From: schillin@spock.usc.edu   
      
   "Cray74@gmail.com"  writes:   
      
   >Would it be possible to set up a circular, polar orbit around Mercury   
   >such that it always followed Mercury's terminator?   
      
   Sun-synchronous orbits like that are possible if the planet has a   
   sufficient equatorial bulge to supply the necessary perturbing force.   
   Earth, for example, has such a bulge, and sun-synchronous orbits   
   are frequently used for a variety of purposes.  Not necessarily   
   following the terminator, though.   
      
   Mercury, may not have enough of an equatorial bulge to make this   
   sort of thing work.  I can't find any data on the matter right this   
   moment, alas, but the planet's slow rotation rate does not make me   
   optimistic.   
      
      
   >Also, could such a "dawn tracking orbit" pass over the same point at   
   >the north and south poles each orbit?   
      
   It can't pass *precisely* over the poles, because it needs to be   
   inclined just a bit off the pure polar case in order to exploit   
   the equatorial perturbations.  Sun-synchronous orbits about the   
   Earth, for example, tend to be at about 98 degrees inclination.   
      
      
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