June 18, 2026 — Southwest Research Institute (SwRI) scientists studying the inner main belt asteroid Donaldjohanson have found that its rotation wobbles. Rather than rolling through space in a steady pattern, Donaldjohanson turns on two axes, rotating end-over-end once every 10.5 Earth days while wobbling around its horizontal axis then once every 26.5 days.
“This is just one of many surprising things learned since NASA’s Lucy spacecraft flew by Donaldjohanson on April 20, 2025,” said SwRI’s Dr. Simone Marchi, deputy principal investigator of the Lucy mission and the study’s lead author. “Lucy images confirmed its elongated shape, initially suggested by Earth-based telescope observations. The flyby revealed that the small asteroid, half a mile in diameter, resembles a peanut, with a two-lobed structure connected by a narrower neck.”
The spacecraft also detected iron-rich clay minerals, formed long ago from the presence of liquid water. These findings indicate that the asteroid likely formed from fragments of a larger, carbon- and water-rich asteroid that broke apart 155 million years ago, following a collision in the main asteroid belt, the region between Mars and Jupiter.
Lucy’s encounter with Donaldjohanson is considered a test run for its primary mission to explore the Trojan asteroids, two swarms of ancient objects that lead and trail Jupiter as it orbits the Sun. Scientists think these populations of space rocks have been preserved since they formed in the early history of the solar system.
“This encounter gave us an opportunity to test our instruments and our procedures to make sure we are ready when we get to Jupiter’s Trojans,” Marchi said. “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 likely to be challenged.”
Donaldjohanson is named for the paleontologist Donald Johanson who discovered “Lucy,” the fossilized skeleton of an early hominin found in Ethiopia in 1974. Lucy is one of the oldest human ancestors ever found and was the inspiration for the Lucy mission’s name.
To read the Science paper titled “The Lucy flyby of (52246) Donaldjohanson: A bilobed asteroid with tumbling rotation,” go to DOI:10.1126/science.aec0503.
For more information, visit Planetary Science or contact Mike W. Thomas, +1 210 522 2255, Communications Department, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166.