From: joe@strout.net   
      
   In article <1115246898.481389.205790@o13g2000cwo.googlegroups.com>,   
    "David Summers"  wrote:   
      
   > The real disadvantage to using microwaves is the spot size on the   
   > spacecraft.  Jordin Kare's analysis showed that once the spacecraft was   
   > most of the way to orbit, an optical system would only be delivering   
   > about half of the energy output onto the vehicle.  With microwaves,   
   > that would be much worse.   
   >   
   > There are probably ways around this though.  Perhaps using some   
   > material with a negative permeability could focus more tightly.  Those   
   > materials exist for microwaves, but are challenging for optical   
   > wavelengths.   
      
   Another possible approach would be to hand off powering of the craft to   
   an orbital source.  This could probably be located in LEO, so it   
   wouldn't need to have an enormous antenna.  But it would itself need a   
   pretty massive power source -- either a huge solar array and well-timed   
   launches, or a smaller array with a large power storage system (perhaps   
   ultracapacitors).  Certainly not cheap infrastructure, but if it's the   
   sort of thing that enables low-cost, high-rate launches, maybe it'd be   
   worth it.   
      
   - Joe   
      
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