NAME:
  moid
 PURPOSE:   (one line only)
  Compute the minimum orbit intersection distance between two orbits
 DESCRIPTION:
 CATEGORY:
  Celestial Mechanics
 CALLING SEQUENCE:
  distance = moid(orb1,orb2)
 INPUTS:
  orb1 - Anonymous structure containing the first orbit.  The following
           tags must be defined in the structure:
             semi - semi-major axis in AU
             ecc  - orbital eccentricity
             inc  - orbital inclination (radians)
             node - Longitude of the ascending node (radians)
             arg  - Argument of perihelion (radians)
             manom - mean anomoly (radians)
             jdepoch - Julian Date of the epoch of the orbit

  orb2 - Structure with the second orbit (same tags as the first orbit)

 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  return value - the minimum orbital intersection distance between the two
                   orbits, in AU.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
  mwb_moid - for internal use only
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2014/01/09