Asteroid 2011 UZ255 passed by the Earth at a distance of about 9 942 000 km (25.86 times the average distance between the Earth and the Moon), at about 8.15 pm GMT on Tuesday 1 July 2014. There was no danger of the asteroid hitting us, though were it to do so it would have presented a real threat. 2011 UZ255 has an estimated equivalent diameter of 43-140 m (i.e. it is estimated that a spherical object with the same volume would be 43-140 m in diameter), and an object towards the upper end of this range would be predicted to be capable of passing through the Earth's atmosphere relatively intact, impacting the ground with an energy equivalent to about 68 megatons of TNT (roughly 4000 times the energy of the Hiroshima bomb). Such an event would result in a crater about 2.25 km across, cause devastation over a wide area around the impact site and would have the potential to affect the climate globally for several years.
The calculated orbit of 2011 UZ255. JPL Small Body Database Browser.
2011 UZ255 was discovered on 30 October 2011 by the University of Arizona's Kitt Peak-Spacewatch Project at the Steward Observatory in the Catalina Mountains north of Tucson.. The designation 2011 UZ255 implies that it was the 6400th asteroid (asteroid Z255) discovered in the second half of October 2011 (period 2011 U).
2011 UZ255 has a 1062 day year orbital period and an eccentric orbit that takes it from 0.86 AU from the Sun (i.e. 86% of the average distance at which the Earth orbits the Sun) to 3.22 AU from the Sun (i.e. 322% of the average distance at which the Earth orbits the Sun, 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). Close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in October 2011 and the next predicted in November 2040.
See also...
Asteroid 2014 MV18 passed by the Earth at a distance of about 2 758 000 km (7.17 times the average distance between the Earth and...
Asteroid 2013 XM24 passed by the Earth at a distance of about 16 920 000 km (44.01 times the average distance between the Earth...
Asteroid 1994 CJ1 passed by the Earth at a distance of about 13 570 000 km (35.29 times the average distance between the Earth and the...
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