Asteroid 2019 UX12 passed by the Earth at a distance of about 381 600 km (0.99 times the average distance between the Earth and the Moon, or 0.26% of the distance between the Earth and the Sun), slightly before 3.10 am GMT on Saturday 26 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 UX12 has an estimated equivalent diameter of 3-11 m (i.e. it is estimated that a spherical object with the same volume would be 3-11 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 30 km above the ground, with only fragmentary material reaching the Earth's surface.
2019 UX12 was discovered on 29 October 2019 (three days after 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 UX12 implies that the asteroid was the 311th 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 X12 = (24 x 12) + 23 = 311) discovered in the second half of October 2019 (period 2010 U).
2019 UX12 has a 375 day orbital period and an eccentric orbit tilted at an angle of 8.22° to the plane of the Solar System, which takes it from 0.74 AU from the Sun (i.e. 74% of he average distance at which the Earth orbits the Sun) to 1.29 AU from the Sun (i.e. 129% of the average distance at which the Earth 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 2019 UX12 occasionally comes close to the Earth, with the last such encounter having happened in October 2018, and the next predicted for October 2020.
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