Asteroid 2021 XC6 passed by the Earth at a distance of about 209 400 km (0.54 times the average distance between the Earth and the Moon, or 0.14% of the distance between the Earth and the Sun), at about 8.10 am GMT on Thursday 16 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 XC6 has an estimated equivalent diameter of 4-14 m (i.e. it is estimated that a spherical object with the same volume would be 4-14 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 28 km above the ground, with only fragmentary material reaching the Earth's surface.
2021 XC6 was discovered on 12 December 2021 (the two 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 XC6 implies that the asteroid was the 153rd object (asteroid C6 - 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 C6 = (25 x 6) + 3 = 153) discovered in the first half of December 2021 (period 2021 X - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).
2021 XC6 has a 306 day (0.84 year) orbital period, with an elliptical orbit tilted at an angle of 9.96° to the plain of the Solar System which takes in to 0.63 AU from the Sun (63% of the distance at which the Earth orbits the Sun and inside the orbit of the planet Venus) and out to 1.17 AU (17% 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 May this year and the next predicted in September 2025. 2021 XC6 also has frequent close encounters with the planet Venus, with the last thought to have occurred in August 2019 and the next predicted for January 2022. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2021 XC6 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid.
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