Inbound evolution of 21 long period comets

We characterize dust activity and nucleus properties of long-period comets during their inbound journey to the inner Solar system, thus supporting the target selection and risk assessment for ESA Comet Interceptor mission. We observed and modelled the dust tails of 21 Oort cloud comets by means of the probabilistic tail model, constraining the nucleus radius $R_n$. About 60 per cent of the dust tails are best fit by anisotropic dust ejection, due to nucleus inhomogeneity built-up by activity occurred before the comet ejection from the outer disc into the Oort cloud. For about half of the sample, the trend of the coma photometry with respect to heliocentric distance is available and consistent with the predictions by the tail model within the high photometry uncertainty. The comets drifting from supervolatile-driven to water-driven activity show a drop of coma photometry, suggesting a nucleus mainly composed of water-poor pebbles, according to the Water Enriched Blocks (WEB) model. The measured dust ejection velocity fits the $R_n$ dependence predicted by well-tested hydrodynamical coma models, suggesting the estimated $R_n$ lower limits being very close to the actual values. We thus find that about 85 per cent of the nuclei have $R_n \lt 1$ km, and about 22 per cent of these are probably very elongated or contact-binary nuclei. All observed comets with a dust tail are consistent with the WEB model and are mainly composed of water-poor pebbles with $R_n \lt 4$ km, posing no dust-related hazard to the Comet Interceptor mission if their nuclei are structured in WEBs.