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

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 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. 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.

Constraints on the Occurrence of 'Oumuamua-Like Objects

“…The mechanical stability of these dust aggregates subjected to tidal torques around perihelion was demonstrated by Flekkøy et al (2019). Furthermore, Seligman et al (2021) demonstrated that the spin-state and thus, the lightcurve, of objects driven by solar radiation pressure is stable. Moreover, the creation of extremelyporous aggregates would not deplete the mass reservoir of rocky and icy material in extrasolar systems.…”

“…6.2. A Solar System Small Body Analogue Seligman et al (2021) showed that carbon monoxide (CO) outgassing could reconcile the energetics of the non-gravitational acceleration with outgassing. While this solar system volatile could place the ISO within known realms of small bodies, the Spitzer non-detection places strict limits on cometary activity from this substance during the observational window.…”

“…Next, we consider other interpretations for 'Oumuamua in Section 6. While we focus on ones involving propulsion via solar radiation pressure (Bialy & Loeb 2018;Moro-Martín 2019a;Luu et al 2020) to provide contrast with exotic ice outgassing, we also discuss the possibility of CO ice as a solar system analogue (Seligman et al 2021) and the tidal fragmentation explanation (Raymond et al 2018;Zhang & Lin 2020), among others. Finally, in Section 7, we discuss the implications of the detection of 2I/Borisov on these analyses and set broad expectations for VRO if any of these hypotheses for 'Oumuamua are correct.…”

Predicting the water content of interstellar objects from galactic star formation histories

“…It is distinctively rich in CO relative to Solar System comets (Bodewits et al 2020;Cordiner et al 2020). Seligman et al (2021) argue that both of these interstellar objects can be plausibly considered as COenriched planetesimals -if 1I had variable activity, a common state for comets.…”

The Population of Interstellar Objects Detectable with the LSST and Accessible for $\textit{In Situ}$ Rendezvous with Various Mission Designs

“…The non-gravitational acceleration has been postulated to be driven by radiation pressure (Micheli et al 2018), which could be explained if 'Oumuamua was an ultra low-density fractal aggregate (Moro-Martín 2019; Luu et al 2020;Sekanina 2019), or a millimeter-thin artificial light sail (Bialy & Loeb 2018). If the non-gravitational acceleration was powered by cometary outgassing (Micheli et al 2018;Seligman et al 2019), it has been demonstrated that the energetics are consistent with the sublimation of H 2 (Füglistaler & Pfenniger 2015;Seligman & Laughlin 2020;, N 2 , or CO (Seligman et al 2021).…”