Revisiting the trajectory of the interstellar object 'Oumuamua: preference for a radially directed non-gravitational acceleration?

Abstract

I present a re-analysis of the available observational constraints on the trajectory of 'Oumuamua, the first confirmed interstellar object discovered in the solar system. 'Oumuamua passed through the inner solar system on a hyperbolic (i.e., unbound) trajectory. Its discovery occurred after perihelion passage, and near the time of its closest approach to Earth. After being observable for approximately four months, the object became too faint and was lost at a heliocentric distance of around 3 au. Intriguingly, analysis of the trajectory of 'Oumuamua revealed that a dynamical model including only gravitational accelerations does not provide a satisfactory fit of the data, and a non-gravitational term must be included. The detected non-gravitational acceleration is compatible with either solar radiation pressure or recoil due to outgassing. It has, however, proved challenging to reconcile either interpretation with the existing quantitative models of such effects without postulating unusual physical properties for 'Oumuamua (such as extremely low density and/or unusual geometry, non-standard chemistry). My analysis independently confirms the detection of the non-gravitational acceleration. After comparing several possible parametrizations for this effects, I find a strong preference for a radially directed non-gravitational acceleration, pointing away from the Sun, and a moderate preference for a power-law scaling with the heliocentric distance, with an exponent between 1 and 2. These results provide valuable constraints on the physical mechanism behind the effect; a conclusive identification, however, is probably not possible on the basis of dynamical arguments alone.