What falling asteroids tell us about their insides

Asteroids are essential pieces of our solar system’s history, but they’re not exactly pristine relics. Especially its surfaces are eroded by solar radiation and marked by meteorite impacts. Detailed studies of the interior of asteroids 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 fall behavior, collision history and interior structure. The key to the study is the discovery that the rotation speeds of asteroids in the main belt, between Mars and Jupiter, do not follow a random distribution.

When the rotation period is compared to the size of the asteroid, asteroids are divided into two distinct populations: slowly rotating ones, which take more than 24 hours to complete one rotation, and fast rotating asteroids, whose rotations take less than 24 hours. Small asteroids are more likely than large ones to spin slowly.

“People found an excess [of] faster spinning asteroids and also an excess [of] slow-rotating asteroids,” with fewer asteroids spinning at a medium speed, he said Wen-Han Zhouplanetary scientist at the University of Tokyo. In his Ph.D. In research at the University of Nice, he and his collaborators realized that many of the slow rotators also rotated: they rotated chaotically, rather than rotating steadily along a clearly defined axis.

“Asteroids are not islands in space; they collide with each other,” Zhou said. “When an asteroid rotates very slowly, a small collision can make it fall. This [also] It causes the materials inside to fall, which will dissipate the energy.”

This dissipation effect comes from internal friction. in your recent Nature Astronomy articleZhou and his colleagues linked an asteroid’s rotation speed, whether it is spinning or spinning gently, its size and its internal structure into a single theoretical framework. In this theory, rapidly spinning asteroids are stable spinners because any collision that topples them also stirs up their insides, causing internal friction that dissipates the chaos and returns the asteroids to stability. Those that spin slowly, however, fall chaotically because their insides do not churn enough to dissipate the energy.

“In my research, I propose that slow rotators are all falling,” Zhou said. “This is a very strong statement, but it is consistent with observation so far.”

Zhou and his collaborators also concluded that all these slow drums are debris pile asteroids: loose aggregates of small pieces loosely held together by mutual gravitation, rather than being monolithic chunks of rock. This has implications for planetary defense.

“Imagine a bunch of Styrofoam pieces stuck together with cohesive forces. If you try to break that, good luck!” saying Alessondra Springmanna Colorado-based asteroid researcher. Springmann has studied near-Earth asteroids using radar in the Arecibo Observatory in Puerto Rico but did not participate in this new investigation.

Scientists have tried. They discovered that crashing a projectile into a pile of rubble, like the one at NASA The Double Asteroid Redirection Test (DART) mission conducted the near-Earth asteroid Dimorphosmight not destroy it. The asteroid could simply reform after being crushed. (Notably, Dimorphos is much smaller than main belt asteroids according to Gaia data.) Radar data on its rotation could tell planetary defenders which methods are useful well in advance. But so could learning more about the insides of an asteroid.

Knowing more “about the internal properties of an asteroid can help us if the time ever comes to redirect an asteroid away from Earth,” Springmann said.

you make me spin

The European Space Agency’s Gaia observatory was built primarily to map the Milky Way. By providing a sensitive wide-angle view of the entire sky, the observatory also provided data on other objects, including asteroids in the main belt of our solar system. To determine the spin rates of asteroids, asteroid researchers turned to Gaia data that shows how reflected light varies over time as objects spin. That’s 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). Essentially, asteroids receive sunlight through their Sun-facing surface, but their irregular surfaces absorb and re-emit that light in more or less random directions. Over many millions of years, that cumulative 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 are piles of rubble.

But slow rotators defied easy explanation.

However, Gaia provided another clue: if the light variations it measured were regular, then the asteroid was a stable spinner. If they were irregular, then the asteroid was falling. Many slow spinners were spinning, while almost all the fast spinners were stable.

Zhou and his collaborators realized that if most or all of the slow spinners are tumblers, that could explain why the observed asteroids split into two distinct populations. The asteroids are too faint for even Gaia to clearly distinguish between rotators and tumblers in all cases, but when the researchers simulated the main belt asteroids on a computer (including the effects of collisions and YORP) they produced something strikingly similar to Gaia’s data.

Along the way, the researchers also realized that the falling behaviors are related to possible internal structural properties, particularly deformability and internal friction, which normally cannot be measured without placing a seismometer on the surface of an asteroid. In other words, these analyzes could reveal the life history and internal properties of asteroids in new ways.

—Matthew R. Francisco (@BowlerHatScience.org), scientific writer

Citation: Francis, MR (2025), What falling asteroids tell us about their insides, Éos, 106, https://doi.org/10.1029/2025EO250414. Published on November 6, 2025.

Text © 2025. The authors. CC BY-NC-ND 3.0
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