LightnessToLoading:

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   Sail loading (mass/area) from lightness parameter. 
   The lightness is the dimensionless ratio of solar radiation pressure force
   to the solar gravitational force.

   Since version 7.
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   Form:
   sigma = LightnessToLoading( beta )
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   Inputs
   ------
   beta          (1,:)   Sail lightness parameter 

   -------
   Outputs
   -------
   sigma         (1,:)    Sail loading (kg/m2)

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   Reference: Colin R. McInnes, "Solar Sailing: Technology, Dynamics and Mission
              Applications", Springer Praxis, London, 1999
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