The potential for quasi-interstellar objects

A population of interstellar objects (ISOs) may exist that originate from the Solar System, rather than from other stars. Such a foreground would challenge straightforward analysis of the ISO sample expected to be gathered by upcoming sky surveys. We assess whether small bodies unbound from the Solar System can experience dynamical evolution in the Galactic potential that places them on re-encounter trajectories. These ‘quasi-interstellar objects’ (quasi-ISOs) primarily depart the Solar System from the recent (∼ 3 Myr ago) erosion of the outer Oort cloud. After orbiting nearby beyond the tidal radius, they re-encounter the Solar System due to dynamical heating in the Galactic potential. Meanwhile, the primary population of ISOs produced by the Solar System early in its life will be too spread-out in the Galaxy to contribute significantly to the observed sample. Caution will thus be needed only with the slowest detected ISOs: any observed quasi-ISOs will typically have low v_∞ values more like those of long-period comets, providing a reliable means of distinguishing them. We predict that the observed ISO sample will be truly Galactic, with a low rate of quasi-ISOs within the upcoming Vera C. Rubin Observatory’s Legacy Survey of Space and Time.