XPost: sci.space.policy, sci.astro   
   From: henry@spsystems.net   
      
   In article <41ed11ff$0$47609$ed2619ec@ptn-nntp-reader02.plus.net>,   
   Ian Stirling   wrote:   
   >There is nothing really prohibiting a mission that just gathers data/sleeps   
   >between communication attempts.   
   >You'd need to add several things.   
   >A RHU, to keep it warm enough for the batteries to work.   
      
   Unfortunately, Huygens already had a whole bunch of RHUs.   
      
   The problem with RHUs is that there is no way to switch them off when your   
   electronics are active and you don't need quite as much heat.  Or when   
   you're closer to the Sun, early in the mission, and need rather less heat.   
      
   Even setting that aside, it's rather tricky to set things up so that the   
   RHUs supply *all* your heat but not too much, especially in an atmosphere   
   whose detailed thermal characteristics are not well known.  (And then   
   there's the possibility that you might be floating in a liquid...)   
      
   In practice, you need some way to *control* the internal temperature.   
   Much the simplest way to do that, unfortunately, is to have the RHUs   
   supply only the very smallest amount of heat you might ever want, and make   
   up the extra with electrical heat.  There are more elegant ways of doing   
   this, but they add complexity and often moving parts, and spacecraft   
   designers tend to distrust them.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)