From: henry@spsystems.net   
      
   In article <42bc0db4$0$1592$ba620dc5@text.nova.planet.nl>,   
   Roland  <"roland_lemmersANTea$PeM"@planet.nl> 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?   
      
   As others have noted, you want a monoatomic material -- something that   
   will produce ions of only one mass, rather than a molecule that might   
   break up and might not -- and you want it heavy.   
      
   Easy ionization is also nice, although the ionization process in most ion   
   thrusters is grossly inefficient anyway and it probably doesn't make a big   
   difference.   
      
   Finally, for practical reasons, it should be something that is neither   
   corrosive nor poisonous.  Cesium, often cited as a likely propellant back   
   before people actually tried to build ion thrusters, is both.  Mercury was   
   used for some time, and has advantages, but it's quite poisonous, and as   
   safety standards were tightened, the cost of ground testing of mercury   
   thrusters became prohibitive.   
      
   Xenon is scarce and expensive, and the high-pressure tanks for storing it   
   are a bit heavy, but otherwise it's a pretty good choice.  The only real   
   problem is that there simply isn't *enough* of it for really large-scale   
   space activity.  Getting a ton of it should not be difficult, if you've   
   got the better part of a million bucks to spend.  But you simply cannot   
   buy a thousand tons of it for delivery next year, no matter how much money   
   you have; the production capacity simply isn't there.  You'd have to build   
   your own extraction plants -- big ones.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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