NASA’s Lucy spacecraft flew past asteroid (52246) Donaldjohanson on April 20, 2025, and sent back close-up images of a body that looks like a lumpy peanut slowly wobbling through the asteroid belt. The flyby data revealed a bilobate shape covered in craters and ridges, along with a strange two-axis tumbling rotation defined by periods of approximately 10.5 days and 26.4 days. Those twin rotation periods, confirmed by a peer-reviewed study published in Science, raise pointed questions about how this small body formed and whether its internal structure is strong enough to hold itself together over time.
Why Donaldjohanson’s wobbling rotation changes what scientists know
Most asteroids spin around a single axis, much like a top. Donaldjohanson does not. Its non-principal-axis rotation means the asteroid tumbles along two separate axes at once, producing a slow, irregular wobble rather than a clean spin. The two measured periods, roughly 10.5 days and 26.4 days, suggest the body never settled into a stable rotation after whatever event shaped it. For a solid, rigid object, internal friction would normally damp out that kind of complex motion over millions of years. The fact that Donaldjohanson still tumbles points toward a loosely bound interior, consistent with what planetary scientists call a rubble pile, a body held together mainly by gravity rather than material strength.
That distinction matters because rubble piles respond differently to collisions, tidal forces, and thermal effects than monolithic rocks do. If Donaldjohanson’s two lobes came together in a low-velocity merger, as its peanut-like shape implies, the lobes may have partially decoupled their rotational states. A strengthless interior would allow each lobe to retain some independent angular momentum, producing exactly the kind of dual-period tumble that Lucy observed. Ground-based light-curve campaigns could, in principle, detect gradual changes in Donaldjohanson’s spin axis over the next several years, offering a way to test whether the wobble is damping or persisting.
Donaldjohanson’s complex motion also feeds into broader debates about how small bodies respond to subtle forces such as the YORP effect, in which sunlight absorbed and re-radiated by an irregular surface can slowly alter an asteroid’s spin. A rubble-pile contact binary with a long-lived tumble may evolve differently under YORP than a compact, single-lobed rock, potentially reshaping models of how often asteroids split apart, re-merge, or shed material into space. The Lucy flyby thus provides a rare real-world test case for theories that, until now, have relied heavily on computer simulations and sparse light-curve data.
Lucy’s flyby images and the 3D shape model
The close-up photographs returned by Lucy showed two distinct lobes joined at a narrow neck, a geometry that earned the peanut comparison. Surface features include craters of varying size and linear ridges, details consistent with a body that has been battered by smaller impacts over a long history. In some views, one lobe appears more heavily cratered than the other, hinting that the two components may have experienced different collisional environments before they came together.
A team of researchers reconstructed a detailed three-dimensional model from multi-angle images captured during the flyby, giving scientists a volumetric picture of Donaldjohanson for the first time. The model confirms the bilobate structure and provides the geometric framework needed to analyze the asteroid’s mass distribution and rotational dynamics. By tracking how sunlight and shadow move across the modeled surface, the team can also refine estimates of the spin state and better understand which regions were imaged at high resolution and which remain poorly seen.
The peer-reviewed findings, reported in Science, formally classify Donaldjohanson as a bilobed body in a tumbling rotation state. That classification places it in a small but growing catalog of contact-binary asteroids and comets visited by spacecraft, a group that includes comet 67P/Churyumov-Gerasimenko, imaged by the European Space Agency’s Rosetta mission. Each new example sharpens the statistical picture of how common these double-lobed forms are among small solar system bodies and what physical processes create them.
With a robust shape model in hand, scientists can now perform numerical simulations that track Donaldjohanson’s motion backward and forward in time. By adjusting assumptions about internal density and cohesion, they can ask how easily the two lobes might separate during close encounters, how resistant the neck region is to tidal stresses, and whether the current tumble could have been triggered by a relatively recent impact. These experiments will not provide definitive answers without direct mass measurements, but they do narrow the range of plausible histories.
Gaps in density, composition, and long-term spin stability
For all that the flyby revealed about shape and rotation, several key measurements are still missing from the public record. No primary-source values for Donaldjohanson’s mass, bulk density, or surface albedo have appeared in the NASA releases or the Science paper abstract available so far. Without density, scientists cannot confirm whether the interior is porous rubble or something closer to solid rock, a distinction that directly controls how the tumbling motion will evolve. A very low density would strongly favor a rubble-pile structure, while a higher value would point toward a more coherent interior with significant material strength.
Spectral data on surface composition, which would indicate whether the asteroid is carbonaceous, silicate-rich, or something else, are also absent from the published materials. Composition matters because it influences both internal strength and how the surface responds to heating and cooling. Dark, carbon-rich material absorbs more sunlight and may experience stronger thermal stresses, potentially driving microfracturing that weakens the structure over time. Brighter, more reflective minerals could behave differently, altering expectations for how quickly the wobble should damp.
Equally missing are direct statements from instrument teams about the expected timescale for the wobble to damp out. If Donaldjohanson is truly a strengthless rubble pile, its tumbling could persist for tens of millions of years. If it has even modest internal cohesion, the wobble should decay faster as internal friction converts rotational energy into heat. Distinguishing between those scenarios requires either a reliable density measurement or repeated observations of the spin axis over time. The latter is something ground-based telescopes could attempt, though the asteroid’s small size and slow rotation make precise light-curve work difficult and time-consuming.
Another open question involves the stability of the neck that joins the two lobes. In some contact binaries, numerical models suggest that even modest changes in spin rate can shift material toward or away from the neck, potentially triggering landslides or partial separation. Without knowing Donaldjohanson’s density profile, scientists can only speculate about how close it might be to such a threshold. Future analyses that combine the existing shape model with plausible internal structures may reveal whether the current configuration is long-lived or precariously close to disruption.
What Lucy leaves behind for future observers
Lucy itself will not return to Donaldjohanson. The spacecraft is continuing through the main asteroid belt toward its primary targets, the Jupiter Trojan asteroids, where it will study bodies thought to be remnants of the early solar system. Donaldjohanson was a bonus target along the way, a dress rehearsal that produced unexpectedly rich science and validated Lucy’s ability to navigate a high-speed flyby while collecting detailed imaging and tracking data.
The next opportunity to learn more about this particular asteroid will likely come from Earth-based observatories or from future missions that target similar contact binaries. Large ground-based telescopes could monitor Donaldjohanson’s brightness over months and years, searching for subtle changes in its light curve that would signal an evolving spin state. Radar observations, while challenging at the asteroid’s distance and size, might someday provide independent constraints on shape and surface properties if orbital circumstances align.
In the meantime, Donaldjohanson’s wobbling, peanut-shaped form has already secured it a place in the broader story of how small bodies assemble and evolve. By highlighting the links between shape, internal structure, and complex rotation, Lucy’s flyby underscores how much remains to be learned from even brief encounters with minor worlds. As researchers sift through the existing data and plan new observations, the asteroid’s unresolved questions about density, composition, and long-term spin stability will continue to shape theories about how rubble piles form, endure, and sometimes fall apart in the crowded lanes of the asteroid belt.
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*This article was researched with the help of AI, with human editors creating the final content.
