"The encounter provided an opportunity to test instruments and procedures for the Trojan asteroid encounters."
— Dr. Simone Marchi, Deputy Principal Investigator, Lucy Mission

Lucy's Flyby Reveals a "Tumbling" Asteroid with a Watery Past

On April 20, 2025, NASA's Lucy spacecraft conducted a flyby of the inner main-belt asteroid (52246) Donaldjohanson at a distance of approximately 650 miles (1,000 km). The encounter provided crucial data on the asteroid's rotation, shape, composition, and formation history. Originally planned as a test for Lucy's primary mission to study the Trojan asteroids near Jupiter, the flyby has yielded significant scientific results.

Shape and Rotation

Observations indicate Donaldjohanson has a bilobate, or peanut-like, shape. This structure consists of two lobes connected by a narrower neck. Scientists infer that the lobes are fragments from an asteroid collision that reaccumulated, forming a contact binary.

The asteroid exhibits a non-principal axis rotation, meaning it tumbles. It rotates end-over-end once every 10.5 Earth days and wobbles along its long axis once every 26.5 Earth days. The asteroid is elongated and approximately half a mile in diameter.

Composition and Water History

Lucy's infrared spectrometer detected iron-bearing phyllosilicates (iron-rich clay minerals) on the asteroid's surface. This indicates that liquid water was present on the body at some point in the past. The alteration was partial, suggesting the period of water exposure was brief and ended early. This composition differs from the magnesium-rich clays found on the asteroids Bennu and Ryugu, which indicate prolonged water interaction.

Origin and Age

The study, published in Science, concludes that Donaldjohanson is a fragment of a larger, carbon- and water-rich parent body that was disrupted by a collision approximately 155 million years ago. This formation age is younger than that of Bennu and Ryugu, which are estimated to be 1-2 billion years old.

Donaldjohanson belongs to the Erigone family of asteroids, whose largest member is 163 Erigone (diameter ~73 km). The presence of volatiles suggests Donaldjohanson formed farther from the sun, likely in the outer asteroid belt, and later migrated inward.

Scientists estimate that the asteroid's rotation has slowed over the last 20-60 million years, likely due to the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, which may have caused material to slide and erode craters.

Differences from Bennu and Ryugu

Differences in water exposure history and age between Donaldjohanson and other studied asteroids (such as Bennu and Ryugu) suggest that the parent bodies of these asteroids may have formed at different times or in different locations before moving to the main belt.

Mission Context

The Donaldjohanson flyby served as a rehearsal for Lucy's primary mission to explore the Trojan asteroids. Lucy's next flyby is of the Trojan asteroid 3548 Eurybates, scheduled for August 2027, which is compositionally similar to Donaldjohanson.

Background on the Asteroid's Name

The asteroid is named after paleontologist Donald Johanson, who discovered the hominin fossil known as "Lucy" in 1974. The Lucy mission's name was inspired by that fossil.