Even small asteroids lead complex lives. During its flyby of the asteroid Donaldjohanson last year, NASA's Lucy spacecraft revealed the asteroid to be a wobbly, peanut-shaped body that has undergone a lot of activity in its relatively short history. Formed as fragments coalesced after a violent collision 155 million years ago, the asteroid was transformed by the small but inexorable force of the Sun's radiation, all while retaining signs of the brief presence of liquid water in its distant past.
Zooming through the main asteroid belt toward one of the Jupiter Trojan asteroid groups, the Lucy spacecraft collected the first close-up images and other data at Donaldjohanson on April 20, 2025, as it passed 650 miles away from the asteroid. The data revealed that, instead of spinning simply around one axis like most other asteroids and planets, Donaldjohanson has a more complicated two-axis rotation. Scientists also saw Donaldjohanson's peanut shape and the craters and ridges on its surface.
Lucy's encounter with the asteroid was planned as a dress rehearsal for the spacecraft and mission team before its primary asteroid encounters, which begin with Lucy's flyby of the Trojan asteroid Eurybates on Aug. 12, 2027. The instruments performed as expected, and, as a bonus, scientists got a rare opportunity to study a previously unexplored asteroid up close and to 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 JAXA's (Japan Aerospace Exploration Agency) Hayabusa2 sample-return mission.
Here's what Lucy's science team has learned so far from Lucy's encounter with Donaldjohanson, as reported on June 18 in the journal Science.
Wobbling rotation
With Earth-based telescopes, observers saw fluctuations in the light Donaldjohanson reflects, regular patterns of peaks and valleys, typical of an elongated object rotating once every 10.5 Earth days. But Lucy's data revealed another pattern: Donaldjohanson appears to be rotating like a wobbly top. Paper authors reported that the asteroid rotates end-over-end once every 10.5 Earth days, and wobbles back and forth around its long axis once every 26.5 days.
Peanut shape
While the Earth-based observations hinted at Donaldjohanson's elongated shape, the Lucy flyby revealed a "bilobate" structure: two lobes connected by a neck, like a peanut. These lobes are likely two fragments from an asteroid collision that gently came together afterward by their mutual gravity.
Donaldjohanson likely rotated at least 10 times faster when it formed, having slowed to its current rate in the last 20 to 60 million years, the team estimates. As it slowed, the balance between the centrifugal force pushing things apart and gravity pulling things together changed and loose rocky material slid down slopes creating the worn-down appearance of many craters, as the flyby images showed.
The paper's authors say that the asteroid's slowing rotation is likely caused by a subtle consequence of solar heating known as the YORP effect. Each part of the asteroid's Sun-warmed surface radiates heat away as infrared light, and that radiation imparts a tiny recoil force to the surface. Because the asteroid's shape isn't symmetric, this results in a net torque, or twist, that can change the asteroid's rotation. Thus, YORP can slow asteroid spins down or speed them up, as in the case of Bennu (once every four hours) and Ryugu (once about every seven hours), which both likely used to rotate much slower than they do today.
Fleeting water
As it passed by Donaldjohanson at 30,000 mph, Lucy recorded the signatures of iron-rich clay minerals on the surface. These clays must have formed in the distant past with the help of liquid water. However, the exposure must have been brief, Lucy scientists concluded, because iron in clays tends to be replaced with other elements, such as magnesium, as water lingers.
Indeed, scientists saw magnesium-rich clays at Bennu and Ryugu, which suggested prolonged water exposure, perhaps lasting millions of years, when they were still part of larger asteroids.
This difference in water exposure history, 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 relocating to the main belt.
Compare, contrast
Donaldjohanson is thought to be made from rocky remnants of a larger, carbon- and water-rich asteroid that collided with another object in the main asteroid belt. Bennu and Ryugu are thought to have formed in the same way and in the same region.
But Donaldjohanson is different. At 155 million years old, it is much younger than Bennu and Ryugu, which formed 1 to 2 billion years ago. Donaldjohanson also has remained in the asteroid belt since birth, while its wandering cousins migrated into orbits around the Sun that bring them close to Earth's orbit about once a year (which made them perfect close targets 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 a 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 population of well-preserved space rocks that formed early in our solar system's history and could help scientists understand how the planets formed and moved around before settling in their current configuration.
About Lucy:
Lucy's principal investigator is based out of the Boulder, Colorado, branch of Southwest Research Institute, headquartered in San Antonio. NASA's Goddard Space Flight Center in Greenbelt, Maryland, provides overall 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. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Discovery Program for the agency's Science Mission Directorate in Washington.
