On the rotation axis of comet Halley

Abstract

The observations of comet Halley by the Halley Multicolour Camera (HMC) on board the European Space Agency's Giotto spacecraft were supported by an extensive ground-based observational programme using the same filter set¹. The intention was to place the transitory but detailed observations of the encounter in the frame of ground-based observations, thereby yielding the evolution of the comet. The importance of this connection has increased because of the limited information on the three-dimensional extent of the cometary nucleus and its environment retrievable from HMC observations, caused by the communication loss just before closest approach. Here we model dust jets visible on the ground-based images, assuming active sources on the rotating nucleus. We found no fit using the rotation axes so far derived from nucleus observations. A mean instantaneous spin axis, at the time of the Giotto encounter, pointing towards α = 328°, δ = -63° is consistent with both ground-based and HMC observations of the dust. We have confirmed a rotation period of about two days.