2025AJ....169..264B

1998 KY₂₆ 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 KY₂₆ 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 KY₂₆ by means of thermal infrared observations. We performed thermal infrared observations of 1998 KY₂₆ 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 KY₂₆ on the resulting images. The upper-limit flux density of 1998 KY₂₆ is derived as 2 mJy at 10.64 μm. From this upper-limit flux density obtained via non-detection, we conclude that the diameter of 1998 KY₂₆ 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 KY₂₆ 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.