(523599) 2003 RM: The asteroid that wanted to be a comet

TL;DR

This paper reports the detection of nongravitational acceleration on asteroid 2003 RM, likely caused by unseen cometary outgassing, despite no direct dust evidence, suggesting it may be a transitional object between asteroids and comets.

Contribution

It provides the first evidence of nongravitational acceleration on a small near-Earth asteroid potentially due to sublimation activity, challenging the traditional asteroid-comet distinction.

Findings

Detected significant nongravitational acceleration on 2003 RM.

The acceleration is inconsistent with typical asteroid effects like Yarkovsky.

Dynamical analysis suggests a main belt origin with possible cometary activity.

Abstract

We report a statistically significant detection of nongravitational acceleration on the sub-kilometer near-Earth asteroid (523599) 2003 RM. Due to its orbit, 2003 RM experiences favorable observing apparitions every 5 years. Thus, since its discovery, 2003 RM has been extensively tracked with ground-based optical facilities in 2003, 2008, 2013, and 2018. We find that the observed plane-of-sky positions cannot be explained with a purely gravity-driven trajectory. Including a transverse nongravitational acceleration allows us to match all observational data, but its magnitude is inconsistent with perturbations typical of asteroids such as the Yarkovsky effect or solar radiation pressure. After ruling out that the orbital deviations are due to a close approach or collision with another asteroid, we hypothesize that this anomalous acceleration is caused by unseen cometary outgassing. A detailed search for evidence of cometary activity with archival and deep observations from Pan-STARRS and the VLT does not reveal any detectable dust production. However, the best-fitting H$_2$O sublimation model allows for brightening due to activity consistent with the scatter of the data. We estimate the production rate required for H$_2$O outgassing to power the acceleration, and find that, assuming a diameter of 300 m, 2003 RM would require Q(H$_2$O)$\sim10^{23}$ molec s$^{-1}$ at perihelion. We investigate the recent dynamical history of 2003 RM and find that the object most likely originated in the mid-to-outer main belt ($\sim86\%$) as opposed to from the Jupiter-family comet region ($\sim11\%$). Further observations, especially in the infrared, could shed light on the nature of this anomalous acceleration.