From: spam@merlyn.demon.co.uk   
      
   JRS:  In article <1106668448.108905.104520@c13g2000cwb.googlegroups.com>   
   , dated Tue, 25 Jan 2005 07:54:08, seen in news:sci.space.science,   
   Cray74@gmail.com  posted :   
   >Henry Spencer wrote:   
   >> Mercury is not an ideal black body in several   
   >> ways, and that can make a big difference.   
   >   
   >Thank for the answer and explanation.   
   >   
   >I was actually thinking of a black sunshield for a   
   >Mercury orbiting object, but the other issues are   
   >noted.   
      
   Henry assumed a spherical body.   
      
   In a non-uniform radiation field, such as that coming from the Sun, the   
   equilibrium temperature of a general unsymmetrical body depends on its   
   orientation.   
      
   A sphere of radius r intercepts pi r^2 of radiation, but radiates from 4   
   pi r^2; a circular plate radiates from 2 pi r^2, but can intercept from   
   pi r^2 down to almost zero.   
      
      
   >Two follow-up questions:   
   >   
   >*If the sunshield is uninsulated, would the IR   
   >radiation from the sunshield warm the objects   
   >behind the sunshield as much as if they were   
   >directly exposed to the sun?   
      
   Only if it were at ground level. If it were at a reasonable height, its   
   back would be radiating out into half of the universe, with Mercury in   
   front of a bit of that; and most of Mercury's re-radiation would also   
   miss the shield.   
      
      
   >*If the sunshield is "perfectly" insulated on   
   >its back (leaving it with half the area to radiate   
   >heat), would its absolute temperature climb by   
   >the fourth root of 2 (1.189) to about 570K?   
      
   Yes and no, since it would not have been at 570/2^0.25 K without   
   insulation.   
      
      
   In this case, a single shield is probably sufficient.  But one can put   
   shields in series; a second shield sees no direct sun, and blocks most   
   of the first from Mercury.   
      
   Much the same is done in cryostats; consider a tall vacuum-insulated   
   test-tube with liquid helium at the bottom, and a room-temperature cap   
   at the top.  Put a few discs at intervals between top and bottom, and   
   boil-off is much reduced.   
      
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