On the stability of spinning asteroids

TL;DR

This paper investigates why large rubble-pile asteroids do not spin faster than a certain limit, attributing it to surface roughness reducing inter-particle forces and affecting their stability.

Contribution

It demonstrates that surface roughness significantly diminishes van der Waals forces between asteroid particles, explaining the observed spin rate limit.

Findings

Surface roughness reduces van der Waals forces by a factor of 100 for micrometer particles.

Interaction force becomes independent of particle size due to surface roughness.

Asteroid fragment interactions are governed by non-gravitational forces similar across scales.

Abstract

Most asteroids with a diameter larger than $\sim 300 \ {\rm m}$ are rubble piles i.e. consisting of more than one solid object. All asteroids are rotating but almost all asteroids larger than $\sim 300 \ {\rm m}$ rotate with a period longer than $2.3 \ {\rm hours}$, which is the critical period where the centrifugal force equals the gravitational force. This indicates that there are nearly no adhesive interaction forces between the asteroid fragments. We show that this is due to the surface roughness of the asteroid particles which reduces the van der Waals interaction between the particles by a factor of $100$ for micrometer sized particles and even more for larger particles. We show that surface roughness results in an interaction force which is independent of the size of the particles, in contrast to the linear size dependency expected for particles with smooth surfaces. Thus, two stone fragments of size $100 \ {\rm nm}$ attract each other with the same non-gravitational force as two fragments of size $10 \ {\rm m}$.