Asteroid 2018 HC1 passed by the Earth at a distance of about 471 700 km (1.23 times the average distance between the Earth and the Moon, or 0.31% of the distance between the Earth and the Sun), slightly before 10.30 pm GMT on Thursday 19 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 HC1 has an estimated equivalent diameter of 8-27 m (i.e. it is estimated that a spherical object with the same volume would be 8-27 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 35 and 18 km above the ground, with only fragmentary material reaching the Earth's surface.
Image of 2018 HC1 taken with the iTelescope T30 500 mm Reflector Deep Field Telescope at Siding Spring Observatory in New South Wales on 21 April 2018. The image is a composite of fifteen five second exposures, the dotted lines being stars which have moved over the course of the exposures and the asteroid the object circled in red at the centre of the image. Steven Tilley/iTelescope/Lagniappe Observing.
2018 HC1 was discovered on 21 April 2018 (two days after its closest approach to the Earth) by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2018 HC1 implies that it was the 28th asteroid (asteroid C1) discovered in the second half of April 2018 (period 2018 H).
2018 HC1 has an 879 day orbital period and an eccentric orbit tilted at an angle of 8.90° to the plane of the Solar System, which takes it from 0.75 AU from the Sun (i.e. 75% of he average distance at which the Earth orbits the Sun, slightly outside the orbit of the planet Venus) to 2.83 AU from the Sun (i.e. 283% of the average distance at which the Earth orbits the Sun, and almost twice as distant 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). This means that close encounters between the asteroid and Earth are common, with the last having occurred in May 1941 and the next predicted in May 2117.
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