Asteroid 2009 UL90 passed the Earth at a distance of 5 855 000 km (about 15.2 times the distance between the Earth and the Moon or 3.91% of the distance between the Earth and the Sun), slightly before 2.55 pm GMT on Monday 12 December 2016. There was no danger of the asteroid hitting us, though if were to do so it would present a significant threat. 2009 UL90 is estimated to be between 360 and 1100 m in diameter, and an asteroid of this size would be expected to pass directly through the atmosphere, striking the Earth's surface and creating a crater between 5 and 15 km in diameter, as well as causing devastation over a wide area and global climatic effects that could last for decades or even centuries.
The calculated orbit of 2008 UL90. Minor Planet Center.
2008 UL90 was discovered on 26 October 2008 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 2009 UL90 implies that the asteroid was the 2261st object (object L90) discovered in the second half of October 2008 (period 2008 UL90).
While 2008 UL90 occasionally comes near to the Earth, it does not actually cross our orbital path. It has an elliptical 212 day orbit, at an angle of 24.3° to the plane of the Solar System, that takes it from 0.43 AU from the Sun (0.46 times the average distance at which the Earth orbits the Sun, slightly inside the orbit of Mercury), to 0.96 AU from the Sun, (0.96 times the distance at which the Earth orbits the Sun). As a Near Earth Object that remains strictly inside the orbit of the Earth it is classed as an Atira Family Asteroid. This means that close encounters between 2008 UL90 and the Earth are very common, with the last having occured in December 2012, and the next predicted for December 2020. The asteroid also has frequent close encounters with the planet Mercury, with the last having occured in December 2014 and the next predicted for November 2017. As an asteroid larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2008 UL90 is also classified as a Potentially Hazardous Asteroid.
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