Sunday 20 May 2018

Asteroid 2018 HL2 passes the Earth.

Asteroid 2018 HL2 passed by the Earth at a distance of about 18 775 000 km (48.9 times the average distance between the Earth and the Moon, or 12.5% of the distance between the Earth and the Sun), slightly before 5.55 pm GMT on Sunday 13 May 2018. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2018 HL2 has an estimated equivalent diameter of 110-340 m (i.e. it is estimated that a spherical object with the same volume would be 110-340 m in diameter), and an object of this size 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 600 to 75 000 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 1.5-5 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 HL2. Minor Planet Center.
2018 HL2 was discovered on 26 April 2018 (17 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 HL2 implies that the asteroid was the 61st object (object L2) discovered in the second half of April 2018 (period 2018 H).   
2018 HL2 has a 788 day orbital period and an eccentric orbit tilted at an angle of 33.5° to the plane of the Solar System, which takes it from 0.89 AU from the Sun (i.e. 89% of he average distance at which the Earth orbits the Sun) to 2.45 AU from the Sun (i.e. 245% of the average distance at which the Earth orbits the Sun, and much 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 an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2018 HL2 is also classified as a Potentially Hazardous Asteroid. 2018 HL2 has occasional close encounters with the planet Earths, with the next predicted for May 2154.
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