SummaryAsteroid 1998 KY26 is a decametre-sized, rapidly rotating body that has been selected as a target for the extended Hayabusa2# mission. Understanding its physical properties is crucial for optimizing mission planning, as it presents both challenges and opportunities for asteroid exploration. This study[1] provides a detailed characterization of 1998 KY26 using a combination of new optical photometric data and radar observations, extending previous analyses of the asteroid's size, spin state, shape, and surface composition[2]. Our results offer new insights into the asteroid's structure and dynamics and highlight the importance of integrating diverse observational techniques to refine the models for small asteroid behavior and non-gravitational forces.Observational Data and MethodologyOur new observations were gathered during the 2024 apparition with the GTC, Gemini South, VLT and Blanco telescopes. For our study, we incorporated the photometric and radar data collected in 1998. Optical data covered a wide range of phase angles to allow for accurate determination of the asteroid's albedo and surface properties. Additionally, the 2024 close approach offered an opportunity to examine potential non-gravitational forces, such as outgassing, which could influence the asteroid's trajectory.Lightcurve inversion techniques were applied to the optical data to refine the asteroid's spin period and shape. The combination of radar data and lightcurve inversion models allowed us to generate a three-dimensional model of 1998 KY26 and derive its size, surface properties, and taxonomic classification.ResultsSpin Period One of the primary results of this study is a revised spin period for 1998 KY26 of 5.35 minutes, significantly shorter than the previously reported value of 10.7 minutes (Figure 1). The reduced spin period is consistent with the asteroid's small size and relatively high rotational stress, which is expected for bodies of this scale.Figure 1: Lightcurve analysis of 1998 KY26 showing the revised spin period of 5.35 minutes, with a comparison to the previous model from Ostro et al. 1999[2]. Size and ShapeUsing the revised spin period and lightcurve data, we derived convex and non-convex shape models which indicates that 1998 KY26 is an irregular, slightly elongated body.We used the new models to fit the 1998 radar data (Figure 2), leading to an updated diameter estimate of 11±2 meters (see Figure 2), which is significantly smaller than the previously reported value of approximately 30 meters. The asteroid's small size presents unique challenges for spacecraft exploration, particularly in terms of navigation and surface operations.Figure 2: Rebinned Doppler-only (cw) observations of 1998 KY26 collected on 8 June 1998, compared with synthetic echoes generated using models derived with lightcurve-inversion methods.Surface Albedo and CompositionFrom the combination of optical photometric data at multiple phase angles and radar observations, we derived a geometric albedo of 0.52±0.08. This high albedo is consistent with that observed in enstatite-rich, Xe-type asteroids, which are known for their bright surfaces.Impact on Mission PlanningThe new spin period, size, and surface properties of 1998 KY26 have significant implications for the extended Hayabusa2# mission. The smaller size of the asteroid will necessitate adjustments to the mission's approach and landing strategies. Additionally, the revised spin period and irregular shape mean that precise navigation will be critical to ensure safe operations near the asteroid.ConclusionThis study provides a comprehensive physical characterization of asteroid 1998 KY26 using a variety of observational techniques. Our results indicate that 1998 KY26 is a small, rapidly rotating body with an irregular shape and a high albedo, consistent with an enstatite-rich composition. We revised its spin period to 5.35 minutes and determined its diameter to be 11±2 meters, which is significantly smaller than previous estimates. We also found no evidence of non-gravitational forces acting on the asteroid, such as outgassing, during the 2024 close approach. These findings provide critical insights into the physical properties of 1998 KY26 and will inform mission planning for the Hayabusa2# spacecraft. Further studies, particularly with regard to non-gravitational forces and surface properties, will be essential for refining our understanding of small, fast-rotating asteroids.References[1] Toni Santana-Ros, Przemyslaw Bartczak, Karri Muinonen et al., "Hayabusa2# mission target 1998 KY26 preview: a small optically bright rapid rotator", 21 January 2025, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-5821856/v1][2] Ostro, S. J., "Radar and Optical Observations of Asteroid 1998 KY26", Science, vol. 285, pp. 557-559, 1999. doi:10.1126/science.285.5427.557.