New Delhi, India

The strange forms of small asteroids Dimorphos and Selam have fascinated astronomers for a long time. A recent study has revealed that these unusual shapes are not as rare as once thought, suggesting that these moonlets would be encountered more commonly. 

Advertisment

As per new computer models, the final form of these moonlets depends upon parameters such as what type of collision occurs with the debris disc and the density of the parent asteroid. 

Formation of Binary asteroids 

Binary asteroids are essentially pairs of asteroids that resemble a miniature Earth-moon system and are common in our cosmic neighbourhood.

Advertisment

For example, NASA's 2022 Double Asteroid Redirection Test mission focused on the Didymos-Dimorphous duo. According to traditional theories, these binary moonlet asteroids form when parent asteroids shed some of their mass, which then come together and shape a smaller satellite. 

The mystery behind moonlet shapes 

Generally, a moonlet asteroid adopts a prolate shape, similar to upright, blunt-ended footballs as they orbit their parent asteroids. Some moonlets, however, have very unusual shapes. For example, Dimorphous was shaped like an oblate spheroid, a sphere flattened at the pole and extended around the centre, resembling a watermelon, before it got impacted by the DART mission. 

Advertisment

Similarly, Selam, a recently discovered moonlet of the Dinkinesh asteroid, consists of two connected rocky spheres. The shape of Salem has also puzzled astronomers, and according to a few of them, the shapes could not be explained easily.

Also Read|NASA astronauts to start livestream from Moon. Here's how it will work|

Shape formation revealed by computer models

To unearth the mystery of these strange shapes, the astronomer John Wimarsson, and his team prepared two sets of detailed computer models. The first set replicates the change of shapes of parent asteroids as they spin rapidly. 

The second set simulated a doughnut-shaped zone of debris disk. The researchers kept track of the interaction and movements of fragments as they experienced collisions and gravitational tugs. Two types of parent asteroids were considered: one representing "rubber-ducky" Ryugu and another Didymos. 

Also Read| NASA issues warning as huge asteroid set to come dangerously close on THIS date|

Factors influencing the final shape of the moonlet

According to a study published in the Journal Icarus on 20 July, the two main factors influencing the final shape are the gravitational force applied by the parent asteroid and the type of collisions in the debris disk. 

The density of the parent asteroid plays a vital role in the shape of the moonlet. Denser asteroids, like Didymos, spin faster and create wider debris disks, causing moonlets to form farther from the parent asteroid. This distance is known as the Roche limit, which helps the moonlet to maintain its shape as it gradually grows through the fusion and collisions with other debris.

Moonlets forming near the parent asteroid tend to be ripped apart by the gravitational force of parent asteroids, making it difficult to retain the prolate shapes. On the other hand, moonlets forming at or far from the Roche limit are likely to acquire oblate shapes as they are less influenced by the gravity of parent asteroids. 

With inputs from agencies