Lucy's First Asteroid Flyby Sheds Light on a Double-lobed Asteroid

On its way to future encounters with Trojan asteroids, the Lucy spacecraft made a practice run past tiny asteroid 52246 Donaldjohanson on April 20, 2025. Today, the Lucy team, led by Simone Marchi (Southwest Research Institute), published the findings from the brief encounter in Science, detailing the eventful history of the little binary world and proving the capability of Lucy’s instruments to gather the data needed to understand its future tiny targets.

The first and most obvious conclusion from Lucy’s images of Donaldjohanson is that it’s made of two chunks stuck together at a narrow neck. Mapping craters across both lobes has made it clear that the pair has been united for a long time. Both lobes are saturated with craters, to the point that any new crater would obliterate older ones. Based on well-understood statistics about the sizes and frequencies of asteroid impacts in the main belt, the Lucy team calculated that the surface is at least 40 million years old. That means that the event that created the fragments that came together to form Donaldjohanson happened more than 40 million years ago.

“More than 40 million years old” isn’t a terribly specific age, but it’s at least consistent with the idea (based on the asteroid’s orbital path) that Donaldjohanson is a member of the Erigone asteroid family. The family consists of fragments from a parent body that was destroyed in a catastrophic collision around 155 million years ago. Lucy’s spectroscopic measurements, which suggest the presence of iron-rich clay minerals, also support the Erigone connection.

The neck that joins the lobes, however, is younger than that. Its surface is much smoother and bears far fewer craters. The Lucy team estimates it’s less than 20 million years old. How can the two lobes be the same age while their connection is younger?

The Lucy team found a clue in Donaldjohanson’s rotation period, measured over a period of two months before the flyby. Lucy confirmed ground-based estimations of a very slow rotation rate of 252.6 hours — but also in the measurements is a second rotation period of 455.2 hours. The asteroid is tumbling – albeit very slowly – in a state called “non-principal axis rotation.”

Such a slow and chaotic rotation state is not original; it happened as a result of forces exerted on the little asteroid by sunlight’s pressure and the asymmetric absorption and re-emission of the Sun’s heat. Contact binaries typically have much higher rotation rates, measured in hours (sometimes, minutes), not days or weeks. At those faster rates, centrifugal acceleration acts to counter self-gravity, pushing material toward the ends of the elongated asteroid.

But as Donaldjohanson’s rotation slowed to its current rate, the centrifugal effect became negligible. Self-gravity took over, pulling material toward the center of mass. Since the center of mass is inside the larger lobe, loose material on the smaller lobe will, if perturbed, slide toward the larger lobe. Indeed, the very smooth surface on the smaller lobe in the neck area facing the larger lobe is a sign of such slope failure. Every time a sizable impact rattles Donaldjohanson, more material flows from the small lobe toward the bigger one, down that smooth slope.

The Lucy team used the shape and size of Donaldjohanson, as measured from the flyby images, to develop a model of its shape. Then, assuming a constant density, they calculated the acceleration due to gravity across its surface to measure the orientation and steepness of its slopes under different rotation rates.

If Donaldjohanson rotated once every 5 hours, material on the surface would spin out from the large lobe toward the small lobe. There’s no evidence on the surface of the asteroid of that ever happening, but then again, any event older than 40 million years has been obliterated by the cumulative effects of impact cratering. A rotation rate of once every 10 hours is the sweet spot where centrifugal and gravitational accelerations would balance in the neck region. Slower than that, and material cascades toward the larger lobe.

So the history of Donaldjohanson is probably like this: 155 million years ago, a big smash in the main asteroid belt destroyed the parent of the Erigone family. Two kilometer-sized fragments, traveling in the same direction, pulled each other in to form a contact binary body spinning once every few hours. Over time, smaller asteroids smacked into the binary, digging craters and spalling off fragments of ejecta, some of which re-landed on the surface. The asteroid developed a deep layer of regolith over its pair of cohesive cores, and now every new crater destroys an older one, churning the surface but not increasing the total number of craters.

At the same time, sunlight and heat acted to slow its rotation. At some point in its history, the rotation rate period slowed to longer than 10 hours, at which point gravity overcame spin and formerly stable slopes in the neck region failed. Landslides fell from the oversteepened slopes on the small lobe, sliding regolith toward the large one. As the rotation period continued to slow and further impacts shook the asteroid, the slope failure continued, building the smooth surface and thick neck that we see today.

It’s neat that we can learn so much story about a world from such a quick flyby. Of course, NASA and the Lucy team knew they could, or they wouldn’t have launched this mission toward multiple flybys of Trojan asteroids. A year from now, Lucy will be approaching (3548) Eurybates-Queta for an even more informative encounter. Eurybates is more than 10 times the size of Donaldjohanson and rotates once in about 9 hours, so Lucy will be able to detect it sooner and see all faces of it multiple times before its August 2027 close approach, and study it for months after the flyby as it dwindles in the distance.