The spin state and homogeneity of comet Halley's nucleus

We determine a model for the spin state of Comet Halley's nucleus that simultaneously satisfies imaging data from the Vega and Giotto encounters, and a wide range of ground-based data including observations of CN-jets, CN-shells, C ₂ production rates, and photometric variability. The model is that of an excited, axially symmetric, rotator whose shape is taken to be that of a prolate spheroid. The motion is assumed to be unaffected by jet-induced torques. The long-axis of the nucleus is inclined to the total angular momentum vector, M, by 66°.0, and rotates around M with a period, P_φ, of 3.69 days. The component of spin around the long axis has a period, P_Ψ, of 7.1 days which compounds with P_φ to produce a total spin period, P_T = 2.84 days. The total spin vector, S, is inclined to M by 21°.4 and freely precesses around the angular momentum vector with a period of 3.69 days. The model has a ratio of maximum to minimum moments of inertia of 2.28, which, when compared to the observed linear dimensions of the nucleus, implies an approximately constant density distribution throughout its interior. When compared to the sense of orbital motion, the spin is direct and M points toward right ascension, declination (1950) = (6°·2, -60°·7). The model is characterized by five, localized, areas on the nucleus which dominate the observed activity. One of these, located near the "waist" of the nucleus, appears to be solely responsible for the initiation of CN-shell boundaries seen propagating through the coma and may also be responsible for photometric activity seen at large (≥5 AU) heliocentric distances on approach to the sun. This behavior may indicate the presence of either a large-scale chemical inhomogeneity in the nucleus or, possibly, an extensive region on the surface with unusual physical structure.