What you're doing is mixing a filter pigment with your media.

Make the spheres of the tail (the container for the media fully transparent.

Then to avoid the coincident surface problem, scale the spheres a little smaller
then the comet sphere.

sphere{ <0,0,0>,0.27 pigment{rgb <1,0,0>}}
 #declare Nr = 1;
 #while (Nr>0)

 sphere{ <0,0,0>,0.26 pigment{rgbt <1,1,1,1>}

        hollow
 interior{
  media{
   scattering{Nr,<1,1,1>}
   intervals 3
   samples 1,1
   }
 }
         translate <Nr,0,0>
       }

 #declare Nr = Nr - 0.1;
 #end

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