The Changing Rotation Period of Comet 67P/Churyumov-Gerasimenko Controlled by Activity

Abstract

Context. The ESA Rosetta spacecraft has been orbiting the nucleus of comet 67P/Churyumov-Gerasimenko since August 2014. The rotation axis of the irregularly shaped nucleus has a large obliquity (52°) and is oriented such that the southern hemisphere is insolated during perihelion. Aims. We calculate the change of the rotation period as a function of the cometary orbital position due to forces exerted by cometary activity. Methods. We use a detailed shape model of 67P with > 10^5 facets. We calculate the efficiency of the facets to exert a torque based on their radial distance from the center of mass and their orientation. We apply the model by Keller et al. (in press) to calculate the diurnal water ice sublimation rate from each facet. The reaction force per facet combined with its torque efficiency creates a torque and changes the angular momentum. The component of the torque parallel to the spin axis changes the rotation period. Results. Our model shows that the rotation period increases slightly during approach of the comet to the sun. It reaches a maximum shortly before equinox and drops rapidly during perihelion passage. The magnitude of the changes depend on the actual sublimation rates. The change in sign mainly depends of the shape of the nucleus and not much on the sublimation variation. The roughness of the nucleus has little influence. Conclusions. For the given geometry of the rotation axis the change of the rotation period is mainly influenced by sublimation activity of the irregular shape of the nucleus. The rotation period increases until shortly before equinox in early May 2015 in good agreement with observations and will then rapidly become shorter.