Spacecraft Flyby Reveals Asteroid Donaldjohanson's Past

A spacecraft flyby of the inner main belt asteroid Donaldjohanson captured details about the small asteroid's surface features and motion that clarify its origin and evolution, report Simone Marchi and colleagues. As part of a 12-year mission to visit 8 asteroids, in April 2025 the Lucy spacecraft – named after a hominin fossil uncovered by paleoanthropologist Donald Johanson – flew past the asteroid. The asteroid's orbit indicates it is a likely member of the Erigone asteroid family, a group of asteroids that were produced by a catastrophic collision that broke up a larger parent body. Images of Donaldjohanson taken during the flyby show that it consists of two cratered lobes connected by a relatively smooth neck. The density of craters is consistent with the age of the Erigone family, which is about 155 million years old. However, craters smaller than 0.4 kilometers in diameter seem to have been preferentially erased from the asteroid, say Marchi et al., who suggest that those craters might have been wiped away by seismic shaking after a more recent impact event. The asteroid has a slow tumbling movement, not simple rotation, which could have been gradually produced by torques driven by thermal radiation. Spectra of the asteroid's surface are similar to other carbonaceous Erigone family asteroids and reveal the presence of iron-bearing phyllosilicates, indicating partial alteration of the minerals by liquid water on the parent body, which likely halted at an early stage due to insufficient water or heat.