there would be on the cable.

an alternative would be having 2 cars of equal mass that go back and forth between the 2 ends and the L1 point, meeting at the middle, transferring contents (if mass to and from the moon was always balanced, which I suppose, if you want more mass going to the moon, you could always transfer moon rocks off, simply for the purpose of keeping masses equal) and then going back to the ends, which would keep the cable balanced.

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Re: Space Elevator Kickstarter

Postby Diadem » Tue Aug 28, 2012 2:35 pm UTC

Would it be possible to hang a cable from the moon, through L1, into LEO? Attach it to a space station there.

Of course such a station wouldn't actually be a low earth orbit, since it's rotating with the moon. So it'll be moving with a considerable velocity with respect to other things in LEO. But its velocity with respect to earth would be quite small, and it would be much more easily reachable from earth than something in L1 or geosynchronous orbit.

The tension on the cable would be significantly more than on a cable just slightly beyond L1. So that's the drawback. But it seems the best way to transport goods to the moon easily.
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Re: Space Elevator Kickstarter

Postby induction » Tue Aug 28, 2012 2:51 pm UTC

The orbital velocity of a space station is determined by its altitude. In order to orbit the Earth with the same orbital period as the moon, the space station would have to be at the same distance from the Earth as the moon is. Otherwise continuous energy expenditure is required to correct the orbital path.

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Re: Space Elevator Kickstarter

Postby bigglesworth » Tue Aug 28, 2012 2:56 pm UTC

I wonder how much energy, though? If that energy could be supplied from a solar power plant on the moon and then taken down the cable , might be worth it?
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Re: Space Elevator Kickstarter

Postby AvatarIII » Tue Aug 28, 2012 3:14 pm UTC

Diadem wrote:Would it be possible to hang a cable from the moon, through L1, into LEO? Attach it to a space station there.

Of course such a station wouldn't actually be a low earth orbit, since it's rotating with the moon. So it'll be moving with a considerable velocity with respect to other things in LEO. But its velocity with respect to earth would be quite small, and it would be much more easily reachable from earth than something in L1 or geosynchronous orbit.

The tension on the cable would be significantly more than on a cable just slightly beyond L1. So that's the drawback. But it seems the best way to transport goods to the moon easily.


I'm not sure if that would be worth the effort of making the cable so much longer, because you'd have to match the velocity of the end of the cable to get onto the space station. which would probably take a lot of energy.

one question though, if there were a space station hanging from the mon towards earth, it would not be in free-fall right? Because it wouldn't be in orbit around the earth, it would be hanging off the moon, if the L1 point is where gravity between the earth and moon cancels each other out, and the station would be the Earth side of L1, would there be gravity?

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Re: Space Elevator Kickstarter

Postby induction » Tue Aug 28, 2012 3:49 pm UTC

bigglesworth wrote:I wonder how much energy, though? If that energy could be supplied from a solar power plant on the moon and then taken down the cable , might be worth it?


Back of the envelope calculation:
The acceleration due to gravity at leo altitude (let's say 7000 km) is about 8.1 m/s^2.

A space station at leo altitude with the moon's rotation period will be traveling at roughly 18.65 m/s (compared to other leo satellites at ~7500 m/s), and will have an acceptable downward acceleration of ~.00005 m/s^2.

So we will need to accelerate the space station 8.1 m/s every second. The international space station has a mass of 420,000 kg, so we will need to provide about 14 million watts to correct its course. Let's say a really good solar panel will give you 300 watts. So that's over 47,000 solar panels installed on the moon. But we will need them on both the near and far side because the near side is in shadow for half the orbit. Assuming we can program them to tilt toward the sun at all times, we can get there with 93,000 solar panels evenly distributed over mid lunar latitudes. Of course, we are ignoring power dissipation as the electricity travels from one side of the moon to the other, and that's small compared to the amount dissipated along the cable to the space station. I also have assumed 100% efficient voltage to thrust conversion, which is highly suspect, to say the least. So we probably want to double the number of panels.

Doable? I don't know.

