Lucy Flyby Shows Donaldjohanson’s 26.5-Day Wobble and Iron-Rich Clays from Ancient Liquid Water

The encounter served as an engineering rehearsal while delivering the first close-range dataset on a main-belt asteroid formed 155 Myr ago by reaccumulation of collision fragments. Ground-based light curves had indicated a 10.5-day period; Lucy’s imaging and tracking instead showed end-over-end rotation at that interval plus a 26.5-day long-axis wobble driven by YORP torques that have slowed the spin over 20–60 Myr. Surface morphology records the resulting regolith migration and crater degradation.

Near-infrared spectra detected iron-rich phyllosilicates whose formation demands sustained liquid water, extending the known distribution of aqueous alteration beyond the sample-return targets Bennu and Ryugu. This places Donaldjohanson in the same primordial reservoir that supplied inner-solar-system water and, by extension, highlights main-belt objects as accessible reservoirs of hydrated minerals for in-situ resource utilization.

Because the data come from a single fast flyby rather than orbital or landed measurements, spatial coverage and mineral quantification remain coarse. A follow-on orbiter or sample return would tighten abundance estimates and test whether the observed wobble and hydration are common among small, collisionally reassembled bodies.