Asteroid 2017 KH5 passed by the Earth at a distance of 509 200 km (1.32 times the average distance between the Earth and the Moon, 0.34% of the average distance between the Earth and the Sun), slightly before 4.25 pm GMT on Thursday 25 May 2017. There was no danger of the asteroid hitting us, though had it done so it would have presented no threat. 2017 KH5 has an estimated equivalent diameter of 5-19 m (i.e. it is estimated that a spherical object with the same volume would be 5-19 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 40 and 25 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2017 KH5. Minor Planet Center.
2017 KH5 was discovered on 25 May 2017 (the day before 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 KH5 implies that the asteroid was the 133rd object (object H5) discovered in the second half of May 2017 (period 2017 K).
2017 KH5 has an 862 day orbital period and an eccentric orbit tilted at an angle of 7.85° to the plane of the Solar System, which takes it from 0.80 AU from the Sun (i.e. 80% of he average distance at which the Earth orbits the Sun) to 2.74 AU from the Sun (i.e. 274% of the average distance at which the Earth orbits the Sun, considerably over twice the distance at which the planet Mars orbits the Sun). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer). This means that close encounters between the asteroid and Earth are common, with the last having occurred in May 1981 and the next predicted for October 2064. 2017 KH5 also has occasional close encounters with the planet Mars, with the next predicted for September 2135.
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