Big problem: how do you prevent the space station and cable from smashing into the satellites going by at 7.5 km/s?

edit: fixed bad math stupidness twice
Last edited by induction on Tue Aug 28, 2012 6:19 pm UTC, edited 2 times in total.

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Re: Space Elevator Kickstarter

Postby Wnderer » Tue Aug 28, 2012 4:03 pm UTC

I don't think the earth end of the lunar space elevator is in orbit. If it was in orbit, the earth end would be weightless and there would be no tension in the cable. How big a space station you could put on the earth end would depend on how far from the lagrange point and how much tension the cable could take.

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Re: Space Elevator Kickstarter

Postby gmalivuk » Tue Aug 28, 2012 4:07 pm UTC

AvatarIII wrote:I'm not sure if that would be worth the effort of making the cable so much longer, because you'd have to match the velocity of the end of the cable to get onto the space station. which would probably take a lot of energy.
Not really. It would just have to go straight up from the ground, instead of also having to reach orbital velocity of 7 or so km/s. (Actually, it would require some energy to slow down, since Earth would be rotating a lot faster than the cable. But not much in comparison to the energy required to add 7 km/s or to get that high in the first place.)

one question though, if there were a space station hanging from the mon towards earth, it would not be in free-fall right? Because it wouldn't be in orbit around the earth, it would be hanging off the moon, if the L1 point is where gravity between the earth and moon cancels each other out, and the station would be the Earth side of L1, would there be gravity?
Yes, that's what it means to "hang down the gravity well". The Moon side of L1 pulls toward the Moon, and the Earth side pulls toward the Earth.

Wnderer wrote:I don't think the earth end of the lunar space elevator is in orbit.
Right. Nothing closer than L1 with the Moon's orbital period is properly in orbit around Earth, because that's the orbital period required to orbit at the distance of the Moon.
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Re: Space Elevator Kickstarter

Postby Tyndmyr » Tue Aug 28, 2012 4:08 pm UTC

Diadem wrote:Would it be possible to hang a cable from the moon, through L1, into LEO? Attach it to a space station there.

Of course such a station wouldn't actually be a low earth orbit, since it's rotating with the moon. So it'll be moving with a considerable velocity with respect to other things in LEO. But its velocity with respect to earth would be quite small, and it would be much more easily reachable from earth than something in L1 or geosynchronous orbit.

The tension on the cable would be significantly more than on a cable just slightly beyond L1. So that's the drawback. But it seems the best way to transport goods to the moon easily.


You will almost certainly want to have at least a little positive tension on the cable. Not too much, as that's going to put a lot of stresses on the system as a whole, but if you have anything less than neutral buoyancy(I suspect this isn't the perfect word, but I'm not sure of another for Lagrange points) around the L2 point(from moon's perspective), the cable falls back down. So, you probably want a certain safety margin the other way.

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Re: Space Elevator Kickstarter

Postby induction » Tue Aug 28, 2012 6:54 pm UTC

Wnderer wrote:I don't think the earth end of the lunar space elevator is in orbit. If it was in orbit, the earth end would be weightless and there would be no tension in the cable. How big a space station you could put on the earth end would depend on how far from the lagrange point and how much tension the cable could take.


I'm no expert in materials like Kevlar, but I think that for any reasonably-sized space-station in LEO, this puts us well beyond the strength of currently available materials.

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Re: Space Elevator Kickstarter

Postby Xeio » Tue Aug 28, 2012 7:09 pm UTC

Tyndmyr wrote:You will almost certainly want to have at least a little positive tension on the cable. Not too much, as that's going to put a lot of stresses on the system as a whole, but if you have anything less than neutral buoyancy(I suspect this isn't the perfect word, but I'm not sure of another for Lagrange points) around the L2 point(from moon's perspective), the cable falls back down. So, you probably want a certain safety margin the other way.
Don't you need, at minimum, enough tension on the cable to lift whatever it is you're trying to lift (plus the transportation mechanism) at ground level without the cable falling back down?

