Weird, 'watermelon shape' asteroids like Dimorphos and Selam may finally have an explanation

The unusual shapes of the tiny asteroids Dimorphos and Selam have perplexed astronomers for years, but a new study finally explains how they got so strange. It also suggests these bizarrely shaped "moonlets" may be more common than scientists thought.

Binary asteroids — pairs of asteroids that are essentially mini versions of the Earth-moon system — are pretty common in our cosmic neighborhood. These include the Didymos-Dimorphos duo that headlined NASA's 2022 Double Asteroid Redirection Test (DART) mission. Previous research suggests that such binary asteroids form when a rubble-pile "parent" asteroid — composed of loosely held rocks — spins so fast that it sheds some of its mass, which coalesces into the second, smaller satellite or "moonlet" asteroid.

Most moonlet asteroids look like upright, blunt-ended Footballs as they orbit their typically top-shaped parents; such moonlets are described as being "prolate." But some have odder shapes. Take Dimorphos — that is, before DART impacted it. It was an "oblate spheroid" — a sphere squished at its poles and stretched along its midriff, like a watermelon. And tiny Selam, the recently discovered satellite of the asteroid Dinkinesh (aka "Dinky"), is even more peculiar, consisting of two connected rocky spheres.

The moonlets' weird shapes have puzzled astronomers, including John Wimarsson, a graduate student at the University of Bern in Switzerland and the new study's lead author. "We have never seen such asteroid satellites before and they cannot be directly explained by traditional binary asteroid formation models," he told Live Science by email.

Related: Earth's gravity knocked pyramid-size asteroid off course during recent ultra-close flyby, NASA images reveal

To understand the asteroids' odd shapes, Wimarsson and his colleagues — from European and American universities — developed two sets of detailed computer models. The first set simulated how the parent asteroids' shapes would change as they spun rapidly and flung out debris. The second set assumed the debris formed a doughnut-shaped zone — called the debris disk — around the parent asteroid. The algorithms then tracked the movement of all the fragments as they experienced gravitational tugs from each other and their parent and collided to form aggregates. The researchers also considered two kinds of parent asteroids, resembling "rubber-ducky" Ryugu and Didymos in size and density.

The results, published online July 20 in the journal Icarus, revealed that there are two primary factors that govern a moonlet asteroid's final shape: the gravitational force the parent exerts, and the nature of collisions the moonlet endures with other rocky objects in the debris disk.