Asteroid 2018 GJ1 passed by the Earth at a distance of about 15 693 000 km (40.8 times the average distance between the Earth and the Moon, or 10.5% of the distance between the Earth and the Sun), slightly after 2.50 pm GMT on Thursday 17 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 GJ1 has an estimated equivalent diameter of 130-410 m (i.e. it is estimated that a spherical object with the same volume would be 130-410 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 1850 to 175 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 2-7 km in diameter and devastation on a global scale, as well as climatic effects that would last decades or even years.
The calculated orbit of 2018 GJ1. Minor Planet Center.
2018 GJ1 was discovered on 8 April 2018 (39 days before 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 GJ1 implies that it was the 34th asteroid (asteroid J1) discovered in the first half of April 2019 (period 2018 G).
2018 GJ1 has an 885 day orbital period and an eccentric orbit tilted at an angle of 6.80° to the plane of the Solar System, which takes it from 0.81 AU from the Sun (i.e. 81% of he average distance at which the Earth orbits the Sun) to 2.80 AU from the Sun (i.e. 280% of the average distance at which the Earth orbits the Sun, and 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). This means that close encounters between the asteroid and Earth are extremely common, with the last having occurred in October 2013 and the next predicted in September 2030. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2018 GJ1 is also classified as a Potentially Hazardous Asteroid.
2018 GJ1 also has frequent close encounters with the planets Venus, which it is thought to have last passed in October 2008, and is next predicted to pass in August 2047, and Mars, which it last came close to in July 1994 and is next predicted to pass in June 2124). Asteroids which make close passes to multiple planets are considered to be in unstable orbits, and are often eventually knocked out of these orbits by these encounters, either being knocked onto a new, more stable orbit, dropped into the Sun, knocked out of the Solar System or occasionally colliding with a planet.
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