Multiple moonlet mergers as the origin of the Dinkinesh-Selam system

On November 1st, 2023, the Lucy spacecraft encountered the main-belt asteroid Dinkinesh, revealing the first confirmed contact binary moon, (152830) Dinkinesh I Selam. Here, we show that Selam likely formed through a series of low-velocity collisions of similarly sized moonlets that once orbited the primary. To gain insight into the processes that shape satellites in small binary systems, we simulate plausible formation scenarios for Selam. The mergers that form each lobe are consistent with collisions that occur beyond four primary radii at impact velocities between 1 and 1.5 times their mutual escape velocity and impact angles between 5 and 15∘. Similar collisions could also explain the oblate shape of asteroid Dimorphos, the secondary in the Didymos system and the target of NASA’s Double Asteroid Redirection Test (DART), though further data from ESA’s Hera mission will provide additional insights. These results indicate that binary asteroid systems can undergo multiple moon-forming events and exhibit a wider diversity of satellite shapes than currently observed. Space missions imaging small asteroid moons revealed the variety in shapes. Here, the authors show that repeated low-speed collisions can explain the shape of Selam, which is the smaller component in Dinkinesh-Selam binary asteroid system.