Asteroids spin. Most of them do so rather slowly, and up until now most theories of asteroid rotation have failed to explain exactly why. A new paper from Wen-Han Zhou at the University of Tokyo and his co-authors might finally be able to fully explain that mystery as well as a few others related to asteroid rotation. Their work was presented at the Joint Meeting of the Europlanet Science Congress and the American Astronomical Society’s Division for Planetary Science in late September and could impact our understanding of how best to defend against a potentially hazardous asteroid.
The key to the paper was the release of a new data set from Gaia, the galaxy mapping mission launched by ESA. As part of its third data release (DR3), it also captured data on thousands of asteroids. Some of these “tumble” by rotating around something other than their principal axis, but others do “spin” around their largest axis. Why some spin and some tumble wasn’t explained by current asteroid rotation models either. Neither was the overabundance of “slow rotators” whose rotational period was much slower than predicted in traditional models.
In addition to the two model discrepancies that had previously been known, a paper released in from J. Durech and J. Hanus from Charles University in Prague, describes a strange quirk of the Gaia data. When plotting the period of an asteroid vs its diameter, there’s a notable “gap” in the data. There are some asteroids in the “fast and small” bucket of high rotational speeds and low diameters, and there is an overabundance of low rotational period asteroids, as was expected from previous observations. But there’s a noticeable “gap” along a line that is slightly increasing in rotational period while also increasing in diameter.
Graph showing the “gap” between the two types of asteroids. Credit – Wen-Han Zhou et al.
According to the authors, solving these three conundrums requires an understanding of impacts and the YORP Effect. Asteroids, even very mature ones, are still consistently hit by other, smaller bodies as they move throughout the solar system. This sets them into a tumbling motion, as the force from the impact is not likely to be directly aligned with their primary axis.
After a long time left to their own devices, the rotation these asteroids experience would slow down due to a combination of internal friction and the Yarovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. The YORP effect describes how an asteroid absorbs heat from the Sun, and then reemits that energy as infrared photos – i.e. heat. For asteroids that are “tumbling” and are relatively uniform in shape, this force can either decrease or increase the speed of their rotation. But for non-uniform asteroids or those that are tumbling in all sorts of directions, the force from the reemitted photons is spread in all directions, essentially canceling itself out.
So what does all this mean? An asteroid’s rotation, whether it be tumbling or spinning, and whether it be fast or slow, can be determined by three different factors. First is the impacts it suffers – where do they happen, and do they force the asteroid to speed up or slow down? Second is the YORP effect – is it contributing to that speed up or slow down or do the shape and rotational pattern of the asteroid essentially cancel it out? Finally, what type of internal structure does the asteroid have, and does friction in that structure dampen the rotational speed enough to offset the energy imparted by the impacts and by the YORP effect?
Fraser discusses the Gaia DR3 release that the authors used for the study with Dr. Martin Barstow.
All of those questions will be critical to answer if we end up discovering a potentially dangerous asteroid. The paper also noted that there is a difference in the “gap” between S-type and C-type asteroids, so understanding what type of asteroid it is that is threatening us is even more critical.
For now, there’s no immediate threat from any known asteroids, such as Gaia and other datasets, like the upcoming Legacy Survey of Space and Time survey at the Vera C. Rubin observatory, will allow us to further categorize and monitor even more asteroids, which might also one day become the critical materials backbone of our space-based expansion.
Learn More:
EPSC-DPS2025 – 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 – Dramatically Decreasing the Time it Takes to Measure Asteroid Distances
UT – Not Just Stars. Gaia Mapped a Diverse and Shifting Universe of Variable Objects