On the Anomalous Acceleration of 1I/2017 U1 ‘Oumuamua

We show that the P ∼ 8 hr photometric period and the astrometrically measured Ang ∼ 2.5 × 10−4 cm s−2 non-gravitational acceleration (at r ∼ 1.4 au) of the interstellar object 1I/2017 ('Oumuamua) can be explained by a nozzle-like venting of volatiles whose activity migrated to track the subsolar location on the object's surface. Adopting the assumption that 'Oumuamua was an elongated a × b × c ellipsoid, this model produces a pendulum-like rotation of the body and implies a long semi-axis $a\sim 5{A}_{\mathrm{ng}}{P}^{2}/4{\pi }^{2}\sim 260\,{\rm{m}}$. This scale agrees with the independent estimates of 'Oumuamua's size that stem from its measured brightness, assuming an albedo of p ∼ 0.1, which is appropriate for ices that have undergone long-duration exposure to the interstellar cosmic-ray flux. Using ray tracing, we generate light curves for ellipsoidal bodies that are subject to both physically consistent subsolar torques and to the time-varying geometry of the Sun–Earth–'Oumuamua configuration. Our synthetic light curves display variations from chaotic tumbling and changing cross-sectional illumination that are consistent with the observations, while avoiding significant secular changes in the photometric periodicity. If our model is correct, 'Oumuamua experienced mass loss that wasted ∼10% of its total mass during the ∼100 days span of its encounter with the inner solar system and had an icy composition with a very low [C/O] ≲ 0.003. Our interpretation of 'Oumuamua's behavior is consistent with the hypothesis that it was ejected from either the outer regions of a planetesimal disk after an encounter with an embedded Mp ∼ MNep planet, or from an exo-Oort cloud.