From: christopher.m.jones@gmail.com   
      
   Marc 182 wrote:   
   > In article , damon1six1   
   > @comcast.not.retro.com says...   
   >>One can reasonably expect several decades of useful   
   >>power, gradually decreasing as the plutonium decays.   
   >   
   > Most of the power decrease is caused by the degradation of the thermal   
   > couples in the radiation environment.  Plutonium 239 has a half life of   
   > over 24 thousand years, fast in geologic time, but infinite in human   
   > engineering terms.   
      
   RTGs do not use Pu-239 but rather Pu-238, which has a   
   half-life of about 88 years.  And is thus many times   
   more radioactive and produces many times more heat   
   output than "ordinary" Plutonium.  The reduction in   
   power output of RTGs is roughly equally due to   
   radioisotope decay and thermocouple degradation.   
   For example, the amount of Pu-238 in the Voyager RTGs   
   has decayed down to about 81% of the amount at launch,   
   however the current power production in those RTGs is   
   down to about 64% of what it was at launch (300 W vs.   
   470 W).  As you can see, the natural decay of the   
   isotope has reduced power output by 19%, whereas the   
   degradation of the thermocouples and related systems   
   has brought about a further 21% reduction in power.   
      
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