Asteroid 2016 XO23 passed by the Earth at a distance of about 14 524 000 km (37.8 times the average distance between the Earth and the Moon, or 9.71% of the distance between the Earth and the Sun),slightly after 12.50 pm GMT on Sunday 26 January 2020. There was no danger of the asteroid hitting us, though were it to do so it would have presented a considerable threat. 2016 XO23 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 1850-175 000 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 centuries.
The calculated orbit of 2016 XO23. Minor Planet Center.
2016 XO23 was discovered on 8 December 2016 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 2016 XO23 implies that the asteroid was the 566th object (asteroid O23 - 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 023 = (24 x 23) + 14 = 122) discovered in the first half of December 2016 (period 2016 X).
2016 XO23 has a 1113 day (3.05 year) orbital period, with an elliptical orbit tilted at an angle of 4.36° to the plain of the Solar System which takes in to 0.80 AU from the Sun (80% of the distance at which the Earth orbits the Sun) and out to 3.40 AU (340% of the distance at which the Earth orbits the sun and more than twice as far from the Sun as the planet Mars). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in January 2017 and the next predicted in March 2023. 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, 2016 XO23 is also classified as a Potentially Hazardous Asteroid.
2016 XO23 also has occasional close encounters with the planets Venus, which it last came close to in December 2007 and is next predicted to pass in August 2056, Mars, which it is predicted to pass in May 2062, and Jupiter, which it last came close to in September 1951 and is expected to pass again in January 2046. 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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