The Size of Particles Populating the Asymmetric Coma of Interstellar Comet 3I/ATLAS

The coma of the second interstellar Comet 3I/ATLAS was asymmetric in shape, demonstrating a strong sunward elongation that was also referred to as its antitail. This feature was tentatively explained by the presence of large, mm-sized or larger particles. We evaluate this hypothesis and demonstrate that this feature also can occur for smaller-size particles as well. Under widely held assumptions on the emanation of particles from a cometary nucleus and the effect of solar-radiation pressure on their motion, the stand-off distance is independent of particle size. The path of micron-sized particles in the sunward direction is as long as that of cm-sized particles. However, in the former case, the particles reach the stand-off point in only a few days; whereas, in the latter case, the time of journey may exceed a year. Fast temporal variations in Comet 3I/ATLAS suggest its coma is populated by fine particles.