Asteroids are primordial pieces of our solar system’s history, but they aren’t exactly pristine relics. Their surfaces in particular are eroded by solar radiation and pockmarked with meteorite impacts. Detailed studies of asteroids’ interiors are also lacking, simply because very few probes have been able to study them up close.
However, a promising new study uses data from the Gaia space observatory to understand the links between asteroid tumbling behavior, collision history, and interior structure. The key to the study is the discovery that rotation speeds of asteroids in the main belt, between Mars and Jupiter, don’t follow a random distribution.
When rotation period is plotted against asteroid size, asteroids fall into two distinct populations: slow spinners, which take more than about 24 hours to complete a rotation, and fast spinners, whose rotations take less than 24 hours. Small asteroids are more likely than large ones to be slow spinners.
“People found an excess [of] faster rotating asteroids and also an excess [of] slow rotating asteroids,” with fewer asteroids rotating at a medium speed, said Wen-Han Zhou, a planetary scientist at the University of Tokyo. In his Ph.D. research at the University of Nice, he and his collaborators realized that many of the slow rotators were also tumbling: rotating chaotically, rather than spinning steadily along a clearly defined axis.
“Asteroids are not islands in space; they collide with each other.”
“Asteroids are not islands in space; they collide with each other,” Zhou said. “When an asteroid spins very slowly, a tiny collision can make it tumble. This [also] tumbles the materials inside, which will dissipate the energy.”
This dissipation effect comes from internal friction. In their recent Nature Astronomy article, Zhou and his colleagues linked an asteroid’s rate of rotation, whether it is tumbling or spinning smoothly, its size, and its internal structure into a single theoretical framework. In this theory, fast rotating asteroids are stable spinners because any collision that sends them tumbling also jumbles their innards, causing internal friction that dissipates the chaos and brings the asteroids back to stability. Slow spinners, however, lock into tumbling chaotically because their guts don’t jumble enough to dissipate the energy.
“In my research, I propose slow rotators are all tumbling,” Zhou said. “This is a very strong statement, but so far it is consistent with observation.”
Zhou and collaborators also concluded that these slow tumblers are all rubble-pile asteroids: loose aggregates of small chunks barely held together by mutual gravitation, rather than being monolithic hunks of rock. This has implications for planetary defense.
“Imagine a bunch of pieces of Styrofoam stuck together with cohesive forces. You try to disrupt that, good luck!” said Alessondra Springmann, an asteroid researcher based in Colorado. Springmann has studied near-Earth asteroids using radar at the Arecibo Observatory in Puerto Rico but was not involved in this new research.
Knowing more “about an asteroid’s internal properties can help us if it ever came time to redirect an asteroid away from Earth.”
Scientists have tried. They found that smashing a projectile into a rubble pile, like NASA’s Double Asteroid Redirection Test (DART) mission did to the near-Earth asteroid Dimorphos, might not destroy it. The asteroid might simply re-form itself after being smashed. (Notably, Dimorphos is much smaller than the main belt asteroids in Gaia’s data.) Radar data on its rotation could tell planetary defenders what methods are useful well in advance. But so, too, could learning more about an asteroid’s innards.
Knowing more “about an asteroid’s internal properties can help us if it ever came time to redirect an asteroid away from Earth,” Springmann said.
You Spin Me Round
The European Space Agency’s Gaia observatory was built primarily to map the Milky Way. Because it provided a sensitive wide-angle view of the whole sky, the observatory also incidentally provided data on other objects, including asteroids in the main belt of our solar system. To determine asteroid spin rates, asteroid researchers turned to Gaia data showing how reflected light varies over time as the objects spin. This is when they found the clear division between fast and slow rotators.
Scientists have a reasonable explanation for the behavior of the fastest spinners: the YORP effect (for Yarkovsky-O’Keefe-Radzievskii-Paddack). In essence, asteroids receive sunlight across their surface facing the Sun, but their uneven surfaces absorb and reemit that light in more or less random directions. Over many millions of years, that accumulated difference in light exposure can cause asteroids to spin until they reach the spin barrier of one rotation every 2.2 hours, at which point they break into pieces if they’re rubble piles.
But slow rotators defied easy explanation.
However, Gaia provided another clue: If the variations in light it measured were regular, then the asteroid was a stable spinner. If they were irregular, then the asteroid was tumbling. Many slow spinners were tumbling, whereas almost every fast rotator was stable.
Zhou and his collaborators realized that if most or all slow spinners are tumblers, it could explain why observed asteroids are split into two distinct populations. Asteroids are too faint for even Gaia to clearly distinguish between rotators and tumblers in every case, but when the researchers simulated main belt asteroids on a computer—including the effects of collisions and YORP—they produced something strikingly similar to the Gaia data.
Along the way, the researchers also realized tumbling behaviors are linked to possible internal structural properties, particularly deformability and internal friction, which are not typically measurable without placing a seismometer on an asteroid’s surface. In other words, these analyses could actually reveal the life history and internal properties of asteroids in new ways.
—Matthew R. Francis (@BowlerHatScience.org), Science Writer
Citation: Francis, M. R. (2025), What tumbling asteroids tell us about their innards,
Eos, 106, https://doi.org/10.1029/2025EO250414. Published on 6 November 2025.
Text © 2025. The authors.
CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
