From: maury_markowitz@hotmail.com   
      
   Mikko wrote:   
   >> But 30,000ft and, say, 600mph isn't much of a boost. It's a long, long,   
   >> long way from orbit. The difficulty of getting to orbit is better   
   >> described by "17500 miles per hour" than any figure of altitude.   
   >>   
   >   
   > Why is that speed needed?   
      
      The simplest way to think of orbit is to use the old line from the   
   Hitchiker's series: orbit is throwing yourself at the ground, and missing.   
      
       Consider you standing in mid air, 100 miles up, starting to fall   
   towards the earth. Simply consider how fast you'd have to be going   
   sideways so you wouldn't hit it, but end up in space "beside" it. The   
   earth is a little more than 3900 miles in radius, so in the time it   
   takes you to fall to the earth, about 15 minutes, you have to go 3900   
   miles "to the side". 3900 x 15 x 5 (periods of 15 in an hour) ~= 17,000   
   miles per hour.   
      
   > But also gravity gets smaller when you get more away from the earth?   
   > How high does one have to go to have only half of gravity?   
      
      A long way. Gravity decreases with r^2, so if the earth is 3900 miles   
   in radius, then the key number is 3900 squared, or 15,210,000. To get   
   one-half the gravity we need that number to be times 2, or 30,420,000,   
   which is 5,515. So basically you need to be 2000 miles out before it   
   becomes 1/2 the gravity, which is a pretty high orbit.   
      
   > What if someone built a 30,000 ft high tube, similar to magnetic trains   
   >   - electrical magnets around it. Then you could just put metallic cargo   
   > inside - without any engine or fuel, and shoot it up. The tube would   
   > have to be high enough that there is no air where the cargo comes out,   
   > and maybe part of the tube would have to be a vacuum.   
      
      Think of you standing on top of that tower. If you jump off, what   
   will happen? Well, you'll slowly but surely end up a splat at the bottom   
   of the tower.   
      
      But also consider what the earth looks like from that point. 30k   
   miles is a long way, the earth is now longer that "big". If you jump to   
   the side hard enough, you'll miss the earth on the way down. That's   
   orbit, albiet a highly eliptical one.   
      
   Maury   
      
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