https://phys.org/news/2018-07-observatories-team-reveal-rare-asteroid.html

On June 21, the asteroid 2017 YE5 made its closest approach to Earth for at least the next 170 years, coming to within 3.7 million miles (6 million kilometers) of Earth, or about 16 times the distance between Earth and the Moon. On June 21 and 22, observations by NASA's Goldstone Solar System Radar (GSSR) in California showed the first signs that 2017 YE5 could be a binary system. The observations revealed two distinct lobes, but the asteroid's orientation was such that scientists could not see if the two bodies were separate or joined. Eventually, the two objects rotated to expose a distinct gap between them.

Scientists at the Arecibo Observatory in Puerto Rico had already planned to observe 2017 YE5, and they were alerted by their colleagues at Goldstone of the asteroid's unique properties. On June 24, the scientists teamed up with researchers at the Green Bank Observatory (GBO) in West Virginia and used the two observatories together in a bi-static radar configuration (in which Arecibo transmits the radar signal and Green Bank receives the return signal). Together, they were able to confirm that 2017 YE5 consists of two separated objects. By June 26, both Goldstone and Arecibo had independently confirmed the asteroid's binary nature.