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## #1 2006-02-16 08:24:15

MathsIsFun
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### Space Elevator

These Guys want to build an elevator into space.

The idea has been around for a while: Start at a satellite, and feed out a cable in both directions (up and down) until one end touches the Earth, then you can just climb the cable to go into space!

The only problem is that a cable can only support so much of its own weight ... Steel is woefully inadequate, but carbon nanotubes show promise.

Waddya reckon?

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

## #2 2006-02-16 08:32:09

mathsyperson
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### Re: Space Elevator

I don't have permission to access "those guys'" server.

This is just speculation, but if you managed to find a material with enough tensile strength, would you need to attach it to a satellite? If you could get it up in the air, wouldn't the centrifugal force caused by the earth's orbit make it stay there?

Why did the vector cross the road?
It wanted to be normal.

## #3 2006-02-16 10:15:04

ryos
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### Re: Space Elevator

You may be right, mathsy.

I personally never understood how a satellite hanging in orbit is supposed to support the weight of a cable reaching to Earth, let alone a payload crawling up it. Wouldn't the downward force drag it out of orbit?

El que pega primero pega dos veces.

## #4 2006-02-16 10:31:22

MathsIsFun
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### Re: Space Elevator

That is why you put a cable in the outwards direction too (with possibly a counterweight on it, to save cable). Once it is all connected up, the orignal "source" satellite is redundant (make a good hotel, possibly).

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

## #5 2006-02-16 11:37:53

mikau
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### Re: Space Elevator

What if you stretched a cable from the earth to the moon? :-P

A logarithm is just a misspelled algorithm.

## #6 2006-02-16 11:56:10

irspow
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### Re: Space Elevator

That might be a bad idea as our angular velocities are not the same.

## #7 2006-02-16 11:58:18

mathsyperson
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### Re: Space Elevator

Wow, good point. We'd reel the moon into us.

Well, it's more likely that the cable would snap, but we might be able to pull it a little bit. And that would screw up all the oceans and stuff.

Why did the vector cross the road?
It wanted to be normal.

## #8 2006-02-16 12:16:04

irspow
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### Re: Space Elevator

Eventually our rotations will match.  So some time in the distant future it would be possible.  As an added bonus, said cable would stop the slow withdrawl of the moon away from the earth.  If it ever got too far away, earth would soon be inhabitable.

## #9 2006-02-16 15:07:11

ryos
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### Re: Space Elevator

Color me stupid, but I still don't understand how a counterweight is any different than a satellite in orbit, as far as the fact that it's not attached to anything, and therefore free to decelerate (and lose altitude and fall as a result) if a force is applied to it.

El que pega primero pega dos veces.

## #10 2006-02-16 15:15:49

mathsyperson
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### Re: Space Elevator

What color is stupid?

Anyway, as far as I can tell, there isn't really a difference between the two. It's just that maybe if something's designed to be a counter-weight, they could make it heavier. Well, technically more massive, but you know what I mean.

Why did the vector cross the road?
It wanted to be normal.

## #11 2006-02-16 15:30:02

ryos
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### Re: Space Elevator

Well, Arthur C. Clarke sure thought it would work. He also thought we'd have intelligent AIs and the ability to send people to Saturn by 2001.

El que pega primero pega dos veces.

## #12 2006-02-16 16:01:05

Ricky
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### Re: Space Elevator

Satillites can have rockets.  Rockets can fire away from earth.

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #13 2006-02-16 17:02:12

MathsIsFun
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### Re: Space Elevator

It works by always keeping the forces in balance. The satellite starts in geosynchronous orbit. It should stay there whether it weighs 100 kg or 100,000kg, right?

Then imagine you attach a km of cable pointing towards the earth - the satellite's center of gravity will change, so you also put some cable pointing away from earth with enough weight on it to bring the center of gravity back again.

Continue this process until you get tired, or one end reaches the planet, then tie it around a rock so you can find it again.

(They may not build it this way, this is just a concept thing)

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

## #14 2006-02-16 17:06:42

MathsIsFun
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### Re: Space Elevator

#### ryos wrote:

Well, Arthur C. Clarke sure thought it would work. He also thought we'd have intelligent AIs and the ability to send people to Saturn by 2001.

Yes, indeed! Well ... his timing may just have been a little off

BTW, he also forecast that man would reach the moon !

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

## #15 2006-02-17 03:49:53

Ricky
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### Re: Space Elevator

Then imagine you attach a km of cable pointing towards the earth - the satellite's center of gravity will change, so you also put some cable pointing away from earth with enough weight on it to bring the center of gravity back again.

Problem with this is that the side going towards earth is under much stronger gravitational forces than the side away from earth.  So you have to have an extremely massive thing on the side away.

The problem only gets worse as you get closer and closer to earth.

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #16 2006-02-17 07:40:35

MathsIsFun
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### Re: Space Elevator

Yes, the forces must be massive.

Those carbon nanotubes will be able to make some amazing structures when they get cheap enough.

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

## #17 2006-02-17 07:44:44

irspow
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### Re: Space Elevator

Everyone also seems to be neglecting the affect of weather and wind currents near the surface of the earth.  While these wouldn't be massive forces on a cable, unless the circumference was very large, it would still be great enough to jerk and jostle the satelite around which isn't under the same influences in the vaccuum of space.  The tension that we created by a counter mass would have to be quite large to minimize these sudden accelerations.

Looking at it in this way, I don't see how any orbit could be maintained without the satelite(?) having a thruster system to constantly counter the forces that it receives from the earth side.

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