Asteroid 2018 XB passed by the Earth at a distance of about 793 000 km (2.06 times the average distance between the Earth and the Moon, or 0.53% of the distance between the Earth and the Sun), before 2.20 pm GMT on Sunday 2 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 XB has an estimated equivalent diameter of 3-12 m (i.e. it is estimated that a spherical object with the same volume would be 3-12 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) in the atmosphere more than 30 km above the ground, with only fragmentary material reaching the Earth's surface.
2018 XB was discovered on 1 December 2018 (the day before its closest approach to the Earth) by the Atlas MLO Telescope at Mauna Loa Observatory in Hawaii. The designation 2018 XB implies that the asteroid was the second object (object B) discovered in the first half of December 2018 (period 2018 X).
2018 XB has an 565 day orbital period and an eccentric orbit tilted at an angle of 4.06° to the plane of the Solar System, which takes it from 0.93 AU from the Sun (i.e. 93% of he average distance at which the Earth orbits the Sun) to 1.73 AU from the Sun (i.e. 173% of the average distance at which the Earth orbits the Sun, further from the Sun than 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 is expected to pass again in September 2021.
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