Rejection of a proposed 7.4-day rotation period of the comet Halley nucleus

Abstract

Preliminary analyses of Vega 1 and Vega 2 images of the comet Halley nucleus have led to a derived rotation period of 53.5±1 hours (2.23±0.04 days) about an axis approximately perpendicular to the long axis of the nucleus,1–3. Images obtained by Giotto4–6 were also found to be consistent with the Vega-derived rotation period. Recently, however, a longer rotation period of 7.4 days has been proposed7, based on an apparent double-peaked periodicity in the production rates of CN, C2 and dust. Although we acknowledge possible photometric evidence for time-variable production rates and appreciate the assumption that this periodicity should somehow be related to the rotation of the nucleus, we conclude, nevertheless, that the imaging observations from the three spacecraft provide a compelling refutation of the proposed 7.4-day rotation period. Of crucial importance to this conclusion are: (1) the non-symmetrical shape of the Halley nucleus; and (2) the independent observations of this irregular body by Vega 1, Vega 2 and Giotto over an interval of 7.7 days. We note, however, that a rotation period of 53.5 hours and a longer period of torque-free precession with the angular momentum vector inclined somewhat to the spin axis might be consistent with both the spacecraft and ground-based observations.