From: henry@spsystems.net   
      
   In article <1120218377.022517.206660@g43g2000cwa.googlegroups.com>,   
     wrote:   
   >> Gravity mapping (via precision spacecraft tracking or an on-board gravity   
   >> gradiometer) has limited performance in such a weak gravitational field...   
   >   
   >Might this make gravity mapping more sensitive: using two or more   
   >probes simultaneously orbiting the asteroid and constantly measuring   
   >each other's distances?  I imagine that must be better than tracking   
   >the probe's position from Earth.   
      
   For large-scale features of the field, it doesn't help much, but for   
   localized features and spacecraft not too far apart, yes, it is better:   
   you're getting differential measurements, rather than having to   
   numerically differentiate during data analysis (which is possible, but   
   terribly sensitive to noise).   
      
   The one downside is that the measurement has to be done on board, which   
   puts troublesome mass and power constraints on the equipment.  That may be   
   a real issue for trying to map an asteroid's field that way, since you'd   
   be looking for very small variations in an already-weak field.   
      
   In case anyone wants to get numerical, for a 10s integration -- roughly   
   appropriate for lunar gravity mapping, perhaps too fast for asteroids -- a   
   range-rate precision of 1mm/s is straightforward and can be done with   
   simple, lightweight equipment, while 0.1mm/s is heavy, complex, and   
   power-hungry, and much beyond that you can forget it.  But these things do   
   get easier with time, as electronics technology improves.   
      
   >How might it compare to an on-board gravity gradiometer?   
      
   Broadly speaking, tracking links win for large-scale features, while a   
   gradiometer is considerably better for local ones.  But there are serious   
   technical problems in trying to put a gradiometer in a spacecraft right   
   now, unless you've got a very deep wallet for a big, complex, costly   
   spacecraft.  The current gradiometer technologies don't lend themselves to   
   cheap missions, although that too shows signs that it might change...   
      
   For a little more detail :-), see Spencer, Carroll, Arkani-Hamed, & Zee,   
   "Lunette:  Lunar Farside Gravity Mapping by Nanosat", to appear in the   
   proceedings of this summer's AIAA/USU Small Satellites Conference.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
   --- SoupGate-Win32 v1.05   
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