(523599) 2003 RM; The Asteroid That Wanted To Be a Comet

We report a statistically signicant 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 nd that the observed plane-of-sky positions cannot be explained with a purely gravity-driven trajectory. Including a transverse non gravitational acceleration allows us to match all observational data, but its magnitude is inconsistent with perturbations typical of asteroids such as the Yarkovsky eect 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-tting H2O sublimation model allows for brightening due to activity consistent with the scatter of the data. We estimate the production rate required for H2O outgassing to power the acceleration, and nd that, assuming a diameter of 300 m, 2003 RM would require Q(H2O) 1023 molec/s at perihelion. We investigate the recent dynamical history of 2003 RM and nd that the object most likely originated in the mid-to-outer main belt ( 86%) as opposed to from the Jupiter-family comet region ( 11%). Further observations, especially in the infrared, could shed light on the nature of this anomalous acceleration.