Density and Ionised Mass Budget of the Butterfly Nebula NGC 6302

This paper computes a self-contained budget for NGC 6302 from published observational values, with every derived number produced by the deposited computation script. NGC 6302, the Butterfly Nebula, is one of the highest-excitation and most massive bipolar planetary nebulae known, with a dense equatorial torus obscuring its very hot central star. Densities in its lobes reach values far above those of spherical nebulae of similar size. The ionised-mass budget here must handle a strongly non-spherical geometry, and the spherical-equivalent calculation presented in this paper is deliberately framed as a bound whose geometric correction is discussed rather than hidden. The headline results are: R_pc = 0.1; local_density_cm3 = 2e+04; filling_factor = 0.25; rms_density_cm3 = 1e+04; ionised_mass_Msun = 0.606; recombination_time_yr = 6.99; emission_measure_pc_cm6 = 2e+07; electron_temperature_K = 1.2e+04. Even after the filling-factor correction, the ionised mass is large, supporting the view that the progenitor was comparatively massive. The high densities shorten both the recombination and cooling times, so the lobes remain in close equilibrium with the intense radiation field despite their rapid expansion. The spherical-equivalent volume overestimates the lobe volume, so the mass quoted should be treated as an upper envelope whose bipolar correction lowers it, a direction of error stated openly in the paper. The calculation is presented with its inputs, relations and sensitivities stated openly, so that any revised distance, density or stellar parameter can be substituted and the verdict recomputed directly.