ROTATION STATES OF THE NUCLEUS OF COMET HALLEY COMPATIBLE WITH SPACECRAFT IMAGES

The spacecraft images of Comet Halley's nucleus have been analyzed to characterize the positions of the nucleus during the Vega 1, Vega 2, and Giotto flybys. Then the mean instantaneous rotations axes compatible with each pair of positions were calculated. The comparison with ground-based estimates of the angular momentum or the instantaneous rotation axis of the nucleus shows that the instantaneous rotation axis during the space-craft encounters was moving. Thus it demonstrates that the rotation state of the nucleus cannot be simple. However, the quick prograde rotation of 54 hr always appears as the dominant motion. The 7.4-day period could be explained by oscillations around the long axis and a nodding motion, with small amplitudes. © 1990.