Non-Gravitational Effects on a Binary Asteroid: Implications for the Long-term Dynamics of Asteroid Didymos-Dimorphos

The AIDA planetary defense program, consisting of the U.S. DART and Europe’s Hera missions, is currently underway. In September 2022, the DART spacecraft successfully conducted an impact experiment on Dimorphos, the satellite of the binary asteroid Didymos (Daly et al., 2023). The Hera mission is scheduled to arrive at Didymos–Dimorphos in December 2026 and will closely observe the artificial crater created by DART. Japan's thermal infrared camera (TIRI), installed on the Hera spacecraft, aims to reveal the thermal properties of the asteroid surface, such as thermal inertia, porosity, and surface roughness.

The evolution of an asteroid's orbits and rotation is affected by gravitational forces from the Sun and planets and by non-gravitational forces due to thermal radiation pressure. We aim to construct theoretical and numerical models of non-gravitational effects acting on binary asteroids and validate them with the Hera mission.

The orbital evolution of a binary asteroid’s satellite is mainly driven by tidal forces and the binary YORP (BYORP) effect. In many binary systems, the primary spins relatively fast, and the satellite’s semi-major axis increases due to tidal evolution. On the other hand, the BYORP effect is a torque generated by the thermal radiation of a synchronously rotating satellite, which can alter the semi-major axis depending on the irregularity of the satellite’s shape (Ćuk & Burns, 2005).

In this study, we newly focused on the Yarkovsky effect acting on the satellite of a binary asteroid. In particular, we analyzed the Yarkovsky-Schach effect, a mechanism that has been studied in the orbital evolution of Saturn’s ring particles and is considered to similarly affect satellites in binary asteroid systems (Rubincam, 2006; Vokrouhlický et al., 2007). We developed both analytical and numerical models of the Yarkovsky-Schach effect acting on a binary asteroid satellite (Zhou, Vokrouhlický, Kanamaru, et al., 2024).

For numerical simulations of the non-gravitational forces acting on the satellite, we used the thermophysical computation library AsteroidThermoPhysicalModels.jl, which is also applicable to binary systems. The results show that satellites experiencing frequent eclipses (in the case of prograde rotation) tend to evolve toward tidally locked orbits due to the Yarkovsky-Schach effect. This orbital evolution mechanism offers essential insights into the statistical properties and the dynamical lifetimes of binary asteroids.