The changing rotation period of comet 67P/Churyumov-Gerasimenko controlled by its 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 in the rotation period as a function of the cometary orbital position due to forces exerted by cometary activity. Methods. We used a detailed shape model of 67P with >10 5 facets. We calculated the efficiency of the facets to exert a torque based on their radial distance from the center of mass and their orientation. We applied our thermal model 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 the approach of the comet to the Sun. It reaches a maximum shortly before equinox and drops rapidly during perihelion passage. The magnitude of the change depends on the actual sublimation rates. The change in sign mainly depends on 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 in the rotation period is mainly influenced by the 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 become shorter rapidly.

Estimate of the erosion rate from H2O mass-loss measurements from SWAN/SOHO in previous perihelions of comet 67P/Churyumov-Gerasimenko and connection with observed rotation rate variations

“…Keller et al (2015b) have made a study parallel to ours on the rotation rate changes of this comet through sublimation activity, with the actual shape model of the nucleus. They reached the following interesting conclusions: 1.…”

Three-dimensional direct simulation Monte-Carlo modeling of the coma of comet 67P/Churyumov-Gerasimenko observed by the VIRTIS and ROSINA instruments on board Rosetta

“…The model from Cowan & A'Hearn (1979), assuming that the spin axis tilt is perpendicular to the comet-Sun line with an active area of 2%, shows a weaker dependence with heliocentric distance. One could explain this by an increase of active area as the comet gets closer to the Sun, which could be due to erosion on the southern latitudes of the nucleus (Keller et al 2015) enabling easier sublimation as these regions are illuminated.…”

“…These results are consistent with the observations made by the different instruments on board Rosetta (Biver et al 2015;Gulkis et al 2015;Hässig et al 2015;Bockelée-Morvan et al 2015;Lee et al 2015;Luspay-Kuti et al 2015;Migliorini et al 2016). Calculations by Keller et al (2015) show that the total energy input close to the Hapi region is enhanced by reflected light and IR radiation from the walls of the local concavity, which could explain the H 2 O distribution of activity. Conversely, the identification of two areas in the Imhotep region, where exposed water…”

The global meter-level shape model of comet 67P/Churyumov-Gerasimenko

“…Later, the presence of this precession was confirmed by Gutiérrez et al (2016) and Jorda et al (2016). Additionally, a significant change from a constant to a varying rotation period, starting in late 2014 and caused by effects from sublimation activity, had been predicted early in the mission by Keller et al (2015) based on the SPG SHAP4S shape model. This change in the rotation period of 67P was confirmed later within different analyses of image data that were taken around the comet's perihelion (e.g., Jorda et al 2016).…”