A Pit Stop on a Grand Tour
NASA’s Lucy spacecraft is on a historic 12-year mission to explore the Trojan asteroids, two swarms of ancient space rocks that orbit the sun alongside Jupiter. These asteroids are considered pristine fossils from the dawn of our solar system, and studying
them promises to unlock secrets about how the planets formed. But before reaching its main targets, Lucy had a scheduled practice run. On November 1, 2023, it flew past a small main-belt asteroid named Dinkinesh to test its autonomous tracking systems. What was planned as a routine engineering check quickly turned into a major scientific discovery. As the first images came back to Earth, the mission team realized Dinkinesh was not alone; it had a tiny moon, which they later named Selam.
The First-Ever Contact Binary Moon
The surprises didn't stop there. As Lucy flew past the system and looked back, its cameras revealed Selam’s true nature from a new angle. It wasn’t a single, potato-shaped rock as one might expect. Instead, it was a “contact binary”—the first of its kind ever seen orbiting another asteroid. The moon consisted of two distinct lobes of nearly equal size, each about 220 meters in diameter, that appeared to be gently resting against each other. This bizarre, snowman-like silhouette was unlike anything astronomers had witnessed up close. The discovery was a scientific puzzle. Contact binaries themselves are common in the solar system, but finding one acting as a satellite to another asteroid was a complete novelty, raising immediate questions about its origin.
Unraveling a Violent Past
A team of scientists led by the Southwest Research Institute began to piece together the history of this strange system. They concluded that Selam’s story didn’t begin with a gentle formation but with a cataclysmic event on its parent, Dinkinesh. Their theory suggests that over millions of years, the slight but persistent push of sunlight on Dinkinesh’s surface caused it to spin faster and faster. Eventually, the rotational stress became too much for the asteroid to handle. A massive chunk, perhaps as much as a quarter of Dinkinesh’s total mass, broke away and was flung into space. This structural failure left behind a giant trough and a distinctive ridge on Dinkinesh's surface that are still visible today.
From Debris to a Double Moon
The material ejected from Dinkinesh didn’t simply drift away. While some of it fell back onto the main asteroid to form the observed surface ridge, the rest entered orbit. From this cloud of debris, Selam was born. However, it wasn't a simple process. Using sophisticated computer simulations, researchers determined that the two lobes of Selam didn't just form and find each other. Instead, it's believed that the debris first coalesced into several smaller 'moonlets'. Over time, these moonlets engaged in a series of slow-motion collisions, sticking together rather than shattering. This gradual, gentle assembly eventually resulted in the two primary chunks that now make up Selam, which finally came together in a soft impact to form the contact binary seen by Lucy.
Why This Cosmic Detective Work Matters
Figuring out the origin of one tiny, odd-shaped moon may seem niche, but it has broad implications for planetary science. The Dinkinesh-Selam system provides the first real-world model for a key theory of how binary asteroids form and evolve. It demonstrates how spin-up, structural failure, and re-accretion work in practice, giving scientists crucial data points to refine their understanding of planetary formation. These processes of objects colliding, breaking apart, and sticking together are the very foundation of how everything from asteroids to planets like Earth came to be. Selam's existence shows that nature has more creative and complex ways of building celestial bodies than previously thought.
