NASA
An analysis carried out with the help of the Lucy probe reveals that the asteroid Donaldjohnson is relatively young and once had liquid water.
Even small asteroids have complex histories. During its pass by the asteroid Donaldjohanson last year, NASA’s Lucy probe revealed that the object is an unstable body, shaped like a peanutwhich has seen a lot of activity in its relatively short history.
Formed from the coalescence of fragments following a violent collision 155 million years ago, the asteroid was transformed by the small but inexorable force of solar radiation, while maintaining signs of the brief presence of liquid water in its distant past.
As it crossed the main asteroid belt toward one of Jupiter’s Trojan asteroid clusters, the Lucy spacecraft collected the first close-up images and other data of Donaldjohanson on April 20, 2025, as it passed the just over a thousand kilometers away of the asteroid. The data revealed that, instead of simply rotating around one axis like most other asteroids and planets, Donaldjohanson has a more complex rotation on two axes. Scientists also observed Donaldjohanson’s peanut shape, as well as the craters and ridges on its surface.
Lucy’s encounter with the asteroid was planned as a dress rehearsal for the spaceship and to the mission team ahead of its main asteroid encounters, which will begin with Lucy’s flyby of the Trojan asteroid Eurybates on August 12, 2027. The instruments worked as expected, and as a bonus, scientists had the rare opportunity to study a previously unexplored asteroid up close and compare with two asteroids of similar compositionsbut with different histories: Bennu, target of NASA’s OSIRIS-REx sample collection mission, and Ryugu, site of JAXA’s (Japan Aerospace Exploration Agency) Hayabusa2 sample collection mission.
Here’s what Lucy’s science team has learned so far thanks to the probe’s encounter with Donaldjohanson, as per June 18 in Science magazine.
Oscillating rotation
Using ground-based telescopes, observers detected fluctuations in the light reflected by Donaldjohanson, 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: Donaldjohanson appears to be spinning like an unstable top. The authors of the scientific paper reported that the asteroid rotates end-to-end once every 10.5 Earth days and oscillates back and forth around its longitudinal axis once every 26.5 days.
Peanut shape
Although observations from Earth suggested Donaldjohanson’s elongated shape, Lucy’s approach revealed a “bilobed” structure“: two lobes connected by a neck, like a peanut. These lobes are probably two fragments resulting from an asteroid collision that later came together smoothly due to their mutual gravity.
Donaldjohanson probably it was spinning at least 10 times faster when it formed, having slowed to its current speed over the past 20 to 60 million years, according to the team’s estimates. As it slowed, the balance between the centrifugal force pushing materials apart and gravity pulling them together changed, and loose rock material slid down the slopes, creating the weathered appearance of many craters, as images from the pass showed.
The authors of the scientific article state that the slowing of the asteroid’s rotation is probably caused by a subtle consequence of solar heating known as YORP effect. Every part of the asteroid’s surface, heated by the Sun, radiates heat in the form of infrared light, and this radiation exerts a tiny recoil force on the surface. Given that the shape of the asteroid is not symmetrical, this results in a torque that can change the rotation of the asteroid. Thus, the YORP effect can slow down or speed up the rotation of asteroids, as in the case of Bennu (once every four hours) and Ryugu (once every about seven hours), which probably used to rotate much more slowly than they do today.
Water, for a moment
While passing Donaldjohanson at more than 48,000 km/h, Lucy recorded, on the surface, signs of iron-rich clay minerals. These clays must have formed in the distant past with the help of liquid water. However, the exposure must have been brief, the Lucy scientists concluded, because the iron in the clays tends to be replaced by other elements, such as magnesium, as the water remains in place.
In fact, scientists observed magnesium-rich clays in Bennu and Ryugu, which suggested prolonged exposure to waterperhaps lasting millions of years, when they were still part of larger asteroids.
This difference in water exposure history, as well as other characteristics, could 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.
Compare, contrast
Donaldjohanson is thought to be made up of the 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, is much younger than Bennu and Ryuguwhich formed 1 to 2 billion years ago. Donaldjohanson has also remained in the asteroid belt since its formation, while its “wandering cousins” have migrated to orbits around the Sun that bring them closer to Earth’s orbit about once a year (making them perfect close-in targets for sample-collection missions).
“It is useful for scientists to compare Donaldjohanson with asteroids like Bennu and Ryugu, which are apparently similar asteroidsbecause each subtle difference is another clue to the story of our origin”, said Simone Marchi, deputy principal investigator of the Lucy mission and lead author of the study at the SwRI (Southwest Research Institute) office in Boulder, Colorado, USA.
“Once we start to learn more about the Trojans – a completely different population of space rocks with very different histories – our understanding of the formation of the Solar System is destined to be put to the test“, stated Marchi.
Named after a fossilized skeleton of a human ancestor discovered in Ethiopia in 1974, NASA’s Lucy mission will be the first to explore Jupiter’s Trojan asteroids, a population of well-preserved space rocks that formed early in the history of our Solar System and could help scientists understand how planets formed and moved before settling into their current configuration.
