Non-gravitational acceleration indicative of cometary activity of near-Earth object

Abstract

The activity of comets when they approach the Sun usually manifests visibly with the appearance of a dust and gas coma, and in turn non-gravitational accelerations perturbing their motion. However, there exist objects that are morphologically inactive and yet exhibit non-gravitational accelerations. Here we show that astrometry from 1998 to 2025 of near-Earth object (875163) 1998 SH2 reveals orbital perturbations consistent with cometary outgassing. Although no cometary activity was evident in archival images, large-aperture telescopic observations show a weak low-surface-brightness tail and prove that 1998 SH2 is indeed a cometary object. When cometary activity is weak, outgassing may remain undetected for decades. Our results show that non-gravitational perturbations apparent over long astrometric data arcs provide a diagnostic for identifying comets and suggest that more near-Earth objects currently classified as asteroids could in fact be comets. Therefore, our findings prompt searches to detect additional near-Earth objects exhibiting weak cometary activity. The motion of these weak comets can be more strongly perturbed than that of asteroids, influencing assessment of their Earth impact hazard and our understanding of the delivery of water to Earth.