Eight billion asteroids in the Oort cloud

TL;DR

This paper suggests that a significant fraction of the Oort cloud consists of rocky asteroids formed close to the Sun, which could be detected by LSST and impact our understanding of Solar system history.

Contribution

It introduces simulations showing that about 4% of Oort cloud objects are rocky asteroids formed near the Sun, a novel insight into the cloud's composition.

Findings

Approximately 4% of Oort cloud objects are rocky asteroids.

LSST could detect about a dozen Oort cloud asteroids in ten years.

Impact hazards from these asteroids are rare, occurring roughly once per billion years.

Abstract

The Oort cloud is usually thought of as a collection of icy comets inhabiting the outer reaches of the Solar system, but this picture is incomplete. We use simulations of the formation of the Oort cloud to show that ~4% of the small bodies in the Oort cloud should have formed within 2.5 au of the Sun, and hence be ice-free rock-iron bodies. If we assume these Oort cloud asteroids have the same size distribution as their cometary counterparts, the Large Synoptic Survey Telescope should find roughly a dozen Oort cloud asteroids during ten years of operations. Measurement of the asteroid fraction within the Oort cloud can serve as an excellent test of the Solar system's formation and dynamical history. Oort cloud asteroids could be of particular concern as impact hazards as their high mass density, high impact velocity, and low visibility make them both hard to detect and hard to divert or destroy. However, they should be a rare class of object, and we estimate globally catastrophic collisions should only occur about once per billion years.