Asteroid 2018 AG4 passed by the Earth at a distance of about 539 000 km (1.40 times the average distance between the Earth and the Moon, or 0.36% of the distance between the Earth and the Sun), slightly after 10.40 am GMT on Wednesday 17 January 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 AG4 has an estimated equivalent diameter of 11-36 m (i.e. it is estimated that a spherical object with the same volume would be 11-36 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 30 and 12 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2018 AG4. Minor Planet Center.
2018 AG4 was discovered on 14 January 2018 (three 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 AG4 implies that it was the 107th asteroid (asteroid G4) discovered in the first half of January 2018 (period 2018 A).
2018 AG4 has a 514 day orbital period and an eccentric orbit tilted at an angle of 10.7° 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 1.62 AU from the Sun (i.e. 162% of the average distance at which the Earth orbits the Sun, and more distant 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 the asteroid has occasional close encounters with the Earth, with the last thought to have occurred in July 2011 and the next predicted in July this year. The asteroid also has occasional close encounters with the planet Mars, with the last thought to have encountered in November 2012 and the next predicted for July this year.
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