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Re: Space Elevator Kickstarter

Postby Tyndmyr » Tue Aug 28, 2012 7:14 pm UTC

Xeio wrote:
Tyndmyr wrote:You will almost certainly want to have at least a little positive tension on the cable. Not too much, as that's going to put a lot of stresses on the system as a whole, but if you have anything less than neutral buoyancy(I suspect this isn't the perfect word, but I'm not sure of another for Lagrange points) around the L2 point(from moon's perspective), the cable falls back down. So, you probably want a certain safety margin the other way.
Don't you need, at minimum, enough tension on the cable to lift whatever it is you're trying to lift (plus the transportation mechanism) at ground level without the cable falling back down?


Yup. That's probably not a ton, but you'll need the minimum, which is set by whatever your maximum load+carriage is, plus some element of safety margin. Considering that the load, if it includes people, is going to need at least a modicum of shielding, you'll probably want a fair bit up there.

Multiple cables, of course, is certainly one way to mitigate this to a degree.

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Re: Space Elevator Kickstarter

Postby Diadem » Tue Aug 28, 2012 7:53 pm UTC

induction wrote:The orbital velocity of a space station is determined by its altitude. In order to orbit the Earth with the same orbital period as the moon, the space station would have to be at the same distance from the Earth as the moon is. Otherwise continuous energy expenditure is required to correct the orbital path.

The space station is attached to the moon, so it has the same orbital period by definition. Like I said, this means it's not in a proper orbit around earth. So you need a force to keep it there. This force is, of course, provided by the cable, pulling at the moon.

It works in theory. The only question is: How strong does the cable have to be?

AvatarIII wrote:I'm not sure if that would be worth the effort of making the cable so much longer, because you'd have to match the velocity of the end of the cable to get onto the space station. which would probably take a lot of energy.

No that's the major advantage of my scheme. You don't. The cable is moving very fast with respect to LEO, but it's moving very slowly with respect to earth. In fact it's rotating slower than the earth is. So anything launched up from earth needs to overcome a lower velocity difference than it would otherwise have to. So the space station would be even easier to reach from earth than something like the international space space, which is in a true LEO.

one question though, if there were a space station hanging from the mon towards earth, it would not be in free-fall right? Because it wouldn't be in orbit around the earth, it would be hanging off the moon, if the L1 point is where gravity between the earth and moon cancels each other out, and the station would be the Earth side of L1, would there be gravity?

Good point, there would be gravity. A significant amount even, almost 90% of standard gravity. So living there would be much easier and much more comfortable than on a normal space station.

I hadn't thought of that myself, but that's another major advantage of my scheme.

Now I just need someone to calculate the necessary cable strength.
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Re: Space Elevator Kickstarter

Postby Tyndmyr » Tue Aug 28, 2012 7:58 pm UTC

The gravity would largely depend on the distance beyond L1. As long as it's still fairly close to the L1 point(almost required, I imagine, by tensile strengths), gravity should be fairly low.

That said, having low, but non-zero gravity might not be that bad of a thing.

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Re: Space Elevator Kickstarter

Postby induction » Tue Aug 28, 2012 8:00 pm UTC

Diadem wrote:Now I just need someone to calculate the necessary cable strength.


For the ISS (420,000 kg) in LEO, you're looking at 3.4 mega-newtons (or about 382 tons) not counting the cable itself. Current materials have breaking lengths of several hundred km and would break under their own weight at much shorter distances than L1 to LEO (about 320,000 km).

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Re: Space Elevator Kickstarter

Postby gmalivuk » Tue Aug 28, 2012 8:28 pm UTC

Tyndmyr wrote:The gravity would largely depend on the distance beyond L1.
Wouldn't it depend *entirely* on that? After all, it's not like the orbital period or other distances involved are variables we can change, is it?
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Re: Space Elevator Kickstarter

Postby Tyndmyr » Tue Aug 28, 2012 8:40 pm UTC

gmalivuk wrote:
Tyndmyr wrote:The gravity would largely depend on the distance beyond L1.
Wouldn't it depend *entirely* on that? After all, it's not like the orbital period or other distances involved are variables we can change, is it?


There are some other factors like local gravity to the station itself, and subjective gravity might be subject to mechanical perturbations(ie, car stops moving up the thread at the top). However, yeah, these are pretty small factors, and can generally be ignored. The largely was simply a "technically correct" sort of thing in case we got into the weeds.

