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

Abstract

We show that the P ∼ 8h photometric period and the astrometrically measured A_(ng) ∼2.5 × 10^(−4) cms^(−2) non-gravitational acceleration (at r ∼ 1.4AU) of the interstellar object 1I/2017 (`Oumuamua) can be explained by a nozzle-like venting of volatiles whose activity migrated to track the sub-solar 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 ∼ 5A_(ng)P^2/4π^2 ∼ 260m. This scale agrees with the independent estimates of `Oumuamua's size that stem from its measured brightness, assuming an albedo of p ∼ 0.1, appropriate to 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 sub-solar 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 ∼100d 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 M_p ∼ M_(Nep) planet or from an exo-Oort cloud.