NASA’s Lucy spacecraft is on its way to the Jupiter Trojan asteroids, but en route it’s exploring some other destinations. First up was Dinkinesh, a small asteroid 480 million kilometers (300 million miles) away from Earth in the main asteroid belt beyond Mars.
Lucy passed Dinkinesh on Wednesday, Nov. 1, 2023, and discovered what first appeared to be a tiny asteroid moonlet in orbit around it. Further imaging revealed something even more surprising: a binary pair of moonlets close enough to touch one another. This is the first time such a pair — called a contact binary — has been seen orbiting another asteroid.
The yet-to-be-named moonlets are each estimated to measure only about 220 meters (about 0.15 miles) in diameter — even dinkier than Dinkinesh itself, which is estimated to be about 790 meters (0.5 miles) at its widest.
Here’s everything you need to know about the asteroid family, the mission that visited them, and what comes next.
The Lucy mission
NASA's Lucy mission will visit a particular group of asteroids called Trojans that share Jupiter's orbit around the Sun. The Trojans may have formed farther away before getting caught by Jupiter's gravity, and likely have different blends of the Solar System's starting materials than other asteroids we've previously visited.
Lucy, named after the fossilized skeleton that helped scientists learn where humans fit into the evolutionary chain of life, launched in 2021 and will visit seven Trojan asteroids between 2027 and 2033. Just like its ancestral namesake, Lucy aims to give us a more complete picture of our origin story.
Lucy flew past Dinkinesh and its satellite asteroids on its way to the Jupiter Trojans. This particular flyby had the goal of testing the spacecraft’s autonomous tracking capabilities, which will be important when it conducts flybys of its main target asteroids.
The Terminal Tracking System is a pair of cameras that image targets as Lucy approaches, providing detailed position data that allow the spacecraft’s instruments to autonomously determine when to start collecting their scientific data, and allow them to stay locked on their target during the entire flyby.
Because Dinkinesh is much smaller than any of Lucy’s other targets, the tracking system had to perform an even more difficult task than it will during its main mission — and it succeeded. The spacecraft made its flyby at a distance of only 425 kilometers (264 miles) from the asteroid. This meant that Lucy was able to collect images and data about Dinkinesh and its satellite while proving its tracking capabilities, allowing the mission’s science team to learn more about this asteroid system.
What we know about Dinkinesh and its satellites
Dinkinesh was originally discovered in 1999 by the Lincoln Near-Earth Asteroid Research (LINEAR) sky survey at Socorro, New Mexico. It orbits the Sun in the main asteroid belt, following a 3.24-year elliptical orbit with an average distance of 2.19 astronomical units (328 million kilometers, or 204 million miles) from the Sun. Dinkinesh may have begun as part of the larger asteroid 8 Flora before being ejected by an impact from another large object, since it shares similar orbital characteristics with that parent asteroid and other smaller asteroids that are thought to have come from it.
The asteroid was named after it was selected for exploration by the Lucy spacecraft. The name comes from Dink’inesh, which is the Ethiopian name for the Lucy fossil after which NASA's Lucy mission is named. The name means "you are wonderful" in the Amharic language.
The Lucy mission team started to suspect that Dinkinesh might have a satellite after noticing regular, repeating changes in its brightness as the spacecraft approached. This was confirmed when Lucy took its first close-up look at its target. In the first images sent back from the flyby, it looked like a single tiny asteroid was in orbit around Dinkinesh. Only when the team saw additional images, captured in the minutes around the encounter, did they notice the second moonlet. It appears that the first images happened to capture the two lobes of the contact binary lying one behind the other from Lucy's point of view.
Lucy’s flyby observations showed that Dinkinesh is similar in shape to near-Earth asteroids Bennu and Ryugu, which are both known to be rubble pile asteroids (loose collections of rocks and dust held together by gravity, rather than solid objects), suggesting that it might have the same structure. This gives researchers an opportunity to study a similar kind of asteroid in a different part of the Solar System, and could also shed light on whether rubble pile asteroids change once they leave the main belt and enter near-Earth space.
The Dinkinesh system also has similarities to another near-Earth pair: asteroid Didymos and its moonlet Dimorphos, which were impacted by the DART spacecraft in 2022. Although the contact binary satellite is different from the lone Dimorphos, this still gives scientists the chance to compare the nature of binary asteroids in different environments.
Dinkinesh and its moonlets are the smallest main-belt asteroids ever visited by a spacecraft. Smaller asteroids have been explored by spacecraft, but only near-Earth ones.
What’s next for Lucy
Lucy is already zipping through space toward its next target, the main belt asteroid Donaldjohanson (named after the paleontologist who co-discovered the Lucy fossil), which it will reach in 2025. This flyby is intended to be another test of the Terminal Tracking System to prepare for encounters with the mission’s main targets, the Jupiter Trojan asteroids, starting in 2027. But as the Dinkinesh flyby has proved, even a technology test can yield exciting insights into the bodies of our Solar System.