Asteroid 2018 XG4 passed by the Earth at a distance of about 1 046 000 km (2.72 times the average distance between the Earth and the Moon, or 0.70% of the distance between the Earth and the Sun), slightly after 1.25 am GMT on Saturday 15 December 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 XG4 has an estimated equivalent diameter of 5-18 m (i.e. it is estimated that a spherical object with the same volume would be 5-18 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 between 40 and 24 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2018 XG4. Minor Planet Center.
2018 XG4 was discovered on 14 December 2018 (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 2018 XG4 implies that the asteroid was the 107th object (object G4) discovered in the first half of December 2018 (period 2018 X).
2018 XG4 has an 1000 day orbital period and an eccentric orbit tilted at an angle of 6.55° to the plane of the Solar System, which takes it from 0.98 AU from the Sun (i.e. 98% of he average distance at which the Earth orbits the Sun) to 2.93 AU from the Sun (i.e. 2.93% of the average distance at which the Earth orbits the Sun, ,almost twice as far 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).
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