Asteroid Collisions and Shape Evolution

“Asteroids” are small celestial bodies composed mainly of rocks, and most of them have sizes smaller than 100 km. Recently, space probes have revealed the detailed shapes of some of the asteroids in the Solar System. Their various shapes, such as a sea otter, a spinning top, an elongated shape, and a snowman, caught people’s attention. One of the most important processes giving rise to these various shapes is the collisional destruction of asteroids. A collision between asteroids breaks them into small pieces of rock. The resulting fragments then gather together through gravitational attraction to form “rubble-pile” bodies. The asteroid Itokawa is thought to have formed through this process. The friction between the fragments helps maintain Itokawa’s irregular two-lobed shape, affectionately likened to a sea-otter.
In this movie, we depict the collisional destruction of asteroids and the formation of rubble-pile bodies via the reaccumulation of fragments through a numerical simulation conducted with the supercomputer “ATERUI II.”

Two asteroids with diameters of 100 km just before they collide. The collision is not head on, but slightly oblique.
The impact breaks the asteroids into small fragments. The fragments spread out in a plane parallel to the impact plane, which produces a sheet-like structure composed of fragments. One particle in this movie, with a size of about 1 km, represents an aggregate of fragments.
The mutual gravity between the fragments causes the sheet-like structure to break apart, producing many filamentary structures. The filaments collapse into high-density regions, which form many spherical bodies. Moreover, gentle coalescences of the spherical bodies produce bodies with various shapes.
This scene shows an edge-on view of the sheet-like structure.
Then we move between the produced bodies. Let’s enjoy watching the accretion process of the fragments and the dynamical coalescence of the bodies.
The filamentary structure of the fragments collapses into many bodies. The size of one of the larger bodies is about 10 km.
When produced spherical bodies coalesce, the tidal force from the larger body disrupts the smaller body, and then the fragmented smaller body accretes onto the larger body.
The impact speeds of the gravitational coalescences are slow, so some bodies maintain their spherical shapes after coalescing, which produces two-lobed shapes resembling peanuts.
In this scene, a smaller body collides with a larger body, and the collision fragments the smaller body. The fragments then accrete onto the larger body due to the gravity of the larger body.
The diameters of the bodies produced in the simulation are about 10 km. Although these diameters are two orders of magnitude larger than that of the asteroid Itokawa, some of the produced bodies in our simulation reproduce the sea-otter-like shape.

Details of Numerical Simulation

PurposeTo understand the shapes of rubble-pile bodies produced through gravitational reaccumulation of fragments after a catastrophic collision of asteroids.
Calculation MethodSPH method considering elastic force of rocky material, fracturing of rocky material, friction between fractured material, and self-gravity.
Number of SPH ParticlesAbout 4 million
Initial ConditionsAn impact between two asteroids with diameters of 100 km. The impact velocity is 350 m/s, and the impact angle is 15 degrees (an impact angle of 0 degrees would represent a head-on collision).
ComputerCray XC50 "ATERUI II" (CfCA, NAOJ)
Time Scale100,000 sec (about one day)
Spatial Scale~100 km (sizes of the asteroids) - ~1000 km (size of the sheet of fragments) Size of one particle (an aggregate of fragments) ~ 1 km
ResercherKeisuke Sugiura (ELSI, Tokyo Institute of Technology)
ReferencesSugiura, K., Kobayashi, H., Inutsuka, S. (2020) High-resolution simulations of catastrophic disruptions: Resultant shape distributions. PSS, 181, 104807

Details of Visualization

This video was created using the visualization application for particle data "Zindaiji." The color of the objects is gray, inspired by the color of basalt. In addition, the shading around the debris is highlighted to make it easier to see the impacted debris in three dimensions. Starting from 0:30, the time in the video slows to 1/5th the speed of the first part to make it easier to see the local phenomena.


  • Simulation: Keisuke Sugiura
  • Visualization: Satoki Hasegawa
  • Four-Dimensional Digital Universe Project, NAOJ

360p (.mov, zip file : 410.1 MB) (.wmv, zip file : 205.1 MB)

720p (.mov, zip file : 1.2 GB) (.wmv, zip file : 204.6 MB)

1080p (.mov, zip file : 2.4 GB) (.wmv, zip file : 203.2 MB)


  • 2023.1 Version 1.0 was released.