Asteroid 2019 UL passed by the Earth at a distance of about 477 800 km (1.24 times the average distance between the Earth and the Moon, or 0.31% of the distance between the Earth and the Sun), slightly after 4.15 pm GMT on Wednesday 16 October 2019. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2019 UL has an estimated equivalent diameter of 3-10 m (i.e. it is estimated that a spherical object with the same volume would be 3-10 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 32 km above the ground, with only fragmentary material reaching the Earth's surface.
2019 UL was discovered on 19 October 2019 (three days before its closest encounter with 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 2019 UL implies that the asteroid was the eleventh object (asteroid L - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that L = 11) discovered in the second half of October 2019 (period 2010 U).
2019 UL has a 282 day orbital period, with an elliptical orbit tilted at an angle of 1.92° to the plain of the Solar System which takes in to 0.67 AU from the Sun (67% of the distance at which the Earth orbits the Sun, and slightly less than the distance at which Venus orbits the Sun) and out to 1.01 AU (1% further away from the Sun than the Earth). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in November 2016 and the next predicted in October 2022. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2019 UL spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid. This also means that the asteroid has occasional close encounters with the planet Venus, with the last calculated to have occurred in December 2017, and the next predicted for December next year.
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