On April 20, 2025, NASA’s Lucy spacecraft swept within roughly 600 miles of asteroid (52246) Donaldjohanson and returned images of a body that looks like a lumpy peanut still tumbling from a violent collision billions of years ago. The flyby data, now published in a peer-reviewed analysis, show the asteroid rotating end-over-end every 10.5 days while simultaneously wobbling on a separate 26.5-day cycle. Those two overlapping periods mark a rare state called non-principal-axis rotation, and they offer a direct window into how leftover building blocks of the solar system break apart, reassemble, and slowly evolve under the influence of sunlight.
Why Donaldjohanson’s double tumble changes collision models
Most small asteroids spin around a single stable axis. Donaldjohanson does not. Its two-period tumble means the body has never fully dissipated the rotational energy injected by whatever impact fused its two lobes together. That energy should drain over time through internal friction, so the fact that the wobble persists tells researchers something about the asteroid’s interior: it is likely too rigid, too small, or too loosely packed to damp the motion quickly. The peer-reviewed results published in Science detail both the 10.5-day end-over-end period and the 26.5-day precession period, establishing the clearest measurement yet of this kind of complex spin in a main-belt asteroid of Donaldjohanson’s size class.
A secondary force may also be at work. Solar photons striking an irregularly shaped body create a tiny but persistent torque known as the YORP effect. For a peanut-shaped object with two unequal lobes, that torque is asymmetric, and over decades it can either amplify or suppress a wobble. If YORP is actively maintaining Donaldjohanson’s tumble, the wobble amplitude should shift measurably within roughly five years. Repeated ground-based light-curve campaigns using facilities like the SOAR telescope and the Zwicky Transient Facility, both of which already tracked Donaldjohanson before the flyby, could detect such a change. That prediction is testable with existing instruments, which makes Donaldjohanson a useful laboratory for separating collision-driven spin from radiation-driven spin evolution.
Lucy’s 600-mile pass and the ground data that prepared for it
Lucy’s encounter with Donaldjohanson was an opportunistic stop on its longer journey toward Jupiter’s Trojan asteroids. The spacecraft’s instruments captured high-resolution images that revealed a bilobed contact-binary shape, with craters and ridges marking each lobe. NASA’s Scientific Visualization Studio released 3D shape reconstructions built from those images, confirming the peanut geometry and the non-principal-axis spin. The closest-approach distance of approximately 600 miles gave Lucy enough resolution to map surface geology while keeping the flyby within safe navigation margins.
The spacecraft data did not arrive in a vacuum. Before the encounter, astronomers had assembled rotational light curves from the SOAR telescope in Chile and archival photometry from the ZTF survey. That ground-based work, published separately in Icarus, established baseline brightness variations that hinted at complex rotation but could not resolve the shape. Lucy’s flyby turned those hints into confirmed measurements, linking the light-curve signal to specific surface features and lobe geometry. The combination of Earth-based and spacecraft observations gives future modelers two independent datasets to cross-check against each other.
Following the encounter, the International Astronomical Union approved official names for regions on Donaldjohanson’s surface, as documented by NASA’s mission team. Those designations allow researchers worldwide to reference the same craters and ridges with standardized terminology, a practical step that accelerates collaborative analysis of the flyby dataset.
Unanswered questions about Donaldjohanson’s interior and spin future
The published results describe shape, rotation, and surface geology in detail, but they leave significant gaps. Full calibrated data from Lucy’s instruments on surface composition and internal density distribution have not yet been released beyond the initial shape models. Without composition measurements, it is difficult to determine whether Donaldjohanson’s lobes are made of the same material or whether one lobe is denser than the other, a distinction that would affect how quickly internal friction should damp the tumble.
Long-term predictions for the tumbling state also lack formal modeling outputs. Researchers have not yet published dynamical simulations that project how the 10.5-day and 26.5-day periods will change over centuries under combined tidal dissipation and YORP torque. Such simulations require accurate density and porosity inputs that the current dataset does not fully constrain. Until those models appear, the question of whether Donaldjohanson’s wobble is growing, shrinking, or holding steady remains open.
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*This article was researched with the help of AI, with human editors creating the final content.
