As part of the preparations for its primary mission—studying Jupiter’s Trojan asteroids—NASA’s Lucy spacecraft successfully conducted a reconnaissance flyby of the asteroid Donaldjohanson. The close approach, which took place on April 20, 2025, allowed scientists to obtain the first detailed images and scientific data from this celestial body, with the spacecraft passing just 1,050 kilometers above its surface. The findings, published in the journal Science, revealed that even small asteroids can possess a complex and eventful history.
Instead of rotating simply around a single axis, as most planets and asteroids do, Donaldjohanson exhibits a chaotic motion resembling that of a “drunken” or “wobbling” top. Data from Lucy confirmed that the asteroid completes one full rotation on its axis every 10.5 Earth days, while its long axis undergoes an additional oscillatory motion with a period of 26.5 days.
Close-up images also revealed its distinctive “peanut” or dumbbell-like shape. This “bipartite” structure directly points to its origins. Scientists believe that Donaldjohanson formed around 155 million years ago, following a collision between two larger asteroids. Their fragments merged under the influence of mutual gravitational attraction, forming a single body.
Over its relatively short cosmic history, the asteroid has undergone significant changes. Initially, it rotated at least ten times faster, but its spin has slowed over the last 20 to 60 million years. This deceleration is attributed to the YORP effect—a phenomenon where thermal radiation from an unevenly Sun-heated surface generates a weak but persistent torque. This shift in force equilibrium caused loose material on the slopes to shift, smoothing out craters and giving the asteroid its current “weathered” appearance.
Lucy’s spectrometric instruments also detected the presence of iron-rich clay minerals on Donaldjohanson’s surface. This indicates that liquid water once existed deep within the asteroid or its parent body in the distant past. However, unlike the asteroids Bennu and Ryugu, which contain magnesium-rich clays indicative of prolonged contact with water, Donaldjohanson’s iron-bearing clays suggest only a brief interaction with moisture.
Comparing Donaldjohanson with the asteroids Bennu and Ryugu—from which samples were returned to Earth by the OSIRIS-REx and Hayabusa2 missions—offers scientists valuable insights into the evolution of the Solar System. All three asteroids share a similar composition and likely originated from the fragmentation of larger, carbon- and water-rich bodies in the main asteroid belt. Yet Donaldjohanson is significantly younger (155 million years, compared to 1–2 billion years) and, unlike its “cousins,” has never left the main belt.
This close encounter served as a dress rehearsal for the main phase of the mission. On August 12, 2027, Lucy will perform its first flyby of the Trojan asteroid Eurybates.
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