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Whether an asteroid is spinning neatly on its axis or tumbling chaotically, and how fast it is doing so, has been shown to be dependent on how frequently it has experienced collisions. The findings, presented at the recent EPSC-DPS2025 Joint Meeting in Helsinki, are based on data from the European Space Agency's Gaia mission and provide a means of determining an asteroid's physical properties—information that is vital for successfully deflecting asteroids on a collision course with Earth.

"By leveraging Gaia's unique dataset, advanced modeling 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 at .

During its survey of the entire sky, the Gaia mission produced a huge dataset of asteroid rotations based on their light curves, which describe how the light reflected by an asteroid changes over time as it rotates. When the asteroid data is plotted on a graph of the rotation period versus diameter, something startling stands out—there's a gap, or dividing line that appears to split two distinct populations.

Now a study led by Zhou, much of which was conducted while he was at the Observatoire de la Côte d'Azur in France, has revealed the reason for this gap—and in doing so solved some longstanding 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."

By applying machine learning to Gaia's asteroid catalog and then comparing the results to their model's prediction, Zhou's team found that the location of the gap matched what their model predicted almost perfectly.

Below the gap are slowly tumbling asteroids with rotational periods of less than 30 hours, while above the gap are the faster 'pure' spinners.

For decades, astronomers have been puzzled about why there are so many asteroids tumbling chaotically rather than spinning around a single rotational axis, and why smaller asteroids are more likely to be tumbling slowly.

Zhou's study shows that collisions and the effects of sunlight are key. Tumbling motion usually starts when an asteroid spins slowly. Its slow rotation means that it is more easily disturbed by collisions, which can knock the asteroid into a chaotic tumble.

Ordinarily, the subtle force of sunlight would be expected to cause the asteroid to stop tumbling and spin up. The surface of an asteroid absorbs heat from the sun and re-emits it in different directions. The emitted photons give the asteroid a tiny push, one that builds up over time, and depending on the asteroid, can either speed up its rotation or slow it down. For an asteroid that is spinning smoothly on its axis, the directions in which sunlight is absorbed and re-emitted remains constant, allowing the strength of the push from sunlight to build up.

However, for tumbling asteroids, the effect of sunlight is much weaker. Because they are rotating chaotically, different parts of the surface are absorbing and re-emitting heat at any given time. Rather than giving the asteroid a consistent push, the effect of absorbing and re-emitting sunlight is smoothed out, so there's no preferred push in any direction. As a result, slowly tumbling asteroids change their spin very slowly, and become stuck in the slow-rotation zone below the gap in the observational data from Gaia.

There's a practical usefulness to this discovery. By understanding how the rigidity of asteroids' interior structure relates to their rotation, it's possible to use that knowledge to infer the internal properties of the asteroids. From the Gaia data, the findings support the picture of asteroids as loosely held together rubble piles, with lots of holes and cavities blanketed in thick, dusty regolith.

Understanding the properties of asteroids also has repercussions for how to deflect a hazardous asteroid on a collision course, because a rubble pile asteroid would react to a kinetic impact like NASA's DART differently than a solid, rigid body. Thanks to these findings, astronomers could soon have an extensive catalog of the internal structures of potentially hazardous asteroids, which could hold the key of how to deflect them.

"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.