Asteroid 2015 LQ21 passed by the Earth at a distance of 4 983 000 km (13.0 times the average distance between the Earth and the Moon, or 3.33% of the average distance between the Earth and the Sun), slightly before 7.00 pm GMT on Thursday 18 June 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented only a minor threat. 2015 LQ21 has an estimated equivalent diameter of 24-75 m (i.e. it is estimated that a spherical object with the same volume would be 24-75 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) in the atmosphere between 20 and 2 km above the ground, with only fragmentary material reaching the Earth's surface, although since an object at the upper end of this range would be expected to release an amount of energy equivalent to about 20 megatons of TNT (roughly 1200 times the energy released by the Hiroshima bomb), then being directly underneath it might be fairly unpleasant.
The calculated orbit of 2015 LQ21. JPL Small Body Database.
2015 LQ21 was discovered on 14 June 2015 (four days before its closest approach to the Earth) by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2015 LQ21 implies that it was the 541st asteroid (asteroid Q21) discovered in the first half of June 2015 (period 2015 L).
While 2015 LQ21 occasionally comes near to the Earth, it does not actually cross our orbital path. It has an elliptical 1231 day orbit, at an angle of 1.42° to the plane of the Solar System, that takes it from 1.02 AU from the Sun (1.02 times the average distance at which the Earth orbits the Sun), slightly outside our orbit, to 3.47 AU from the Sun, (3.47 times the distance at which the Earth orbits the Sun and considerably more than twice the distance at which the planet Mars orbits the Sun). 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 2015 MA passed by the Earth at a distance of 656 700 km (1.71 times the average distance between the Earth and the Moon, or 0.44% of the average distance between the Earth and the Sun), slightly before 2.10 am GMT on Wednesday 17 June 2015. There...
Asteroid (1566) 1949 MA Icarus passed by the Earth at a distance of 8 054 000 km (20.9 times the average distance between the Earth and the Moon, or 5.38 % of the average distance between the Earth and the...
Asteroid 2015 KQ120 passed by the Earth at a distance of 3 279 000 km (8.53 times the average distance between the Earth and the Moon, or 2.19% of the average distance between the Earth and the Sun), slightly after 8.35 pm GMT on Sunday 31 May...
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