Gravity field and solar component of the precession rate and nutation coefficients of Comet 67P Churyumov-Gerasimenko

TL;DR

This study models the gravity field and solar-induced precession and nutation of Comet 67P, providing insights into its rotational dynamics and moments of inertia based on Rosetta data.

Contribution

It introduces a method to derive the comet's gravity field and rotational parameters, including the solar component of precession and nutation, using polyhedral models and adapted Earth rotation theory.

Findings

Gravity field characterized by specific Stokes coefficients

Precession rate due to solar torques estimated at 20-30 arcseconds per year

Obliquity between rotation axis and orbit normal is approximately 52 degrees

Abstract

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 NAV-CAM 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) C20 = -6.74 [-7.93] * 10^-2, C22 = 2.60 [2.71] * 10^-2 consistent with normalized principal moments of inertia A/MR^2 = 0.13 [0.11], B/MR^2 = 0.23 [0.22], with polar moment c = C/MR^2 = 0.25, depending on the choice of the polyhedron model. The obliquity between the rotation axis and the mean orbit normal is 52 degree, and the precession rate only due to solar torques becomes 20-30 "/y. Oscillations in longitude caused by the gravitational pull of the Sun turn out to be of the order of 1', oscillations in obliquity can be estimated to be of the order of 0.5'.