A routine asteroid flyby reveals a bizarre, contact-binary moon orbiting in deep space
NASA's Lucy spacecraft was meant to test its tracking systems on a small asteroid named Dinkinesh. Instead, it uncovered a complex, dynamic world. The asteroid hosts a moon named Selam, which is a contact binary—two objects fused together—marking the first time such a structure has been seen orbiting another asteroid.
On November 1, 2023, the Lucy spacecraft performed a high-speed flyby of the asteroid Dinkinesh, located in the main belt between Mars and Jupiter. While the primary mission objective was to calibrate the spacecraft's Terminal Tracking System at 10,000 miles per hour, the encounter yielded unexpected scientific discoveries. Initial brightness variations observed during the approach hinted at the presence of a satellite, but the true nature of the system only emerged after the team processed images captured during the encounter.
The data revealed that Dinkinesh possesses a moon named Selam. Remarkably, Selam is a contact binary, consisting of two lobes touching one another. This configuration is the first of its kind ever observed orbiting an asteroid. Furthermore, surface imaging of Dinkinesh showed a trough and a ridge, suggesting the asteroid underwent a significant structural failure. Researchers believe that thermal radiation caused the asteroid to rotate faster over millions of years, building up centrifugal stress until the body shifted, shedding material that eventually formed the ridge and the satellite.
This discovery provides rare insight into the internal strength of small solar system bodies. Unlike a fluid pile of sand, Dinkinesh demonstrated enough structural integrity to hold together until a sudden, earthquake-like failure occurred. Understanding how these objects behave under stress is critical for scientists, as it informs theories on how the early solar system's smaller objects collided to form planets like Earth. The mission, managed by NASA's Goddard Space Flight Center, is only beginning; Lucy is scheduled to explore at least 11 asteroids over a 12-year journey, with a gravity assist from Earth planned for December 2024 before it proceeds to the Trojan asteroids in 2027.
