Asteroid 2021 WC1 passed by the Earth at a distance of about 203 500 km (0.53 times the average distance between the Earth and the Moon, or 0.14% of the distance between the Earth and the Sun), slightly after 3.00 pm GMT on Sunday 28 November 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 WC1 has an estimated equivalent diameter of 4-12 m (i.e. it is estimated that a spherical object with the same volume would be 4-12 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 42 and 30 km above the ground, with only fragmentary material reaching the Earth's surface.
2021 WC1 was discovered on 27 November 2021 (the day 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 WC1 implies that the asteroid was the 28th object (asteroid C1 - 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 C1 = 25 + 3 = 28) discovered in the second half of November 2021 (period 2021 W - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).
2021 WC1 has a 590 day (1.61 year) orbital period, with an elliptical orbit tilted at an angle of 9.01° to the plain of the Solar System which takes in to 0.81 AU from the Sun (81% of the distance at which the Earth orbits the Sun) and out to 1.94 AU (194% of the distance at which the Earth orbits the Sun, and more than the distance at which the planet Mars 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 WC1 has occasional close encounters with the Earth, with the last thought to have happened in May 2017 and the next predicted in April 2080. 2021 WC1 also has occasional close encounters with the planet Mars, which it last came close to in September 2012, and which it is expected to approach again in December 2023.
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