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

We show that the P similar to 8 hr photometric period and the astrometrically measured Ang similar to 2.5 x 10(-4) cm s(-2) non-gravitational acceleration (at r similar to 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 x b x c ellipsoid, this model produces a pendulum-like rotation of the body and implies a long semi-axis a similar to 5A(ng)P(2)/4 pi(2) similar to 260 m. This scale agrees with the independent estimates of 'Oumuamua's size that stem from its measured brightness, assuming an albedo of p similar to 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 similar to 10% of its total mass during the similar to 100 days span of its encounter with the inner solar system and had an icy composition with a very low [C/O] less than or similar to 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 M-p similar to M-Nep planet, or from an exo-Oort cloud.