Asteroid 2017 UL6 passed by the Earth at a distance of about 59 585 km (0.15 times the average distance between the Earth and the Moon, 0.04% of the distance between the Earth and the Sun, and about 14 200 km above satellites in geostationary orbit), slightly before 11.25 am GMT on Saturday 28 October 2017. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2017 UL6 has an estimated equivalent diameter of less than 2 m (i.e. it is estimated that a spherical object with the same volume would be less than 2 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 more than 50 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2017 UL6. Minor Planet Center.
2017 UL6 was discovered on 27 October 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 UL6 implies that the asteroid was the 161st object (object L6) discovered in the second half of October 2017 (period 2017 U).
2017 UL6 has a 504 day orbital period and an eccentric orbit tilted at an angle of 4.05° to the plane of the Solar System, which takes it from 0.88 AU from the Sun (i.e. 88% of he average distance at which the Earth orbits the Sun) to 1.59 AU from the Sun (i.e. 159% of the average distance at which the Earth orbits the Sun, slightly more than the distance at which the planet Mars orbits). 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). Close encounters between the 2017 UL6 and Earth are common, with the last thought to have happened in March 2011 next predicted in July 2018.
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