The asteroid 2013 SB21 passed by the Earth at a distance of 5 085 000 km (slightly over 13 times the distance between the Earth and the Moon) at about 9.25 am on Tuesday 15 October 2013. There was no danger of the asteroid hitting us, and even if it had of done it would have presented no danger; 2013 SB21 is calculated to be between 7 and 24 m in diameter, and an object of this size would be expected to break up in the atmosphere between 36 and 21 km above the Earth's surface, with only fragmentary material reaching the ground.
The orbit of 2013 SB21. JPL Small Body Database Browser.
2013 SB21 was discovered on 25 September 2013 by he University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2013 SB21 implies that the asteroid was the 527th object discovered in the second half of September 2013 (period 2013 S).
While 2013 SB21 regularly comes near to the Earth, it does not actually cross our orbital path. It has an elliptical three year orbit that takes it from 1.03 AU from the Sun (1.03 times the distance at which the Earth orbits the Sun), slightly outside our orbit to 1.64 AU from the Sun, slightly outside the orbit of 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 TF 135 passes by the Earth, Asteroid 2010 SG15 passes by the Earth, Asteroid 2013 TQ135 flies past the Earth, Asteroid 2013 SE21 passes the Earth and Asteroid 2013 SR passes the Earth.
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