Keck and Gemini characterization of $Hayabusa2\#$ rendezvous target 1998 KY$_{26}$

Abstract

Near-earth object (NEO) 1998 KY$_{26}$ is a target of the $Hayabusa2\#$ spacecraft, which it will rendezvous with in July 2031. The asteroid is a rapid rotator and has a large out-of-plane nongravitational acceleration. We present deep $g$ and $R$ band imaging obtained with the Keck I/Low Resolution Imaging Spectrometer and visible spectroscopy from Gemini North/Gemini Multi-Object Spectrograph taken of 1998 KY$_{26}$ on 2024 June 8-9 when the asteroid was $\sim$0.037 au from the Earth. The asteroid lacks evidence of a dust coma in the deep images and its spectrum most closely resembles Xe-type asteroids, possessing a spectral slope of 6.71$\pm$0.43 $\%$ 100 nm$^{-1}$, and colors $g$-$r$ = 0.63$\pm$0.03, $r$-$i$ = 0.15$\pm$0.03, $i$-$z$ = 0.05$\pm$0.04, and implies a diameter of $\sim$10 m. From our images, we compute a 3$\sigma$ upper limit on the dust production of 1998 KY$_{26}$ of $<$10$^{-5}$ kg s$^{-1}$, $<$10$^{-2}$ kg s$^{-1}$, and $<$10$^{-1}$ kg s$^{-1}$ assuming $\mathrm{\mu}$m, mm, and cm size dust particles. Additionally, we compare the orbit of 1998 KY$_{26}$ and large nongravitational parameters asteroids to NEO population models and find that the majority, including 1998 KY$_{26}$, likely originated from the inner Main Belt, while the second most numerous group originates from the outer Main Belt, followed by a third group originating from the Jupiter Family Comet population. Given its inner Main Belt origin, its Xe-type spectrum, and rapid rotation, we hypothesize that the nongravitational acceleration of 1998 KY$_{26}$ may be caused by the shedding of large dust grains from its surface due to its rotation rather than H$_2$O vapor outgassing.