💥 What is the origin of Selam, the moon of asteroid Dinkinesh?

On November 1, 2023, NASA's Lucy spacecraft, en route to Jupiter's Trojan asteroids, flew by asteroid Dinkinesh and made a remarkable discovery: it is actually a binary asteroid whose small moon, Selam, is itself made of two lobes stuck together, an object called a contact binary. This is the first confirmed observation of such an object orbiting an asteroid.

Simulations presented in the paper co-authored by scientists from CNRS Earth & Universe (see box), show that Selam most likely formed when several small satellites collided at low speed and ended up merging, rather than breaking apart. This type of slow collision is common around asteroids, where gravity is very weak.


Why is this result important? Because it helps us better understand how asteroids and their moons form and evolve, but also how these small bodies react to impacts. This knowledge is essential not only for tracing the history of the Solar System but also for planetary defense: knowing how an asteroid is structured allows us to anticipate its behavior in the event of an impact or a deflection attempt.

Similar mechanisms could explain the peculiar shape of Dimorphos, the small moon of the binary asteroid Didymos that was intentionally hit by NASA's DART mission. The European Space Agency's Hera mission, currently en route, will complement these observations and improve our understanding of the formation and evolution of asteroid moons.


This discovery shows that asteroid systems are more complex and diverse than previously thought, and that they can form their satellites repeatedly. Each new space mission thus provides us with valuable keys to understanding our cosmic environment and better protecting Earth.

This study benefited from the support of the IDEX JEDI of Université Côte d'Azur supported by the French National Research Agency (ANR-15-IDEX-01) and CNES.