An easy-to-use model for the optical thickness and ambient illumination within cometary dust comae

We define a procedure which allows estimation of the optical thickness of a cometary dust coma and the ambient illumination of the nucleus for any given comet, if estimates of the nucleus radius and the dust activity (Afρ) are available. The calculation is performed for a singly scattering coma with a cos(ϑ) distribution of dust over its day side. We find that the ambient illumination is of the same order as the incident sunlight if the optical thickness is of order one. The optical thickness increases, all else equal, linearly with the nucleus radius. Therefore the effect of the presence of the coma may be neglected for small (≈ 1 km diameter) comets, but is important for comets such as 1P/Halley and Hale–Bopp