The Return of Rosetta's Comet: Photometric and Polarimetric Observations of Comet 67P/Chruyumov-Gerasimenko in its 2021-22 Apparition.

Remote observations of the light scattered by cometary comae make it possible to constrain the physical properties of the dust particles lifted into the comet atmosphere. Ground-based photometric and polarimetric observations of the light scattered by comet 67P/Churyumov-Gerasimenko (67P) present a unique opportunity to directly compare remote measurements to those performed in-situ by the ESA/Rosetta mission. The polarisation of the light scattered by cometary dust varies as a function of phase-angle (the Sun-comet-Earth angle), as well as properties of the dust particles: their size, structure and composition. Therefore, our polarimetric maps can be used to monitor the spatial and temporal evolution of the coma dust properties up to fifty thousand kilometres, while the phase-angle dependence of linear polarisation can be compared to that of other comets, and measurements from light scattering experiments and cometary models. Here, we present photometric and polarimetric observations of 67P from 2021 performed between 1-6 months pre-perihelion with FORS2 @ VLT, as well as 6 months post-perihelion observations from 2015-2016 with FORS2 @ VLT and ISIS @ WHT. This data, of unprecedentedly high S/N ratio, covers a phase-angle range of ~4-50° and heliocentric distances from 1.2-2.5 AU. Complementing previous measurements with these data points we can see that the polarimetric phase curve of 67P resembles that of a high-polarisation dusty comet. Various jet-like structures in the inner coma are visible in processed intensity images. Polarimetric maps, however, show a homogenous distribution of polarisation throughout the coma and tail, and show no evidence of dust particle evolution over the entire observing run.