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Re: Space Elevator Kickstarter

Postby Arancaytar » Wed Aug 29, 2012 10:16 am UTC

poxic wrote:
Iulus Cofield wrote:Wouldn't a space elevator between the earth and the moon go, like, really fast?

Wouldn't it, like, get snapped by the movement of the moon orbiting the Earth?


Mostly the Earth's rotation would do that...
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Re: Space Elevator Kickstarter

Postby AvatarIII » Wed Aug 29, 2012 10:32 am UTC

that's why it wouldn't be tethered to the Earth, only to the moon, it would dangle above the Earth, that's the point

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Re: Space Elevator Kickstarter

Postby poxic » Fri Aug 31, 2012 1:49 am UTC

While whipping around the equator (or another latitude) at about a thousand miles an hour? Sounds perfectly sane.
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Re: Space Elevator Kickstarter

Postby gmalivuk » Fri Aug 31, 2012 2:12 am UTC

Which is more than ten thousand miles an hour slower than all the stuff we already have orbiting us.
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Re: Space Elevator Kickstarter

Postby poxic » Fri Aug 31, 2012 2:16 am UTC

Oh, so the Earth-end is at a reasonable orbiting distance? I missed something in the conversation then. *goes back to reading*
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Re: Space Elevator Kickstarter

Postby Adacore » Fri Aug 31, 2012 8:29 am UTC

poxic wrote:Oh, so the Earth-end is at a reasonable orbiting distance? I missed something in the conversation then. *goes back to reading*

I think the proposed idea was earth end at LEO level. I suppose the fuel costs for keeping it in the right place might not be too bad, if you can just send power up the cable from a lunar power station?

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Re: Space Elevator Kickstarter

Postby AvatarIII » Fri Aug 31, 2012 8:41 am UTC

it would need constant correcting though, so it depends on how much power is needed,
The L1, L2, and L3 lagrange points are all what’s called metastable. The forces of gravity and orbital motion are precisely balanced at these points, but even a slight nudge will send any object at them drifting off. Think of a ball balanced precisely on top of a hill; though the ball is stable and in equilibrium, even the slightest push will send it rolling off. Because satellites even in the vacuum of space aren’t completely devoid of forces acting on them (such as the solar wind, micrometeorites, and even light pressure), anything placed at the first three lagrange points will need periodic course corrections to keep them in place.

http://orbitalvector.com/Space%20Struct ... CTURES.htm

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Re: Space Elevator Kickstarter

Postby Tyndmyr » Fri Aug 31, 2012 12:42 pm UTC

poxic wrote:While whipping around the equator (or another latitude) at about a thousand miles an hour? Sounds perfectly sane.


I would assume the cable couldn't possibly extend into atmo, on account of that whole "burned to a crisp" problem.

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Re: Space Elevator Kickstarter

Postby SlyReaper » Fri Aug 31, 2012 1:15 pm UTC

AvatarIII wrote:it would need constant correcting though, so it depends on how much power is needed,
The L1, L2, and L3 lagrange points are all what’s called metastable. The forces of gravity and orbital motion are precisely balanced at these points, but even a slight nudge will send any object at them drifting off. Think of a ball balanced precisely on top of a hill; though the ball is stable and in equilibrium, even the slightest push will send it rolling off. Because satellites even in the vacuum of space aren’t completely devoid of forces acting on them (such as the solar wind, micrometeorites, and even light pressure), anything placed at the first three lagrange points will need periodic course corrections to keep them in place.

http://orbitalvector.com/Space%20Struct ... CTURES.htm

If it's extending to such a length that the center of weight is well beyond L1, and the end is in the vicinity of LEO, then the Earth's own gravity should provide that corrective force. You might get some weird bending in the cable, as the cable desperately tries to avoid the local maximum of L1, but if we're assuming a cable made of unobtanium anyway, that shouldn't be a problem. What would be a problem is earth-orbiting satellites colliding with it.
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Re: Space Elevator Kickstarter

Postby gmalivuk » Fri Aug 31, 2012 1:29 pm UTC

Tyndmyr wrote:I would assume the cable couldn't possibly extend into atmo, on account of that whole "burned to a crisp" problem.
Except, things don't burn to a crisp at a mere 1000mph.

