From: bob_jenkins@burtleburtle.net   
      
   Space elevators won't work.  Well maybe they will but they're more complicated   
   than we've been thinking.   
      
   A space elevator would be anchored at one point on earth, usually over the   
   ocean on the equator, then would extend straight up to about 1/5th the   
   distance to the moon.   
      
   A minor overlooked matter is the moon.  It's 1/81th the mass of the earth, and   
   on average 10x further from the elevator than the earth, so the elevator would   
   be pulled up at 1/8000th its weight when the moon is overhead.  So the   
   connection to earth has    
   to be strong enough to resist a force of 1/8000th the elevator's total mass   
   pulling up.  Like tides, it'll be 1/8000th heavier when the moon is opposite   
   the elevator, so it's more like 1/4000th fluctuation daily.   
      
   But the major overlooked matter is that the earth's equator is not aligned   
   with the moon's orbit.  It's off by 18 to 29 degrees.  Two issues.   
      
   1. The elevator's orbit wants to be in the plane of the moon's orbit, because   
   the moon tugs it that way once a day.  On planets without a moon, the sun will   
   have a similar effect.  The plane of the earth's orbit around the sun and the   
   plane of the moon's    
   orbit around the earth are pretty close, off by about 5 degrees.  If the   
   elevator isn't in the moon's plane, it'll fall towards it, and keep falling   
   past it then fall back, oscillating back and forth.  "Precession."  From earth   
   this looks like tilting    
   north, then south, then back to north, once a day.  If you start with an   
   elevator pointed up, after a few years it'll have fallen 2*29 degrees to the   
   side, forming a 32 degree angle with the ground, then it'll rise back to   
   straight up, back and forth.     
   If you want to prevent it from precessing, you have to actively push it   
   sideways, which is a thrust of about 1/100000th the elevator's mass times g   
   all the time.  I can't see doing that.  So instead you'd build the elevator in   
   the moon's orbital plane in    
   the first place, and it would tip north and south daily by 18 to 29 degrees.   
      
   2. The elevator no longer goes straight up from the nearest point on earth:   
   the connection oscillates north and south from the nearest point between -18   
   to -29 and 18 to 29 degrees daily.  This implies a change of up to 800km of   
   elevator length twice    
   daily.  Perhaps you could spool it up then reel it out again.  You could drive   
   a power plant off of reeling it out.  The elevator cable will be moving pretty   
   fast: 800km in 12 hours is an average of 66km/hour.  Alternatively, you could   
   keep the elevator    
   connection fixed, just tugging harder when the attachment point is pulling in   
   and tugging less when the attachment point is going out.   
      
   Any pictures of a space elevator going straight up from earth are as wrong as   
   a stopped clock: they're only right for an instant twice daily.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)