Figure 1. Images of asteroids (left to right, Bennu, Ryugu, and Didymos). These asteroids are reported to be characterized by their rubble-pile structure, top-shaped figure, and equatorial bulge; Didymous has a satellite Dimorphous. The relative size of each asteroid (and satellite) is not shown exactly for the sake of clarity.　Credit: NASA, ESA, JAXA, modified by Ryuki Hyodo.

On Earth, an avalanche in the Alps would not collapse Waikiki Beach. However, this common knowledge only applies on Earth (with a diameter of about 12,000 km), and the story becomes completely different for asteroids less than about 1 km in diameter.

There are numerous asteroids close to the Earth. The smaller they are, the more numerous they are. Asteroids with diameters of about 1 km or less include rubble-pile asteroids, which are not monolithic but are made up of smaller rocks that are gravitationally assembled together. Many of the rubble-pile asteroids are top-shaped asteroids with an equatorial bulge (Fig. 1). For example, Ryugu, Bennu (visited by NASA’s OSIRIS-REx²), and Didymos (NASA’s DART and ESA-JAXA’s HERA³) are all rubble-pile asteroids with such features. In addition, Didymos is orbited by a satellite named Dimorphos.

Such asteroids are thought to have brought water and building materials for life to ancient Earth. Today, their collisions with the Earth could cause a critical situation for life on Earth. The orbital evolution of asteroids greatly depends on their shape, constituent materials, and rotation state. Therefore, understanding the shape evolution and formation process of asteroids is important not only to investigate the origin of the Earth and life but also for planetary defence (protecting the Earth from asteroid impact).

However, until now, it has been technically difficult to directly account for effects such as friction and cohesion between particles in simulations, and it has not been possible to account for all of the observed features simultaneously. In this study, we used advanced numerical simulations and the supercomputer⁴ to study the shape evolution of rubble-pile asteroids during their rotational acceleration. The dependence on the physical properties of the constituent particles (especially frictional forces) was also investigated.

Our results revealed the following evolution (see Figure 2 and movies: ‘Avalanche on an asteroid!? (Diagonal view)’ and ‘Avalanche on an asteroid!? (Viewed from above)’).

A. When the rotation period of an asteroid becomes smaller than a critical value (e.g., the rotation period of about 3 hours) 5 with a fast-enough spin acceleration and when the friction of constituent particles is large enough, a surface landslide (global avalanche) occurs symmetrically with the rotation axis. This changes the shape of the asteroid from spherical to top-shaped. (The Earth’s rotation period is almost 24 hours, so 3 hours is a very fast rotation.)

B. The surface material of the rubble-pile asteroid ejected by the avalanche is distributed in a disk-like structure on the asteroid’s equatorial plane.

C. The particle disk spreads both inward and outward by collision and self-gravity. Particles spreading outward accumulate by their own gravity, forming a rubble-pile satellite. Particles spreading inward selectively re-accrete in the equatorial region of the top-shaped rubble-pile asteroid. This forms an equatorial bulge.

D. Depending on the shape and surface conditions (particle size distribution and material properties) of the top-shaped rubble-pile asteroid and the rubble-pile satellite, the satellite’s orbit could be greatly expanded, and the satellite could eventually be lost. In other cases, the satellite’s orbit could stabilize without expanding. (Although this is not the main topic of this study and requires further research)

The above processes may explain the top-shape figure, the equatorial bulge, and the presence or absence of satellites observed around the rubble-pile asteroids. The “degree” of each step strongly depends on the initial shape of the asteroid, the spin-up process, various physical properties of its constituent particles, and their heterogeneity within the asteroid. Combining theoretical models (numerical simulations) such as those in this study with information on asteroid systems discovered through asteroid exploration can therefore combine to provide a clearer picture of each asteroid’s past and future. Future detailed studies of each asteroid are awaited.

Figure 2. Schematic illustration of the results of this study. (a,b) A spherical rubble-pile asteroid rotating with a rotation period of about 3 hours becomes a top-shaped asteroid by a global avalanche symmetrically with the rotation axis. (c) The surface material of the rubble-pile asteroid ejected by the avalanche forms a particle disk. The particle disk spreads and evolves. (d) Satellites are formed by the diffusion of the disk. An equatorial bulge is also formed through the re-accretion of particles in the asteroid’s equatorial region. (e,f) Satellites could be lost or settle into a relatively stable-state in the long-term dynamical evolution; Ryugu and Bennu may correspond to (e); the Didymos-Dimorphos system may correspond to (f). Credit:  Hyodo & Sugiura 2022, ApJL