The aim of this study is first to determine the gravity field of the comet 67P/Churyumov–Gerasimenko and second to derive the solar component of the precession rate and nutation coefficients of the spin axis of the comet nucleus , i.e .
without the direct , usually larger , effect of outgassing .
The gravity field , and related moments of inertia , are obtained from two polyhedra , that are provided by the OSIRIS and NAVCAM experiments on Rosetta , and are based on the assumption of uniform density for the comet nucleus .
We also calculate the forced precession rate as well as the nutation coefficients on the basis of Kinoshita ’ s theory of rotation of the rigid Earth and adapted it to be able to indirectly include the effect of outgassing on the rotational parameters .
The 2nd degree denormalized Stokes coefficients of comet 67P/C-G turn out to be ( bracketed numbers refer to second shape model ) C _ { 20 } \simeq - 6.74 [ -7.93 ] \times 10 ^ { -2 } , C _ { 22 } \simeq 2.60 [ 2.71 ] \times 10 ^ { -2 } consistent with normalized principal moments of inertia A / MR ^ { 2 } \simeq 0.13 [ 0.11 ] , B / MR ^ { 2 } \simeq 0.23 [ 0.22 ] , with polar moment c = C / MR ^ { 2 } \simeq 0.25 , depending on the choice of the polyhedron model .
The obliquity between the rotation axis and the mean orbit normal is \varepsilon \simeq 52 ^ { o } , and the precession rate only due to solar torques becomes \dot { \psi } \in [ 20 , 30 ] ^ { \prime \prime } / y .
Oscillations in longitude caused by the gravitational pull of the Sun turn out to be of the order of \Delta \psi \simeq 1 ^ { \prime } , oscillations in obliquity can be estimated to be of the order of \Delta \varepsilon \simeq 0.5 ^ { \prime } .