The Shape of (486958) Arrokoth

Abstract

Here we present an updated shape model of (486958) Arrokoth, the bilobate Kuiper Belt Object (KBO) which the NASA New Horizons spacecraft flew past in 2019. This updated shape model uses all of the resolved images of Arrokoth obtained by the New Horizons LOng Range Reconnaissance Imager (LORRI). We developed an updated shape modeling algorithm which allowed the shape and rotational pole of Arrokoth to be fit to much better quality with an efficient use of GPU-accelerated features. The resulting model of Arrokoth's contact binary shape is significantly thicker and of larger volume than the one previously published immediately after the flyby by Spencer et al (2020). We show that Arrokoth's smaller lobe Weeyo is roughly spherical in shape, while the larger lobe Wenu is more flattened, with the volume ratio between the lobes being roughly 2:1. Owing to Wenu's oblate shape, Arrokoth's rotational lightcurve would have significantly lower mean reflectance when viewed from subobserver latitudes that would have shown lightcurve variation. We discuss the impact this may have on estimates of the frequency of contact binaries in the Kuiper Belt. We also discuss the implications of this shape for the formation of Arrokoth, particularly in the context of the Streaming Instability.