Asteroid 2011 UG20 passed by the Earth at a distance of about 10 222 000 km (26.6 times the average distance between the Earth and the Moon, or 6.83% of the distance between the Earth and the Sun), slightly after 7.15 pm GMT on Sunday 15 October 2017. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2011 UG20 has an estimated equivalent diameter of 99-310 m (i.e. it is estimated that a spherical object with the same volume would be 99-310 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 225-88 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 1-5 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 2011 UG20. Minor Planet Center.
2011 UG20 was discovered on 18 October 2011 by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2011 UG20 implies that it was the 507th asteroid (asteroid G20) discovered in the second half of October 2011 (period 2011 U).
2011 UG20 has a 436 day orbital period and an eccentric orbit tilted at an angle of 19.0° to the plane of the Solar System, which takes it from 0.35 AU from the Sun (i.e. 35% of he average distance at which the Earth orbits the Sun, somewhat less the distance at which the planet Mercury orbits the Sun) to 1.89 AU from the Sun (i.e. 1.89% 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 close encounters between the asteroid and Earth are extremely common, with the last having occurred in June 2014 and the next predicted in May 2020. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2011 UG20 is also classified as a Potentially Hazardous Asteroid.
2011 UG20 also has frequent close encounters with the planets Mercury, which it is thought to have last passed in October 2010, and is next predicted to pass in August 2034, and Venus, which it last came close to in November 2005 and is next predicted to pass in December 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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