When NASA's Lucy spacecraft zoomed past the asteroid Dinkinesh in 2024, it found something that shouldn't exist according to decades of established science: a moon called Selam, made of two oddly matched lobes pressed together like a cosmic dumpling, orbiting far farther out than physics said was possible. That peculiar discovery has just sparked a revolution in how scientists understand the deep histories of binary asteroid systems—and the answer lies in an epic multigenerational tale of collision and rebirth.
For generations, astronomers had a tidy explanation for how moons form around asteroids. A spinning primary asteroid would shed material like a spinning top flinging off water, and that loose debris would clump together into an elongated satellite orbiting at the Roche limit, the gravitational boundary where tidal forces would normally tear it apart. Simple. Elegant. Wrong, at least for systems like Dinkinesh's.
The puzzle deepened because some asteroids carry more than one moon, and their configurations didn't match the standard model. A new study published in Nature Communications now provides the missing piece: asteroids live far longer than the time it takes for a single satellite-shedding event to occur. That means a binary system can shed material multiple times over its lifetime, and each new generation of debris interacts gravitationally with whatever moons already exist. Using high-fidelity computer simulations, researchers led by Wen-Yue Dai discovered that when an older satellite has migrated to a medium-distance orbit by the time the next shedding event happens, the system enters what they call an "interaction regime"—a gravitational gauntlet of collisions and mergers.
In this regime, newly born satellites and pre-existing moons collide at low velocities and stick together, a process that can forge contact binaries like Selam. The simulations showed that low-velocity mergers reliably produce the two-lobed configuration we see in the Dinkinesh system. Remarkably, Selam's current orbital parameters fall squarely within this interaction regime, suggesting a plausible backstory: a first shedding event created a moon that drifted outward; a second event spawned another satellite; the two met gently and merged into the double-lobed world we observe today.
The implications reach far beyond Dinkinesh. Triple asteroid systems such as 2001 SN263 and Balam likely share similar multigenerational histories, the researchers found. Even the active asteroid 311P/PANSTARRS, target of China's upcoming Tianwen-2 mission, may owe its repeated mass-ejection events to the tidal disruptions of satellites born from this complex evolutionary process.
Most strikingly, the team discovered that approximately 44 percent of known binary asteroid systems show configurations consistent with a multigenerational satellite history—an astonishingly large fraction that suggests such exotic origins are commonplace, not rare. As space missions continue to visit asteroids across the solar system, scientists now expect to encounter many more contact binaries and other puzzling configurations. Rather than contradicting our understanding of the cosmos, systems like Dinkinesh reveal that asteroids have richer, messier stories to tell—narratives written in stone, gravity, and the patient drift of ancient moons.