Asteroid 2017 KY4 passed by the Earth at a distance of 948 300 km (2.47 times the average distance between the Earth and the Moon, 0.63% of the average distance between the Earth and the Sun), slightly before 3.50 pm GMT on Saturday 20 May 2017. There was no danger of the asteroid hitting us, though had it done so it would have presented no threat. 2017 KY4 has an estimated equivalent diameter of 22-68 m (i.e. it is estimated that a spherical object with the same volume would be 22-68 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) in the atmosphere between 20 and 3 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2017 KY4. Minor Planet Center.
2017 KY4 was discovered on 22 May 2017 (two days after its closest approach to the Earth) by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2017 KY4 implies that the asteroid was the 124th object (object Y4) discovered in the second half of May 2017 (period 2017 K).
2017 KY4 has an 406 day orbital period and an eccentric orbit tilted at an angle of 17.5° to the plane of the Solar System, which takes it from 1.002 AU from the Sun (i.e. 100.2% of he average distance at which the Earth orbits the Sun) to 1.15 AU from the Sun (i.e. 115% of the average distance at which the Earth orbits the Sun). It is therefore classed as an Amor Group Asteroid (an asteroid which comes close to the Earth, but which is always outside the Earth's orbit). This means that close encounters between the asteroid and Earth are fairly common, with the most recent having occurred in November 2007 next predicted in November this year.
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