Unlocking Asteroid Dinkinesh’s Dynamic History: The Mission of NASA’s Lucy Spacecraft

WASHINGTON—A small asteroid named Dinkinesh, visited by NASA’s Lucy spacecraft in November, has a surprisingly dynamic history, as scientists have discovered. Along with its moonlet Selam, these two bodies have uniquely merged into one.

Dinkinesh and Selam are the smallest asteroids from the main asteroid belt located between Mars and Jupiter in our solar system ever observed up close by a spacecraft. Lucy’s observations of Dinkinesh revealed ridges, trough structures, and other features that suggest a complex past for the asteroid and its companion.

NASA launched Lucy in 2021 for a 12-year mission to study asteroids, focusing on Jupiter’s Trojan asteroids. These are two groups of space rocks that lead and follow the large planet in its orbit around the sun. During its journey, Lucy flew past Dinkinesh and Selam at the inner edge of the main asteroid belt.

Dinkinesh has a diameter of about 720 meters. Selam consists of two lobes of similar size—one around 230 meters wide and the other about 210 meters wide. Selam orbits Dinkinesh at a distance of approximately 3.1 km, completing one orbit every 53 hours.

The researchers suggest that a significant piece of rock broke off from Dinkinesh at some point, accounting for about a quarter of its total size. This event caused a trough on the asteroid’s surface and scattered debris into space. Some of this debris fell back onto Dinkinesh, forming boulders that created a ridge structure, while other material combined to form Selam.

Selam is classified as a contact-binary moonlet.

“When referring to small bodies in the solar system, a contact-binary is when it appears that a single body is composed of two objects that collided gently enough not to become disrupted,” explained planetary scientist Katherine Kretke of the Southwest Research Institute (SwRI) in Colorado, a co-author of the study published in the journal Nature.

“They are relatively common in the solar system, but Selam was the first time a contact-binary has been observed orbiting another asteroid,” Ms. Kretke said.

Dinkinesh orbits the sun at roughly 2.2 times the distance of Earth’s orbit.

“During their lifetime, small asteroids may shed material, which later ends up forming a small satellite or satellites. The complex shape of Selam indicates that this process may occur multiple times,” noted SwRI planetary scientist and Lucy mission deputy principal investigator Simone Marchi, another study co-author.

Lucy’s next asteroid visit will be Donaldjohanson in 2025 in the main asteroid belt, part of a total of 11 asteroids on its agenda. The visit to Dinkinesh was a late addition to Lucy’s itinerary.

“Dinkinesh was a test fly-by for the Lucy mission that allowed us to exercise some of the procedures that will be used later in the mission when we get to the Trojan asteroids,” Mr. Marchi said. “Lucy performed flawlessly and as planned.”