NASA’s Lucy spacecraft has confirmed that the main-belt asteroid Donaldjohanson exhibits a complex “tumbling” rotation, spinning on two axes while traveling through space. Data published June 18, 2026, in the journal Science reveals the asteroid rotates end-over-end every 10.5 days, while simultaneously wobbling around a horizontal axis every 26.5 days, according to Dr. Simone Marchi of the Southwest Research Institute.
Why does Donaldjohanson wobble through space?
The asteroid’s non-steady rotation is a direct result of its physical structure and history. According to the Lucy mission team, the object is composed of two distinct, heavily cratered lobes connected by a narrow, smoother neck. This “peanut” shape, measuring roughly 8.8 km by 4.4 km by 3.1 km, creates an uneven distribution of mass. Dr. Marchi states that this bilobed configuration, confirmed by images taken during the April 20, 2025, flyby, prevents the asteroid from maintaining a simple, singular rotation pattern.
The crater density on Donaldjohanson suggests it is a remnant of the Erigone asteroid family. This group formed 155 million years ago following a catastrophic collision that shattered a much larger parent body.
What do iron-rich clays tell us about early solar history?
Lucy’s instrumentation detected iron-rich clay minerals on the surface of Donaldjohanson, providing evidence of past exposure to liquid water. Researchers conclude these minerals formed within a larger, water-rich parent body before the 155-million-year-old breakup event. By identifying these materials, scientists can trace the migration of volatile-rich objects in the early Solar System. According to Dr. Marchi, comparing these findings to other “peanut-shaped” asteroids like Bennu and Ryugu allows researchers to refine models of how planetary building blocks were distributed during the system’s formation.
How does this mission prepare NASA for the Trojan asteroids?
The flyby of Donaldjohanson serves as a high-stakes rehearsal for Lucy’s primary objective: exploring the Trojan asteroids. These two swarms of ancient objects orbit the Sun in the same path as Jupiter, effectively acting as time capsules from the era of planet formation. While Donaldjohanson is a fragment of a relatively recent collision, the Trojans are expected to have vastly different histories. Dr. Marchi notes that once the spacecraft reaches these populations, the data will likely challenge existing theories regarding the movement and evolution of objects in the outer Solar System.
Scientists often look for “bilobed” structures—like the one found on Donaldjohanson—because they suggest the object formed through a gentle collision of two smaller bodies or the fragmentation of a single, larger parent. Comparing these shapes helps categorize asteroid families.
Frequently Asked Questions
- What is the shape of Donaldjohanson? It is an elongated, bilobed asteroid measuring 8.8 km by 4.4 km by 3.1 km, often described as resembling a peanut.
- How long does it take for the asteroid to rotate? It has a complex rotation: it turns end-over-end every 10.5 days and wobbles on a horizontal axis every 26.5 days.
- What did the Lucy spacecraft find on the surface? The spacecraft detected iron-rich clay minerals, which indicate the asteroid’s parent body once contained liquid water.
- Why is this mission important? It provides a baseline for comparing main-belt asteroids with the Trojan asteroids, which are expected to reveal more about the early history of our Solar System.
The findings from the Lucy mission were published in the journal Science (doi: 10.1126/science.aec0503). To stay updated on the latest discoveries from the Lucy mission as it heads toward the Trojan asteroids, subscribe to our weekly space newsletter or explore our archives on planetary science.