NASA’s Lucy finds a wobbling peanut-shaped asteroid with signs of ancient water

The asteroid formed about 155 million years ago when fragments from a violent collision gradually came together. Since then, a subtle but persistent force generated by sunlight has altered its rotation, while traces of ancient water remain preserved in its rocky surface.

On April 20, 2025, Lucy passed within 650 miles of Donaldjohanson while traveling through the main asteroid belt on its way to a group of Jupiter Trojan asteroids. During the flyby, the spacecraft captured the first close-up images and collected detailed scientific measurements. Those observations revealed an asteroid that does not rotate in the simple way scientists expected.

Lucy also provided detailed views of Donaldjohanson's unusual shape, along with craters, ridges, and other surface features that help tell the story of its evolution.

Lucy's Asteroid Flyby Reveals an Unusual Rotation

The encounter served as a practice run for Lucy's future visits to the Trojan asteroids, beginning with its flyby of Eurybates on Aug. 12, 2027. While testing spacecraft systems and mission operations, scientists gained a valuable opportunity to study a previously unexplored asteroid and compare it with Bennu and Ryugu, two asteroids that have been examined up close through sample-return missions.

Researchers reported their findings on June 18 in the journal Science.

Before Lucy's arrival, astronomers studying Donaldjohanson from Earth noticed a repeating pattern in the asteroid's brightness. Those observations suggested it was an elongated object completing one rotation every 10.5 Earth days.

The spacecraft's close-up measurements revealed a more complicated reality.

Instead of rotating around a single axis like most asteroids and planets, Donaldjohanson behaves more like a wobbling spinning top. Scientists found that it turns end-over-end once every 10.5 days while also rocking back and forth around its long axis once every 26.5 days.

Peanut-Shaped Asteroid Formed From Ancient Collision

Earth-based observations had already hinted that Donaldjohanson was elongated, but Lucy showed that the asteroid is actually composed of two connected lobes joined by a narrow neck.

Scientists describe this type of structure as bilobate. It likely formed when two fragments produced by an earlier collision drifted together and merged under their own gravity.

Researchers estimate that the asteroid was spinning at least ten times faster shortly after it formed. Over the last 20 to 60 million years, however, that rotation gradually slowed.

As the spin rate decreased, the balance between centrifugal forces and gravity changed. Loose rock and debris shifted down slopes, reshaping parts of the surface and contributing to the softened appearance of many craters visible in Lucy's images.

The team believes this slowdown was caused by the YORP effect, a subtle process driven by sunlight.

When sunlight warms an asteroid, the surface releases that energy as infrared radiation. Although the resulting recoil force is extremely small, it acts continuously over millions of years. Because Donaldjohanson's shape is uneven, those tiny forces do not cancel out completely and instead create a twisting effect that gradually alters the asteroid's rotation.

The same process can either slow or accelerate an asteroid's spin. Bennu, which rotates once every four hours, and Ryugu, which rotates roughly once every seven hours, were likely spinning much more slowly in the distant past before YORP sped them up.

Evidence of Ancient Water on Donaldjohanson

As Lucy raced past Donaldjohanson at about 30,000 mph, its instruments detected iron-rich clay minerals on the asteroid's surface.

These minerals could only have formed in the presence of liquid water. However, scientists believe the water exposure was relatively brief.

Over time, prolonged interaction with water tends to replace iron within clay minerals with other elements such as magnesium. Because the clays on Donaldjohanson remain iron-rich, researchers concluded that liquid water was present only for a limited period.

The situation appears different for Bennu and Ryugu. Both asteroids contain magnesium-rich clays, suggesting they experienced much longer periods of water exposure, possibly lasting millions of years while they were still parts of larger parent bodies.

Those differences may indicate that the parent asteroids formed at different times or in different regions of the solar system before eventually moving into the main asteroid belt.

Comparing Donaldjohanson, Bennu, and Ryugu

Scientists believe Donaldjohanson originated from the rocky remains of a larger asteroid rich in carbon and water that was shattered in a collision within the main asteroid belt.

Bennu and Ryugu likely formed through a similar process and in the same general region. Yet important differences set them apart.

Donaldjohanson is only about 155 million years old, making it much younger than Bennu and Ryugu, which are estimated to have formed 1 to 2 billion years ago.

Its orbital history is also different. Donaldjohanson has remained in the asteroid belt since its formation, while Bennu and Ryugu eventually migrated into near-Earth orbits that periodically bring them close to our planet. Those trajectories made them ideal destinations for sample-return missions.

"It's helpful for scientists to compare Donaldjohanson with asteroids like Bennu and Ryugu, which are seemingly similar asteroids, because every subtle difference is another clue to our origin story," said Simone Marchi, Lucy deputy principal investigator and lead author of the study at the Boulder, Colorado, office of the Southwest Research Institute.

"Once we start learning more about the Trojans, a completely different population of space rocks with very different histories, our understanding of solar system formation is destined to be challenged," said Marchi.

Named after the famous fossilized human ancestor discovered in Ethiopia in 1974, Lucy is on its way to become the first mission to explore Jupiter's Trojan asteroids. These ancient and relatively unchanged objects formed early in solar system history and may help scientists better understand how the planets formed and migrated before settling into their present-day positions.

About the Lucy Mission

Lucy's principal investigator is based at the Boulder, Colorado, office of the Southwest Research Institute, which is headquartered in San Antonio. NASA's Goddard Space Flight Center in Greenbelt, Maryland, oversees mission management, systems engineering, and safety and mission assurance. Lockheed Martin Space in Littleton, Colorado, built the spacecraft.

Lucy is the 13th mission in NASA's Discovery Program. The program is managed by NASA's Marshall Space Flight Center in Huntsville, Alabama, for the agency's Science Mission Directorate in Washington.