Diverse configurations of binary asteroids explained by multi-generation satellites

Introduction

Near-Earth asteroids and small main-belt asteroids (MBAs) with diameters between 200 m and 20 km are thought to be rubble piles. Most of their satellites are believed to be born from rotational mass shedding of the primary body, where the spin-up of a primary asteroid triggers mass shedding, creating a transient debris disk that finally re-accumulates to form satellite(s). In this scenario, a prolate satellite in a compact orbit is expected from theoretical predictions (Walsh et al, 2008). However, recent space missions have revealed a remarkable diversity of binary configurations. One striking anomaly is the (152830) Dinkinesh system, which hosts a contact-binary satellite named Selam in a wide orbit (Levison et al, 2025). In addition, several asteroids in this size range have been found to have more than one satellite. These findings reveal a diversity in the orbital architecture, satellite shape, and the number of satellite(s) when multiple asteroid systems are considered.

Existing models in the literature (Jacobson & Scheeres, 2011; Madeira et al, 2023) prefer to reconstruct this diversity through a single mass shedding event, but recent simulations (Agrusa et al, 2024) suggest that the actual picture of satellite formation is quite different, and the morphological properties of Selam have still not been convincingly reproduced. In this work, by considering multiple mass shedding and thus multi-generation of satellites, we propose a unified and self-consistent framework capable of covering all aspects of this configuration diversity.

Results

Through gravitational N-body simulations, we find that if multiple episodes of mass shedding and multi-generations of satellites are considered, the pre-existing satellite can strongly influence the subsequent satellite formation. Taking into account the orbital migration of the pre-existing satellite, this leads the system evolution after a subsequent mass shedding to different pathways, where the observed diversity in binary asteroid configurations can be naturally produced. Furthermore, a dynamical atlas of binary asteroid evolution is presented.

We suggest that the Selam-like contact binary satellite is more likely to originate from two separate mass shedding events and the subsequent inter-satellite collision, in which satellite migration also plays an important role. This also suggests a new formation mode for contact binary asteroids like Itokawa. We also find that ~ 44% of known binaries are located in the parameter ranges corresponding to multi-satellite histories, suggesting that the mechanisms shown in this work are prevalent in the evolution of binaries. Consequently, Selam-like satellites may be not rare, and satellites with even stranger shapes such as a contact triple, may also be found in the future.

This work has been supported by the National Natural Science Foundation of China grant No. 12372047.

References

Walsh, D. C. Richardson, P. Michel, Rotational breakup as the origin of small binary asteroids. Nature 454 (7201), 188–191 (2008).

Levison, et al., A contact binary satellite of the asteroid (152830) Dinkinesh. Nature 629 (8014), 1015–1020 (2024).

Jacobson, S. A. & Scheeres, D. J. Dynamics of Rotationally Fissioned Asteroids: Source of Observed Small Asteroid Systems. Icarus 214, 161–178 (2011).

Madeira, G., Charnoz, S. & Hyodo, R. Dynamical Origin of Dimorphos from Fast Spinning Didymos. Icarus 394, 115428 (2023).

Agrusa, H. F. et al. Direct N-body Simulations of Satellite Formation around Small Asteroids: Insights from DART’s Encounter with the Didymos System. The Planetary Science Journal 5, 54 (2024).