Comet 67P/Churyumov-Gerasimenko rotation changes derived from sublimation-induced torques

Abstract

Context. The change in rotation period and the orientation of the rotation axis of comet 67P/Churyumov-Gerasimenko (67P) can be deduced with high precision from images taken by the scientific imaging instruments on board the Rosetta mission. Non-gravitational forces are a natural explanation for these data. Aims. We describe observed changes in orientation of the rotation axis and the rotation period of 67P. We explain them based on a sublimation model with a best fit for the surface active fraction (model P). Torque effects of periodically changing gas emissions on the surface are considered. Methods. We solved the equation of state for the angular momentum in the inertial and the body-fixed frames and provide an analytic theory of the rotation changes in terms of Fourier coefficients, which are generally applicable to periodically forced rigid-body dynamics. Results. The torque-induced changes in rotation state constrain the physical properties of the surface, the sublimation rate, and the local active fraction of the surface. Conclusions. We determine a distribution of the local surface active fraction in agreement with the rotation properties, period, and orientation of 67P. The torque movement confirms that the sublimation increases faster than the insolation toward perihelion. The derived relatively uniform activity pattern is discussed in terms of related surface features.