From: ram@zedat.fu-berlin.de   
      
   Jos Bergervoet  writes:   
   >This is incorrect. Lowering the cable can give it all kinds of   
   >wildly swinging motion, depending on how fast the cable is   
   >lowered. If it is lowered very slowly this might be avoided,   
   >but then the result is that the endpoint of the cable keeps   
   >the same *angular* velocity around the planet, not the same   
   >velocity normal to the surface.   
      
     When a mass connected with a cable is emitted with low speed   
     at the earth-side of a spacecraft in orbit, it might not   
     "lower" at all since it still is in orbit. (Ok, I see that   
     you write the same thing below.) So, the gravity of the   
     earth is "defeated" by the large velocity of the mass, whose   
     direction is tangentially to the Earth's surface.   
      
   >And this angular velocity can (accidentally or by clever choice   
   >of the orbit) be the same as the rotation of the planet, i.e.   
   >starting with a geostationary orbit and slowly lowering the   
   >cable would avoid the whole problem. (NB: when you start you'd   
   >have to help the cable a little, it does not fall by itself.)   
      
     If the angular velocity of the ship in geostationary orbit   
     is equal to the angular velocity of the part of the Earth's   
     surface below it, then the linear velocity of the spacecraft   
     (and the lowered mass) still is larger than the linear   
     velocity of the Earth's surface below it (because the radius   
     of the lowered mass is larger).   
      
     Since this linear tangential speed (the linear tangential   
     momentum of the lowered mass) is a conserved quantity, it   
     must be reduced somehow (via the cable?). (The Earth's   
     gravitational field is only altering the direction of the   
     linear tangential speed, but not its absolute value [in the   
     simplified case of a circular orbit].)   
      
   >But of course that is not how it is done! It has to be fast   
   >and we want it to work from any orbit. A better way would then   
   >be to give the cable a certain controlled swing, like a   
   >skilled crane driver would do when lowering a load, so that   
   >upon reaching its position it will land with almost no   
   >horizontal speed.   
      
     Yes, but then the direction of the cable would not always be   
     towards the center of the Earth, and it might be difficult   
     to control the normal part of the velocity of the lowered   
     mass via the cable as intended.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)