Size Constraint on Hayabusa2 Extended Mission Rendezvous Target 1998 KY₂₆ via VLT/VISIR Nondetection

Abstract 1998 KY 26 is a tiny near-Earth asteroid ( H = 26.1) discovered in 1998. It has been selected as the target of the Hayabusa2 extended mission, which will rendezvous with 1998 KY 26 in 2031. However, one of the most basic physical properties, size, remains poorly constrained, posing potential challenges for spacecraft operations. We aimed at constraining the size of 1998 KY 26 by means of thermal infrared observations. We performed thermal infrared observations of 1998 KY 26 using the ESO Very Large Telescope/VISIR on three consecutive nights in 2024 May. After stacking all frames, we find no apparent detection of 1998 KY 26 on the resulting images. The upper-limit flux density of 1998 KY 26 is derived as 2 mJy at 10.64 μ m. From this upper-limit flux density obtained via non-detection, we conclude that the diameter of 1998 KY 26 is smaller than 17 m with thermophysical modeling. This upper limit size is smaller than the radar derived 30 (±10) m. Our size constraint on 1998 KY 26 is essential for the operation of the Hayabusa2 spacecraft during proximity operations using remote sensing instruments as well as a possible impact experiment using the remaining projectile.