Asteroid 2018 WZ1 passed by the Earth at a distance of about 109 100 km (0.27 times the average distance between the Earth and the Moon, or 0.07% of the distance between the Earth and the Sun), before 10.30 am GMT on Tuesday 27 November 2018. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2018 WZ1 has an estimated equivalent diameter of 2-7 m (i.e. it is estimated that a spherical object with the same volume would be 2-7 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 36 km above the ground, with only fragmentary material reaching the Earth's surface.
2018 WZ1 was discovered on 28 November 2018 (the day after 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 2018 WZ1implies that the asteroid was the 50th object (object Z1) discovered in the second half of November 2018 (period 2018 W).
2018 WZ1 has an 820 day orbital period and an eccentric orbit tilted at an angle of 6.01° to the plane of the Solar System, which takes it from 0.80 AU from the Sun (i.e. 80% of he average distance at which the Earth orbits the Sun) to 2.63 AU from the Sun (i.e. 263% of the average distance at which the Earth orbits the Sun, much further from the Sun as 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). As such the asteroid has occasional close encounters with the planet Earth, which it last came close to in July 2016, and is expected to pass again in December 2092. The asteroid also has occasional close encounters with the planet Venus, which it is expected to next pass in January 2095 and Mars which it is next predicted to pass in November 2090.
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