Asteroids may look like simple space rocks, but their movements hide complex physics.
Some spin neatly on their axes, while others tumble chaotically, flipping in unpredictable ways.
For decades, astronomers have wondered why so many asteroids spin irregularly and why smaller ones are more likely to tumble.
Now, thanks to data from the European Space Agency’s Gaia space telescope, researchers have solved this mystery and found a new way to probe what lies inside these ancient remnants of the solar system.
At the recent EPSC-DPS 2025 Joint Meeting in Helsinki, Dr. Wen-Han Zhou of the University of Tokyo presented new findings based on Gaia’s vast asteroid catalog.
By combining advanced modeling, artificial intelligence, and Gaia’s uniquely detailed light curve data—which track how an asteroid reflects sunlight as it rotates—Zhou’s team identified a striking pattern.
When asteroid rotation speeds are plotted against their sizes, the data show a clear gap dividing two distinct groups of asteroids.
This dividing line had puzzled scientists for years. Zhou and his colleagues built a new model of asteroid spin evolution that explains it.
Their work shows that asteroid rotation is shaped by two competing processes: collisions in the asteroid belt, which can knock asteroids into chaotic tumbling states, and internal friction, which gradually calms the motion, smoothing it back into a stable spin.
When these forces balance out, they naturally create the gap observed in Gaia’s data.
Below the gap lie slowly tumbling asteroids, rotating in less than 30 hours. Above the gap are the faster and more stable spinners.
Smaller asteroids, the team found, are especially prone to tumbling because their slow spins make them more easily disturbed by collisions.
Sunlight also plays a role. As asteroids absorb heat from the sun and re-emit it, they experience a tiny push, known as the YORP effect. For smooth spinners, this push builds consistently over time, gradually speeding them up or slowing them down.
But for tumbling asteroids, the effect is weaker.
Because they are rotating chaotically, different parts of their surface absorb and emit heat at different times, canceling out the push. As a result, tumbling asteroids often remain stuck in their slow, chaotic rotations.
The discovery is more than an academic breakthrough. By linking asteroid rotation to their interior properties, scientists can use spin patterns as a tool to infer what asteroids are made of.
Gaia’s findings suggest that many asteroids are “rubble piles”—loose collections of rock and dust held together by weak gravity, full of holes and cavities rather than solid stone.
This knowledge is crucial for planetary defense. A rubble pile asteroid would respond very differently to a deflection attempt, such as NASA’s DART mission, compared to a solid rock. Understanding the internal structures of asteroids could therefore determine the best way to protect Earth from a potential impact.
“With future surveys like the Rubin Observatory’s Legacy Survey of Space and Time (LSST), we’ll soon be able to apply this method to millions more asteroids,” Zhou said. “That will refine our understanding of how asteroids evolve and what they’re made of.”
Gaia has not only explained why asteroids tumble but also opened a new window into their hidden interiors.
These insights will help us better understand the solar system’s building blocks—and may one day help us avert a catastrophic collision.
Source: KSR.
