Size and Physical Properties of the Comet Hale–Bopp (C/1995 O1) Nucleus

It is possible to estimate the real size of the nucleus from the equation of the energy balance, using some parameters obtained through space experiments tied with the exploration of comet Halley. We have also taken the albedo value 0.04 (for that part of the nucleus surface which is covered with mineral crust and does not sublimate). Representing the nucleus of comet Halley as a triaxial ellipsoid a· b· c = 15· 8· 7.5 km we can calculate the parameter of its shape B = RequSF/V = 3.20, where Requ= (a· b· c)1/3, SF - the total surface of the ellipsoid, V - its volume. Parameter B was also used for comet Hale-Bopp’s nucleus. The equation of the energy balance for comet Hale-Bopp’s nucleus is analyzed for the moment when the comet passed perihelion and the sublimation rate (for water) was 1031 molecules· s−1≈ 3·108g· s−1. The energy balance equation contains the following components: energy coming from the Sun and absorbed by the comet nucleus, sublimation energy and energy of heat radiation of that part (1 - γ) of the nucleus surface which does not sublimate and is covered with mineral crust; the mean temperature of the nucleus surface (of the mineral crust) at perihelion according to our calculations is 330 K. As a result the dependence of the value Requ on γ, the fraction of the nucleus surface which is a source of sublimation, was obtained. The minimal value of the nuclear radius Requ (for γ = 1.0, i.e., the total surface sublimation) equals 14.6 km; for γ = 0.5 the value Requ = 20.7 km. For γ = 0.1 (comet Halley’s nucleus had approximately this value of γ) Requ = 46.3 km.; the thickness of the mineral crust equals ≈ 1 cm for the heat conductivity coefficient λ≈ 2· 104 erg· cm−1· s−1· K−1.