Asteroid 2013 SE21 passed the Earth at a distance of 11 370 000 km (slightly under 30 times as distant as the Moon) slightly after 1.00 pm GMT on Saturday 12 October 2013. There was no chance of the object hitting the Earth, and had it done so it would only have presented a minimal risk to us. 2013 SE21 has an estimated diameter of between 31 and 99 m. An object towards the upper end of this range could potentially penetrate the Earth's atmosphere, resulting in the creation of a crater over a kilometer wide and devastation over a fairly wide area, as well as affecting the climate for several years, but it would not be large enough to trigger a global extinction event, and would be more likely to hit the sea or an area of desert or other wilderness than land in the middle of a city.
2013 SE21 was discovered on 27 September 2013 by the University of Hawaii's PANSTARRS telescope on Mount Haleakala. The name 2013 SE21 indicates that it was the 530th such object discovered in the second half of September 2013 (period 2013 S).
While 2013 SE21 occasionally comes near to the Earth, it does not actually cross our orbital path. It has an elliptical 3.9 year orbit that takes it from 1.06 AU from the Sun (1.06 times the distance at which the Earth orbits the Sun), slightly outside our orbit, to 3.9 AU from the Sun, more than twice as far from the Sun as the planet Mars, so unless an encounter with another body causes it's orbital path to alter in a very specific way (highly unlikely) there is no chance of it hitting the Earth. As a Near Earth Object that remains strictly outside the orbit of the Earth it is classed as an Amor Family Asteroid.
See also Asteroid 2013 SR passes the Earth, Asteroid 2013 TH69 passes by the Earth, Asteroid 2013 TN127 flies past the Earth, Asteroid 2013 TT5 passes by the Earth and Asteroid 2013 TL127 flies past the Earth.
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