Why Asteroid Spins Have Confounded Scientists
For years, astronomers have observed asteroids rotating at a wide range of speeds, with many behaving oddly—tumbling instead of simply spinning around a single axis. Traditional theories could not fully explain why some asteroids slow down, while others tumble, nor why a surprising number are slower than expected. A fresh set of insights now emerges from a collaboration led by Wen-Han Zhou at the University of Tokyo, drawing on a wealth of data from Gaia’s third data release (DR3).
Gaia DR3: A New Window into Small-Body Dynamics
Gaia’s DR3 dataset includes observations of thousands of asteroids, providing precise measurements of their rotation periods and shapes. The new analysis harnesses this data to test how impacts, thermal forces, and internal structure shape an asteroid’s rotational fate. In particular, researchers observe a “gap” pattern when plotting spin rate versus size, hinting at underlying physical processes that previous models missed.
The Three Pillars of Rotation: Impacts, YORP, and Internal Damping
Researchers argue that an asteroid’s rotation is governed by three major factors:
– Impacts: Repeated collisions with smaller bodies inject angular momentum, sometimes speeding up rotation or nudging the body into a tumble depending on impact location and energy.
– YORP Effect: The Yarkovsky–O’Keefe–Radzievskii–Paddack effect describes how absorbed solar energy is re-emitted as heat. For simple, uniform shapes, this can accelerate or decelerate spin. For irregular or tumbling bodies, the re-emitted photons are distributed in many directions, often canceling each other out and dampening net changes in rotation.
– Internal Structure and Damping: The asteroid’s interior—its rigidity, friction, and energy dissipation—determines how quickly rotation speeds decay or persist after perturbations. High internal friction can offset gains from impacts and YORP, producing slower, steadier spins or stabilizing tumbling motion over long timescales.
Why the “Gap” Matters
The observed gap in Gaia DR3—where fast, small asteroids and slow, larger ones are plentiful but a diagonal region remains sparse—suggests a transition zone in rotational behavior. The new model links this gap to an interplay of the three pillars: some sizes and shapes are more susceptible to YORP, while others are more prone to sustained tumbling after impacts. Understanding this transition helps explain why slow rotators abound and why tumbling is common even among mature asteroids.
Implications for Planetary Defense
Knowing how an asteroid’s rotation evolves is not a purely academic pursuit. If we ever identify a potentially hazardous asteroid, its spin state will influence how we might alter its trajectory or cause controlled disruption. The Gaian insights show that simply measuring current rotation is insufficient; researchers must consider how impacts, YORP, and internal damping will reshape rotation over time. The study also notes a difference in rotational behavior between asteroid types (e.g., S-type vs. C-type), highlighting the importance of tailoring defense strategies to the specific object encountered.
What’s Next in Asteroid Spin Research
As ground- and space-based surveys prepare to collect even more data—such as the Legacy Survey of Space and Time at the Vera C. Rubin Observatory—scientists expect to refine the model further. These observations could help categorize asteroids by internal structure and confirm how the three factors interact across different sizes and compositions. The goal is a robust predictive framework that can anticipate both the spin evolution and stability of any asteroid the solar system might present.
Learn More
EPSC-DPS2025 presentation: Gaia Solves Mystery of Tumbling Asteroids and Finds a New Way to Probe Their Interiors. W.H. Zhou et al. Understanding the Long-term Rotational Evolution of Asteroids with Gaia. UT briefing on faster asteroid distance measurements.
