Asteroid 2000 AZ93 (264357) passed by the Earth at a distance of about 8 564 000 km (22.3 times the average distance between the Earth and the Moon, or 5.72% of the distance between the Earth and the Sun), slightly before 8.40 pm GMT on Monday 16 December 2019. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2000 AZ93 (264357) has an estimated equivalent diameter of 120-380 m (i.e. it is estimated that a spherical object with the same volume would be 120-380 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 3500-165 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 1-6 km in diameter and devastation on a global scale, as well as climatic effects that would last decades.
The calculated orbit of 2000 AZ93 (264357). JPL Small Body Database.
2000 AZ93 (264357) was discovered on 17 January by the Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research Laboratory in Socorro, New Mexico. The designation 2000 AZ93 implies that it was the 2256th asteroid (asteroid Z93 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that Z93 = 24 + (24 X 93) = 2256) discovered in the sixth and seventh weeks of 2010 (period 2010 C), while the designation 264357 implies that it was 264 357th asteroid ever discovered (asteroids are not given this longer designation immediately to avoid naming double or false sightings).
2000 AZ93 (264357) has a 335 day orbital period, with an elliptical orbit tilted at an angle of 8.58° to the plain of the Solar System which takes in to 0.48 AU from the Sun (48% of the distance at which the Earth orbits the Sun, and slightly more than the distance at which Mercury orbits the Sun) and out to 1.01 AU (1% further away from the Sun than the Earth). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in May 2018 and the next predicted in October 2021. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2000 AZ93 (264357) spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, (494999) 2010 JU39 is also classified as a Potentially Hazardous Asteroid.
2000 AZ93 (264357) also has frequent close encounters with the planets Mercury, which it last came close to in February 2017 and is next predicted to pass in May 2029, and Venus, which it last came close to in June 2011 and is predicted to pass again in May 2024. 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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