Gaia solves the mystery of tumbling asteroids and reveals what’s inside them

"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," said Dr. Wen-Han Zhou of the University of Tokyo, who presented the results at EPSC-DPS2025.

Gaia's all-sky survey built a vast catalog of asteroid light curves, which track changes in brightness as asteroids spin. When the researchers compared these rotation patterns to asteroid size, they noticed a surprising gap separating two distinct groups.

Led by Zhou, who conducted much of the research at the Observatoire de la Côte d'Azur in France, the team determined the origin of this divide and resolved several long-standing mysteries about asteroid rotation.

"We built a new model of asteroid-spin evolution that considers the tug of war between two key processes, namely collisions in the Asteroid Belt that can jolt asteroids into a tumbling state, and internal friction, which gradually smooths their spin back to a stable rotation," said Zhou. "When these two effects balance, they create a natural dividing line in the asteroid population."

Using machine learning, Zhou's team compared Gaia's asteroid data with their theoretical model and found that the position of the gap matched the model's predictions almost perfectly.

Asteroids below the gap tend to rotate irregularly and slowly, with spin periods under 30 hours. Those above it rotate faster and more steadily.

Scientists have long wondered why so many asteroids wobble rather than spin smoothly, and why smaller ones are especially prone to slow, erratic motion. Zhou's analysis shows that both collisions and sunlight contribute. Tumbling begins when a slowly rotating asteroid is nudged off balance by an impact.

Ordinarily, sunlight would be expected to speed up an asteroid's spin over time. As sunlight warms the asteroid's surface, the absorbed heat is later re-emitted as infrared radiation, producing a faint thrust that gradually alters its rotation. For asteroids spinning on a single axis, this process happens consistently, so the effect accumulates and can increase the spin rate.

Tumbling asteroids, however, experience this process unevenly. Because their rotation is chaotic, heat is absorbed and released from constantly changing regions. The resulting forces cancel out, preventing any consistent buildup of momentum. These objects therefore remain in the slow-rotation zone identified in Gaia's data.

This discovery offers more than just theoretical insight. By linking rotation behavior to internal structure, researchers can infer whether an asteroid is solid or made of loosely bound rubble. The Gaia data suggest that many are porous, with cavities and thick blankets of dust and rock (regolith).

Knowing an asteroid's internal structure is critical for planetary defense. A rubble pile would respond very differently to an impact, such as NASA's DART test, than a dense, solid body. With this method, astronomers could soon build a detailed catalogue of asteroid interiors -- knowledge that may one day be essential for safely deflecting a dangerous object.

"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.