Asteroid 2020 UU4 passed by the Earth at a distance of about 710 600 km (1.85 times the average distance between the Earth and the Moon, or 0.48% of the distance between the Earth and the Sun), slightly after 3.30 am GMT on Tuesday 27 October 2020. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2020 UU4 has an estimated equivalent diameter of 8-26 m (i.e. it is estimated that a spherical object with the same volume would be 8-26 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) between 35 and 20 km above the ground, with only fragmentary material reaching the Earth's surface.
2020 UU4 was discovered on 23 October 2020 (four days 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 2020 UU4 implies that the asteroid was the 116th object (asteroid U4 - 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 U4 = (24 x 4) + 20 = 116) discovered in the second half of October 2020 (period 2020 U - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).
2020 UU4 has a 795 day (2.18 year) orbital period, with an elliptical orbit tilted at an angle of 4.37° to the plain of the Solar System which takes in to 0.85 AU from the Sun (85% of the distance at which the Earth orbits the Sun) and out to 2.50 AU (2.50% of the distance at which the Earth orbits the Sun, and outside the orbit of 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).
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