Asteroid 2013 SL20 passed by the Earth at a distance of 6 723 000 km (slightly under seventeen and a half times as far away as the Moon), slightly before 12.35 pm on Tuesday 15 October 2013. There was no danger of the asteroid hitting us, and even if it had done so it would have presented only a minor hazard; 2013 SL 20 is estimated to be between 24 m and 75 m in diameter, and an object of this size would be predicted to break up in the atmosphere between 20 and 3 km above the ground, with only fragmentary material reaching the ground, though it is likely that an object towards the upper end of this range would cause considerable localized damage on the Earth's surface.
The calculated orbit of 2013 SL20. JPL Small Body Database Browser.
2013 SL20 was discovered on 25 September 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 511th object discovered in the second half of September 2013 (period 2013 S).
2013 SL20 has a 410 day orbital period and an eliptical orbit that takes it from 0.65 AU from the Sun (i.e. 65% of the average distance at which the Earth orbits the Sun, slightly inside the orbit of Venus) to 1.52 AU from the Sun (i.e. 152% of the average distance at which the Earth orbits the Sun, slightly outside outside the orbit of Mars). 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 2013 TX68 passes the Earth, Asteroid 2013 TT5 passes by the Earth, Asteroid 2013 TL127 flies past the Earth, Asteroid 2013 RN9 to fly past the Earth on Wednesday 9 October 2013 and Asteroid 2013 TO4 to pass the Earth on Tuesday 8 October 2013.
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