NASA's Lucy Reveals Asteroid Donaldjohanson Is a Wobbling, Peanut-Shaped Relic of a 155-Million-Year-Old Collision With Traces of Past Water

Overview

NASA’s Lucy spacecraft has returned a detailed portrait of asteroid Donaldjohanson, the small main-belt rock it flew past in April 2025, revealing a wobbling, peanut-shaped object whose surface preserves traces of ancient liquid water. The findings were reported on June 18 in the journal Science, according to NASA Science.

The study, led by Lucy deputy principal investigator Simone Marchi at the Boulder, Colorado, office of the Southwest Research Institute, characterizes Donaldjohanson as a fragment of a much larger body that broke apart in a violent collision roughly 155 million years ago, as reported by ScienceDaily.

What We Know

The asteroid has a “bilobate” structure — two lobes connected by a narrow neck, like a peanut, according to NASA Science. The two lobes are heavily cratered and joined by a relatively smooth neck, as reported by Gizmodo. NASA describes the asteroid as about 5 miles (8 kilometers) wide; Sci.News reports its full dimensions as 8.8 km by 4.4 km by 3.1 km.

Its rotation is unusual. Rather than spinning around a single axis, Donaldjohanson turns end-over-end once every 10.5 Earth days while also wobbling back and forth around its long axis once every 26.5 days — behaving like a wobbly top, according to NASA Science. The slow rotation, ScienceDaily reports, was driven by the YORP effect, a subtle process powered by sunlight, as reported by ScienceDaily.

Lucy made its closest approach on April 20, 2025, passing within 650 miles of the asteroid while traveling at roughly 30,000 mph, according to ScienceDaily.

The most intriguing result concerns the asteroid’s chemistry. Lucy detected iron-rich clay minerals on the surface — minerals that could only have formed in the presence of liquid water, according to NASA Science. The Southwest Research Institute, which leads the mission, notes the clays are similar to those found in carbon-rich meteorites, according to the SwRI release on EurekAlert. Unlike the asteroids Bennu and Ryugu, which show signs of prolonged water exposure, Donaldjohanson’s exposure to water appears to have been brief, according to NASA Science.

The asteroid is likely a member of the Erigone asteroid family — a collection of nearly 1,800 identified objects produced when a larger parent body was shattered, as reported by Gizmodo. That parent body measured around 50 miles wide (80 kilometers) before it was struck by a 12-mile-wide (20-kilometer-wide) space rock about 155 million years ago and broke apart, according to Gizmodo.

“It’s helpful for scientists to compare Donaldjohanson with asteroids like Bennu and Ryugu, because every subtle difference is another clue to our origin story,” said Marchi, according to NASA Science.

What We Don’t Know

The published dimensions vary slightly between accounts, and the flyby was a brief encounter rather than an orbital survey, leaving the asteroid’s deeper structure and full composition uncharacterized. The precise sequence by which the parent body’s water-altered material ended up in Donaldjohanson — and what its brief water exposure implies about the early solar system — remains a subject of ongoing analysis.

Analysis

Donaldjohanson was always intended as a rehearsal rather than a primary objective. Lucy will begin its tour of the Trojan asteroids in 2027, visiting Eurybates and its moon Queta, followed by Polymele and its moon, Leucus; Orus; and the binary asteroid system Patroclus and Menoetius, according to Gizmodo. The detailed Science results — catalogued under DOI 10.1126/science.aec0503, according to the SwRI release on EurekAlert — confirm that the spacecraft’s instruments can resolve fine structural and compositional detail during a fast flyby, an encouraging validation ahead of the mission’s main targets among the Jupiter Trojans.