Understanding asteroid spin: a balance of forces
Asteroids come in a wide range of shapes and spin states. Some rotate smoothly like a top, while others tumble chaotically. Recent research presented at the Europlanet Science Congress in Helsinki sheds light on what drives these different spin patterns. By examining data from the European Space Agency’s Gaia mission and combining it with new computer models, scientists are beginning to map the physical processes that govern asteroid rotation.
The core finding is that asteroid spin results from a cosmic tug of war between two main forces: collisions with other space rocks that push an asteroid into unstable motion, and internal friction that gradually damps or stabilizes its rotation. When these effects balance, a characteristic dividing line emerges in the asteroid population. Machine learning helped reveal a gap separating fast-spinning bodies from slow, tumbling ones. Slower rotators are more susceptible to becoming unstable after impacts, while faster rotators resist disturbances more effectively.
Another important player is sunlight. As an asteroid’s surface heats and cools, tiny photons exert a minuscule thrust—an effect known as the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect. For smoothly spinning asteroids, these subtle thrusts align and slowly alter the rotation rate. In tumbling bodies, however, the tiny pushes tend to cancel, helping preserve chaotic motion. A notable takeaway is that many asteroids may be “rubble piles”—loosely bound aggregates of rock and dust. This porous structure would respond to impacts differently than a solid rock, influencing both rotation and the outcome of any deflection mission.
These insights matter because they help scientists infer an asteroid’s interior and mechanical properties from its spin. As astronomical surveys grow, the approach can be applied to millions of objects, refining our understanding of how these ancient bodies evolve and what they are made of. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) will be a game changer, enabling researchers to extend this method to an enormous population of asteroids and improve risk assessment and defense planning.
Where to hit: identifying safe deflection targets
Knowing how an asteroid spins is only part of the defense equation. The other critical question is: where on the asteroid should a spacecraft strike to alter its trajectory without causing unintended consequences?
A team led by Rahil Makadia explored this by analyzing how a kinetic-impact mission would change an asteroid’s motion. They built probability maps of asteroid surfaces to identify the safest and most effective impact sites. The goal is to avoid “gravitational keyholes”—tiny regions where a planet’s gravity would subtly bend an asteroid’s orbit so that Earth could be at risk again years or decades later. Even a well-intentioned impact could, if misapplied, steer the object toward a dangerous return path.
The researchers used lessons from NASA’s DART mission, which impacted the asteroid Dimorphos in 2022, and ran hundreds of millions of trajectory simulations. By varying impact speed, angle, timing, and the asteroid’s rotation, they predicted how different strikes would alter Bennu’s orbit and whether the new path would intersect a keyhole. The result is a practical map that highlights “bright crosshair” regions offering the best chances to push an asteroid away from Earth and away from future keyhole encounters.
What makes this approach powerful is that it can operate with ground-based observations when a rapid response is required, though rendezvous missions remain preferable for precision. By combining spin physics with surface-targeting strategies, planetary defense planners gain a more reliable way to deflect a threat while minimizing the chance of a later return impact.
Implications for planetary defense
These advances—connecting spin dynamics to interior structure, and coupling that knowledge with safe impact-zone mapping—enhance our preparedness for potential asteroid threats. As data from Gaia and future observatories accumulate, scientists will sharpen their models and broaden the range of asteroids that can be assessed, ultimately contributing to smarter, safer deflection strategies.
