From: dietz@dls.net   
      
   Roland wrote:   
   > The header of the message pretty much says what my question is: Why is   
   > Xenon the propellant of choice for an ion thruster?   
   > What makes it a better choice than for instance nitrogen or oxygen?   
      
   It requires less energy to ionize per unit mass.   
   Also, for a fixed exhaust velocity and grid spacing,   
   the space charge limit on thrust density goes as   
   the square of the ion mass/charge ratio.   
      
   	Paul   
      
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