Understanding asteroid spin: the tug-of-war between collisions and internal friction
Asteroids don’t all rotate the same way. Some spin smoothly like a top, others tumble chaotically, and a few seem to drift through space with unpredictable wobble. Recent work presented at the Europlanet Science Congress in Helsinki sheds light on why this happens and what it means for planetary defense. At the heart of the story is how an asteroid’s spin results from two competing processes: external jolts from collisions and the slow, internal damping that tries to straighten rotation over time.
Researchers led by Wen-Han Zhou of the University of Tokyo used data from the European Space Agency’s Gaia mission—a vast catalog of asteroid light patterns—to map spin behavior across thousands of bodies. Machine learning revealed a striking dividing line in the population: fast rotators and slow, tumbling asteroids occupy different parts of the spectrum, with the latter being more prone to chaos after impacts.
The study also highlights the role of Yarkovsky-powered tiny thrusts. As an asteroid’s sunlit surface heats and cools, it emits heat asymmetrically, creating minute pushes that accumulate over time. For smoothly spinning objects, these forces add up in a preferred direction and gradually alter spin rates. But for tumbling bodies, the alternating thrusts tend to cancel, helping maintain a slow, unpredictable rotation. These findings imply that many asteroids may be rubble piles—loose aggregates of rock and dust bound by gravity—rather than solid monoliths. This distinction matters for deflection missions, as fragile rubble piles respond differently to impacts than dense rock.
With Gaia’s treasure trove of observational data and advanced modeling, Zhou’s team is building a framework to infer the interior structure and evolutionary history of asteroids. The researchers say the method will scale with upcoming surveys, such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time, enabling analysis of millions of objects and refining our understanding of how these ancient bodies rotate and evolve.
Identifying safe strike locations: how to move an asteroid without increasing risk
Knowing how an asteroid spins is only part of the defense equation. The next critical question is where to strike for a deflection mission to be effective and safe in the long term. Rahil Makadia and colleagues at the University of Illinois Urbana-Champaign tackled this by developing probability maps that indicate safe impact sites on an asteroid’s surface.
The central danger with any kinetic impact is the possibility that the asteroid could be nudged into a gravitational “keyhole”—a tiny region in space where the planet’s gravity could steer it toward Earth on a future pass. An ill-chosen hit could inadvertently set the asteroid on a trajectory that returns years or decades later for another encounter. Makadia emphasizes that even deliberate deflection must be planned to avoid the boomerang effect.
The team built simulations inspired by NASA’s DART mission, which deliberately collided with the asteroid Dimorphos in 2022. By varying impact speed, angle, timing, and target location—while also accounting for rotation—they generated hundreds of millions of scenarios. Each scenario produced a projected orbital change and an assessment of whether a dangerous keyhole could be encountered later.
Applied to Bennu, the well-studied near-Earth asteroid observed by the OSIRIS-REx mission, the probability maps showed which surface areas would yield a robust long-term deflection without increasing future risk. Bright crosshair regions in the maps mark optimal strike zones where a kinetic impact would push Bennu away from Earth and away from any future keyholes. Less favorable zones could raise the odds of a late, ground-bound return.
Remarkably, Makadia notes that, at least in a preliminary sense, this analysis can be conducted using ground-based observations, outside of a full rendezvous mission. That said, a dedicated mission provides the most reliable control over the deflection outcome.
Why this matters for Earth’s safety
Taken together, these studies offer a more complete picture of asteroid behavior: from the physical processes shaping spin to the strategic choices that govern where to deliver a deflection kick. Understanding spin helps scientists anticipate how an asteroid will respond to a deflection attempt, while precise targeting minimizes the risk of inadvertently increasing the threat to Earth.
As astronomical surveys expand in scope and detail, these methods will enable faster, more reliable assessments of potential hazards. In a world where even small miscalculations could have long-term consequences, having a robust, data-driven defense plan is essential for planetary protection.
