Dark Comets? II: Detection of Non-Gravitational Accelerations in a Sample of Inactive Small Bodies

In this paper, we report statistically significant detections of out-of-plane non-gravitational accelerations based on astrometric data in the photometrically inactive objects 1998 KY26, 2005 VL1, 2016 NJ33, 2010 VL65, and 2010 RF12. The magnitudes of the non-gravitational accelerations are greater than those typically induced by the Yarkovsky effect or radiation pressure. Therefore, we hypothe24 size that the accelerations are driven by outgassing and calculate nominal production rates of H2O implied for each object. We attempt to reconcile outgassing induced acceleration with the lack of visible comae or photometric activity via rapid rotation and low levels of gas production. First, we argue that rapidly rotating objects do not retain surface rubble that is not bound by tensile strength. This occurs when the rotational period PRot < PCrit ≃ 2.3 hours for a spherical object with bulk density ρBulk = 1 g cm−3. At these rotational speeds — faster than any measured of cometary nuclei — dust production is strictly set by volatile mass loss modulated by the primordial dust to gas ratio. This yields negligible dust production of ∼ 10−4 g s−1 in each object, comparable to that of (101955) Bennu, which was only detectable in-situ by OSIRIS-REx. These dust production rates correspond to changes in brightness of order ∼ 0.0025%, which are undetectable in extant photometric data. We assess the observability of each of these targets before 2024, and find that the orbit of 1998 KY26 — which is also the target for the extended Hayabusa2 mission — exhibits favorable viewing geometry during this period. This effect should be more apparent in small objects that are easier to spin up, and therefore we expect to find more small active objects without comae in the Solar System small body populations. These results also have direct ramifications on the interpretation of the non-gravitational forces acting on the first interstellar object, 1I/‘Oumuamua.