From: henry@spsystems.net   
      
   In article ,   
   Rodney Kelp  wrote:   
   >Secondary question: Could a satellite be put in lunar orbit that would   
   >always face the earth?   
      
   Well, kind of...   
      
   You can't arrange an actual lunar orbit that's always going "around" the   
   Moon, never behind it, as seen from Earth.  That would require the plane   
   of the orbit to rotate once a month, to keep it in the right orientation   
   as the Moon goes around the Earth.  As with non-equatorial geostationary   
   orbits around Earth, you can't do that without continuous thrust.  Left to   
   its own devices, the plane of a lunar orbit is pretty much fixed in space.   
   (The plane of most *Earth* orbits slowly precesses around Earth's axis,   
   but the Moon doesn't have enough of an equatorial bulge to do that.)   
      
   However, there is a point between Earth and Moon -- the Earth-Moon L1   
   point -- where a spacecraft can "hang motionless" on the Earth-Moon axis.   
   (Roughly speaking, the Moon's gravity plus centrifugal force balance out   
   Earth's gravity there.)  In practice, this point is not entirely stable   
   and regular small stationkeeping corrections are needed, but that's not   
   a big deal.   
      
   There is another such point above the lunar farside, the L2 point.   
      
   And finally, there are "orbits" around such points -- more or less   
   orbiting around the Earth-Moon axis -- which likewise are almost stable   
   and only need small corrections.  And such an orbit can give the   
   appearance, at first glance, of an orbit always going "around" the Moon,   
   although in fact the orbit is either somewhat nearer than the Moon or   
   somewhat farther away.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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