Research progress on internal characteristics of asteroids and comets: implications of Tianwen-2 radar observations

Asteroids and comets, which have long been of interest to astronomers, are important components of the solar system. They contain rich primitive information about the early solar system, which provide a unique pathway for studying the formation and evolution of the solar system, and even the origin of life on planet Earth. In this review, the formation, evolution processes and the internal characteristics of the asteroids and comets in the solar system are presented. The focus of this review is on the internal characteristics especially the dielectric properties of asteroids and comets by radar observations. On this basis, the possible internal structure of these celestial bodies is discussed. Combined with the existing theories, simulations and experiments, the possible internal structure of near-Earth asteroid 2016 HO₃ is a monolith while structure of active asteroid 311P/PANSTARRS remains unknown. In 2025, China will launch Tianwen-2 asteroid-comet exploration mission to detect 2016 HO₃ and 311P/PANSTARRS, which will carry a penetrating radar to observe the internal characteristics of these two objects.