NASA’s Lucy reveals a wobbly, peanut-shaped asteroid

Even small asteroids lead complex lives. During its flyby of asteroid Donald Johansson last year, NASA’s Lucy spacecraft revealed that the asteroid is a wobbly, peanut-shaped object that has seen a lot of activity in its relatively short history. Formed from fragments after a violent collision 155 million years ago, the asteroid was transformed by the small but stubborn force of the sun’s radiation, all while retaining signs of the brief presence of liquid water in its distant past.

By zooming across the main asteroid belt toward one of Jupiter’s Trojan asteroid clusters, the Lucy spacecraft collected the first close-up images and other data at Donald Johansson on April 20, 2025, as it passed within 650 miles of the asteroid. The data revealed that instead of simply rotating around a single axis like most asteroids and other planets, Donald Johansson has a more complex rotation around two axes. Scientists also saw the shape of Donald Johansson’s peanut and the craters and ridges on its surface.

Lucy’s encounter with the asteroid was planned as a rehearsal for the spacecraft and mission team before its initial encounters with the asteroid, which begin with Lucy’s flyby of the Trojan Eurybates on August 12, 2027. The instruments worked as expected, and as a bonus, scientists got a rare opportunity to study a previously unexplored asteroid up close and compare it to two asteroids with similar compositions but different histories: Bennu, the target of NASA’s OSIRIS-REx sample return mission, and Ryugu, the site of the Japan Aerospace Exploration Agency’s (JAXA) Hayabusa2 sample return mission.

Here’s what Lucy’s science team has learned so far from Lucy’s encounter with Donald Johansson Reported June 18 in Science magazine.

Using ground-based telescopes, observers saw fluctuations in the light reflected by Donald Johansson, and regular patterns of peaks and valleys, typical of an elongated object that rotates once every 10.5 Earth days. But Lucy’s data revealed another pattern: Donald Johansson appears to be spinning like a wobbly top. The paper’s authors report that the asteroid rotates end-to-end once every 10.5 Earth days, and oscillates back and forth on its long axis once every 26.5 Earth days.

While ground observations hinted at Donald Johansson’s elongated shape, Lucy’s flyby revealed a “bilobed” structure: two lobes connected by a neck, like a peanut. These two lobes were likely fragments from an asteroid impact, and they came together gently afterward due to their mutual attraction.

Donald Johansson was likely rotating at least 10 times faster when it formed, having slowed to its current rate over the past 20 to 60 million years, the team estimates. As it slowed, the balance between the centrifugal force pushing things apart and gravity holding things together changed, and loose rocky material slid down the slopes, creating a rickety appearance for many of the craters, flight images show.

The authors of the paper say the slowing of the asteroid’s rotation is likely caused by a hidden consequence of solar heating known as the YORP effect. Every part of the asteroid’s surface heated by the Sun radiates heat in the form of infrared light, and this radiation imparts a small rebound force to the surface. Because the asteroid’s shape is asymmetrical, this results in a net torque, or twist, that can change the asteroid’s spin. Thus, YORP can slow down or speed up the rotation of asteroids, as in the case of Bennu (about once every four hours) and Ryugu (about once every seven hours), both of which likely rotated much more slowly than they do today.

As it passed by Donald Johansson at 30,000 miles per hour, Lucy recorded fingerprints of iron-rich clay minerals on the surface. These clays must have been formed in the distant past with the help of liquid water. However, the Lucy scientists concluded that exposure should be short, because the iron in the clay tends to be replaced by other elements, such as magnesium, as the water remains.

In fact, scientists have seen magnesium-rich clays on Bennu and Ryugu, suggesting they were exposed to water over a long period, perhaps millions of years, when they were still part of larger asteroids.

This difference in water exposure histories, and other characteristics, may mean that the parent bodies of these asteroids formed at different times or in different regions of the solar system before moving into the main belt.

Donald Johansson is believed to consist of the rocky remains of a larger, carbon- and water-rich asteroid that collided with another body in the main asteroid belt. Bennu and Ryugu are believed to have formed in the same way and in the same region.

But Donald Johansson is different. At 155 million years old, it is much younger than Bennu and Ryugu, which formed one to two billion years ago. Donald Johansson has also remained in the asteroid belt since its birth, while its roving cousins ​​have migrated to orbits around the Sun that bring them close to Earth’s orbit about once a year (making them ideal close-in targets for sample return missions).

“It’s useful for scientists to compare Donald Johansson to asteroids like Bennu and Ryugu, which appear to be similar asteroids, because every subtle difference is another clue to our origin story,” said Simone Marchi, deputy principal investigator for Lucy and lead author of the study in the Boulder, Colo., office of the Southwest Research Institute.

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

Named after the fossilized skeleton of a human ancestor discovered in Ethiopia in 1974, NASA’s Lucy will be the first mission to explore Jupiter’s Trojan asteroids, a group of well-preserved space rocks that formed early in the history of our solar system and can help scientists understand how planets formed and moved before settling into their current configuration.

Download story graphics from NASA’s Scientific Visualization Studio.

Principal Investigator Lucey is based in Boulder, Colorado, and the Southwest Research Institute affiliate is based in San Antonio. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, provides comprehensive mission management, systems engineering, safety, and mission assurance. Lockheed Martin Space in Littleton, Colorado, built the spacecraft. Lucy is the thirteenth mission in NASA’s exploration program. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the discovery program for the agency’s Science Mission Directorate in Washington.

For more information about NASA’s Lucy mission, visit: