XPost: sci.space.policy   
   From: underway@comcast.net   
      
   "Ool"  writes:   
      
   > I'd be surprised if an airbag system would do a lot of good.  Since   
   > there's no air you can't use any aero-braking methods to slow down,   
   > so, unlike the Mars probes, Moon probes would have to stand on top of   
   > a descent stage rocket anyway, rather than hang from a parachute.  If   
   > such a rocket can slow the probe down enough for airbags to work, it   
   > could slow it down enough for a simple soft touchdown, too, I bet.   
   >   
   > I may be wrong, but I don't think airbags would be practical on the   
   > Moon.  They'd be much heavier than the extra fuel for a soft landing.   
      
   Maybe, but I don't buy your reasoning.  The purpose of the bags is not   
   to slow the craft down for landing.  However that is done, it is   
   supposed to leave the bagged craft at near-zero velocity, just above   
   the surface.  They can save money by building a lousy system that   
   can't be trusted to leave it very close, so they design it to stop   
   many meters above the surface and hope it stops somewhere between the   
   surface and too high for the bags to work.  The main reason for the   
   bags is so the craft doesn't have to be capable (and expensive) enough   
   to guide itself to a good landing and settle down on its legs on   
   good-enough ground.  Bag landers don't have to worry about moving   
   sideways to avoid hills, cliffs, big rocks, etc.   
      
   But many missions can't put up with the limitations of bags even now,   
   despite the cost savings.  And as rocket landing systems get more   
   modular and mass-produced, the cost difference will decrease.   
      
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