XPost: sci.space.policy   
   From: quibbler247@yahoo.com   
      
   In article ,   
   kysy@erikseen.com.retro.com says...   
   > By looking at the MERs the solar panels have no future in robotic   
   > exploration.   
      
   Sorry, but the MERs show quite the opposite.  Those solar panels are   
   extremely cheap and lightweight compared to RTGs.  They produce quite   
   respectable amounts of power and could obviously produce more if they had   
   been made to track on one or two axes.  For that matter, a solar thermal   
   concentrator could be used to drive a direct heat engine during the day,   
   making solar an extremely efficient option.   
      
   > They need to be faced towards the sun to produce anything   
      
   Well most all solar collectors and PV will produce around double the   
   power if they track.  However, actually many systems including amorphous   
   PV and flat panel collectors can work quite well with diffuse light.   
      
   > and   
   > they require lots of space when deployed.   
      
   Perhaps.  I'm not sure that this is a problem in most cases.   
      
      
   > You also need heavy onboard   
   > batteries to cover the blackouts.   
      
   Yes, for things like rovers perhaps.  For normal probes they can rely on   
   sun virtually all the time.  It is also worth noting that one could use a   
   small 1 or 2 kilo RTG as a backup power source, rather than a real   
   battery.  It will provide plenty of waste heat in cold areas like mars   
   and as well as reasonable amounts of standby power.  As you probably know   
   the MERs do have about 8 tiny radio-isotopic heaters on board.   
      
      
   > I seriously doubt solar panels can come   
   > anywhere near RTG in watts per kg comparison,   
      
   I gave the figures.  RTGs standardly produce 500W thermal per kg of fuel.   
   That is standardly only converted at around 10% efficiency or less with   
   thermal diodes.  OTOH, PV can easily get several KW electrical per pound   
   of solar panel, especially if the panels are equipped with tracking   
   capabilities.  Even cutting this capacity factor by about 3 for 8 hours   
   of sunlight and assuming the lower insolation of mars means that PV and   
   other solar technologies come out ahead.   
      
   However, I think that a combination of solar and RTG could work quite   
   well.  Things like amorphous PV can be coated onto many external surfaces   
   and provide basic DC power needs during the day.  Like I said before,   
   you'd have to be very far from the sun, or in a very special environment,   
   before an RTG only approach would be the best one.   
      
   > especially if they operate on   
   > the planet with night/day cycles.   
      
   Of course there are appropriate technologies.  It would be nice to have   
   an RTG so that a rover could keep operating at night.  But I don't see   
   any fundamental technology breakthrough on the horizon that will make   
   RTGs substantially cheaper.  OTOH, solar continues to improve in   
   efficiency and drop in cost virtually every five minutes.   
      
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