  (Context) The change of the rotation period and the orientation of the
rotation axis of comet 67P/Churyumov-Gerasimenko (67P/C-G) is deducible from
images taken by the scientific imaging instruments on-board the Rosetta mission
with high precision. Non gravitational forces are a natural explanation for
these data. (Aims) We describe observed changes for the orientation of the
rotation axis and the rotation period of 67P/C-G. For these changes we give an
explanation 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 solve 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,
generally applicable to periodically forced rigid body dynamics. (Results) The
torque induced changes of the 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/C-G. The torque movement confirms that the sublimation increases faster
than the insolation towards perihelion. The derived relatively uniform activity
pattern is discussed in terms of related surface features.
