On The Spin Dynamics of Elongated Minor Bodies with Applications to a Possible Solar System Analogue Composition for `Oumuamua

TL;DR

This paper investigates the spin dynamics of interstellar object `Oumuamua, demonstrating that transient jets and radiation pressure can explain its observed behavior, with implications for its composition and activity variations.

Contribution

It provides a detailed analysis of `Oumuamua's spin and non-gravitational acceleration, proposing that surface activity and radiation pressure can account for observations, challenging previous assumptions about its composition.

Findings

Transient jets do not cause a secular spin increase.

Radiation pressure can produce a steady spin-state.

Surface outgassing from modest CO ice coverage can explain acceleration.

Abstract

The first interstellar object, 1I/2017 U1 (`Oumuamua), exhibited several unique properties, including an extreme aspect ratio, a lack of typical cometary volatiles, and a deviation from a Keplerian trajectory. Several authors have hypothesized that the non-gravitational acceleration was caused by either cometary outgassing or radiation pressure. Here, we investigate the spin dynamics of `Oumuamua under the action of high surface area fractional activity and radiation pressure. We demonstrate that a series of transient jets that migrate across the illuminated surface will not produce a secular increase in the spin rate. We produce 3D tumbling simulations that approximate the dynamics of a surface covering jet, and show that the resulting synthetic light curve and periodogram are reasonably consistent with the observations. Moreover, we demonstrate that radiation pressure also produces a steady spin-state. While carbon monoxide (CO) has been dismissed as a possible accelerant because of its non-detection in emission by $\textit{Spitzer}$, we show that outgassing from a surface characterized by a modest covering fraction of CO ice can satisfy the non-ballistic dynamics for a plausible range of assumed bulk densities and surface albedos. $\textit{Spitzer}$ upper limits on CO emission are, however, inconsistent with the CO production necessary to provide the acceleration. Nonetheless, an ad hoc but physically plausible explanation is that the activity level varied greatly during the time that the trajectory was monitored. We reproduce the astrometric analysis presented in Micheli et al. (2018), and verify that the non-gravitational acceleration was consistent with stochastic changes in outgassing.