Asteroid 2018 UO1 passed by the Earth at a distance of about 658 080 km (1.71 times the average distance between the Earth and the Moon, or 0.44% of the distance between the Earth and the Sun), slightly after 7.05 am GMT on Thursday 18 October 2018. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2018 UO1 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 23 and 40 km above the ground, with only fragmentary material reaching the Earth's surface.
2018 UO1 was discovered on 20 October 2018 (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 2018 UO1 implies that the asteroid was the 39th object (object O1) discovered in the second half of October 2018 (period 2018 U).
2018 UO1 has an 1114 day orbital period and an eccentric orbit tilted at an angle of 6.50° to the plane of the Solar System, which takes it from 0.66 AU from the Sun (i.e. 66% of he average distance at which the Earth orbits the Sun, and slightly inside the orbit of the planet Venus) to 3.55 AU from the Sun (i.e. 355% of the average distance at which the Earth orbits the Sun, and more than twice as far from the Sun as the planet Mars). 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). As such the asteroid has occasional close encounters with the planet Earth, which it last came close to in April 1954, as well as Venus, which it last came close to in September this year.
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