A new study published in Nature reveals that NASA’s Lucy spacecraft has has made a remarkable discovery about a tiny asteroid named Dinkinesh. As Lucy flew past Dinkinesh, also known as “Dinky”, scientists found that this asteroid has a unique companion—a pair of small moons stuck together, known as a contact binary. This unexpected discovery provides new insights into how asteroids and other space objects form and change over time.

Dinkinesh is a small asteroid, approximately 2,362 feet (720 meters) in diameter, orbiting the Sun in the asteroid belt between Mars and Jupiter. Adding Dinkinesh to Lucy’s mission plan was primarily to evaluate the spacecraft’s navigation systems. The asteroid’s S-type classification indicates it is composed of silicate minerals and nickel-iron, typical for bodies in this part of the asteroid belt.

In November 2023, Lucy made a close pass by Dinkinesh, getting as close as 268 miles (431 kilometers) to the asteroid. The spacecraft captured detailed images of Dinkinesh, revealing a far more complex surface than scientists had anticipated. They discovered a large groove, or trough, and a prominent ridge on the asteroid’s surface. These features indicated that Dinkinesh had experienced significant geological changes, likely due to its small size and the effects of solar radiation.

“There’s a lot more complexity in these small bodies than we originally thought,” said Jessica Sunshine, a co-author of the paper. “With the additional observations taken by the spacecraft, we were able to better analyze features such as Dinkinesh’s rotation speed and Selam’s orbit pattern. We also have a better understanding of what materials they’re possibly made of, bringing us a step closer to learning just how terrestrial bodies are created.”

But the biggest surprise came when Lucy’s images revealed that Dinkinesh wasn’t alone.

A tiny binary

Orbiting the asteroid at a distance of about two miles (three kilometers) was a tiny moon, now named Selam. What’s more, Selam turned out to be a contact binary, with its two lobes measuring 689 feet (210 meters) and 755 feet (230 meters) in diameter. This is the first time such a formation has been confirmed in space.

Scientists believe that Dinkinesh’s unusual features and the formation of Selam are due to a phenomenon called the YORP (Yarkovsky–O’Keefe–Radzievskii–Paddack) effect. The YORP effect causes small space objects to spin faster over time because of uneven heating from the Sun. As Dinkinesh spun faster, it likely started to break apart, shedding material that eventually formed Selam. Some material that didn’t form the moon may have fallen back onto Dinkinesh, creating the ridge observed on its surface. This spin-up process caused the asteroid to shed parts of itself, leading to the creation of its moon.

This discovery is significant because it helps scientists understand how small space objects evolve. The presence of Selam suggests that small asteroids can undergo complex processes, leading to the formation of unusual structures. By studying Dinkinesh and Selam, scientists can learn more about the early stages of planet formation, as similar processes might have occurred when our own planet, Earth, was forming.

“One of the things that’s critical to understanding how planets like Earth got here is understanding how objects behave when they hit each other, and to understand that we need to understand their strength,” said lead scientist Hal Levison of Southwest Research Institute, Boulder, Colorado, principal investigator for the Lucy mission. “Basically, the planets formed when [smaller objects like asteroids] orbiting the Sun ran into each other. Whether objects break apart when they hit or stick together has a lot to do with their strength and internal structure.”

The detailed images from Lucy allowed scientists to create precise shape models of Dinkinesh and Selam. Despite their small sizes, the imaging was possible for several minutes before and after the close encounter. The rotation of Dinkinesh and the positions of Selam were observed, providing critical data on their dynamics.

After the encounter, analyzing the changing brightness of Dinkinesh and Selam provided information about how long they take to rotate and orbit. The light curve showed periodic dips due to mutual eclipses, confirming that Selam is tidally locked, meaning one side always faces Dinkinesh, with an orbital period of about 52.7 hours.

Dinkinesh’s significant trough and ridge, along with Selam’s dual lobes, suggest that these bodies are not solid rocks but rather have a porous, rubble-pile structure. This means they comprise many smaller pieces held together by gravity. Dinkinesh’s bulk density was estimated at 2,400 kg/m³, consistent with similar asteroids.

Just how Dinky’s unusual dual moons formed remains a mystery, but Sunshine said that the team’s findings open the door to comparative studies with similar celestial bodies.

“I’m personally very excited to compare the Didymos binary system with this one, especially as they appear to share many similarities such as size, general shape and possibly composition despite being in totally different parts of the solar system,” said Sunshine, who was also on NASA’s DART research team and helped detail the DART spacecraft’s successful deflection of Didymos’ small moon called Dimorphos. “The Didymos binary system is located in a near-Earth environment while the Dinkinesh system is located much farther away from Earth in the main asteroid belt. They have very different features but we think they may have undergone similar processes to become what we know of them today.”

Lucy’s mission is far from over. After studying Dinkinesh and Selam, Lucy will continue exploring other asteroids. These observations will help scientists gather more information about the building blocks of our solar system and how planets like Earth came into existence.