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Comet 67P/Churyumov–Gerasimenko (67P) become observable for the first time in 2021 since the Rosetta rendezvous in 2014–2016. Here, we present pre-perihelion polarimetric measurements of 67P from 2021 performed with the Very Large Telescope (VLT), as well as post-perihelion polarimetric measurements from 2015 to 2016 obtained with the VLT and the William Herschel Telescope. This new data covers a phase angle range of ∼4○−50○ and presents polarimetric measurements of unprecedentedly high S/N ratio. Complementing previous measurements, the polarimetric phase curve of 67P resembles that of other Jupiter family comets and high-polarization, dusty comets. Comparing pre- and post-perihelion data sets, we find only a marginal difference between the polarimetric phase curves. In our imaging maps, we detect various linear structures produced by the dust in the inner coma of the comet. Despite this, we find a homogeneous spread of polarization around the photocentre throughout the coma and tail, in contrast to previous studies. Finally, we explore the consequences of image misalignments on both polarimetric maps and aperture polarimetric measurements. [ABSTRACT FROM AUTHOR]

Context. Comets undergo resurfacing due to solar radiation, while their primordial interiors remain unchanged. Multi-epoch observations of comets enable us to characterize a change in sublimation pattern as a function of heliocentric distance, which in turn provides information on the dust environments of comets. Aims. We aim to constrain the size and porosity of ejected dust particles from comet 252P/LINEAR and their evolution near perihelion via near-infrared (NIR) multiband polarimetry. A close approach of the comet to the Earth in March 2016 (~0.036 au) provided a rare opportunity for the sampling of the comet at high spatial resolution. Methods. We made NIR JHKS-band (1.25–2.25 μm) polarimetric observations of the comet for 12 days near perihelion, interspersed between broadband optical (0.48–0.80 μm) imaging observations over four months. In addition, a dynamical simulation of the comet was performed 1000 yr backward in time. Results. We detected two discontinuous brightness enhancements of 252P/LINEAR. Before the first enhancement, the NIR polarization degrees of the comet were far lower than those of ordinary comets at a given phase angle. Soon after the activation, however, they increased by ~13% at most, showing unusual blue polarimetric color over the J and H bands (−2.55% μm−1 on average) and bluing of the dust color in both J−H and H−KS. Throughout the event, the polarization vector was marginally aligned perpendicular to the scattering plane (i.e., θr = 4.6°–10.9°). The subsequent postperihelion reactivation of the comet lasted for approximately 1.5 months, with a factor of ~30 times pre-activation dust mass-loss rates in the RC band. Conclusions. The marked increase in the polarization degree with blue NIR polarimetric color is reminiscent of the behavior of a fragmenting comet D/1999 S4 (LINEAR). The most plausible scenario for the observed polarimetric properties of 252P/LINEAR is an ejection of predominantly large (well within the geometrical optics regime) and compact dust particles from the desiccated surface layer. We conjecture that the more intense solar heating that the comet has received in the near-Earth orbit would cause the paucity of small fluffy dust particles around the nucleus of the comet. [ABSTRACT FROM AUTHOR]