From: dietz@dls.net   
      
   Mike Lepore wrote:   
   > Someone please tell me why spacecraft are designed to reenter the earth's   
   > atmosphere at high speed. Isn't there some way to come down slowly,   
   > so the heat shields wouldn't be needed?   
      
   They are initially traveling very fast, since they are either in orbit   
   or are coming from far away and have fallen into Earth's   
   gravity well.   
      
   Slowing without drag in the atmosphere would mean using rockets,   
   which would require a prohibitively large quantity of propellant.   
      
    > Has anyone modeled the idea of   
   > unfolding some large wings to add a lot of surface area, or using   
   > propellers to resist falling, or parachutes? Thank you.   
      
   Certainly.  The time required to brake during reentry is   
   increases as the lift/drag ratio increases, so this can be   
   used to prolong the reentry.  The altitude also is dependent   
   on the ballistic coefficient (mass/area) of the vehicle,   
   allowing a broad, light vehicle to slow higher in the   
   atmosphere, spreading the heat over a larger area.  But the   
   energy still has to be dissipated somehow.   
      
   	Paul   
      
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