Dr Wen-Han Zhou of the University of Tokyo, who presented the results, said, "By leveraging Gaia's unique dataset, advanced modelling and A.I. tools, we've revealed the hidden physics shaping asteroid rotation, and opened a new window into the interiors of these ancient worlds."
Gaia's all-sky survey has captured vast amounts of light-curve data, showing how asteroid brightness changes as they spin. When plotted as rotation period versus diameter, the data reveals a distinct gap dividing two populations. Zhou's team, supported by work at the Observatoire de la Cote d'Azur, found that this division marks a balance between two competing processes: collisions that induce tumbling, and internal friction that stabilizes rotation.
Using machine learning to analyze Gaia's asteroid catalogue, the team confirmed that this dividing line matches their model's predictions. Below the gap lie slow, tumbling asteroids with rotational periods under 30 hours, while above it are faster, stable spinners. The study provides a physical explanation for why many smaller asteroids rotate irregularly rather than smoothly.
Collisions and solar radiation both play a role. Slowly spinning asteroids are more easily knocked into chaotic rotation, while the sunlight-driven YORP effect - small photon pushes from re-emitted heat - fails to stabilize them because their random tumbling cancels out the directional thrust. This dynamic traps such asteroids in the slow-rotation region seen in Gaia's data.
The findings go beyond theory. By linking rotation behavior to internal rigidity, researchers can infer whether an asteroid is a solid body or a loose "rubble pile" of debris. Gaia's observations strongly support the rubble-pile model, suggesting that most asteroids consist of porous aggregates cloaked in thick regolith. This insight is critical for future planetary defense missions, as rubble-pile asteroids would respond differently to kinetic impactors like NASA's DART mission compared to solid ones.
"With forthcoming surveys like the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), we'll be able to apply this method to millions more asteroids, refining our understanding of their evolution and make-up," said Zhou.
Research Report:Understanding the Long-Term Rotational Evolution of Asteroids with Gaia
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