Asteroid 2018 HM passed by the Earth at a distance of about 1 096 000 km (2.85 times the average distance between the Earth and the Moon, or 0.72% of the distance between the Earth and the Sun), slightly before 3.50 pm GMT on Sunday 15 April 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 HM has an estimated equivalent diameter of 6-21 m (i.e. it is estimated that a spherical object with the same volume would be 6-21 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 38 and 20 km above the ground, with only fragmentary material reaching the Earth's surface.
2018 HM was discovered on 18 April 2018 (three 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 2018 HM implies that the asteroid was the 12th object (object M) discovered in the second half of April 2018 (period 2018 H).
2018 HM has a 381 day orbital period and an eccentric orbit tilted at an angle of 17.6° to the plane of the Solar System, which takes it from 0.94 AU from the Sun (i.e. 94% of he average distance at which the Earth orbits the Sun) to 1.11 AU from the Sun (i.e. 111% of the average distance at which the Earth 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 the asteroid has occasional close encounters with the Earth, with the last thought to have happened in October 2017 and the next predicted in September this year.
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