An object the size of a mountain, 2.5 kilometers across, would spin on itself in just... five minutes.
This astonishing rotation was measured for asteroid 491384. A second object, also more than two kilometers across, would have a comparable rotation.
The Vera C. Rubin Observatory is located atop Cerro Pachón at an altitude of 2,600 m in Chile. It benefits from exceptionally high-quality skies throughout the year.
Credit: Hernan Stockebrand.
Astronomers spotted these objects in data from the Vera C. Rubin Observatory in Chile. Its powerful telescope regularly photographs large regions of the sky. Because asteroids are not perfectly round, their brightness varies as they rotate. These changes make it possible to estimate their rotation.
Obtaining a reliable measurement remains difficult when photographs are taken at widely spaced times. The researchers therefore improved their calculations to distinguish the correct rotation period from several possibilities. Their analysis revealed 66 asteroids that rotate particularly quickly.
The case of 491384 is surprising because of its size. Asteroids are generally not made of a single solid rock. They are more like collections of rocks and dust held together by their own gravity. Beyond a certain rotation speed, their weak attraction is no longer sufficient to keep all the pieces together.
There is therefore an approximate rotation limit, around two hours for many asteroids of this type. Small solid objects can rotate much faster. But a several-kilometer-wide block completing one rotation in five minutes is much harder to explain.
To remain intact, asteroid 491384 must have considerable internal strength: it could be a single rock with greater cohesion than a simple pile of debris. However, the observations do not yet make it possible to see its structure directly.
According to the study, this result makes 491384 the largest known asteroid in this category of extreme rotations.
For now, the study is available as a preprint, without completed independent scientific peer review. As Vera C. Rubin continues its surveys, other similar objects could emerge and help clarify what asteroids are really made of and why some remain intact despite rotating so rapidly.