Asteroid 2013 TK passed by the Earth at a distance of 18 150 000 km (over 47 times as distant as the Moon) slightly after 3.25 am GMT on Thursday 17 October 2013. There was no danger of the asteroid hitting us, though it if it were to do so (which is highly unlikely) then it would present some risk; 2013 TK is thought to be between 340 and 110 m in diameter, and an object towards the upper end of this range would be capable of penetrating the Earth's atmosphere to arrive at the planet's surface reasonably intact. Such an event would lead to the formation of a crater over a kilometer in diameter, as well as causing devastation over a fairly large region and climatic effects that would be expected to last for several years, though it would not be sufficient to cause a global extinction event.
2013 TK was discovered on 1 October 2013 by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2013 SL20 implies that the asteroid was the 10th object discovered in the first half of October 2013 (period 2013 T).
While 2013 TK 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 1.06 AU from the Sun (1.06 times the distance at which the Earth orbits the Sun), slightly outside our orbit to 3.93 AU from the Sun, considerably 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 SB21 passes the Earth, Asteroid 2013 TF 135 passes by the Earth, Asteroid 2010 SG15 passes by the Earth, Asteroid 2013 TQ135 flies past the Earth and Asteroid 2013 SE21 passes the Earth.
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