EPSC-DPS2025: Gaia Solves Mystery of Tumbling Asteroids and Finds a New Way to Probe Their Interiors

Joint Meeting of the Europlanet Science Congress and the American Astronomical Society’s Division for Planetary Science (EPSC-DPS2025) Press Release

Whether an asteroid is spinning neatly on its axis or tumbling chaotically, and how fast it is doing so, has been shown to be dependent on how frequently it has experienced collisions. The findings, presented at the recent EPSC-DPS2025 Joint Meeting in Helsinki, are based on data from the European Space Agency’s Gaia mission and provide a means of determining an asteroid’s physical properties – information that is vital for successfully deflecting asteroids on a collision course with Earth.

“By leveraging Gaia’s unique dataset, advanced modelling and A.I. tools, we’ve revealed the hidden physics shaping asteroid rotation, and opened a new window into the interiors of these ancient worlds,” said Dr Wen-Han Zhou of the University of Tokyo, who presented the results at EPSC-DPS2025.

During its survey of the entire sky, the Gaia mission produced a huge dataset of asteroid rotations based on their light curves, which describe how the light reflected by an asteroid changes over time as it rotates. When the asteroid data is plotted on a graph of the rotation period versus diameter, something startling stands out – there’s a gap, or dividing line that appears to split two distinct populations. 

Now a study led by Zhou, much of which was conducted while he was at the Observatoire de la Côte d’Azur in France, has revealed the reason for this gap – and in doing so solved some longstanding mysteries about asteroid rotation.

“We built a new model of asteroid-spin evolution that considers the tug of war between two key processes, namely collisions in the Asteroid Belt that can jolt asteroids into a tumbling state, and internal friction, which gradually smooths their spin back to a stable rotation,” said Zhou. “When these two effects balance, they create a natural dividing line in the asteroid population.”

By applying machine learning to Gaia’s asteroid catalogue and then comparing the results to their model’s prediction, Zhou’s team found that the location of the gap matched what their model predicted almost perfectly.

Below the gap are slowly tumbling asteroids with rotational periods of less than 30 hours, while above the gap are the faster ‘pure’ spinners. 

For decades, astronomers have been puzzled about why there are so many asteroids tumbling chaotically rather than spinning around a single rotational axis, and why smaller asteroids are more likely to be tumbling slowly.

Zhou’s study shows that collisions and the effects of sunlight are key. Tumbling motion usually starts when an asteroid spins slowly. Its slow rotation means that it is more easily disturbed by collisions, which can knock the asteroid into a chaotic tumble.

Ordinarily, the subtle force of sunlight would be expected to cause the asteroid to stop tumbling and spin up. The surface of an asteroid absorbs heat from the Sun and re-emits it in different directions. The emitted photons give the asteroid a tiny push, one that builds up over time, and depending on the asteroid can either speed up its rotation or slow it down. For an asteroid that is spinning smoothly on its axis, the directions in which sunlight is absorbed and re-emitted remains constant, allowing the strength of the push from sunlight to build up.

However, for tumbling asteroids the effect of sunlight is much weaker. Because they are rotating chaotically, different parts of the surface are absorbing and re-emitting heat at any given time. Rather than giving the asteroid a consistent push, the effect of absorbing and re-emitting sunlight is smoothed out, so there’s no preferred push in any direction. As a result, slowly tumbling asteroids change their spin very slowly, and become stuck in the slow-rotation zone below the gap in the observational data from Gaia.

There’s a practical usefulness to this discovery. By understanding how the rigidity of asteroids’ interior structure relates to their rotation, it’s possible to use that knowledge to infer the internal properties of the asteroids. From the Gaia data, the findings support the picture of asteroids as loosely held together rubble piles, with lots of holes and cavities blanketed in thick, dusty regolith.

Understanding the properties of asteroids also has repercussions for how to deflect a hazardous asteroid on a collision course, because a rubble pile asteroid would react to a kinetic impact like NASA’s DART differently than a solid, rigid body. Thanks to these findings, astronomers could soon have an extensive catalogue of the internal structures of potentially hazardous asteroids, which could hold the key of how to deflect them.

“With forthcoming surveys like the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), we’ll be able to apply this method to millions more asteroids, refining our understanding of their evolution and make-up,” said Zhou.

Further information

EPSC-DPS2025-893 Understanding the Long-Term Rotational Evolution of Asteroids with Gaia

Wen-Han Zhou, Patrick Michel, Marco Delbo, Wenchao Wang, Bonny Y. Wang, Josef Durech and Josef Hanuš. https://doi.org/10.5194/epsc-dps2025-893

Paper: Wen-Han Zhou et al; Confined Tumbling State as the Origin of the Excess of Slowly Rotating Asteroids, Nature Astronomy, 9, 493–500 (2025)

Images

https://www.europlanet.org/wp-content/uploads/2025/09/asteroid_Collision.jpg

Gaia data.jpg

Contacts

Dr Wen-Han Zhou
The University of Tokyo
wenhan.zhou@oca.eu

EPSC-DPS2025 Press Office
press@europlanet.org 

Notes for Editors

About the Joint Meeting of the Europlanet Science Congress and the Division of Planetary Sciences (EPSC-DPS) 

The Europlanet Science Congress (EPSC), established in 2006 as the European Planetary Science Congress, is the largest planetary science meeting in Europe. It covers the entire range of planetary sciences, with an extensive mix of talks, workshops and poster sessions, as well as providing a unique space for networking and exchanges of experiences.

EPSC joined forces for the first time with the American Astronomical Society’s Division for Planetary Sciences (DPS) for a joint meeting in Nantes, France, in 2011. This was followed by DPS-EPSC 2016 in Pasadena, EPSC-DPS 2019 in Geneva, and the return to the United States for the DPS-EPSC 2023 meeting in San Antonio. This year will mark the third iteration of a joint European-based meeting. The intent of the joint meetings is not only to connect the European and North American planetary science communities, but also to consolidate two major meetings and motivate planetary scientists from all over the globe to attend.

Follow on social media (Bluesky, X and LinkedIn) with the hashtag #EPSC-DPS2025 for updates on the meeting.

About Europlanet

Europlanet (europlanet.org) is a non-profit association and membership organisation that provides the planetary science community with access to research infrastructure, services and training. The Europlanet Association Sans But Lucratif (AISBL), established in 2023, builds on the heritage of a series of projects funded by the European Commission between 2005 and 2024 (Grant Numbers 871149, 654208, 228319 and RICA-CT-2004-001637) to support the planetary science community in Europe and around the world. 

About the DPS

The Division for Planetary Sciences (DPS), founded in 1968, is the largest special-interest Division of the American Astronomical Society (AAS). Members of the DPS study the bodies of our own solar system, from planets and moons to comets and asteroids, and all other solar-system objects and processes. With the discovery that planets exist around other stars, the DPS has expanded its scope to include the study of extrasolar planetary systems as well. The American Astronomical Society (AAS), established in 1899, is the major organization of professional astronomers in North America. The mission of the AAS is to enhance and share humanity’s scientific understanding of the universe as a diverse and inclusive astronomical community, which it achieves through publishing, meeting organization, science advocacy, education and outreach, and training and professional development.