On 2020 April 29, the near-Earth object (52768) 1998 OR2 experienced a close approach to Earth at a distance of 16.4 lunar distances (LD). 1998 OR2 is a potentially hazardous asteroid of absolute magnitude H = 16.04 that can currently come as close to Earth as 3.4 LD. We report here observations of this object in polarimetry, photometry, and radar. Our observations show that the physical characteristics of 1998 OR2 are similar to those of both M- and S-type asteroids. Arecibo’s radar observations provide a high radar albedo of σˆOC= 0.29 ± 0.08, suggesting that metals are present in 1998 OR2 near-surface. We find a circular polarization ratio of μ _c = 0.291 ± 0.012, and the delay-Doppler images show that the surface of 1998 OR2 is a top-shape asteroid with large-scale structures such as large craters and concavities. The polarimetric observations display a consistent variation of the polarimetric response as a function of the rotational phase, suggesting that the surface of 1998 OR2 is heterogeneous. Color observations suggest an X-complex taxonomy in the Bus–DeMeo classification. Combining optical polarization, radar, and two epochs from the NEOWISE satellite observations, we derived an equivalent diameter of D = 1.80 ± 0.1 km and a visual albedo p _v = 0.21 ± 0.02. Photometric and radar data provide a sidereal rotation period of P = 4.10872 ± 0.00001 hr, a pole orientation of (332.°3 ± 5°, 20.°7 ± 5°), and a shape model with dimensions of (2.08−0.10+0.10,1.93−0.10+0.10,1.60−0.05+0.05) km.

中文翻译：

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近地小行星的表面异质性、物理和形状模型 (52768) 1998 OR2

2020年4月29日，近地天体（52768）1998 OR2以16.4个月球距离（LD）近距离接近地球。1998 OR2 是一颗绝对星等具有潜在危险的小行星H= 16.04 目前可以接近地球 3.4 LD。我们在此报告在偏振测量、光度测量和雷达方面对该物体的观察结果。我们的观察表明，1998 OR2 的物理特征与 M 型和 S 型小行星相似。阿雷西博的雷达观测提供了高雷达反照率 σ^奥克= 0.29 ± 0.08，表明 1998 OR2 近地表存在金属。我们发现圆偏振比为μ _C = 0.291 ± 0.012，延迟多普勒图像显示1998 OR2表面是一颗顶部形状的小行星，具有大陨石坑、凹陷等大规模结构。旋光观测显示旋光响应随旋转相位的变化而变化，表明 1998 OR2 的表面是异质的。颜色观察表明 Bus-DeMeo 分类中存在 X 复合分类法。结合光学偏振、雷达和 NEOWISE 卫星观测的两个历元，我们得出了等效直径D= 1.80 ± 0.1 km 和目视反照率p _v = 0.21 ± 0.02。光度和雷达数据提供的恒星自转周期为磷= 4.10872 ± 0.00001 hr，极方位为 (332.°3 ± 5°, 20.°7 ± 5°)，形状模型尺寸为 （2.08-0.10+0.10,1.93-0.10+0.10,1.60-0.05+0.05） 公里。