File:1999KW4 Arecibo 2019May29-31.png

English: Radar images from Arecibo Observatory on May 29, 30, and 31 unambiguously reveal the binary nature of near-Earth asteroid 66391 (1999 KW4). The images have resolution of 30 meters per pixel in the vertical direction and 0.3 Hz in the horizontal direction and are sums of all data collected at this resolution on each day. The primary component, the larger and wider body at the center of each frame, is more than one kilometer in diameter, while its satellite is about one-third the size of the primary. The width of the echoes depends on the rotation period of the objects: the primary rotates in less than 3 hours, while the rotation of the satellite is likely tidally locked to its orbital period of ~17 hours. The echo of the satellite is also smeared in each frame by its motion in its orbit about the primary. The properties gleaned from the echoes of the two components and their relative orientations will constrain the mass and density of the bodies. As the 1999 KW4 system receded from Earth, the image quality of the data decreased such that later dates required coarser resolution than shown here.