SlyReaper wrote:If it's extending to such a length that the center of weight is well beyond L1, and the end is in the vicinity of LEO, then the Earth's own gravity should provide that corrective force. You might get some weird bending in the cable, as the cable desperately tries to avoid the local maximum of L1
L1 isn't a maximum, though, it's a saddle point. It's a minimum in the tangential direction, and a maximum in the radial direction. Which, as you correctly point out, isn't actually a problem because gravity will stabilize it in that direction. (In this regard it is similar to tidal force, as any particular point in orbit can be thought of as unstable in the rotating reference frame, because anything further in (but still at rest in the rotating frame) will tend to fall to Earth, while anything farther out will be flung away. And this is why a traditional geosynchronous space elevator would be held taut and stable.)
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Re: Space Elevator Kickstarter

Postby Tyndmyr » Fri Aug 31, 2012 2:15 pm UTC

gmalivuk wrote:
Tyndmyr wrote:I would assume the cable couldn't possibly extend into atmo, on account of that whole "burned to a crisp" problem.
Except, things don't burn to a crisp at a mere 1000mph.


That was perhaps a bit of hyperbole, but still, the kind of cable that can withstand being supersonic for it's entire lifespan, in atmo, is probably not something we have. It introduces a great many complications, and frankly, you just don't need the cable to be that long, and it offers fairly little. If you're dumping stuff to earth, you don't need the cable to actually get into our atmosphere to do it.

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Re: Space Elevator Kickstarter

Postby gmalivuk » Fri Aug 31, 2012 2:21 pm UTC

Compared to being able to support its own weight strung from the Moon to almost Earth, surviving supersonic speeds seems rather trivial, actually.
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Re: Space Elevator Kickstarter

Postby induction » Fri Aug 31, 2012 2:57 pm UTC

Other engineering issues:

The moon's orbital distance at apogee is about 43,000 km further from the Earth than it is at perigee, so some kind of intelligent winching system on the moon is required, or a highly variable-distance space station orbit.

The angle of the moon's orbit is tilted by 20 to 30 degrees with respect to the equator, and geostationary orbits are at ~42,000 km. So if we go with the variable-distance orbit, the space station will cross the geostationary orbit at the nodes, and we have to figure out how not to smash our communications satellites with the SS. Even with a winch, we'll have the same problems with the cable. Generally, the space station will be traveling at a very different speed than anything near it, so collisions in general have to be prevented.

The lunar libration will generate forces on the cable that will have to be studied in order to avoid standing waves, instabilities, and increased stress on the cable.

If we choose to send power down the cable, we could try to take advantage of the low temperatures and make the connection superconducting, but you can't stress such a cable very much because plastic deformation of the conductor will generally occur at lower stresses than that of the sheath. Also the breaking length of the cable will be reduced with the added weight of the conductor.

Instead of dangling from the moon, the space station could use thrust to hold itself in orbit, but that would be prohibitively expensive since it's essentially standing still in orbit, and doesn't have much centrifugal force to help out (It's moving at < 19 m/s compared to a the 7500 m/s that would keep it in a gravitational orbit). So it's continually thrusting upward at ~8 m/s just to stay still. That's as bad as using continual thrust as a propulsion mechanism on a space journey (worse, actually, because you're sitting in a gravity well; you'd get more for your money on a space journey).

Maybe instead of LEO, we should be aiming for an orbital distance just further than geostationary.

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Re: Space Elevator Kickstarter

Postby Wnderer » Fri Aug 31, 2012 8:36 pm UTC

induction wrote:
Maybe instead of LEO, we should be aiming for an orbital distance just further than geostationary.


I like it. Then we can build a space elevator on Earth and it would align once a day with the Lunar elevator.

