Weird 'watermelon-shaped' 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.

- Abha Jain

Binary asteroids - pairs of asteroids that are essentially mini versions of the Earth-Moon system - are pretty common in our cosmic neighbourhood. 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 rugby balls 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, is even more peculiar, consisting of two connected rocky spheres.