Confined tumbling state as the origin of the excess of slowly rotating asteroids

The rotational distribution of asteroids as a function of their size is used as a diagnostic of their physical properties and evolution. Recent photometric surveys from the Gaia mission, allowing the observation of asteroids with long spin periods (for example ≳24 h), found an excess of slow rotators and a gap separating them from faster rotators, which is unexplained by current theories. Here we developed an asteroid rotational evolution model capable of reproducing the observed distribution. We suggest that this distribution is regulated by the competition between collisions and internal friction dampening of tumblers—asteroids with unstable rotation vectors—and that the slow rotator group is populated mainly by tumblers. We constrain the product of the rigidity and quality factor, which relates to the body’s viscosity, to μQ ≈ 4 × 10⁹ Pa. This number, two orders of magnitude smaller than the one assumed for monolithic boulders, implies that rubble-pile asteroids could have a porous structure or a thick regolith layer and undergo stronger tidal effects.