Spin Evolution and Cometary Interpretation of the Interstellar Minor Object 1I/2017 'Oumuamua

TL;DR

This paper critically examines the cometary interpretation of 'Oumuamua's non-gravitational acceleration, arguing that its stable rotation and lack of outgassing signs make a cometary nature unlikely.

Contribution

It provides a new analysis linking 'Oumuamua's rotational stability to its non-cometary nature, challenging previous comet-based explanations for its acceleration.

Findings

Rapid spin evolution would occur if outgassing caused the acceleration

'Oumuamua's stable rotation suggests minimal outgassing activity

The cometary interpretation of 'Oumuamua's acceleration is likely incorrect

Abstract

Observations of the first interstellar minor object 1I/2017 'Oumuamua did not reveal direct signs of outgassing that would have been natural if it had volatile-rich composition. However, a recent measurement by Micheli et al (2018) of a substantial non-gravitational acceleration affecting the orbit of this object has been interpreted as resulting from its cometary activity, which must be rather vigorous. Here we critically re-assess this interpretation by exploring the implications of measured non-gravitational acceleration for the 'Oumuamua's rotational state. We show that outgassing torques should drive rapid evolution of 'Oumuamua's spin (on a timescale of a few days), assuming torque asymmetry typical for the Solar System comets. However, given the highly elongated shape of the object, its torque asymmetry is likely higher, implying even faster evolution. This would have resulted in rapid rotational fission of 'Oumuamua during its journey through the Solar System and is clearly incompatible with the relative stability of its rotational state inferred from photometric variability. Based on these arguments, as well as the lack of direct signs of outgassing, we conclude that the classification of 'Oumuamua as a comet (invoked to explain its claimed anomalous acceleration) is questionable.