The latest scientific results of NASA's "Lucy" probe show that an asteroid shaped like a "peanut" and swinging abnormally in space not only records traces of violent collisions and long years of evolution, but also retains evidence of the brief existence of ancient liquid water on its parent body, providing important clues for studying the early evolution of the solar system.
On its way through the main asteroid belt, "Lucy" passed the asteroid "Donaldjohanson" from a distance of about 650 miles (about 1,000 kilometers) on April 20, 2025, and obtained the first high-resolution images and scientific data of this target that has never been detected at close range before. Observations show that this asteroid with a diameter of about 8 kilometers exhibits a "peanut"-shaped bilobed structure with two connected "heads" and a narrow middle neck. The surface is covered with craters and grooves, and shows complex non-principal axis rotation motion, rather than rotating smoothly around a single axis like most planets and asteroids.
Previously, ground telescopes could only estimate that this long and narrow celestial body rotates once every 10.5 Earth days through changes in brightness. However, precise measurements of "Lucy" found that its true motion is more like a constantly rolling top: on the one hand, it flips "end-to-end" as a whole with a cycle of about 10.5 days; on the other hand, it swings back and forth around its long axis with a cycle of about 26.5 days, forming a so-called "non-major axis rotation" or "tumbling" state. Scientists pointed out through shape models and gravity slope analysis that there are a large number of relatively steep areas and relatively gentle terrain on its surface, reflecting the long-term effects of internal structure and rotational evolution that have shaped its current appearance.
Shape analysis shows that "Donald Johnson" is a two-lobed asteroid formed by the re-aggregation of two fragments under the influence of gravity. It has a narrow neck and an overall appearance that resembles a "peanut". The research team believes that the two leaves were fragments produced in an asteroid collision and gradually rejoined under the gravitational pull about 155 million years ago. Model estimates show that the asteroid's rotation speed was at least ten times faster than it is now at the beginning of its formation, and it gradually "slowed down" over the past 20 to 60 million years. The slowdown in rotation caused changes in the balance of gravity and centrifugal force. Loose rocks and debris on the surface slowly slid down the slope, thus "smoothing" part of the crater edge, making it look softer.
Research points out that this rotation "brake" is likely to be driven by the so-called "YORP effect" - a physical process driven by sunlight that is extremely weak but can significantly change the rotation of small celestial bodies on a scale of millions of years. When sunlight shines on the surface of an irregularly shaped asteroid, the energy absorbed in different areas will produce a very small reaction force when it is released in the form of infrared radiation, which accumulates over a long period of time to form a torque on the rotation, causing its speed to change. This effect may either decelerate or accelerate on different asteroids. For example, the asteroid "Bennu" currently rotates once every 4 hours, and "Ryugu" rotates once every 7 hours. Studies believe that they both once rotated more slowly and gradually "accelerated" through the YORP effect.
In addition to rotation and shape, "Lucy" used an infrared spectrometer during this flyby to discover iron-rich clay minerals on the surface of "Donald Johnson." These minerals have also been found in carbon-rich meteorites and are thought to only form in the presence of liquid water. However, spectral analysis shows that the iron element in these clays has not been replaced by other elements such as magnesium on a large scale, which means that liquid water exists within its parent body for a relatively short time and does not last for millions of years. This is in stark contrast to the magnesium-rich clays found on Bennu and Ryugu, which point to a longer hydration process, possibly millions of years, that occurred when they were still part of the larger parent asteroid.
Since "Donald Johnson" is considered to be a collection of debris formed after the collision and fragmentation of a large carbon- and water-containing asteroid in the main asteroid belt, and "Bennu" and "Ryuugu" are also considered to have similar sources of carbon-rich and water-containing parent bodies, the differences between the three provide important constraints on the distribution and evolution of early material in the solar system. The research team pointed out that "Donald Johnson" is only about 155 million years old, which is much younger than "Bennu" and "Dragon Palace" that were formed about 1 billion to 2 billion years ago, and have always stayed in the main asteroid belt. The latter two gradually migrated to orbits closer to the Earth during the evolution process, making it an ideal target for sample return missions.
Relevant research results were published in the journal Science on June 18, 2026, titled "Lucy Flyby (52246) Donaldjohanson: A Two-lobed Asteroid with Tumbling Rotation". The paper was led by Simone Marchi, Lucy's deputy principal investigator at the Boulder branch of the Southwest Research Institute, and was completed in conjunction with researchers from multiple institutions. It analyzed in detail the asteroid's shape model, rotation state, surface composition and evolutionary history.
The "Lucy" mission is named after the human ancestor fossil "Lucy" discovered in Ethiopia in 1974, symbolizing that it will search for "clues of life" in the early solar system in the "fossil group" of planet formation - Jupiter's Trojan asteroids. As planned, Lucy will be the first spacecraft to fly to Jupiter's Trojan asteroids. These ancient and well-preserved objects formed in the earliest stages of the solar system and are regarded as key samples for studying how planets formed, migrated and eventually settled in their current orbits. This flyby of "Donald Johnson" was regarded by the mission team as a "full-process exercise" before officially entering the Trojan target. It included a comprehensive test of the cooperative working of navigation, imaging and scientific payloads, and accumulated valuable experience for subsequent missions such as the flyby of the Trojan asteroid "Eurybates" on August 12, 2027.
The "Lucy" mission is led by the principal investigator of the Southwest Research Institute's Boulder branch as the chief scientist, NASA Goddard Space Flight Center is responsible for mission management, systems engineering, and safety and quality assurance, and Lockheed Martin Space Systems is responsible for aircraft development. The mission is the 13th project in NASA's Discovery Program and is managed on behalf of the Science Mission Directorate at NASA's Marshall Space Flight Center in Huntsville, Alabama. As "Lucy" continues to approach the Trojan asteroid group during its long journey, scientists expect that more small celestial bodies of various shapes and the clues to water and organic matter they carry will continue to refresh mankind's understanding of the origin and evolutionary history of the solar system.