Saturday 12 January 2019

Asteroid 2018 XH passes the Earth.

Asteroid 2018 XH passed by the Earth at a distance of about 14 773 000 km (38.5 times the average distance between the Earth and the Moon, or 9.87% of the distance between the Earth and the Sun), slightly before 9.25 am GMT on Saturday 5 January 2019. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2018 XH has an estimated equivalent diameter of 75-240 m (i.e. it is estimated that a spherical object with the same volume would be 75-240 m in diameter), and an object at the upper end of this range would be predicted to be capable of passing through the Earth's atmosphere relatively intact, impacting the ground directly with an explosion that would be about 29 400 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater over 3.5 km in diameter and devastation on a global scale, as well as climatic effects that would last years or even decades.

 The calculated orbit of 2018 XH. JPL Small Body Database.

2018 XH was discovered on 3 December 2018 by the European Space Agency's Optical Ground Station on Tenerife in the Canary Islands. The designation 2018 XH implies that it was the eighth asteroid (asteroid H - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, so that H = 8) discovered in the first half of December 2018 (period 2018 X - the year being split into 24 half-months represented by letters).

2018 XH has an 615 day orbital period and an eccentric orbit tilted at an angle of 1.23° to the plane of the Solar System, which takes it from 0.99 AU from the Sun (i.e. 99% of the the average distance at which the Earth orbits the Sun) to 1.84 AU from the Sun (i.e. 184% of the average distance at which the Earth orbits the Sun, more the distance at which Mars orbits). 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 extremely common, with the last having occurred in May 2014 and the next predicted in March 2024. 2018 XH also has occasional close encounter with the planet Mars, with the last having happened in January 2017. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2018 XH is also classified as a Potentially Hazardous Asteroid.
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