Researchers reveal the reason for the strange shape of the asteroid Dimorphos

Scientists have succeeded in finding an explanation for the peculiar shapes of the asteroids Dimorphos and Selam which have long puzzled the astronomical community. New research published in the journal *Icarus* suggests that the “watermelon-like” shape of these small asteroids may be more common than previously thought.

Reporting from Space.com (14/8), Binary asteroids, in which two asteroids orbit each other, are very common in the solar system. One famous example is the Didymos-Dimorphos pair, which is the main object of NASA’s Double Asteroid Redirection Test (DART) mission in 2022.

Usually, moons or small asteroids that orbit their parent asteroids are prolate in shape, like an upright soccer ball. However, Dimorphos, before being impacted by the DART mission, had an oblate shape that was more like a sphere compressed at the poles and expanded at the center. Selam, a recently discovered small satellite of the asteroid Dinkinesh, has an even stranger shape, consisting of two rock balls joined together.

The research was led by John Wimarsson, a graduate student at the University of Bern, Switzerland. His team used computer models to simulate how a fast-moving parent asteroid threw up debris that then formed a donut-shaped zone around it, called a debris disk. This debris then collects to form moons of various shapes.

The simulation results show that the two main factors influencing the final shape of the moon are the gravity of the parent asteroid and the type of collision the moon experiences with other debris in the disk. At a certain distance, called the Roche limit, the parent asteroid’s gravity balances the internal forces of the moon, so that the moon becomes prolate. However, if a moon forms outside the Roche limit, the parent’s gravity is not strong enough to maintain the prolate shape, so the moon tends to be oblate.

The study also found that the way in which two precursor moons collide has a significant effect on the final shape of the moon. If the collision occurs from the side, the moon will tend to be oblate in shape. However, if the collision occurs along the long axis, the moon will have two lobes that form, similar to the shape of Selam.

These findings suggest that unusual asteroid shapes such as Dimorphos and Selam may be more common in the universe than previously thought. However, oblate-shaped asteroids are often unable to be detected by current observational technology, and many may have been missed in previous studies.