2017 YE5 is a binary pair of asteroids of approximately equal size and mass, each about 0.9 km (0.56 mi) in diameter. Classified as a near-Earth asteroid and potentially hazardous object of the Apollo group, 2017 YE5 was discovered by amateur astronomer Claudine Rinner at the Oukaïmeden Observatory on 21 December 2017. On 21 June 2018, the pair of asteroids passed within 15.5 lunar distances or approximately 6 million km (3.7 million mi) from Earth. During the close encounter, 2017 YE5 was resolved in high detail by concurrent radar observations by the Arecibo and Green Bank observatories, along with individual observations by the Goldstone Solar System Radar. 2017 YE5 is likely an extinct or dormant comet due to its distant elliptical orbit and dark red surface.
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2017 YE5 in the context of Binary asteroid
A binary asteroid is a system of two asteroids orbiting their common barycenter. The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993. Since then numerous binary asteroids and several triple asteroids have been detected.
The mass ratio of the two components – called the "primary" and "secondary" of a binary system – is an important characteristic. Most binary asteroids have a large mass ratio, i.e. a relatively small satellite in orbit around the main component. Systems with one or more small moons – also called "companions" or simply "satellites" – include 87 Sylvia, 107 Camilla and 45 Eugenia (all triples), 121 Hermione, 130 Elektra (a quadruple), 22 Kalliope, 283 Emma, 379 Huenna, 243 Ida and 4337 Arecibo (in order of decreasing primary size). Some binary systems have a mass ratio near unity, i.e., two components of similar mass. They include 90 Antiope, 2006 VW139, 2017 YE5 and 69230 Hermes, with average component diameters of 86, 1.8, 0.9 and 0.8 km, respectively.