Asteroid 2021 YB passed by the Earth at a distance of about 555 000 km (1.44 times the average distance between the Earth and the Moon, or 0.37% of the distance between the Earth and the Sun), slightly before 0.20 am GMT on Thursday 23 December 2021. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2021 YB has an estimated equivalent diameter of 6-19 m (i.e. it is estimated that a spherical object with the same volume would be 6-19 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) between 35 and 24 km above the ground, with only fragmentary material reaching the Earth's surface.
2021 YB was discovered on 17 December 2021 (six 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 2021 YB implies that the asteroid was the second object (asteroid B - in numbering asteroids the letters A-Z, excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 26, A2 = 51, etc., which means that B = 2) discovered in the second half of December 2021 (period 2021 Y - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).
2021 YB has a 375 day (1.03 year) orbital period, with an elliptical orbit tilted at an angle of 5.22° 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 1.19 AU (119% of the 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 2021 YB has occasional close encounters with the Earth, with the last thought to have happened in December 2020 and the next predicted in November 2022.
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