Some of the most familiar objects in our solar system are hiding extraordinary secrets. Binary asteroids, once thought to host a single moon, are revealing unexpectedly complex histories, with satellites that collide, merge, and evolve over millions of years. One striking example is Selam, the double-lobed moon orbiting Dinkinesh, discovered by NASA’s Lucy mission. Its unusual shape and orbit challenge decades of conventional thinking about how asteroid moons form. Recent research published in Nature Communications provides a framework to explain these puzzling configurations and suggests that multisatellite histories may be far more common than previously believed.
The Mystery of Contact-Binary Satellites
Binary asteroids, where a smaller moon orbits a larger primary asteroid, have long been explained by the idea that fast-spinning asteroids shed material that forms a single satellite near the Roche limit. This simple picture was challenged when NASA’s Lucy mission encountered Dinkinesh and its satellite Selam, which instead appeared as a contact binary composed of two roughly equal-sized lobes, orbiting much farther from the primary than classical models predicted. This discovery raised the question: how do such unusual satellites form, and why are some asteroid systems far more intricate than the standard paradigm suggested?
Multigenerational Shedding Events
The research team addressed this puzzle by studying the long-term evolution of rubble-pile asteroids, which have lifespans much longer than the timescales of their rotational mass-shedding episodes. Their simulations revealed that an asteroid can undergo multiple shedding events across millions of years, each producing new satellite material that interacts gravitationally with moons already in orbit. The older satellites may have migrated outward due to tidal or thermal forces, which can lead to unpredictable encounters with newly formed moons. This multigenerational process naturally explains why some satellites appear as contact binaries or exhibit unusual orbital distances.
Interaction Regimes Shape Satellite Evolution
By using high-fidelity N-body simulations, the researchers identified three distinct evolutionary regimes for binary asteroid systems. Among these, the “interaction regime” is particularly transformative. When an older satellite has migrated to a medium-distance orbit, newly formed debris can collide or gravitationally interact with it, triggering tidal disruptions, low-velocity mergers, and gravitational scattering. This complex interplay accounts for the diverse shapes and orbits observed in certain asteroids today, including the double-lobed Selam, whose formation fits precisely within this interaction regime scenario.
Evidence From Other Asteroid Systems
The study, published in Nature Communications, extends beyond Dinkinesh, highlighting other systems with similar multigenerational histories. Triple asteroid systems such as 2001 SN263 and Balam may also have formed through repeated shedding events and subsequent satellite interactions. Even active asteroids, like 311P/PANSTARRS, which undergo repeated small ejections of material, may demonstrate the same tidal disruption phenomena on a smaller scale. Remarkably, researchers estimate that roughly 44% of observed binary systems exhibit characteristics consistent with a multigenerational satellite past, indicating that such histories are widespread across the inner solar system.