Asteroid 2013 XU 21 passed the Earth at a distance of 2 364 000 km (roughly 6.13 times the distance between the Earth and the Moon) at about 8.20 am GMT on Saturday 14 December 2013. There was no danger of the asteroid hitting us, and had it done so it would have presented little danger. 2013 XU21 is estimated to be between 11 and 34 m in diameter, and such an object would be expected to break up in the atmosphere between 31 and 10 km above the Earth's surface, with only fragmentary material reaching the ground.
2013 XU21 was discovered on 13 December 2013 (i.e. the day before its closest approach to the Earth) by the University of Arizona's Catalina Sky Survey in the Catalina Mountains north of Tucson. The designation 2013 XU21 implies that it was the 545th asteroid discovered in the first half of December 2013 (period 2013 X).
2013 XU21 has a 3.88 year orbital period and an eliptical orbit that takes it from 0.94 AU from the Sun (i.e. 94% of the distance between the Earth and the Sun) to 4.00 AU from the Sun (i.e. 400% of the average distance at which the Earth orbits the Sun, considerably more than twice the distance at which 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 3023 WV43 passes the Earth, Asteroid 2013 UE3 passes the Earth, Asteroid 2011 JY1 passes the Earth, Asteroid 2013 VX4 passes the Earth and Asteroid 2013 VO4 passes the Earth.
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