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tokens: distance and shows a greater emission of gas and dust. Here, solar gravity and solar radiation pressure are shaping the morphology of the dust coma, and its tail is in fact obvious and oriented in nearly antisolar direction. Dust emission in comet 2I/Borisov seems to be less collimated than C/2014 B1, with the dust particles widely scattered in antisolar direction, possibly due to the combination of a lower ejection speed and the effect of a greater radiation pressure. Our estimate of the size of dust particles in the range of 30–100 μ m radius is consistent with that recently reported by Jewitt Luu ( 2019 ). However, it should be considered that in both cases the estimates apply to particles displaced to the end of the visible tail in order to reproduce its apparent length. In fact, using higher β values in the Finson–Probstein diagram, in the region close to the nucleus the simulation approaches the wide scattering of the emitted dust observed in the HST images, which might be suggestive of the presence also of dust particles with a smaller size. 4.2 An emissive rotating structure Assuming that the nucleus of comet 2I/Borisov is rotating and that the geometric position of the spin axis is such to favour a prolonged insolation, the dust emission from a single source located on the sunlit hemisphere should be modulated by the insolation and the resulting features of the inner coma should provide indications on the orientation of the spin axis. In the case of two emissive structures with similar morphology that are observed in approximately opposite directions, it could be supposed that both originated from the same active source on a nucleus that is rotating with a spin axis coinciding with the axis of the angle between the two structures and, if so, that they would be more evident when their position corresponds to the projection on the plane of the sky for the greater condensation of material on the perpendicular to the view (Sekanina 1987 ). The finding of a jet at PA 180 ○ and of its counterpart at PA 10 ○ suggests that the active area originating the structures is located in a near-equatorial position, and that the nucleus is probably rotating with a spin axis lying near the plane of the sky and geometrically oriented at PA ∼100 ○ or PA ∼100 ○ + 180 ○ (Fig. 3 ). Unfortunately, from the HST images it is not possible to determine the direction of rotation of the nucleus. To further verify our findings, we run a computer model of the inner coma using a proprietary software (P. Pellissier, 2009) specifically designed to reproduce Earth-based observations of the dust coma structures and of the development of the tail. The parameters needed to run the model are the properties of the dust particles, those of the nucleus, and the geometric conditions of the Sun–Earth–comet system. The following physical parameters of the dust particles, compatible with the observations of Rosetta on comet 67P (Fulle et al. 2015 ; Rotundi et al. 2015 ;