The interstellar object 'Oumuamua as a fractal dust aggregate

TL;DR

This paper proposes that 'Oumuamua's unusual acceleration and spin-down can be explained by radiation pressure acting on a fractal, ultra-low density dust aggregate, supported by observational and mechanical analysis.

Contribution

It introduces the fractal dust aggregate model for 'Oumuamua, explaining its acceleration and spin-down through radiation pressure and analyzing its mechanical stability.

Findings

'Oumuamua's rotation period decreased, consistent with YORP effect on a fractal structure.

A fractal structure with ultra-low density can withstand rotational and tidal forces.

The model supports radiation pressure as the primary acceleration mechanism.

Abstract

The first known interstellar object 'Oumuamua exhibited a nongravitational acceleration that appeared inconsistent with cometary outgassing, leaving radiation pressure as the most likely force. Bar the alien lightsail hypothesis, an ultra-low density due to a fractal structure might also explain the acceleration of 'Oumuamua by radiation pressure (Moro-Martin 2019). In this paper we report a decrease in 'Oumuamua's rotation period based on ground-based observations, and show that this spin-down can be explained by the YORP effect if 'Oumuamua is indeed a fractal body with the ultra-low density of $10^{-2}$ kg m$^{-3}$. We also investigate the mechanical consequences of 'Oumuamua as a fractal body subjected to rotational and tidal forces, and show that a fractal structure can survive these mechanical forces.