Asteroid 2013 UB1 passed by the Earth at distance of roughly 10 850 000 km (over 28 times the distance between the Earth and the Moon) at about 3.25 am GMT on Monday 6 January 2013. There was no danger of the asteroid hitting us, though had it done so it would have presented a significant threat. 2013 UB1 is estimated to be between 120 and 390 m in diameter, large enough to punch directly through the Earth's atmosphere and impact the planet's surface, resulting in an explosion between 30 000 and 1 750 000 times as powerful as the Hiroshima bomb, creating a crater 1.5-6.0 km in diameter and causing devastation over a wide area, as well as climatic effects that could last for decades.
2013 UB1 was discovered on 21 October 2013 by the University of Hawaii's PANSTARRS telescope on Mount Haleakala. The designation 2013 UB1 implies that it was the 27th asteroid discovered in the second half of October 2013 (period 2013 U).
2013 UB1 has a 3.82 year orbital period and an eccentric orbit tilted to the plain of the Solar System, that takes it from 0.91 AU from the Sun (i.e. 91% of the average distance at which the Earth orbits the Sun) to 3.98 AU from the Sun (i.e. 398% of the average distance at which the Earth orbits the Sun, considerably more than the twice distance at which the planet Mars orbits the Sun). 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).
See also Asteroid 2014 AF16 passes the Earth, Asteroid 2014 AA passes the Earth at a distance of about 570 km, Asteroid 2013 YB14 passes the Earth before being discovered, Asteroid 2013 YB and Asteroid 2012 CL19 passes the Earth.
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