New computer models, however, indicate that the final form of these moonlets is dependent on parameters such as the density of the parent asteroid and the type of collisions that occur within the debris disc. In addition to explaining Dimorphos and Selam’s peculiar morphologies, this work raises the possibility that such abnormalities could be common.
Binary asteroids and their formation
Binary asteroids, which are essentially pairs of asteroids resembling a miniature Earth-moon system, are prevalent in our cosmic neighbourhood. For instance, the Didymos-Dimorphos duo was the focus of NASA’s 2022 Double Asteroid Redirection Test (DART) mission.
Traditional theories suggest that these binary asteroids form when a rapidly spinning “rubble-pile” parent asteroid—composed of loosely held rocks—sheds some of its mass, which then coalesces into a smaller satellite or “moonlet” asteroid.
The mystery of moonlet shapes
Typically, moonlet asteroids adopt prolate shapes, resembling upright, blunt-ended footballs as they orbit their usually top-shaped parent asteroids. However, some moonlets display more unusual shapes. For example, Dimorphos was an “oblate spheroid”—a sphere squished at its poles and stretched along its midriff, like a watermelon—before the DART mission impacted it.
Similarly, Selam, a recently discovered moonlet of the asteroid Dinkinesh (also known as “Dinky”), consists of two connected rocky spheres. These peculiar shapes have puzzled astronomers, including John Wimarsson, a graduate student at the University of Bern and lead author of the new study. According to Wimarsson, these shapes cannot be easily explained by traditional binary asteroid formation models.
Computer models reveal shape formation
To unravel the mystery behind these strange shapes, Wimarsson and his team, comprising researchers from European and American universities, developed two sets of detailed computer models. The first set simulated how the shapes of parent asteroids change as they spin rapidly and shed debris.
The second set modelled a doughnut-shaped zone of debris—known as the debris disk—around the parent asteroid. The researchers tracked the movement and interactions of the fragments as they experienced gravitational tugs and collisions, leading to the formation of aggregates. They considered two types of parent asteroids for their simulations: one resembling the “rubber-ducky” Ryugu and another like Didymos.
Factors influencing asteroid shapes: Density and Roche limit
The study, published online on July 20 in the journal Icarus, identified two main factors influencing a moonlet asteroid’s final shape: the gravitational force exerted by the parent asteroid and the nature of collisions with other objects in the debris disk.
The density of the parent asteroid plays a crucial role. Denser asteroids, such as Didymos, spin faster, creating wider debris disks and causing moonlets to form farther from the parent asteroid. This distance, known as the Roche limit, helps maintain the moonlet’s shape as it gradually grows through collisions and fusion with other debris.
Moonlets forming at or beyond the Roche limit tend to acquire oblate shapes because they are less influenced by the parent asteroid’s gravity. As they collide with other debris, they grow more uniformly compared to their prolate counterparts.
Conversely, moonlets forming too close to the parent asteroid are ripped apart by its gravity, making them less likely to retain prolate shapes. Such moonlets are more prone to becoming oblate spheroids after collisions with precursor moonlets.
Collision angles and final moonlet shapes
The angle at which precursor moonlets collide also impacts their final shapes. Collisions occurring side-to-side and aligning along the short axes result in a more oblate shape, while edge-to-edge collisions aligning the longest axes create bilobate (two-lobed) objects, similar to the moonlet Selam.
