NASA’s Lucy probe revealed, dduring its passage close to the asteroide Donaldjohanson in 2025, that the planetoid is an unstable, peanut-shaped body that has experienced a lot of activity in its history.
Formed by fragments after a violent collision 155 million years ago, the asteroid has been transformed by the small but inexorable force of solar radiation, while retaining traces of the brief presence of liquid water in its remote past.
Traveling at high speed through the main asteroid belt toward one of Jupiter’s Trojan asteroid clusters, the Lucy probe collected the first close-up images and other data of Donaldjohanson on April 20, 2025, when it passed 1,046 kilometers from the asteroid.
The data revealed that, rather than simply rotating around one axis like most other asteroids and planets, Donaldjohanson has a more complex rotation on two axes. Scientists also observed the peanut shape of Donaldjohanson, as well as 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 their main asteroid encounters, which begin with Lucy’s pass by the Trojan asteroid Eurybates on August 12, 2027.
With ground-based telescopes, observers saw fluctuations in the light reflected by Donaldjohanson, regular patterns of peaks and valleystypical of an elongated object that rotates once every 10.5 Earth days.
But data from the Lucy probe revealed another pattern: Donaldjohanson appears to spin like an unstable top. The paper’s authors reported that the asteroid rotates around itself once every 10.5 Earth days and wobbles back and forth around its longitudinal axis once every 26.5 days.
Although observations made from Earth already indicated Donaldjohanson’s elongated shape, the passage of the Lucy probe revealed a : two lobes connected by a “neck”, like a peanut. These lobes are likely two fragments from an asteroid collision that gently came closer together due to mutual gravity.
A it was probably spinning at least 10 times faster when it formed, having slowed to its current rate 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 sideways and the gravity pulling them in changed, and the loose rock material slid down the slopescreating the weathered appearance of many craters, as images from the nearby pass showed.
The paper’s authors state that the slowing of the asteroid’s rotation is likely caused by a subtle 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 exerts a small recoil force on the surface.
Because the asteroid’s shape is not symmetrical, this results in a net torque, or twist, that can change its rotation. Thus, the YORP effect can decrease or increase the rotation of asteroids, as in the case of Bennu — once every four hours — and Ryugu — once every seven hours or so — which probably used to spin much more slowly than they do today.
As she passed Donaldjohanson at 30,000 mph, Lucy recorded the signatures of iron-rich clay minerals on the surface. These clays must have formed in the remote past with the help of liquid water.
However, the exposure must have been brief, the Lucy scientists concluded, because the iron in clays tends to be replaced by other elementslike magnesium, as the water remains longer.
In fact, scientists have observed magnesium-rich clays on Bennu and Ryugu, which suggests prolonged exposure to water, perhaps lasting millions of years, when they were still part of larger asteroids.
This difference in water exposure history, among other characteristics, could 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.
Donaldjohanson is believed to be formed from the rocky remains of a larger, carbon- and water-rich asteroid that collided with another object in the main asteroid belt. Bennu and Ryugu would have formed in the same way and in the same region.
At 155 million years old, Donaldjohanson is much younger than Bennu and Ryuguwhich formed between 1 and 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 targets for sample-collection missions).
Named after the fossilized skeleton of a human ancestor discovered in Ethiopia in 1974, NASA’s Lucy probe 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 planets formed and moved around before settling into their current configuration.
*Under AR supervision.