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Re: Space Elevator Kickstarter

Postby SlyReaper » Fri Aug 31, 2012 8:48 pm UTC

Space elevator extending from the moon's surface to just above geosynchronous orbit altitude above Earth. I shall name it... the elevator of Damocles.
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Re: Space Elevator Kickstarter

Postby Coyne » Sat Sep 01, 2012 3:29 am UTC

Zamfir wrote:There's no lunar equivalent to a geostationary orbit, I think, because its too close to earth. Instead you would have to build to a Lagrange point and somewhat beyond. L1 and L2 are something like 60,000 km from the moon. Which is more than earth-geostationary, but the stresses in the cable would be far lower because of the lower gravity.

Though L1 and L2 are unstable points, and L4 and L5 are more like 300,000 km away. But perhaps a cable would act as stabilizer, I don't know that.


L1 would be the place to go, because you'd have to build beyond it to support the whole thing anyway. Not sure how far. But for a given length, the cable would have to be something like 1/36th the strength; probably much less.

The fact that the moon doesn't rotate with respect to L1 would help immensely, so the cable position could easily be stabilized in "L1 orbit" by the fact that it is anchored to the moon.

The biggest problems are getting the material there, of course, but you actually only have to get it to L1; you can just lower/raise the cable from there.
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Re: Space Elevator Kickstarter

Postby Tirian » Sat Sep 01, 2012 9:32 am UTC

This might be a noob question, but wouldn't the Earth-Moon L1 point be one of the most valuable chunks of real estate in the solar system and not really the sort of place that we'd "let" a private group set up one end of an insanely long cable?

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Re: Space Elevator Kickstarter

Postby Elvish Pillager » Sat Sep 01, 2012 1:34 pm UTC

Who's "we" exactly?

The moon police?
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Re: Space Elevator Kickstarter

Postby Wnderer » Sat Sep 01, 2012 2:36 pm UTC

SlyReaper wrote:Space elevator extending from the moon's surface to just above geosynchronous orbit altitude above Earth. I shall name it... the elevator of Damocles.


The Elevator of Damocles. Sounds like the title of Sci Fi book. What does happen if the cable breaks loose and falls to the Earth? Is it going fast enough to burn up in the atmosphere? Or will it just melt into big balls that will hit the Earth like cannon balls? Or maybe just break up into large spears skewering the planet ?

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Re: Space Elevator Kickstarter

Postby SlyReaper » Sat Sep 01, 2012 2:44 pm UTC

Wnderer wrote:
SlyReaper wrote:Space elevator extending from the moon's surface to just above geosynchronous orbit altitude above Earth. I shall name it... the elevator of Damocles.


The Elevator of Damocles. Sounds like the title of Sci Fi book. What does happen if the cable breaks loose and falls to the Earth? Is it going fast enough to burn up in the atmosphere? Or will it just melt into big balls that will hit the Earth like cannon balls? Or maybe just break up into large spears skewering the planet ?


Well the thing is, it would be suspended in such a way that it would be moving very slowly for it's altitude. It would be like skydiving from thousands of miles up - no worries about re-entry burn up. It would be subjected to some forces and heat as it entered the atmosphere, but nowhere near as much as it would if it were re-entering from orbit. So I would expect a great deal of the structure to survive and impact the Earth's surface.

And all you would need to initiate this catastrophe would be one bastard in a black hat with a pair of pruning shears on the moon.

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Tirian
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Re: Space Elevator Kickstarter

Postby Tirian » Sat Sep 01, 2012 4:10 pm UTC

Elvish Pillager wrote:Who's "we" exactly?

The moon police?


I was thinking more of the Chinese military or NASA defending the satellite that's already there. If you'll pardon the pun, building something at a Lagrange point isn't a decision that is made in a vacuum.

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Re: Space Elevator Kickstarter

Postby poxic » Sat Sep 01, 2012 6:31 pm UTC

SlyReaper wrote:And all you would need to initiate this catastrophe would be one bastard in a black hat with a pair of pruning shears on the moon.

Awfully mighty pruning shears, but still.
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Re: Space Elevator Kickstarter

Postby bigglesworth » Sat Sep 01, 2012 7:24 pm UTC

Tirian wrote:not really the sort of place that we'd "let" a private group set up one end of an insanely long cable?
Anything of this size is always going to happen only with the cooperation of governments. Just like nuclear power plants or the current set of satellites.
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