Gegenschein Surface Brightness Budget from Backscattering by Interplanetary Dust

This paper examines Predicted versus observed brightness of the antisolar brightening, testing the backscattering phase function. Using only published input values and transparent formulae, the calculation obtains the following headline results: observed_gegenschein_brightness_S10 = 170; model_brightness_S10 = 185; model_to_observed_ratio = 1.09; adopted_grain_albedo = 0.08; opposition_enhancement_factor = 1.35; scattering_optical_depth = 7e-08. A phase function with a modest opposition enhancement reproduces the observed gegenschein brightness to within about ten percent, without invoking a separate dust concentration at the antisolar point. The method, inputs and limitations are stated in full so that any reader can reproduce or rescale every number from the deposited calculation script. Agreement and disagreement with independent published estimates are reported at their actual size, and the conclusions are limited to what the budget arithmetic supports.